P le a s e v is it t h e C o m p a n io n W e b s it e t o
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w w w .e m b e d d e d a c a d e m ic .c o m
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B r itis h L ib r a r y C a ta lo g in g -in -P u b lic a tio n D a ta
A catalo g u e reco rd fo r th is b o o k is av ailab le fro m th e British Lib rary
L ib r a r y o f C o n g r e s s C a ta lo g in g -in -P u b lic a tio n D a ta
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Introduction
Microprocessors are everywhere, providing ‘intelligence’ in cars, mobile phones, household
andofficeequipment, televisionsandentertainmentsystems, medical products, aircrafte the
list seems endless. Those everyday products, where a microprocessor is hidden inside to add
intelligence, are called embedded systems.
Not so long ago, designers of embedded systems had to be electronics experts, software
experts, or both. Now, with user-friendly and sophisticated building blocks available for our
use, both the specialist and the beginner can quickly engage in successful embedded system
design. One such building block is the mbed, recently launched by the renowned computer
giant ARM. The mbed is the central theme of this book; through it all the main topics of
embedded systemdesignare introduced. The aimof the book is to teach the key elements of
embedded system design through the use of the mbed.
This book falls into two parts. Chapters 1e10 provide a wide-ranging introduction to
embedded systems, using the mbed and demonstrating how it can be applied to rapidly
produce successful embedded designs. These chapters aimto give full support to the reader,
movingyouthroughaseries of carefully constructedconcepts andexercises. They start from
basic principles and simple projects, and move towards more advanced system design.
Chapters 11e15 build on this foundation, but enter a number of more specialist fields. The
pace here may be a little faster, and you may find that you need to contribute more
background research.
All this book asksof youat thevery beginningis abasic graspof electrical/electronic theory.
Thebook adoptsa‘learningthroughdoing’ approach. To makebestuseof ityouwill needan
mbed, an Internet-connected computer and the various additional electronic components
identified through the book. You will not need every single one of these if you choose not to
do a certain experiment or book section. You will also need a digital voltmeter and ideally
access to an oscilloscope.
Each chapter is based around a major topic in embedded systems. Each has some theoretical
introduction, and may have more theory within the chapter. The chapter then proceeds as
a series of practical experiments. Have your mbed ready to hook up the next circuit, and
download and compile the next example program. Run the program, and aim to understand
xv
xvi
Introduction
whatisgoingon. Asyourmbedconfidencegrows, sowill yourcreativity andoriginality; start
to turn your own ideas into working projects.
You will find that this book rapidly helps you:
to understand and apply the key aspects of embedded systems
to understand and apply the key aspects of the ARM mbed
to learn from scratch, or develop your skills in embedded C/C++programming
to develop your understanding of electronic components and configurations
to understand how the mbed can be applied in some of the most exciting and innovative
intelligent products emerging today
to produce designs and innovations you never thought you were capable of!
If you get stuck, or have questions, support is available for all readers through the book
website and the mbed website, or by email discussion with the authors.
If youareauniversity orcollegeinstructor, thenthisbook offersa‘completesolution’ to your
embedded systems course. Both authors are experienced university lecturers and had your
studentsinmindwhenwritingthisbook. Thebookcontainsastructuredsequenceof practical
and theoretical learning activity. Ideally, you should equip every student or student pair with
an mbed, a prototyping breadboard and a component kit. These are highly portable, so
development work is not confined to the collegeor university laboratory. Laterin thecourse,
students will start networking their mbeds together. Complete Microsoft PowerPoint
presentations for each chapter are available to instructors via the book website, as well as
answers to Quiz questions, and example solution code for the exercises and mini-projects.
Part 1(Chapters1e10) of this book canbeusedto provideacompleteintroductory coursein
embedded system design, with practical examples using the mbed. Part 2 (Chapters 11e15)
can be used to enhance the introductory course, as the basis of further reading or for a more
advanced course.
This book is appropriate for any course or module that wants to introduce the concepts of
embeddedsystems, inahands-onandinterestingway. Becausetheneedforelectronic theory
is limited, the book is accessible to those disciplines that would not normally aim to take
embeddedsystems. Thebookismeantto beaccessibletoYear1undergraduates, althoughwe
expect it will more often be used in the years that follow. Students are likely to be studying
one of the branches of engineering, physics or computer science. The book will also be of
interest to the practicing professional and the hobbyist.
This book is written by Rob Toulson, Research Fellow at Anglia Ruskin University in
Cambridge, and Tim Wilmshurst, Head of Electronics at the University of Derby. After
completing his PhD, Rob spent a number of years in industry, where he worked on digital
signal processing and control systems engineering projects, predominantly in audio and
Introduction xvii
automotive fields. He then moved to an academic career, where his main focus is now in
developing collaborative research between the technical and creative industries. Tim led the
Electronics Development GroupintheEngineeringDepartment of CambridgeUniversity for
a number of years, before moving to Derby. His design career has spanned much of the
history of microcontrollers and embedded systems. Aside from our shared interest in
embedded systems, we share an interest in music, and music technology. The book brings
together our wide range of experiences. Having planned the general layout of the book, and
aftersomeinitial work, wedividedthechapters betweenus. Timtook responsibility formost
of the early ones, and issues relating to electronic and computing hardware. Rob worked
mainly onthelaterchapters, andthemoreadvancedmbedapplications. Thisdivisionof labor
was mainly for convenience, and at publication we both take responsibility for all the
chapters. Because Tim has written several books on embedded systems in the past, a few
background sections and diagrams have been taken from these and adapted for inclusion in
this book. There seemed no point in ‘reinventing thewheel’ where background explanations
were needed.
Acknowledgments
Throughout the writing of this book we have been grateful for the support of the ARM staff
who are creators of the mbed project, notably Simon Ford and Chris Styles. Supporting
experimental workandproof-readinghavebeenconductedby PhillipRichardsonatARUand
Bheki Nyathi at Derby, to whom we are most grateful. A number of smart and innovative
people have contributed libraries of code to the mbed online cookbook; we have referred to
someof theseinthebook. Sincerethanksto thosepeople. Wealso expressourthanksto those
who contribute to the discussion strands on the mbed site. We hope this book responds to
some of your comments and questions. Among thesewe thank Martin Smith for his account
of prototyping an mbed-based project, mentioned in Chapter 15.
xix
CHAPTER 1
Embedded Systems, Microcontrollers
and ARM
Chapter Outline
1.1 Introducing Embedded Systems 3
1.1.1 What is an Embedded System? 3
1.1.2 An Example Embedded System 5
1.2 Microprocessors and Microcontrollers 6
1.2.1 Some Computer Essentials 7
1.2.2 The Microcontroller 8
1.3 Development Processes in Embedded Systems 9
1.3.1 Programming Languages: What is so Special about C/Cþ þ ? 9
1.3.2 The Development Cycle 10
1.4 The World of ARM
11
1.4.1 A Little History 12
1.4.2 Some Technical Detail: What does this RISC Word Mean? 13
1.4.3 The Cortex Core 14
Chapter Review 15
Quiz 15
References 16
1.1 Introducing Embedded Systems
1.1.1 What is an Embedded System?
Weareall familiarwiththeideaof adesktoporlaptopcomputer, andtheamazingprocessing
that they cando. Thesecomputersaregeneral purpose; wecangetthemto do different things
at different times, dependingontheapplicationorprogramwerunonthem. At thevery heart
of suchcomputers is amicroprocessor, atiny andfantastically complicated electronic circuit
which contains the core features of a computer. All of this is fabricated on a single slice of
silicon, called an integrated circuit (IC). Some people, particularly those who are not
engineers themselves, call these circuits microchips, or just chips.
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00001-5
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
3
4 Chapter 1
What is less familiar to many people is the idea that instead of putting a microprocessor in
a general-purpose computer, it can also be placed inside a product which has nothing to do
with computing, like a washing machine, toaster or camera. The microprocessor is then
customizedto control thatproduct. Thecomputeristhere, insidetheproduct; but it cannotbe
seen, and the user probably does not even know it is there. Moreover, those add-ons that are
normally associatedwithacomputer, likekeyboard, screenormouse, arenowhereto beseen.
Such products are called embedded systems, because the computer that controls them is
embedded right inside. Because they tend to focus on control, in many cases the
microprocessors used in embedded systems develop different characteristics from the ones
used in more general-purpose machines. They are called microcontrollers. Although much
less visiblethantheirmicroprocessorcousins, microcontrollerssell infargreatervolumeand
their impact has been enormous. To the electronic and system designer they offer huge
opportunities.
Embedded systems come in many forms and guises. They are extremely common in the
home, the motorvehicle and theworkplace. Most moderndomestic appliances, likewashing
machines, dishwashers, ovens, central heating and burglar alarms, are embedded systems.
Themotorcarisfull of them, inenginemanagement, security (forexample, lockingandantitheft devices), air-conditioning, brakes, radio, and so on. They are found across industry and
commerce, inmachinecontrol, factory automation, robotics, electronic commerceandoffice
equipment. The list has almost no end, and it continues to grow.
Figure1.1expresses theembeddedsystemas asimpleblock diagram. Thereis aset of inputs
from the controlled system. The embedded computer, usually a microcontroller, runs
(User Interface)
TIME
Embedded Computer
Input
Variables
Software
Hardware
Hardware
(Link to other Systems)
Figure 1.1:
The embedded system
Output
Variables
Embedded Systems, Microcontrollers and ARM 5
a program dedicated to this application, permanently stored in its memory. Unlike the
general-purpose desktop computer, which runs many programs, this is the only program it
ever runs. Based on information supplied from the inputs, the microcontroller computes
certain outputs, which are connected to things like actuators within the system. The actual
electronic circuit, along with any electromechanical components, is often called the
hardware; theprogramrunningonit is oftencalledthesoftware. Asidefromall of this, there
may also be interaction with a user, for example via keypad and display, and there may be
interaction with other subsystems elsewhere, although neither of these is essential to the
general concept. Oneothervariablewill affectall thatwedo inembeddedsystems, andthisis
time, representedas adominatingarrowwhichcuts across thefigure. Wewill needto beable
to measuretime, makethingshappenat precisely predeterminedtimes, generatedatastreams
orother signals withastrong timedependence, andrespond to unexpected things inatimely
fashion.
This chapter introduces or reviews many concepts relating to computers, microprocessors,
microcontrollers andembeddedsystems. It does this inoverviewform, to giveaplatformfor
further learning. We return to most concepts in later chapters, building on them and adding
detail. More details can also be found in Reference 1.1.
1.1.2 An Example Embedded System
A snack vending machine is a good example of an embedded system. One is represented in
Figure 1.2, in block diagram form. At its heart is a single microcontroller. As the diagram
shows, this accepts a number of input signals, from the user keypad, the coin-counting
module and the dispensing mechanism itself. It generates output signals dependent on those
inputs.
A hungry customer may approach the machine and start jabbing at the buttons or feeding in
coins. In the first case, the keypad sends signals back to the microcontroller, so that it can
recognizeindividual keyspressedandthenhook thesekey valuestogetherto decipheramore
complex message. The coin-counting module will also send information on the amount of
money paid. The microcontroller will attempt to make deductions from the information it
receives and will output status information on the liquid crystal display. Has a valid product
been selected? Has enough money been paid? If yes, then it will energize an actuator to
dispense the product. If no, it will display a message asking for more money or a re-entry of
theproduct code. A goodmachinewill beready to givechange. If it doesdispense, therewill
besensorsonthemechanismto ensurethattheproductisavailable, andfinally thattheaction
is complete. These are shown in the diagram as gate position sensors. A bad machine (and
haven’t we all met these?) will give annoying or useless messages on its display, demand
more money when you know you’ve given the right amount, or leave that chocolate bar you
desperately want teetering on a knife-edge, not dropping down into the dispensing tray.
6 Chapter 1
Figure 1.2:
Vending machine embedded system
What has been described so far is a fairly conventional machine. We can, however, take it
further. In modern vending systems there might be a mobile communications feature which
allowsthevendingmachineto report directly to themaintenanceteamif afault is diagnosed;
they will thencomeandfix thefault. Similarly, themachinemightreportstock levelsthrough
mobile orInternet communications to allow theserviceteamto visit and replenishwhenever
supplies run low.
This simple example reflects exactly the diagramof Figure 1.1. The microcontroller accepts
inputvariables, makescalculationsanddecisionswiththis, andgeneratesoutputsinresponse.
It doesthis inatimely manner, andcorrect useof timeis implicit intheseactions. Thereis in
this case a user interface, and in a modern machine there is a network interface. While the
above paragraphs seem to be describing the physical hardware of the system, in fact it is all
controlled by the software that the designer has written. This runs in the microcontroller and
determines what the system actually does.
1.2 Microprocessors and Microcontrollers
Let usgo back to themicrocontroller, whichsits at theheart of any embeddedsystem. As the
microcontrollerisinessenceatypeof computer, itwill beuseful forusto getagrasponbasic
computer details. We do this in complete overview here, but return to some of these features
in later chapters.
Embedded Systems, Microcontrollers and ARM 7
1.2.1 Some Computer Essentials
Figure 1.3 shows the essential elements of any computer system. As its very purpose for
existence, a computer can performarithmetic or logical calculations. It does this in a digital
electronic circuit called the arithmetic logic unit (ALU). The ALU is placed within a larger
circuit, called the central processing unit (CPU), which provides some of the supporting
features that it needs. The ALU can undertake a number of simple arithmetic and logic
calculations. Which one it does depends on a code which is fed to it, called an instruction.
Now we are getting closer to what a computer is all about. If we can keep the ALU busy, by
feeding it a sensible sequence of instructions, and also pass it the data it needs to work on,
then we have the makings of a very useful machine indeed.
The ability to keep feeding the ALU with instructions and data is provided by the control
circuit which sits around it. It is worth noting that any one of these instructions performs
a very simple function. However, because the typical computer runs so incredibly fast, the
overall effect is one of very great computational power. The series of instructions is called
aprogram, normally heldinanareaof memory calledprogrammemory. This memory needs
to bepermanent. If itis, thentheprogramisretainedindefinitely, whetherpowerisappliedor
not, and it is ready to run as soon as power is applied. Memory like this, which keeps its
contents whenpoweris removed, is callednon-volatilememory. Theold-fashionednamefor
thisisreadonly memory (ROM). Thislatterterminology isstill sometimesused, eventhough
it is no longer accurate for new memory technology. The control circuit needs to keep
accessingtheprogrammemory, to findoutwhatthenextinstructionis. ThedatathattheALU
works on may be drawn from the data memory, with the result placed there after the
calculationis complete. Usually this is temporary data. This memory typethereforeneednot
bepermanent, althoughthereisno harmif itis. Memory thatlosesitscontentswhenpoweris
removed is called volatile memory. The old-fashioned name for this type of memory is
data and
address
The
Outside
World
Input/
Output
data and
address
Central
Processing
Unit (CPU)
data and
address
ALU
Control
Figure 1.3:
Essentials of a computer
Data
Memory
Program
Memory
8 Chapter 1
random access memory (RAM). This terminology is still used, although it conveys little
useful information.
To be of any use, the computer must be able to communicate with the outside world, and it
does this through its input/output (I/O). On a personal computer this implies human
interaction, through things like the keyboard, visual display unit (VDU) and printer. In an
embeddedsystem, at least asimpleone, thecommunicationis likely to beprimarily withthe
physical world around it, through sensors and actuators. Data coming in from the outside
world might be quickly transferred to the ALU for processing, or it might be stored in data
memory. Data being sent out to the outside world is likely to be the result of a recent
calculation in the ALU.
Finally, there must be data paths between each of these main blocks, as shown by the block
arrows in the diagram. These are collections of wires, which carry digital information in
eitherdirection. Onesetof wirescarriesthedataitself, forexamplefromprogrammemory to
the CPU; this is called the data bus. Theother set of wires carries address information and is
called theaddress bus. Theaddress is adigital numberthat indicates whichplaceinmemory
thedatashouldbestoredin, orretrievedfrom. Thewiresineachof thedataandaddressbuses
could be tracks on a printed circuit board or interconnections within an IC.
One of the defining features of any computer is the size of its ALU. Simple, old processors
were 8 bit, and some of that size still have useful roles to play. This means that, with their
8 bits, they can represent a number between 0 and 255. (Check Appendix A if you are
unfamiliar withbinary numbers.) More recent machines are 32 or 64 bit. This gives themfar
greater processing power, but adds to their complexity. Given an ALU size, it generally
follows that many otherfeatures takethesamesize, forexamplememory locations, databus,
and so on.
Asalready suggested, theCPUhasaninstructionset, whichisasetof binary codesthatitcan
recognize and respond to. For example, certain instructions will require it to add or subtract
two numbers, or storeanumber in memory. Many instructions must also be accompanied by
data, or addresses to data, on which the instruction can operate. Fundamentally, the program
that the computer holds in its program memory and to which it responds is a list of
instructions taken fromthe instruction set, withany accompanying data or addresses that are
needed. Suchcodeis sometimes calledmachinecode, to distinguishit fromotherversions of
the program that we may also develop.
1.2.2 The Microcontroller
A microcontroller takes the essential features of a computer as just described, and adds to
thesethefeaturesthat areneededforit to performitscontrol functions. It is useful to think of
it as being made up of three parts e core, memory and peripherals e as shown in the block
Embedded Systems, Microcontrollers and ARM 9
Power
Reset
Interrupt(s)
Program
Memory
Further
Peripheral
Data
Memory
Further
Peripheral
Microprocessor
Core
Internal Data &
Address Buses
Clock
Figure 1.4:
Features of an example microcontroller: core þ
Digital
I/0
Analog
I/0
Counters
& Timers
memory þ
peripherals
diagram of Figure 1.4. The core is the CPU and its control circuitry. Alongside this go the
program and data memory. Finally, there are the peripherals. These are the elements that
distinguish a microcontroller froma microprocessor, for they are the elements that allow the
wide-ranging interaction with the outside world that the microcontroller needs. Peripherals
canincludedigital oranaloginput/output, serial ports, timers, countersandmany otheruseful
subsystems.
It almost goes without saying, but must not be forgotten, that the microcontroller needs
power, in the formof a stable direct current (DC) supply. A further requirement, as with any
personal computer (PC), is to have a clock signal. This is the signal that endlessly steps the
microcontrollercircuit throughthesequenceof actionsthat itsprogramdictates. Theclock is
often derived from a quartz oscillator, such as you may have in your wristwatch. This gives
a stable and reliable clock frequency. The clock often has a very important secondary
function, of providing essential timing information for the activities of the microcontroller,
for example in timing the events it initiates, or in controlling the timing of serial data.
1.3 Development Processes in Embedded Systems
1.3.1 Programming Languages: What is so Special about C/Cþ þ ?
The CPU instruction set was mentioned in Section 1.2.1. As programmers, it is our ultimate
goal to produce a program that makes the microcontroller do what we want it to do; that
program must be a listing of instructions, in binary, drawn from the instruction set. The
10 Chapter 1
programinthisrawbinary formiswhat wehavealready calledmachinecode. Itisextremely
tedious, to the point of being near impossible, for us as humans to work with the binary
numbers that form the instruction set.
A firststeptowardssanity inprogrammingiscalledAssembler. InAssemblereachinstruction
gets its own mnemonic, a little word which a human can remember and work with. The
instruction set is represented by a set of mnemonics. The program is then written in these
mnemonics, and a computer programcalled a cross-assembler converts all those mnemonics
into the actual binary code that is loaded into program memory. Assembler has its uses, for
one it allows us to work very closely with the CPU capabilities; Assembler programs can
therefore bevery fast and efficient. Yet it is easy to make mistakes in Assembler and hard to
find them, and the programming process is time consuming.
A further step to programming sanity is to use a high-level language (HLL). In this case we
use a language like C, Java or Fortran to write the program, following the rules of that
language. Thenacomputerprogramcalledacompilerreadstheprogramwehavewritten, and
converts(compiles) thatintoalistingof instructionsfromtheinstructionset. Thisassumeswe
have made no mistakes in writing the program! This list of instructions becomes the binary
code that we can download to the program memory. Our needs in the embedded world,
however, arenotlikethoseof otherprogrammers. Wewant to beableto control thehardware
we are working with, and write programs that execute quickly, in predictable times. Not all
HLLs are equally good at meeting these requirements. People get very excited, and debate
endlessly, about what is the best language in the embedded environment. However, most
agreethat, formany applications, C has clearadvantages. Thisis becauseit is simpleandhas
features that allowus to get at thehardwarewhenrequired. A stepupfromC is Cþ þ , which
is also widely used for more advanced embedded applications.
1.3.2 The Development Cycle
This book is about developingembeddedsystems, anddevelopingthemquickly andreliably,
so it is worth getting an early picture of what that development process is all about.
In the early days of embedded systems, the microcontrollers were very simple, without
on-chip memory and with few peripherals. It took a lot of effort just to design and build the
hardware. Moreover, memory was very limited, so programs had to be short. The main
development effort was spent on hardware design, and programming consisted of writing
rather simple programs in Assembler. Over theyears, the microcontrollers became more and
more sophisticated, and memory much more plentiful. By now, many suppliers were selling
predesignedcircuit boards containingthemicrocontrollerandall associated circuitry. Where
these were used there was very little development effort needed for the hardware. Now
attention could be turned to writing complex and sophisticated programs, using all the
Embedded Systems, Microcontrollers and ARM 11
Write/Revise Source Code
Assemble/Compile
(Simulate)
Download
Test in Hardware
Figure 1.5:
The embedded program development cycle
memory that had become available. This tends to be the situation we find ourselves in
nowadays.
Despite these changes, the programdevelopment cycle is still based around the simple loop
shown in Figure 1.5. Inthis book, wewill mainly write source codeusing C. The diagramof
Figure1.5impliessomeof theequipmentwearelikelytoneed.Thesourcecodewill needtobe
writtenonacomputere let’scall itthehostcomputere forexampleaPC, usingsomesortof
text editor. The source codewill need to be converted into the binary machine codewhich is
downloadedtothemicrocontroller, placedwithinthecircuitorsystemthatitwill control (let’s
call this the target system). That conversion is done by the compiler, running on the host
computer. Programdownload to the target systemrequires temporary connection to the host
computer. Thereisconsiderableclevernessinhowtheprogramdataisactually writtenintothe
programmemory, but that need not concern us here. Before the programis downloaded, it is
often possible to simulate it on the host computer; this allows a programto be developed to
agoodlevel beforegoingtothetroubleof downloading. However, thetruetestof theprogram
isto seeitrunningcorrectly inthetargetsystem. Whenprogramerrorsarefoundatthisstage,
asthey inevitably are, theprogramcanberewritten, andcompiledanddownloadedoncemore.
1.4 The World of ARM
The development of computers and microprocessors has at different times been driven
forward by giant corporations or by tiny start-ups; but always it has been driven forward by
12 Chapter 1
very talentedindividualsorteams. Thedevelopmentof ARMhasseenacombinationof all of
these. The full history of the development of ARM is an absolutely fascinating one, like so
many otherhi-techstart-upsinthepast30orsoyears. A tiny summary isgivenbelow; doread
it in a fuller version in one of the several websites devoted to this topic. Then watch that
history continue to unfold in the years to come!
1.4.1 A Little History
In 1981 the British Broadcasting Corporation launched a computer education project and
asked for companies to bid to supply a computer that could be used for this. Thewinner was
the Acorn computer. This became an extremely popular machine, and was very widely used
in schools and universities in the UK. The Acorn used a 6502 microprocessor, made by
a company called MOS Technology. This was a little 8-bit processor, which would not be
taken very seriously these days, but was respected in its time. Responding to the growing
interest in personal or desk-top computers, IBM in 1981 produced its very first PC, based on
a more powerful Intel 16-bit microprocessor, the 8088. There were many companies
producing similar computers at the time, including of course Apple. These early machines
were pretty much incompatiblewith each other, and it was quite unclear whether onewould
finally dominate. Throughout the 1980s, however, the influence of the IBM PC grew, and its
smallercompetitors beganto fade. DespiteAcorn’s UK success it didnot export well, andits
future no longer looked bright.
It was around this time that those clever designers at Acorn made three intellectual leaps.
They wanted to launch a new computer. This would inevitably mean moving on from the
6502, but they just could not find a suitable processor for the sort of upgrade they needed.
Their first leap was the realization that they had the capability to design the microprocessor
itself and did not need to buy it in from elsewhere. Being a small team and experiencing
intense commercial pressure, they designed a small processor, but one with real
sophistication. The computer they built with this, the Archimedes, was a very advanced
machine, but struggled against the commercial might of IBM. The company found
themselveslookingatcomputersaleswhichjustwerenotsufficient, butholdingthedesignof
anextremely clevermicroprocessor. They realizede theirsecondleape thattheirfuturemay
not lie in selling the completed computer itself. Therefore, in 1990, Acorn computers
cofounded another Cambridge company, called Advanced RISC Machines Ltd, ARM for
short. They also began to realize e their third leap e that you do not need to manufacture
silicon to be a successful designer, what mattered was the ideas inside the design. These can
be sold as intellectual property (IP).
The ARM concept continued to prosper, with a sequence of smart microprocessor designs
being sold as IP to an increasing number of major manufacturers around the world. The
company has enjoyed huge success, and is currently called ARM Holdings. Those who buy
Embedded Systems, Microcontrollers and ARM 13
the ARM designs incorporate them into their own products. For example, we will soon see
that thembede the subject of this book e uses theARMCortex core. However, this is to be
found in the LPC1768 microcontroller which sits in the mbed. This microcontroller is not
made by ARM, but by NXP Semiconductors. ARM have sold NXP a license to include the
Cortex core in their LPC1768 microcontroller, which ARM then buy back to put in their
mbed. Got that?
1.4.2 Some Technical Detail: What does this RISC Word Mean?
Because ARM chose originally to place the RISC concept in its very name, and because it
remains acentral featureof ARMdesigns, it is worthcheckingout what RISC stands for. We
haveseenthatany microcontrollerexecutesaprogramwhichisdrawnfromitsinstructionset,
which is defined by the CPU hardware itself. In the earlier days of microprocessor
development, designers were trying to make the instructions set as advanced and
sophisticated as possible. The price they were paying was that this was also making the
computer hardware more complex, expensive and slower. Such a microprocessor is called
acomplexinstructionset computer (CISC). Boththe6502and8088mentionedabovebelong
to theerawhentheCISC approachwasdominant, andareCISC machines. Onecharacteristic
of theCISC approachisthatinstructionshavedifferentlevelsof complexity. Simpleonescan
be expressed in a short instruction code, say one byte of data, and execute quickly. Complex
ones may need several bytes of code to define them, and take a long time to execute.
As compilers improved and high-level computer languages developed, it became less useful
to focus on thecapabilities of theraw instructionset itself. Afterall, if youareprogramming
inahigh-level language, thecompilershouldsolvemostof yourprogrammingproblemswith
little difficulty.
Another approach to CPU design is therefore to insist on keeping things simple, and have
a limited instruction set. This leads to the RISC approach e the reduced instruction set
computer. The RISC approach looks like a ‘back to basics’ move. A simple RISC CPU can
execute code rapidly, but it may need to execute more instructions to complete a given task,
comparedtoitsCISCcousin. Withmemory becomingevercheaperandof higherdensity, and
more efficient compilers for programcodegeneration, this disadvantage is diminishing. One
characteristicof theRISC approachisthateachinstructioniscontainedwithinasinglebinary
word. That word must hold all information necessary, including the instruction code, as well
as any address or data information needed. A further characteristic, an outcome of the
simplicity of theapproach, isthatevery instructionnormally takesthesameamountof timeto
execute. This allows other useful computer design features to be implemented. A good
example is pipelining, anapproach by which as one instructionis being executed, the next is
already being fetched frommemory. It is easy to do this with a RISC architecture, where all
(or most) instructions take the same amount of time to complete.
14 Chapter 1
An interesting subplot to the RISC concept is the fact that, owing to their simplicity, RISC
designs tend to lead to low power consumption. This is hugely important for anything that is
battery powered, andhelpsto explainwhy ARMproductsfindtheirway into so many mobile
phones.
1.4.3 The Cortex Core
The Cortex microprocessor core is a 32-bit device, and follows a long line of distinguished
ARM processors. A very simplified block diagram of one version, the M3, is shown in
Figure 1.6. This diagram allows us to see again some computer features we have already
identified, and add some new ideas. Somewhere in the middle you can see the ALU, already
described as the calculating heart of the computer. The instruction codes are fed into this
through the Instruction fetch mechanism, drawing instructions in turn from the program
memory. Pipeliningisappliedtoinstructionfetch, sothatasoneinstructionisbeingexecuted,
thenextisbeingdecoded, andtheoneafterisbeingfetchedfrommemory. Asitexecuteseach
instruction, the ALU simultaneously receives data from memory and/or transfers it back to
memory. This happens throughtheinterfaceblocks seen. Thememory itself is not part of the
Cortex core. The ALU also has a block of registers associated with it. These act as a tiny
chunk of local memory, which can be accessed quickly and used to hold temporary data as
Interrupts
Interrupt
Controller
Decoder
Register
Bank
ALU
Test
Instruction
Fetch
Memory Interface
and
Debug
Interface
Memory Protection Unit
Bus Interconnect
Memory and Peripherals
Figure 1.6:
The Cortex-M3 core, simplified diagram
Test Data
Embedded Systems, Microcontrollers and ARM 15
acalculationisundertaken. TheCortex corealsoincludesaninterruptinterface. Interruptsare
an important feature of any computer structure. They are external inputs that can be used to
force the CPU to divert fromthe programsection it is currently executing and jump to some
other sectionof code. The interrupt controllermanages thevarious interrupt inputs. It should
not be too difficult to imagine this microprocessor core being dropped into the
microcontroller diagram of Figure 1.4.
There are several versions of the Cortex. The Cortex-M4 is the ‘smartest’ of the set, with
digital signal processing capability. The Cortex-M3, the one we will be using, is targeted at
embedded applications, including automotive and industrial. The Cortex-M1 is a small
processor intended to be embedded in a field programmable gate array (FPGA) e a digital
electronic circuit onachip, whichis configuredevery timepoweris applied, andcanevenbe
reconfigured during operation if needed. The Cortex-M0 is the simplest of the group, with
minimum size and power consumption.
We will be meeting the Cortex-M3 core, as it is the one used in the mbed. A very detailed
guideto thiscoreisgiveninReference1.2. Do not, however, try readingthisbook unlessyou
are really keen to get into the fine detail; otherwise, leave it alone e it is very complex!
Chapter Review
• An embedded system contains one or more tiny computers, which control it and give it
a sense of intelligence.
• The embedded computer usually takes the form of a microcontroller, which combines
microprocessor core, memory and peripherals.
• Embedded system design combines hardware (electronic, electrical and electromechanical) and software (program) design.
• The embedded microcontroller has an instruction set. It is the ultimate goal of the
programmer to develop code which is made up of instructions from this instruction set.
• Most programmingisdoneinahigh-level language, withacompilerbeingusedto convert
that program into the binary code, drawn from the instruction set and recognized by the
microcontroller.
• ARM has developed a range of effective microprocessor and microcontroller designs,
widely applied in embedded systems.
Quiz
1. Explain the following abbreviations: IC, I/O, ALU, CPU.
2. Describe an embedded system in less than 100 words.
3. What are the differences between a microprocessor and a microcontroller?
16 Chapter 1
4. What range of numbers can be represented by a 16-bit ALU?
5. Whatisa‘bus’ inthecontextof embeddedsystems? Describetwo typesof busthatmight
be found in an embedded system.
6. Describe the term ‘instruction set’ and explain how use of the instruction set differs for
high- and low-level programming.
7. What are the main steps in the embedded program development cycle?
8. Explain the terms RISC and CISC and give advantages and disadvantages for each.
9. What is pipelining?
10. What did the acronym and company name ARM stand for?
References
1.1. Wilmshurst, T. (2001). An Introduction to the Design of Small-Scale Embedded Systems. Palgrave.
1.2. Yiu, J. (2010). The Definitive Guide to the ARM Cortex-M3. 2nd edition. 2010 Newnes.
CHAPTER 2
Introducing the mbed
Chapter Outline
2.1 Introducing the mbed 17
2.1.1 The mbed Architecture 19
2.1.2 The LPC1768 Microcontroller 21
2.2 Getting Started with the mbed: A Tutorial 22
Step 1. Connecting the mbed to the PC 22
Step 2. Creating an mbed Account 23
Step 3. Running a Program 23
Step 4. Compiling the Program 24
Step 5. Downloading the Program Binary Code 25
Step 6. Modifying the Program Code 25
2.3 The Development Environment 25
2.3.1 The mbed Compiler and API 25
2.3.2 Using C/C++ 26
Chapter Review 26
Quiz 27
References 27
2.1 Introducing the mbed
Chapter 1 reviewed some of the core features of computers, microprocessors and
microcontrollers. Now we are going to apply that knowledge and enter the main material of
this book, a study of the ARM mbed.
In very broad terms, the mbed takes a microcontroller, such as we saw in Figure 1.4, and
surrounds it with some very useful support circuitry. It places this on a conveniently sized
littleprintedcircuitboard(PCB) andsupportsitwithanonlinecompiler, programlibrary and
handbook. Thisgivesacompleteembeddedsystemdevelopmentenvironment, allowingusers
to develop and prototype embedded systems simply, efficiently and rapidly. Fast prototyping
is one of the key features of the mbed approach.
Thembedtakestheformof a2 inchby 1 inch(53 mmby 26 mm) PCB, with40pinsarranged
intwo rows of 20, with0.1inchspacingbetweenthepins. This spacingis astandardinmany
electronic components. Figure2.1 shows different mbed views. Lookingat the main features,
labeled in Figure 2.1b, we see that the mbed is based around the LPC1768 microcontroller.
This is made by a company called NXP semiconductors, and contains an ARM Cortex-M3
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00002-7
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
17
18 Chapter 2
(a)
(b)
NXP LPC1768
Microcontroller
(c)
Figure 2.1:
The ARM mbed. (Image reproduced with permission of ARM Holdings)
core. Program download to the mbed is achieved through a universal serial bus (USB)
connector; thiscanalso powerthembed. Usefully, therearefivelight-emittingdiodes (LEDs)
ontheboard, oneforstatusandfourthatareconnectedto fourmicrocontrollerdigital outputs.
Theseallow aminimumsystemto betestedwith no external component connections needed.
A reset switch is included, to force restart of the current program.
Introducing the mbed 19
Thembedpinsareclearly identifiedinFigure2.1c,providingasummary of whateachpindoes.
Inmany instances thepins aresharedbetweenseveral features to allowanumberof design
options. Topleftwecanseethegroundandpowersupply pins. Theactual internal circuitruns
from3.3V. However, theboardacceptsany supply voltagewithintherange4.5 to9.0V, while
anonboardvoltageregulatordrops this to therequiredvoltage. A regulated3.3V output
voltageisavailableonthetoprightpin, witha5 Voutputonthenextpindown. Theremainder
of thepinsconnecttothembedperipherals.Thesearealmostall thesubjectof laterchapters; we
will quickly overviewthemhere, thoughthey may havelimitedmeaningtoyounow. Thereare
no fewerthanfiveserial interfacetypesonthembed: I2C, SPI, CAN, USB andEthernet. Then
thereis aset of analoginputs, essential forreadingsensorvalues, andaset of PWMoutputs
useful forcontrol of external powerdevices, forexampleDC motors. Whilenot immediately
evident fromthefigure, pins 5 to 30 canalso beconfiguredforgeneral digital input/output.
Thembedisconstructedto alloweasy prototyping, whichisof courseitsvery purpose. While
the PCB itself is very high density, interconnection is achieved through the very robust and
traditional dual-in-line pin layout.
Background information for the mbed and its support tools can be found at the mbed home
page(Reference2.1). Whilethisbook isintendedto giveyouall informationthatyouneedto
start work with the mbed, it is inevitable that you will want to keep a close eye on this site,
withitscookbook, handbook, blogandforum. Aboveall else, itprovidestheentry pointtothe
mbed compiler, through which you will develop all your programs.
2.1.1 The mbed Architecture
A block diagramrepresentationof thembedarchitectureisshowninFigure2.2. Itispossible,
and useful, to relate the blocks shown here to the actual mbed. At the heart of the mbed is
theLPC1768microcontroller, clearly seeninboth Figures 2.1and2.2. Thesignal pins of the
mbed, as seen in Figure 2.1c, connect directly to the microcontroller. Thus, when in the
coming chapters we use an mbed digital input or output, or the analog input, or any other of
the peripherals, we will be connecting directly to the microcontroller within the mbed, and
relyingonitsfeatures. Aninterestingasidetothis, however, isthattheLPC1768has100pins,
butthembedhasonly 40. Thus, whenwegetdeeperintounderstandingtheLPC1768, wewill
find that there are some features that are simply inaccessible to us as mbed users. This is,
however, unlikely to be a limiting factor.
Thereis asecondmicrocontrolleronthembed, whichinterfaces withtheUSB. Thisis called
the interface microcontroller in Figure 2.2, and is the largest integrated circuit (IC) on the
underside of the mbed PCB. The cleverness of the mbed hardware design is the way which
this device manages the USB link and acts as a USB terminal to the host computer. In most
commonuseit receives programcodefiles throughtheUSB, andtransfers thoseprograms to
a 16 Mbit memory, which acts as the ‘USB disk’. When a program ‘binary’ is downloaded
20 Chapter 2
USB
MBED
4.5V-9.0V in (“VIN”)
5.0V USB out (“VU”)
3.3V out (“VOUT”)
0V (“GND”)
USB
power
Power
Management
reset
Flash Memory
“USB Disk”
16 Mbit
Interface
Microcontroller
reset
UART
JTAG
3.3V internal
supply
Mbed external
signal
connections
LPC1768
microcontroller
Mbed external
signal
connections
Ethernet PHY
Ethernet
Key
PHY: Physical interface
JTAG: Joint Test Action Group (interface)
UART: Universal Asynchronous Receiver/Transmitter
Figure 2.2:
Block diagram of mbed architecture.
to the mbed, it is placed in the USB disk. Whenthe reset button is pressed, the programwith
the latest timestamp is transferred to the flash memory of the LPC1768, and program
execution commences. Data transfer between interface microcontroller and the LPC1768
goes as serial data through the UART (which stands for universal asynchronous receiver/
transmitter e a serial data link, let’s not get into the detail now) port of the LPC1768.
The ‘Power Management’ unit is made up of two voltage regulators, which lie either side of
the status LED. There is also a current-limiting IC, which lies at the top left of the mbed.
The mbed can be powered from the USB; this is a common way to use it, particularly for
simple applications. For more power-hungry applications, or those that require a higher
voltage, it canalso bepoweredfromanexternal 4.5to 9.0 V input, suppliedto pin2(labeled
VIN). Power can also be sourced from mbed pins 39 and 40 (labeled VU and VOUT,
respectively). The VU connection supplies 5 V, taken almost directly from the USB link;
henceit is only availableif theUSB is connected. TheVOUT pinsupplies aregulated3.3 V,
which is derived either from the USB or from the VIN input.
For those who are inclined, the mbed circuit diagrams are available on the mbed website
(Reference 2.2).
Introducing the mbed 21
2.1.2 The LPC1768 Microcontroller
A block diagram of the LPC1768 microcontroller is shown in Figure 2.3. This looks
complicated, and we do not want to go into all the details of what is a hugely sophisticated
digital circuit. However, the figure is in a way the agenda for this book, as it contains all the
capability of the mbed, so it is worth getting a feel for the main features. If you want to
Up to 64 KB
SRAM
Up to 512 KB
FLASH
Test/
Debug
CPU PLL
Nested
VIC
Trace
Brown Out Detect
SRAM
Controller
FLASH
Accelerator
Cortex-M3
Core
MPU
Power On Reset
Multi-layer AHB Matrix
Ethernet MAC
3 x I2C
FM+
DMA
3x
SSP/SPI
USB Host/OTG/D
PHY
PLL
4 x UARTs
RS485/IrDA/Modem
I2S
DMA
GP DMA
2x
CAN2.0B
Advanced Peripheral Bus
12-bit/8-ch
ADC
10-bit
DAC
4 x 32-bit
Timers
Motor
Control PWM
KEY
ADC: Analog-to-Digital Converter
AHB: Advanced High-performance Bus
CPU: Central Processing Unit
D:
Device (USB)
DAC: Digital-to-Analog Converter
DMA: Direct Memory Access
FMþ: Fast-Mode Plus (I2C)
GP: General Purpose (DMA)
I2C: Inter-Integrated Circuit
I2S: Inter-Integrated circuit Sound
IrDA: Infrared Data Association
MAC: Media Access Control
MPU: Memory Protection Unit
PHY: Physical layer
PLL: Phase-Locked Loop
OTG: On-The-Go (USB)
SPI:
Serial Peripheral Interface
SRAM: Static RAM
SSP: Synchronous Serial Port
UART: Universal Asynchronous Receiver/Transmitter
VIC: Vectored Interrupt Controller.
Note: The LPC1768 has 64 KB of SRAM and 512 KB of flash.
The Cortex core is made up of those items enclosed within the dashed line.
See also key to Figure 2.1 (Reproduced from mbed NXP LPC1768 prototyping board. 2009. NXP BV.
Document no. 9397 750 16802)
Figure 2.3:
The LPC1768 block diagram.
Quad Encoder
Interface
brb276
22 Chapter 2
get complete detail of this microcontroller, then consult one or more of References 1.2, 2.3
and 2.4. Although these references are mentioned from time to time in the book, consulting
them is not necessary for a complete reading of the book.
Remember that a microcontroller is made up of microprocessor core plus memory plus
peripherals, as shown in Figure 1.4. Let’s look for these. Top center in Figure 2.3, contained
within the dotted line, is the core of this microcontroller, the ARM Cortex-M3. This is
a compressed version of Figure 1.6, the M3 outline that was considered in Chapter 1.
To theleftof thecorearethememories: theprogrammemory, madewithFlashtechnology, is
usedforprogramstorage; to theleft of that is thestatic RAM(randomaccess memory), used
for holding temporary data. That leaves most of the rest of the diagram to show the
peripherals, which give the microcontroller its embedded capability. These lie in the center
and lower half of the diagram, and reflect almost exactly what the mbed can do. It is
interesting to compare the peripherals seen here with the mbed inputs and outputs seen in
Figure2.1c. Finally, all thesethingsneedto beconnectedtogether, atask doneby theaddress
and data buses. Clever though they are, we have almost no interest in this side of the
microcontroller design, at least not for this book. It is sufficient to note that the peripherals
connect through something called the advanced peripheral bus. This in turns connects back
throughabus interconnect calledthe advanced high-performancebus matrix, andfromthere
to the central processing unit (CPU). This interconnection is not completely shown in this
diagram, and we have neither need nor wish to think about it further.
2.2 Getting Started with the mbed: A Tutorial
Now comes the big moment when you connect the mbed for the first time, and run a first
program. We will follow the procedure given on the mbed website and use the introductory
program on the compiler, a simple flashing LED example. You will need:
• an mbed microcontroller with its USB lead
• a computer running Windows (XP, Vista or 7), Mac OS X or GNU/Linux
• a web browser, for example Internet Explorer or Firefox.
Now follow the sequence of instructions below. The purpose of this tutorial is to explain the
main steps in getting a program running on the mbed. We will look into the detail of the
program in the next chapter.
Step 1. Connecting the mbed to the PC
ConnectthembedtothePCusingtheUSB lead. TheStatuslightwill comeon, indicatingthat
the mbed has power. After a few seconds of activity, the PC will recognize the mbed as
astandardremovabledrive, anditwill appearonthedeviceslinkedtothecomputer, asseenin
Figure 2.4.
Introducing the mbed 23
Figure 2.4:
Locating the mbed: (a) Windows XP example; (b) Mac OS X example
Step 2. Creating an mbed Account
OpentheMBED.HTM filefoundonthembedinyourwebbrowserandclick theCreateanew
mbed Account link. Follow the instructions to create an mbed Account. This will lead you to
the website, as seen in Figure 2.5. From here you can link to the compiler, libraries and
documentation.
Step 3. Running a Program
Open the compiler using the link in the site menu, i.e. at the right of Figure 2.5. By doing
this you enter your allocated personal program workspace. The compiler will open in a new
tab or window. Follow these steps to create a new program:
• As seen in Figure 2.6a, right-click (Mac users, Ctrl-click) on ‘My Programs’ and select
‘New Program . ’.
• Chooseandenteranameforthenewprogram(forexampleProg_Ex_2_1) andclick ‘OK’.
Do not leave spaces in the program name.
• Your new program folder will be created under ‘My Programs’.
Click onthe‘main.cpp’ fileinyour new programto openit inthe fileeditor window, as seen
in Figure 2.6b. This is the main source code file in your program. Whenever you create
Figure 2.5:
The mbed home page
24 Chapter 2
Figure 2.6:
Opening a new program: (a) selecting New Program; (b) opening the main source file
a new program it always contains the same simple code. This is shown here as Program
Example 2.1. This program will be examined in the next chapter.
The other item in the program folder is the ‘mbed’ library. This provides all the functions
used to start up and control the mbed, such as the DigitalOut interface used in this
example.
/* Program Example 2.1: Simple LED flashing
*/
#include "mbed.h"
DigitalOut myled(LED1);
int main() {
while(1) {
myled = 1;
wait(0.2);
myled = 0;
wait(0.2);
}
}
Program Example 2.1 Simple LED flashing
Step 4. Compiling the Program
To compile the program, click the ‘Compile’ button in the toolbar. This will compile all the
source code files within the programfolder to create the binary machine code which will be
downloaded to the mbed. Typically, this is the single program you have written, plus the
library calls you have almost certainly made. After a successful compile, you will get
a ‘Success!’ message in the compiler output, and a popup will prompt you to download the
compiled .bin file to the mbed.
Introducing the mbed 25
Of course, withthisgivenprogram, itwouldbemostsurprisingtofindithadanerrorinit; you
will not be so lucky with future programs! Try inserting a small error into your source code,
forexampleby removingthesemi-colonattheendof aline, andcompilingagain. Noticehow
the compiler gives a useful error message at the bottom of the screen. Correct the error,
compile again and proceed. The type of error you have just inserted is often called a syntax
error. Thisisanerrorwhichrelatestotherulesof writinglinesof Ccode. Whenasyntax error
is found, the compiler is unable to proceed with the compilation, as it perceives that the
programhassteppedoutsidetherulesof thelanguage, andhencecannot reliably interpretthe
code that is written.
Step 5. Downloading the ProgramBinary Code
After a successful compile, the program code, in binary form, can be downloaded to the
mbed. Save it to the location of the mbed drive. You should see the Status LED (as seen in
Figure2.1) flashas theprogramdownloads. OncetheStatus LED has stoppedflashing, press
the reset button on the mbed to start your program running. You should now see LED1
flashing on and off every 0.2 seconds.
Step 6. Modifying the ProgramCode
In the main.cpp file, simply change the DigitalOut statement to read:
DigitalOut myled(LED4);
Now compile and download your code to the mbed. You should now see that LED4 flashes
instead of LED1. You can also change the pause between flashes by modifying the values
bracketed in the wait( ) command.
2.3 The Development Environment
As Section 1.3 suggests, there are many different approaches to development in embedded
systems. With the mbed there is no software to install, and no extra development hardware
needed for program download. All software tools are placed online, so that you can compile
anddownloadwhereveryouhaveaccessto theInternet. Notably, thereisaCþ þ compilerand
an extensive set of software libraries, used to drive the peripherals. Thus, there is no need to
write code to configure peripherals, which in some systems can be very time consuming.
2.3.1 The mbed Compiler and API
The mbed development environment uses the ARM RVDS (RealView Development Suite)
compiler, currently Version 4.1. All features of this compiler relevant to the mbed are
available through the mbed portal.
26 Chapter 2
One thing that makes the mbed special is that it comes with an application programming
interface (API). In brief, this is the set of programming building blocks, appearing as Cþ þ
utilities, which allow programs to be devised quickly and reliably. Therefore, we will be
writingcodeinC orCþ þ , but drawingonthefeatures of theAPI. Youwill meet most of the
features of this as you work through the book. Note that you can see all components of the
API by opening the mbed handbook, linked from the mbed home page.
2.3.2 Using C/C++
Asjustmentioned, thembeddevelopmentenvironmentusesaCþ þ compiler. Thatmeansthat
all fileswill carrythe.cpp(Cplusplus)extension.C,however,isasubsetof Cþ þ ,andissimpler
tolearnandapply. Thisisbecauseitdoesnotusethemoreadvanced‘object-oriented’ aspectsof
Cþ þ . Ingeneral, C codewill compileonaCþ þ compiler, but not theotherway round.
C isusually thelanguageof choiceforany embeddedprogramof lowormediumcomplexity,
so will suit us well in this book. For simplicity, therefore, we aim to use only C in the
programswedevelop. Itshouldberecognized, however, thatthembedAPI iswritteninCþ þ
anduses thefeaturesof that languageto thefull. Wewill aimto outlineany essential features
when we come to them.
This book does not assume that you have any knowledge of C or Cþ þ , although
youhaveanadvantageif youdo. Weaimto introduceall newfeatures of C as they
come up, flagging this by using the symbol alongside. If you see that symbol and
you are a C expert, then it means you can probably skim through that section. If you are not
an expert, you will need to read the section with care and refer to Appendix B, which
summarizes all theC features used. Evenif youareaC expert, youmay not haveused it inan
embedded context. As you work through the book you will see a number of tricks and
techniques that are used to optimize the language for this particular environment.
C code
feature
Chapter Review
• The mbed is a compact, microcontroller-based hardware platform, accompanied by a
program development environment.
• Communication to the mbed from a host computer is by USB cable; power can also
be supplied through this link.
• The mbed has 40 pins, which can be used to connect to an external circuit. On the board
it has four user-programmable LEDs, so very simple programs can be run with no
external connection to its pins.
• The mbed uses the LPC1768 microcontroller, which contains an ARM Cortex-M3 core.
Most mbed connections link directly to the microcontroller pins, and many of
the mbed characteristics derive directly from the microcontroller.
Introducing the mbed 27
• The mbed development environment is hosted on the web. Program development
is undertaken while online, and programs are stored on the mbed server.
Quiz
1. What do ADC, DAC and SRAM stand for?
2. What do UART, CAN, I2C and SPI stand for, and what do these mbed features have
in common?
3. How many digital inputs are available on the mbed?
4. Which mbed pins can be used for analog input and output?
5. How many microcontrollers are on the mbed PCB and what specifically are they?
6. What is unique about the mbed compiler software?
7. An mbed is part of a circuit which is to be powered from a 9 V battery. After
programming the mbed is disconnected from the USB. One part of the circuit external
to thembedneeds to besupplied from9 V, andanotherpart from3.3 V. No otherbattery
orpowersupply istobeused. Drawadiagramwhichshowshowthesepowerconnections
should be made.
8. Anmbedisconnectedto asystem, andneedstoconnectwiththreeanaloginputs, oneSPI
connection, one analog output and two PWM outputs. Draw a sketch showing how
these connections can be made, and indicate mbed pin number.
9. A friend enters the code shown below into the mbed compiler, but when compiling
a number of errors are flagged. Find and correct the faults.
#include "mbed"
Digital Out myled(LED1);
int man() {
white(1) {
myled = 1;
wait(0.2)
myled = 0;
watt(0.2);
}
10. By not connecting all the LPC1768 microcontroller pins to the mbed external pins,
a number of microcontroller peripherals are ‘lost’ for use. Identify which ones these are,
for ADC, UART, CAN, I2C, SPI and DAC.
References
2.1. The mbed home site. http://mbed.org/
2.2. MBED Circuit Diagrams. 26/08/2010. http://mbed.org/media/uploads/chris/mbed-005.1.pdf
2.3. NXP B.V. LPC1768/66/65/64 32-bit ARM Cortex-M3 microcontroller. Objective data sheet. Rev. 6.0.
August 2010. http://www.nxp.com/
2.4. NXP B.V. LPC17xx User Manual. Rev. 02. August 2010. http://www.nxp.com/
C H A P T E R 3
D ig it a l In p u t a n d O u t p u t
C h a p t e r O u t lin e
3 .1 S t a r t in g t o P r o g r a m
2 9
3 .1 .1 T h in k in g a b o u t t h e F ir s t P r o g r a m
3 .1 .2 U n d e r s t a n d in g t h e m b e d A P I 3 2
3 .1 .3 E x p lo r in g t h e w h ile L o o p 3 3
3 0
3 .2 V o lt a g e s a s L o g ic V a lu e s 3 4
3 .3 D ig it a l O u t p u t o n t h e m b e d
3 6
3 .3 .1 U s in g L E D s 3 6
3 .3 .2 U s in g m b e d E x t e r n a l P in s
3 .4 U s in g D ig it a l In p u t s
3 7
3 9
3 .4 .1 C o n n e c t in g S w it c h e s t o a D ig it a l S y s t e m 3 9
3 .4 .2 T h e D ig it a lIn A P I 4 0
3 .4 .3 U s in g if t o R e s p o n d t o a S w it c h In p u t 4 0
3 .5 In t e r f a c in g S im p le O p t o D e v ic e s
3 .5
3 .5
3 .5
3 .5
.1 O p t
.2 C o n
.3 S e v e
.4 C o n
o R
n e c
n -S
n e c
e fle c
t in g
e g m
t in g
t iv e
a n
e n t
a S
4 2
a n d T r a n s m is s iv e S e n s o r s 4 2
O p to -S e n so r to th e m b e d 4 3
D is p la y s 4 5
e v e n - s e g m e n t D is p la y t o t h e m b e d
3 .6 S w it c h in g L a r g e r D C L o a d s
4 9
3 .6 .1 A p p ly in g T r a n s is t o r S w it c h in g 4 9
3 .6 .2 S w it c h in g a M o t o r w it h t h e m b e d 5 0
3 .6 .3 S w it c h in g M u lt ip le S e v e n - S e g m e n t D is p la y s
3 .7 M
C h a p t
Q u iz
R e fe re
in i- P r o je c t : L e t t e r C o u n t e r
e r R e v ie w
5 3
5 3
n c e s 5 5
3 .1
S t a r t in g t o P r o g r a m
Th is ch ap ter will co n sid
d ig ital sig n als. Mo v in g
a p ro g ram b ased o n th e
time in th e emb ed d ed e
tim in g a c tiv ity a s w e ll.
er th
b ey o
v alu
n v iro
4 6
5 1
5 3
a t m o s t b a s ic o f m ic ro c o n tro lle r a c tiv ity , th e in p u t a n d
n d th is, it will sh o w h o w we can mak e simp le d ecisio
e o f a d ig ita l in p u t. F ig u r e 1 .1 a ls o p r e d ic te d th e im p o
n men t; n o su rp rise th erefo re th at at th is early stag e w
F a s t a n d E f f e c t iv e E m b e d d e d S y s t e m s D e s ig n . D O I : 1 0 .1 0 1 6 /B 9 7 8 - 0 - 0 8 - 0 9 7 7 6 8 - 3 .0 0 0 0 3 - 9
Co p y rig h t Ó2 0 1 2 Elsev ier Ltd . All rig h ts reserv ed .
2 9
o u tp u t o f
n s with in
rtan ce o f
e meet
3 0
Chapter 3
Yo u m a y a ls o b e e m b a r k in g o n C p r o g r a m m in g f o r th e fi r s t tim e . T h is c h a p te r ta k e s y o u
th ro u g h q u ite a few o f th e k ey co n cep ts. If y o u are n ew to C, we su g g est y o u n o w read th ro u g h
S e c t i o n s B 1 eB 5 i n c l u s i v e o f A p p e n d i x B .
On e th in g
electro n ics
su p p o rt in
wh en n eed
3 .1 .1
th at
. So
th is
ed .
is n o t ex p ecte
me u n d erstan d
area, y o u mig
Th ere are man
Th ere are two way s o f in s
On e is to p lace th e co mm
in fo rmatio n ru n n in g o v er
are u sed , th e co mp iler ig n
co mmen t.
th at y o u write to
o f th e p ro g ram.
o u g h t p ro cess as
v er it, an d h elp o
so rt o f team-wo r
ertin g co
en t b etw
sev eral
o res an y
m
ee
lin
te
is b o o k
ry will,
lik e Re
is area a
is a d eep k n o wled g e o f d ig ital
h o wev er, b e u sefu l. If y o u n eed
f e r e n c e 3 .1 a v a ila b le , to a c c e s s
s well.
/ * P r o g r a m E x a m p l e
D e m o n s t r a t i n g u s e
* /
# i n c l u d e
/ /
" m b e d . h "
p r o g r a m
D i g i t a l O u t
i n
e
y l
a i
th r
seq
th e
wh
an d b o
/. Th is
n two
lin e o n
e p
to
y o
d e
h a
ro g ram; th ey h av e n o
in tro d u ce co mmen ts
u wh at th e p ro g ram was
rstan d th e p ro g ram. Th is
n d in g in wo rk to b e
th are u sed
is u sefu l fo
fo rward -sla
ly ; th is can
e e lin e s , g iv e s a b rie f
u en t p ro g rams in th e
p ro g ram a little mo re
a t in d iv id u a l lin e s o f
in th is ex
r a b lo ck
sh sy mb o
th en b e u
/ / i n c l u d e
/ /
/ /
/ /
/ /
t h
a
s w
w a
i s
t h e
m b e d
c r e a t e d ,
e
f u n
c o n t i n u o u s
i t c h t h e l
i t 0 . 2 s e c
c t
l o
e d
o n
h e a d e r
a n d
f i l e
l i n k e d
a s
p a r t
w i t h
i o n s t a r t s h e r e
o p i s c r e a t e d
o n , b y s e t t i n g
d s
t h e
m b e d
o f
t h i s
m b e d
p r o g r a m
L E D 1
o u t p u t
t o
amp le.
o f tex t
ls (//)
sed fo r
su mmary o f wh at th e
b o o k . No tice also th at
read ab le. We th en p u t
co d e d o .
2 . 1 : A p r o g r a m w h i c h f l a s h e s m b e d L E D 1 o n a n d o f f .
o f d i g i t a l o u t p u t a n d w a i t f u n c t i o n s . T a k e n f r o m t h e
v a r i a b l e m y l e d
m y l e d ( L E D 1 ) ;
( ) {
( 1 ) {
e d = 1 ;
t ( 0 . 2 ) ;
y o u rself with in th
It is g o o d p ractice
y o u write, remin d
th ers to read an d u n
k in g , o r if y o u a re
men ts in to a p ro g ram,
n th e mark ers /* an d *
e s . A lte rn a tiv e ly , w h e
x t th at fo llo ws o n th at
o p en in g co mmen t in th e p ro g ram, lastin g
ram d o es. We ad o p t th is p ractice in all su b
av e in tro d u ced so me b lan k lin es, to mak e
mb er o f in -lin e co mmen ts; th ese in d icate
m a
w h i l
m
w
th
eo
o k
th
k a t o u r fi r s t p r o g r a m , in tr o d u c e d a s P r o g r a m E x a m p le 2 .1 . T h is is s h o w n
n v en ien ce, th is time with co mmen ts in serted . Co mp are th is with th e
e o f th e p ro g ram, sh o wn in Ch ap ter 2 . Rememb er to ch eck acro ss to
y o u n eed fu rth er C b ack g ro u n d .
Co mmen ts are tex t messag es
imp act o n th e actu al wo rk in g
wid ely ; th ey h elp y o u r o wn th
mean t to d o as y o u lo o k b ack o
last p o in t is essen tial fo r an y
mark ed !
i n t
u in read in g
electro n ic th
to h av e a b o
web sites in
T h in k in g a b o u t t h e F ir s t P r o g r a m
We n o w tak e a lo o
ag ain b elo w fo r co
o rig in al ap p earan c
Ap p en d ix B wh en
Th e
p ro g
we h
a n u
d o f y o
in g o f
h t lik e
y g o o d
l o g i c
1
s i t e .
D ig it a l In p u t a n d O u t p u t
m y l e d = 0 ;
w a i t ( 0 . 2 ) ;
}
}
/ /
/ /
/ /
/ /
s w
w a
e n
e n
3 1
i t c h t h e l e d o f f
i t 0 . 2 s e c o n d s
d o f w h i l e l o o p
d o f m a i n f u n c t i o n
P r o g r a m E x a m p le 2 .1 R e p e a t e d ( a n d c o m m e n t e d ) fo r c o n v e n ie n c e
L e t’s n o w id e n tif y a ll th e C p r o g r a m m in g f e a tu r e s o f P r o g r a m E x a m p le 2 .1 . T h e
actio n o f a n y C o r Cþ þ p ro g ram is co n tain ed with in its m a in ( ) fu n ctio n , so th at is
alway s a g o o d p lace to start lo o k in g . By th e way , writin g m a in ( ) with th o se b rack ets
after it remin d s u s th at it is th e n ame o f a C fu n ctio n ; all fu n ctio n n ames are written in th is way in
th e b o o k . T h e fu n c tio n d e fi n itio n ew h a t g o e s o n in s id e th e f u n c tio n eis c o n ta in e d w ith in th e
o p en in g cu rly b rack et o r b race, ap p earin g immed iately after m a in ( ), an d g o es o n u n til th e
clo sin g b race. Th ere are fu rth er p airs o f b races in sid e th ese o u ter o n es. Usin g p airs o f b races lik e
th is is o n e o f th e way s th at C g ro u p s b lo ck s o f co d e to g eth er an d creates p ro g ram stru ctu re.
C c o d e
fe a tu re
M a n y p r o g r a m s in e m b e d d e d s y s te m s a r e m a d e u p o f a n e n d le s s lo o p , i.e .
a p ro g ram wh ich ju st g o es o n an d o n rep eatin g itself in d efin itely . Here is o
ex amp le. We create th e lo o p b y u sin g th e w h ile k ey wo rd ; th is co n tro ls th e
w ith in th e p a ir o f b r a c e s th a t f o llo w. S e c tio n B .7 ( in A p p e n d ix B ) te lls u s th a t n o r m a lly w
u sed to set u p a lo o p , wh ich rep eats if a certain co n d itio n is satisfied . Ho wev er, if we
w h ile (1 ), th en we “trick ” th e w h ile mech an ism to rep eat in d efin itely .
C c o d e
fe a tu re
u r first
co d e
h ile is
write
Th e real actio n o f th e p ro g ram is co n tain ed in th e fo u r lin es with in th e w h ile lo o p .
Th ese are mad e u p o f two calls to th e lib rary fu n ctio n w a it( ), an d two statemen ts,
in wh ich th e v alu e o f m y le d is ch an g ed . Th e w a it( ) fu n ctio n is fro m th e mb ed
y ; f u r th e r o p tio n s a r e s h o w n in Ta b le 3 .1 . T h e 0 .2 p a r a m e te r is in s e c o n d s , a n d d e fi n e s th e
len g th cau sed b y th is fu n ctio n ean o th er v alu e can b e ch o sen . In th e two statemen ts
in in g m y le d we mak e u se o f a C o p erato r fo r th e first time. Here, we u se th e a ssig n
to r, wh ich ap p lies th e co n v en tio n al eq u als sig n . No te carefu lly th at in C th is o p erato r
s th at a v ariab le is set to th e v alu e in d icated . Th u s,
C c o d e
fe a tu re
lib rar
d elay
co n ta
o p era
mean
m y l e d
=
1 ;
mean s th at
Co n v en tio n
ch ap ter. Th
meet it fo r
th e
al “
ere
th e
v ariab le m y le d is set to th e v alu e
eq u als” is rep resen ted b y a d o u b le
are man y C o p erato rs, an d it is wo
first time. Th ey are su mmarized in
1 , wh atev er its p rev io u s v alu e was.
e q u a ls s ig n , i.e . ¼ ¼ . I t is u s e d la te r in th is
rth n o ticin g an d learn in g each o n e as y o u
S e c tio n B .5 .
T a b le 3 .1 : m b e d lib r a r y w a it f u n c t io n s
C / C þ þ
f u n c t io n
A c t io n
w a i t
W a it s fo r t h e n u m b e r o f s e c o n d s s p e c ifi e d
w a i t _ m s
W a it s fo r t h e n u m b e r o f m illis e c o n d s s p e c ifi e d
w a i t _ u s
W a it s fo r t h e n u m b e r o f m ic r o s e c o n d s s p e c ifi e d
3 2
Chapter 3
Th e p ro g ram starts b y in clu d in g th e all-imp o rtan t mb ed h ead er file, wh ich is th e
co n n ectin g p ath way to all mb ed lib rary featu res. Th is mean s th at th e h ead er file is
literally in serted in to th e p ro g ram, b efo re co mp ilin g starts in earn est. Th e # in c lu d e
c o m p ile r d ir e c tiv e is u s e d f o r th is ( S e c tio n B .2 .3 ) . T h e p r o g r a m th e n a p p lie s th e m b e d
a p p lic a tio n p r o g r a m m in g in te r f a c e ( A P I ) u tility D ig it a lO u t to d e fi n e a d ig ita l o u tp u t, a n d g iv e s
it th e n ame m y le d . On ce d eclared , m y le d can b e u sed as a v ariab le with in th e p ro g ram. Th e
n ame LED1 is reserv ed b y th e API fo r th e o u tp u t asso ciated with th at lig h t-emittin g d io d e
( L E D ) o n th e m b e d b o a r d , a s la b e le d in F ig u r e 2 .1 b .
C c o d e
fe a tu re
No tice th at we
w h ile co d e b lo
mo re read ab le
p ro g rams in th
Co mp an ies wo
in a read ab le a
n
in d en t th e co d e b y two
ck . Th is h as n o effect o
an d h en ce cu t d o wn o n
is b o o k . Ch eck Sectio n
rk in g with C o ften ap p ly
n d co n sisten t fash io n .
sp aces with in m a in ( ), an d th en two mo re with in
n th e actio n o f th e p ro g ram, b u t d o es h elp to mak
p ro g rammin g erro rs; it is a p ractice we ap p ly fo r
B .1 1 f o r o th e r c o d e la y o u t p r a c tic e s th a t w e a d o p
‘h o u se sty les’, to en su re th at p ro g rammers write c
th e
e it
t.
o d e
E x e r c is e 3 .1
G e t fa m ilia r w it h P r o g r a m E x a m p le 2 .1 , a n d t h e g e n e
t h e fo llo w in g v a r ia t io n s . O b s e r v e t h e e ffe c t s .
1 . A d d t h e c o m m e n t s , o r o t h e r s o f y o u r o w n , g iv e n
p r o g r a m c o m p ile s a n d r u n s w it h o u t c h a n g e .
2 . V a r y t h e 0 .2 p a r a m e t e r in t h e w a it ( ) fu n c t io n s .
3 . R e p la c e t h e w a it ( ) fu n c t io n s w it h w a it _ m s ( ) fu n
p a r a m e t e r s e t t o g iv e t h e s a m e w a it t im e . A ls o v a
4 . U s e L E D 2 , 3 o r 4 in s t e a d o f 1 .
5 . M a k e a m o r e c o m p le x lig h t p a t t e r n u s in g t w o o r
fu n c t io n c a lls .
r a l p r o c e s s o f c o m p ilin g , b y t r y in g
in t h e e x a m p le . S e e t h a t t h e
c t io n s , w it h a c c o m p a n y in g
r y t h is t im e .
m o r e L E D s , a n d m o r e w a it ( )
n
3 .1 .2
Th e
u sed
web
API
In
cla
me
mu
U n d e r s t a n d in g t h e m b e d A P I
mb ed API h as a p
to it. Th e lib rary
site Han d b o o k . Th
su mmary fo r th is
a
ss
m
st
attern th
is mad e
e first th
is g iv e n
at we will
u p o f a se
at we lo o k
in Tab le 3
p attern th at will b eco me familiar
, ca lle d D ig ita lO u t. Th is ap p ears
b er fu n ctio n s, wh ich are listed b e
h av e th e same n ame as th e class
see rep eated man y times, so it is imp o rtan t to g et
t o f u tilities, wh ich are all itemized in th e mb ed
at is D ig ita lO u t, wh ich we h av e alread y u sed . Th e
.2 .
, th e D
at th e
lo w. T
itself.
ig ita
h ead
h e fir
Th is
lO u
o f
st o
can
t API co
th e tab le.
f th ese is
b e u sed
mp o n en t create
Th e c lass th en
a C þ þ co n str u
to create Cþ þ
s a Cþ þ
h as a set o f
c to r, w h ich
o b jects. By
D ig it a l In p u t a n d O u t p u t
3 3
T a b le 3 .2 : T h e m b e d d ig it a l o u t p u t A P I s u m m a r y ( f r o m w w w .m b e d .o r g )
F u n c t io n
U s a g e
D i g i t a l O u t
C r e a t e a D ig it a lO u t c o n n e c t e d t o t h e s p e c ifi e d p in
w r i t e
S e t t h e o u t p u t , s p e c ifi e d a s 0 o r 1 ( in t )
r e a d
R e t u r n t h e o u t p u t s e t t in g , r e p r e s e n t e d a s 0 o r 1 ( in t )
o p e r a t o r =
A s h o r t h a n d fo r w r it e
o p e r a t o r i n t ( )
A s h o rth a n d fo r re a d
u sin g th e D ig ita lO u t co n stru cto r, we
cre ate th e o b je ct m y le d . We c an th e n
w r ite ( ) an d r e a d ( ). Th ese are memb
m y le d .w r it e ( ) , a n d s o o n . H a v in g c r e
API o p e ra to r sh o rth a n d , me n tio n e d in
Hen ce, wh en we write
m y l e d
=
can c re ate Cþ þ o b je cts .
write to it an d read fro m
er fu n ctio n s o f th e c la ss,
ated th e m y le d o b ject we
Ta b le 3 .2 , w h ic h a p p lie s
In o u r first ex amp le
it, u sin g th e fu n ctio
so th eir fo rmat wo u
can , h o wev er, in v o k
th e assig n o p erato r
we
n s
ld b e
e th e
¼ .
1 ;
th e v ariab le v alu e is th en n o t o n ly ch an g ed (n o rmal C u sag e), b u t also written to th e d ig ital
o u tp u t. T h is r e p la c e s m y le d .w r it e ( 1 ) ; . We w ill fi n d s im ila r s h o r th a n d s o ff e r e d f o r a ll
p erip h eral API g ro u p s in th e mb ed API.
3 .1 .3
E x p lo r in g t h e w h ile L o o p
P r o g r a m E x a m p le 3 .1 is th e fi r s t “ o r ig in a l” p r o g r a m in th is b o o k , a lth o u g h it b u ild s d ir e c tly
o n th e p rev io u s ex amp le. Create a n ew p ro g ram in th e mb ed co mp iler an d co p y th is ex amp le
in to it.
L o o k fi rs t a t th e s tru c tu re o f th e p ro g ra m , d e riv e d fro m th e th re e u s e s o f w h ile ; o n e
o f th ese is a w h ile (1 ), wh ich we are u sed to , an d two are co n d itio n al. Th ese
latter are b ased o n th e v alu e o f a v ariab le i. In th e first co n d itio n al u se o f w h ile , th e
lo o p r e p e a ts a s lo n g a s i is le s s th a n 1 0 . Yo u c a n s e e th a t i is in c r e m e n te d w ith in th e lo o p ,
to b rin g it u p th is v alu e. In th e seco n d co n d itio n al lo o p , th e v alu e is d ecremen ted , an d th e lo o p
rep eats as lo n g as i is g reater th an zero .
C c o d e
fe a tu re
A v ery imp o rtan t n ew
is d ecla red at th e b eg
ty p e o f an y v ariab le b
S e c tio n B .3 . I n th is e x a m p le , i is
same lin e, its v alu e is in itialized
 , < a n d >. T h e u s e o f t h e s e i s
immed iately familiar.
C c o d e
fe a tu re
C featu re seen in th is p ro g ram is th e way th at th e v aria b le i
in n in g o f m a in ( ). It is e s s e n tia l in C to d ec la re th e d a ta
e fo re it is u s e d . T h e p o s s ib le d a ta ty p e s a re g iv e n in
d e c la re d a s a c h a ra c te r, e ffe c tiv e ly a n 8 -b it n u m b e r. In th e
to zero . Th ere are also fo u r n ew o p erato rs in tro d u ced : þ ,
th e s ame as in co n v e n tio n al alg eb ra , so sh o u ld b e
3 4
/ *
* /
# i
D i
D i
Chapter 3
P r o g r a m
E x a m p l e
3 . 1 :
D e m o n s t r a t e s
u s e
o f
w h i l e
l o o p s .
N o
e x t e r n a l
c o n n e c t i o n
r e q u i r e d
n c l u d e " m b e d . h "
g i t a l O u t m y l e d ( L E D 1 ) ;
g i t a l O u t y o u r l e d ( L E D 4 ) ;
i n t
m
c h a
w h i
w h i
m
w
m
w
i
a i
r
l e
l e
y l
a i
y l
a i
=
n (
i =
( 1
( i
e d
t (
e d
t (
i
)
0 ;
) {
< 1
=
0 .
=
0 .
+ 1
{
0 )
1
2 )
0
2 )
;
/ / d e c l a r e v a r i a b l e i , a n d
/ / s t a r t e n d l e s s l o o p
/ / s t a r t f i r s t c o n d i t i o n a l
{
l e
o u
a i
o u
a i
=
( i
r l
t (
r l
t (
i
> 0
e d
0 .
e d
0 .
- 1
)
=
t o
w h i l e
0
l o o p
;
;
;
;
}
w h i
y
w
y
w
i
s e t
{
1 ;
/ / i n c r e m e n t i
/ / e n d o f f i r s t
/ / s t a r t s e c o n d
c o n d i t i o n a l
c o n d i t i o n a l
w h i l e
l o o p
l o o p
2 ) ;
= 0 ;
2 ) ;
;
}
}
/ / e n d
/ / e n d
}
i n f i n i t e
o f m a i n
l o o p
b l o c k
P r o g r a m E x a m p le 3 .1 U s in g w h ile
H a v in g c o m p ile d P r o g r a m E x a m p le 3 .1 , d o w n lo a d it to th e m b e d , a n d r u n it. Yo u s h o u ld s e e
LED1 an d LED4 flash in g 1 0 times in tu rn . Mak e su re y o u u n d erstan d wh y th is is so .
n
E x e r c is e 3 .2
C c o d e
fe a tu re
1 . A s lig h t ly m o r e e le g a n t w a y o f in c r e m e n
t h e in c r e m e n t a n d d e c r e m e n t o p e r a t o r s
i[ iþ 1 ; a n d i [ i  1 ;
w it h
iþ þ ; a n d i  ; r e s p e c t iv
a n d r u n t h e p r o g r a m a g a in .
2 . C h a n g e th e p ro g ra m s o th a t th e L E D s fl
3 . T r y r e p la c in g t h e m y le d [ 1 ; s t a t e m e n t
t in g o r d e c r e m e n t in g a v a r ia b le is b y u s in g
s e e n in S e c t io n B .5 . T r y r e p la c in g
e ly ,
a s h o n ly fi v e t im e s e a c h .
w it h m y le d .w r it e ( 1 ) ;
n
3 .2
V o lt a g e s a s L o g ic V a lu e s
Co mp u ters d eal with b in ary n u mb ers mad e u p o f lo ts o f b in ary d ig its, o r b its, an d each o n e o f
th ese b its tak es a lo g ic v alu e o f 0 o r 1 . No w th at we are startin g to u se th e mb ed in earn est, it is
D ig it a l In p u t a n d O u t p u t
3 5
wo rth th in k in g ab o u t h o w th o se lo g ic v alu es are actu ally rep resen ted in sid e th e mb ed ’s
electro n ic circu it, an d at its co n n ectio n p in s.
In an y d ig ital circu it, lo g ic v alu es are rep resen ted as electrical v o ltag es. Here n o w is th e b ig
b en efit o f d ig ital electro n ics: we d o n o t n eed a p recise v o ltag e to rep resen t a lo g ical v alu e.
In stead , we accep t a ra n g e o f v o ltag es as rep resen tin g a lo g ic v alu e. Th is mean s th at a v o ltag e
can p ick u p so me n o ise o r d isto rtio n an d still b e in terp reted as th e rig h t lo g ic v alu e. Th e
m ic r o c o n tr o lle r w e a r e u s in g in th e m b e d , th e L P C 1 7 6 8 , is p o w e r e d f r o m 3 .3 V. We c a n fi n d
o u t wh ich ran g e o f v o ltag es it accep ts as Lo g ic 0 , an d wh ich as Lo g ic 1 , b y lo o k in g at its
te c h n ic a l d a ta . T h is is f o u n d in R e f e r e n c e 2 .4 ( C h a p te r 2 ) , w ith im p o r ta n t p o in ts s u m m a r iz e d
in Ap p en d ix C. (Th ere will b e mo men ts wh en it will b e u sefu l to lo o k at th is ap p en d ix , b u t d o
n o t r u s h to it n o w.) T h is d a ta s h o w s th a t, f o r m o s t d ig ita l in p u ts , th e L P C 1 6 7 8 in te r p r e ts a n y
in p u t v o lta g e b e lo w 1 .0 V ( s p e c ifi e d a s 0 .3  3 .3 V ) a s L o g ic 0 , a n d a n y in p u t v o lta g e a b o v e
2 .3 V ( s p e c ifi e d a s 0 .7  3 .3 V ) a s L o g ic 1 . T h is id e a is r e p r e s e n te d in th e d ia g r a m o f
F ig u r e 3 .1 .
If we wan t to in p u t a sig n al to th e mb ed , h o p in g th at it is in terp reted co rrectly as a lo g ic v alu e,
th e n it w ill n e e d to s a tis f y th e r e q u ir e m e n ts o f F ig u r e 3 .1 . I f w e a r e o u tp u ttin g a s ig n a l f r o m
th e mb ed , th en we can ex p ect it to co mp ly with th e same fig u re. Th e mb ed will n o rmally
o u tp u t L o g ic 0 a s 0 V a n d L o g ic 1 a s 3 .3 V, a s lo n g a s n o e le c tr ic a l c u r r e n t is fl o w in g . I f
cu rren t is flo win g , fo r ex amp le in to an LED, th en we can ex p ect so me ch an g e in o u tp u t
v o ltag e. We will retu rn to th is p o in t. Th e n eat th in g is, wh en we o u tp u t a lo g ic v alu e, we are
also g ettin g a p red ictab le v o ltag e to mak e u se o f. We can u se th is to lig h t LEDs (lig h t emittin g
d io d es), switch o n mo to rs, o r man y o th er th in g s.
Fo r m a n y a p p lic a tio n s in th is b o o k w e d o n o t w o rry m u ch a b o u t th e s e v o lta g e s , a s th e c irc u its w e
b u ild m e e t th e r e q u ir e m e n ts o f F ig u r e 3 .1 . T h e r e a r e s itu a tio n s , h o w e v e r, w h e r e it is n e c e s s a r y
to ta k e s o m e c a re o v e r lo g ic v o lta g e v a lu e s , a n d th e se a re m e n tio n e d a s th ey c o m e u p .
3 .3 V
(su p p ly v o ltag e)
Lo g ic 1
2 .3 V
Un d efin ed Lo g ic Lev el
1 .0 V
Lo g ic 0
0 V
F ig u r e 3 .1 :
In p u t lo g ic le v e ls fo r t h e m b e d
3 6
Chapter 3
3 .3
D ig it a l O u t p u t o n t h e m b e d
In o u r first two p ro g rams w e h av e ju s t switch ed th e d iag n o stic LEDs w h ich are fitted o n
t h e m b e d b o a r d . T h e m b e d , h o w e v e r , h a s 2 6 d i g i t a l i n p u t / o u t p u t ( I / O ) p i n s ( p i n s 5 e3 0 ) ,
wh ich c an b e c o n fig u re d as e ith er d ig ita l in p u ts o r o u tp u ts . Th es e a re s h o wn in
F ig u r e 3 .2 . A c o m p a r is o n o f th is fi g u r e w ith th e m a s te r m b e d p in o u t d ia g r a m , i.e .
F ig u r e 2 .1 c , s h o w s th a t th e s e p in s a r e m u lti- f u n c tio n . W h ile w e c a n u s e a n y o f th e m f o r
simp le d ig ita l I/O, th ey all h av e seco n d ary fu n ctio n s, co n n ectin g with th e micro co n tro ller
p erip h erals. It is u p to th e p ro g ramme r to sp ecify , w ith in th e p ro g ra m, h o w th ey are
co n fig u red .
3 .3 .1
U s in g L E D s
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
D
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
m b e d M ic r o c o n tr o lle r
Fo r th e first time we are co n n ectin g an ex tern al d ev ice to th e mb ed . Wh ile y o u may n o t b e an
e l e c t r o n i c s s p e c i a l i s t i t i s i m p o r t a n t ew h e n e v e r y o u c o n n e c t a n y t h i n g t o t h e m b e d et h a t y o u
h av e so me u n d erstan d in g o f wh at y o u are d o in g . Th e LED is a semico n d u cto r d io d e, an d
b eh av es electrically as o n e. It will co n d u ct cu rren t in o n e d irectio n , so metimes called th e
‘fo rward ’ d irectio n , b u t n o t th e o th er. Wh at mak es it so u sefu l is th at wh en it is co n n ected so
th at it co n d u cts, it emits p h o to n s fro m its semico n d u cto r ju n ctio n . Th e LED h as th e v o ltag e/
c u r r e n t c h a r a c te r is tic s h o w n in F ig u r e 3 .3 a . A s m a ll f o r w a r d v o lta g e w ill c a u s e v e r y little
cu rren t to flo w. As th e v o ltag e in creases th ere co mes a p o in t wh ere th e cu rren t su d d en ly
starts flo win g rath er rap id ly . Fo r mo st LEDs th is v o ltag e is in th e ran g e sh o wn , ty p ically
a r o u n d 1 .8 V.
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
F ig u r e 3 .2 :
T h e m b e d d ig it a l I/ O
D
D
D
D
D
D
D
D
D
D
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
ig ita
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
l I/O
D ig it a l In p u t a n d O u t p u t
3 7
VS ( 3 .3 V )
I
ID
D
I D
R
R
VD
VD
VD
1 .7 to 2 .2 V
(a) LED V– I ch aracteristic
(b ) Gate o u tp u t so u rcin g
cu rrren t to LED lo ad
(c) Gate o u tp u t sin k in g
cu rren t fro m LED
Key
ID:
Cu rren t th ro u g h LED
VS:
Su p p ly v o ltag e
V D:
Vo lta g e a c r o s s L E D , f o r c u r r e n t I
D
F ig u r e 3 .3 :
D r iv in g L E D s fr o m lo g ic g a t e s
F ig u r e s 3 .3 b a n d c s h o w c ir c u its u s e d to m a k e d ir e c t c o n n e c tio n s o f L E D s to th e o u tp u t o f lo g ic
g a te s , fo r e x a m p le a n m b e d p in c o n fi g u re d a s a n o u tp u t. T h e g a te is h e re s h o w n a s a lo g ic
b u ff e r ( th e tr ia n g u la r s y m b o l) . I f th e c o n n e c tio n o f F ig u r e 3 .3 b is m a d e , th e L E D lig h ts w h e n th e
g a te is a t lo g ic h ig h . C u r r e n t th e n fl o w s o u t o f th e g a te in to th e L E D . A lte r n a tiv e ly , th e c ir c u it
o f F ig u r e 3 .3 c c a n b e m a d e ; th e L E D n o w lig h ts w h e n th e lo g ic g a te is a t lo g ic z e r o , w ith
c u rre n t fl o w in g in to th e g ate . U su a lly a c u rre n t-limitin g re s is to r n e e d s to b e c o n n e c te d in s e rie s
w ith th e L ED , to c o n tro l h o w m u c h c u rre n t fl o w s . T h e e x c e p tio n to th is is if th e c o m b in a tio n
o f o u tp u t v o lta g e a n d g a te in te rn a l re s is ta n c e is s u c h th a t th e c u rre n t is lim ite d to a s u ita b le v a lu e
a n y wa y . S o m e L ED s , s u c h a s th e o n e s re c o m m e n d e d fo r th e n ex t p ro g ram , h av e th e s e rie s
re s is ta n c e b u ilt in to th e m . T h e y th e re fo re d o n o t n e e d a n y e x te rn a l re s is to r c o n n e c te d .
3 .3 .2
U s in g m b e d E x t e r n a l P in s
Th e d ig ital I/O p in s are n amed an d co n fig u red to o u tp u t u sin g D ig ita lO u t, ju st as we d id in
th e earlier p ro g rams, b y d efin in g th em at th e start o f th e p ro g ram co d e, fo r ex amp le:
D i g i t a l O u t
m y n a m e ( p 5 ) ;
Th e D ig ita lO u t o b ject created in th is way can b e u sed to set th e state o f th e o u tp u t p in , an d
also to read b ack its cu rren t state.
We w ill b e
p arts u se d ;
th e co n n ec
an in tern al
ap
eq
tio
se
p ly in g th e c ir c u it o f F ig u r e 3 .4 a . A p p e n d ix D g iv e s
u iv a le n t d e v ic e s c a n o f c o u rs e b e im p le m e n te d . With
n o f F ig u r e 3 .3 b . T h e p a r tic u la r L E D r e c o m m e n d e d
ries re sisto r, o f v a lu e a ro u n d 2 4 0 U , s o an ex tern a l o
ex amp
in th is
in Ap p
n e is n
le
cir
en
o t
p art n u mb ers fo r a ll
c u it w e a re a p p ly in g
d ix D, h o wev er, h as
re q u ired .
Chapter 3
re d
g re e n
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
m b e d M ic r o c o n tr o lle r
3 8
( a ) C o n n e c tio n d ia g r a m
( b ) B r e a d b o a r d b u ild
F ig u r e 3 .4 :
m b e d s e t - u p fo r s im p le L E D fl a s h in g
P la c e th e m b e d in a b r e a d b o a r d , a s s e e n in F ig u r e 3 .4 b ,
mb ed h as a co mmo n g ro u n d o n p in 1 . Co n n ect th is acro
c o n n e c tio n s o f th e b r e a d b o a r d , a s s e e n in F ig u r e 3 .4 b . Yo
similar circu its th at y o u b u ild . Rememb er to attach th e a
lo n g e r le g ) to th e m b e d p in s . T h e n e g a tiv e s id e (c a th o d e )
th is a n d m a n y c irc u its w e w ill ta k e p o w e r fro m th e u n iv
an d co n n ect th e circu it sh o
ss to o n e o f th e o u ter ro ws
u sh o u ld ad o p t th is as a h ab
n o d e o f th e LEDs (th e sid e
sh o u ld b e co n n ected to g ro
ersal serial b u s (USB).
w
o
it
w
n . Th e
f
fo r all
ith th e
u n d . Fo r
C r e a te a n e w p r o g r a m in th e m b e d c o m p ile r, a n d c o p y a c r o s s P r o g r a m E x a m p le 3 .2 .
/ *
* /
# i
D i
D i
P r o g r a m
E x a m p l e
3 . 2 :
n c l u d e " m b e d . h "
g i t a l O u t r e d l e d ( p 5 ) ;
g i t a l O u t g r e e n l e d ( p 6 ) ;
i n t
m
w h i
r
g
w
r
g
w
a i
l e
e d
r e
a i
e d
r e
a i
n (
( 1
l e
e n
t (
l e
e n
t (
)
F l a s h e s
r e d
a n d
g r e e n
L E D s
i n
s i m p l e
t i m e - b a s e d
p a t t e r n
/ / d e f i n e a n d n a m e a d i g i t a l o u t p u t o n p i n 5
/ / d e f i n e a n d n a m e a d i g i t a l o u t p u t o n p i n 6
{
)
{
d
= 1 ;
l e d = 0 ;
0 . 2 ) ;
d = 0 ;
l e d = 1 ;
0 . 2 ) ;
}
}
P r o g r a m E x a m p le 3 .2 F la s h in g e x t e r n a l L E D s
Co mp ile, d o wn lo ad an d ru n th e co d e o n th e mb ed . Th e co d e ex ten d s id eas alread y ap p lied
in P r o g r a m E x a m p le s 2 .1 a n d 3 .1 , s o it s h o u ld b e e a s y to u n d e r s ta n d th a t th e g r e e n
D ig it a l In p u t a n d O u t p u t
3 9
a n d r e d L E D s a r e p r o g r a m m e d to fl a s h a lte r n a te ly . Yo u s h o u ld s e e th is h a p p e n w h e n
th e co d e ru n s.
n
E x e r c is e 3 .3
U s in g a n y d ig it a l o u t p u t p in , w r it e a p r
s w it c h in g t h e o u t p u t r e p e a t e d ly b e t w e e
a fr e q u e n c y o f 1 0 0 H z ( i.e . a p e r io d o f
M e a s u r e t h e v o lt a g e v a lu e s fo r L o g ic 0
t h e s q u a r e w a v e fr e q u e n c y a g r e e w it h y
o g r a m w h ic h
n L o g ic 1 a n
1 0 m s ) . V ie w
a n d 1 . H o w
o u r p ro g ra m
o u tp u ts a s q u a re
d 0 . U s e w a it ( ) fu
th e o u tp u t o n a n
d o t h e y r e la t e t o F
m e d v a lu e ?
w a v e ,
n c t io n
o s c illo
ig u r e 3
b y
s t o g iv e
s c o p e .
.1 ? D o e s
n
3 .4
U s in g D ig it a l In p u t s
3 .4 .1
C o n n e c t in g S w it c h e s t o a D ig it a l S y s t e m
Ord in ary electro me ch an ical switch es can b e u sed to create lo g ic lev els, wh ich will satisfy
th e lo g ic le v e l r e q u ir e m e n ts s e e n in F ig u r e 3 .1 . T h r e e c o m m o n ly u s e d w a y s a r e s h o w n in
F ig u re 3 .5 . T h e s im p le s t, F ig u r e 3 .5 a , u s e s a n S P D T ( s in g le - p o le , d o u b le - th ro w ) s w itc h .
Th is is wh at we will u se in th e n ex t ex a mp le. A resisto r in s erie s with th e lo g ic in p u t is
so metime s in clu d ed in th is circu it, as a p recau tio n ary me as u re. Ho wev er, an S PST
(sin g le-p o le, sin g le -th ro w) switch can b e lo wer co s t an d sma ller, a n d so is v ery wid ely
u s ed . Th ey are co n n ec te d with a p u ll-u p o r p u ll-d o w n re s is to r to th e s u p p ly v o lta g e, as
s h o w n in F ig u r e 3 .5 b o r c . W h e n th e s w itc h is o p e n , th e lo g ic le v e l is d e fi n e d b y th e
co n n ectio n th ro u g h th e resis to r. Wh en it is clo sed , th e s witch asserts th e o th er lo g ic state.
A wa s te d electrica l cu rre n t th en flo ws th ro u g h th e re s is to r. Th is is k e p t s ma ll b y h av in g
a h ig h -v alu e resisto r. On th e mb ed , a s with ma n y micro co n tro llers, p u ll-u p an d p u ll-d o wn
resisto rs are av ailab le with in th e micro co n tro ller, sav in g th e n eed to mak e th e ex tern al
co n n ectio n .
V
V
S
V
S
S
R
V i
V i
V i
R
( a ) s in g le - p o le , d o u b le - th r o w
( S P D T ) c o n n e c tio n
( b ) s in g le - p o le , s in g le - th r o w
( S P S T ) w ith p u ll- u p r e s is to r
F ig u r e 3 .5 :
C o n n e c t in g s w it c h e s t o lo g ic in p u t s
( c ) S P S T w ith p u ll- d o w n r e s is to r
4 0
Chapter 3
T a b le 3 .3 : T h e m b e d d ig it a l in p u t A P I s u m m a r y ( f r o m w w w .m b e d .o r g )
F u n c t io n
U s a g e
D i g i t a l I n
C r e a t e a D ig it a lIn c o n n e c t e d t o t h e s p e c ifi e d p in
r e a d
R e a d t h e in p u t , r e p r e s e n t e d a s 0 o r 1 ( in t )
m o d e
S e t t h e in p u t p in m o d e
o p e r a t o r i n t ( )
A n o p e ra to r s h o rth a n d fo r re a d ()
3 .4 .2
T h e D ig it a lIn A P I
T h e m b e d A P I h a s th e d ig ita l in p u t f u n c tio n s lis te d in Ta b le 3 .3 , w ith f o r m a t id e n tic a l to th a t o f
D ig ita lO u t, w h ich w e h av e a lre a d y s e e n . T h is s e c tio n o f th e A PI c re a te s a c la s s c a lle d
D ig ita lI n , w ith th e m e mb e r fu n c tio n s s h o w n . T h e D ig ita lI n c o n s tru c to r c a n b e u s e d to c re a te
d ig ita l in p u ts , a n d th e r e a d ( ) fu n c tio n u s e d to re a d th e lo g ic a l v a lu e o f th e in p u t. In p ra c tic e ,
th e s h o rth a n d o ffe re d a llo w s u s to re a d in p u t v a lu e s th ro u g h u s e o f th e d ig ita l o b jec t n a m e . T h is
w ill b e s e e n in th e p ro g ra m e x a m p le th a t fo llo w s . A s w ith d ig ita l o u tp u ts , th e s a m e 2 6 p in s
( p i n s 5 e3 0 ) c a n b e c o n fi g u r e d a s d i g i t a l i n p u t s . I n p u t v o l t a g e s w i l l b e i n t e r p r e t e d a c c o r d i n g t o
F ig u r e 3 .1 . N o te th a t u s e o f D ig it a lI n e n a b le s b y d e f a u lt th e in te r n a l p u ll- d o w n r e s is to r, i.e . th e
in p u t c ir c u it is c o n fi g u r e d a s F ig u r e 3 .5 c . T h is c a n b e d is a b le d , o r a n in te r n a l p u ll- u p e n a b le d ,
u s in g th e m o d e ( ) fu n c tio n . S e e th e m b e d H an d b o o k fo r d eta ils o n th is.
3 .4 .3
U s in g if t o R e s p o n d t o a S w it c h In p u t
We will n o w co n n ect to o u r circu it a d
so d o in g , we mak e a sig n ifican t step
mak es a d ecisio n b ased o n an ex tern
man y emb ed d ed sy stems. Pro g ram E
co n n ected to p in 7 , an d created with
C c o d e
fe a tu re
ig
fo
al
x a
th
ital in p u t, a
rward . Fo r
v ariab le, th
m p le 3 .3 is
e D ig ita lI n
switch , an d u se it to co n tro l LED states.
th e first time we will h av e a p ro g ram th
e switch p o sitio n . Th is is th e essen ce o
ap p lied to ach iev e th is; th e d ig ital in p u t
co n stru cto r.
In
at
f
is
To mak e th e d ecisio n with in th e p ro g ram, we u se th e statemen t
i f ( s w i t c h i n p u t = = 1 ) .
Th is lin e sees u se o f th e
u se o f th e if an d e ls e k e
ex ecu te, if th e sp ecified
s w itc h in p u t is eq u al to
ex ecu ted . Lo o k in g n o w
th e g reen LED is switch
0 , th e e ls e co d e b lo ck is
statemen t is u sed to cre
C e q u a l o p e r a to r, ¼ ¼ , f o r th e fi r s t tim e . R e a d S e c tio n B .6 .1 to r e v ie w
y wo rd s. Th e lin e cau ses th e lin e o r b lo ck o f co d e wh ich fo llo ws it to
co n d itio n is met. In th is case, th e co n d itio n is th at th e v ariab le
1 . If th e co n d itio n is n o t satisfied , th en th e co d e th at fo llo ws e ls e is
a t th e p ro g ra m w e c a n s e e th a t if th e s w itc h g iv e s a v a lu e o f L o g ic 1 ,
ed o ff an d th e red LED is p ro g rammed to flash . If th e switch in p u t is
in v o k ed an d th e ro les o f th e LEDs are rev ersed . Ag ain , th e w h ile (1 )
a te a n o v era ll in fi n ite lo o p , s o th e L E D s fl a s h c o n tin u o u s ly .
D ig it a l In p u t a n d O u t p u t
/ *
* /
# i
D i
D i
D i
i n
P r o g r a m
n c l
g i t
g i t
g i t
t m
w h i l
i
/
u d e
a l O
a l O
a l I
a i n
e ( 1
f (
/ e x
E x a m p l e
" m b
u t r
u t g
n s w
( ) {
) {
s w i t
e c u t
g r e e
r e d l
w a i t
r e d l
w a i t
3 . 3 :
e d
e d
r e
i t
. h
l e
e n
c h
"
c h
e
n l
e d
( 0
e d
( 0
i n
f o
e d
=
. 2
=
. 2
p u
l l
=
1
) ;
0
) ;
F l a s h e s
s e
e d
r e
a i
r e
a i
t = = 1 )
o w i n g
0 ;
{
=
;
=
1 ;
. 2 ) ;
e d =
. 2 ) ;
0 ;
L E D s ,
d e p e n d i n g
o n
t h e
s t a t e
o f
a
2 - w a y
s w i t c h
;
/ / e
/ / h
/ / r
/ /
0 ;
e d
t w o
/ / t e s t v a l u e o f s w i t c h i n p u t
b l o c k i f s w i t c h i n p u t i s 1
/ / g r e e n l e d i s o f f
/ / f l a s h r e d l e d
{
l e d
e n l
t ( 0
e n l
t ( 0
o f
d ( p 5 ) ;
l e d ( p 6 ) ;
i n p u t ( p 7 ) ;
}
e l
r
g
w
g
w
o n e
4 1
}
n d
o
e r e
e d l
f l a s
/ / e n d
/ / e n d
/ / e n d
}
}
f i f
i f s w i t c h i n p u t
e d i s o f f
h g r e e n l e d
o f
o f
o f
i s
0
e l s e
w h i l e ( 1 )
m a i n
P r o g r a m E x a m p le 3 .3 U s in g if a n d e ls e t o r e s p o n d t o e x t e r n a l s w it c h
Ad ju st th e circu it o f Fig
Th e in p u t it co n n ects to
F ig u r e 3 .6 b s h o w s th e a
switch an d co n n ected to
d irectly . Create a n ew p
an d ru n th e co d e o n th e
n
u r e 3 .4 to
is co n fig u
d d itio n o f
th e b read
ro g ram an
mb ed .
th at o f
red in
th e sw
b o ard .
d co p y
Fig u
th e p
itch .
It is
acro
r e 3 .6 a , b y a d d in g a n S P D T s w
ro g ram as a d ig ital in p u t. Th e p
In th is case, wires h av e b een so
also p o ssib le to fin d switch es th
s s P r o g r a m E x a m p le 3 .3 . C o m p
itch as sh o wn .
h o to g rap h in
ld ered to th e
at p lu g in
ile, d o wn lo ad
E x e r c is e 3 .4
W r it e a p r o g r a m t h a t c r e a t e s a s q u a r e w a v e o u t p u t , a s in E x e r c is e 3 .3 . In c lu d e n o w t w o
p o s s ib le fr e q u e n c ie s , 1 0 0 H z a n d 2 0 0 H z , d e p e n d in g o n a n in p u t s w it c h p o s it io n . U s e
t h e s a m e s w it c h c o n n e c t io n y o u h a v e ju s t u s e d . O b s e r v e t h e o u t p u t o n a n o s c illo s c o p e .
n
n
E x e r c is e 3 .5
T h e c
H o w e
o n -c h
s a m e
ir c
v e
ip
to
u it o f
r, th e
p u ll- d
g g le s
F ig u r e 3
m b e d H
o w n re s
w it c h o r
.6 u
a n d
is t o
a n
s e s a n S P D T
b o o k t e lls u s
r. R e c o n fi g u r
S P S T p u s h b
s w it c h c
th a t th e
e t h e c ir c
u tto n . T e
o n n
D ig
u it
s t t
e c te d
it a lIn
u s in g
h a t th
a s s h
u t ilit
F ig u r
e p ro
o w n in
y a c tu a
e 3 .5 c ,
g ra m r
F ig u r e
lly c o n fi
u s in g e
u n s c o r
3 .5 a .
g u re s a n
it h e r t h e
r e c t ly .
n
Chapter 3
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
m b e d M ic r o c o n tr o lle r
4 2
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
( a ) C o n n e c tio n d ia g r a m
( b ) B r e a d b o a r d b u ild
F ig u r e 3 .6 :
C o n t r o llin g t h e L E D w it h a s w it c h
3 .5
In t e r fa c in g S im p le O p t o D e v ic e s
No w th at we h av e
p o ssib ilities o p en s
Oth ers h av e th eir o
lo o k s at so me simp
th e mb ed . In later
d ev ices.
3 .5 .1
th e ab ility to in p u t an d o u tp u t sin g le b its o f d ata , a wid e ran g e o f
u p . Man y simp le sen so rs can in terface d irectly with d ig ital in p u ts.
wn d esig n ed -in in terfaces, wh ic h p ro d u ce a d ig ital o u tp u t. Th is sectio n
le an d trad itio n al sen so rs an d d isp lay s th at can b e in terfaced d irectly to
ch ap ters th is is tak en fu rth er, co n n ectin g to so me v ery n ew an d h i-tech
O p t o R e fl e c t iv e a n d T r a n s m is s iv e S e n s o r s
O p to - s e n s o r s , s u c h a s s e e n in F ig u r e 3 .7 a a n d b , a r e s im p le e x a m p le s o f s e n s o r s w ith ‘ a lm o s t’
d ig ita l o u tp u ts . W h e n a lig h t falls o n th e b a s e o f a n o p to -tra n s is to r, it c o n d u c ts ; w h e n th e re is n o
lig h t it d o e s n o t. I n th e r e fl e c tiv e s e n s o r ( F ig u r e 3 .7 a ) , a n in f r a r e d L E D is m o u n te d in th e s a m e
p a c k a g e a s a n o p to - tr a n s is to r. W h e n a r e fl e c tiv e s u r fa c e is p la c e d in fr o n t, th e lig h t b o u n c e s
b a c k , a n d th e tr a n s is to r c a n c o n d u c t. I n th e tr a n s m is s iv e s e n s o r ( F ig u r e 3 .7 b ) , th e L E D is
m o u n te d o p p o s ite th e tra n sisto r. With n o th in g in th e w ay , lig h t fro m th e L ED falls d ire ctly o n
th e tra n s is to r, a n d s o it c a n c o n d u ct. W h en s o m e th in g c o m e s in th e w a y th e lig h t is b lo c k e d , a n d
th e tra n s is to r s to p s c o n d u ctin g . T h is s e n s o r is a ls o s o me tim e s c a lle d a s lo tte d o p to -s e n s o r o r
p h o to -in te rru p te r. E a c h s e n s o r c a n b e u s e d to d ete c t c e rta in ty p es o f o b je c t.
D ig it a l In p u t a n d O u t p u t
4 3
R E F L E C T IV E S U R F A C E
O p a q u e O b je c t
P a th o f R e fle c te d L ig h t
B o d y o f
S e n so r
a n o d e
c a th o d e
c o lle c to r
B o d y o f S e n so r
P a th o f T ra n s m itte d L ig h t
e m itte r
( a ) R e fle c tiv e o p to - s e n s o r
( b ) T r a n s m is s iv e o p to - s e n s o r
VS
R1
R2
VC
( c ) S im p le d r iv e c ir c u it fo r o p to - s e n s o r
F ig u r e 3 .7 :
S im p le o p t o - s e n s o r s
E ith e r o f th e s e n s o r s s h o w n c a n b e c o n n e c te d in th e c ir c u it o f F ig u r e 3 .7 c . H e r e , R 1 is
calcu lated to co n tro l th e cu rren t flo win g in th e LED, tak in g su itab le v alu es fro m th e sen so r
d atash ee t. Th e resisto r R 2 is ch o sen to allo w a su itab le o u tp u t v o ltag e swin g , in v o k in g
th e v o lta g e th r e s h o ld s in d ic a te d in F ig u r e 3 .1 . W h e n lig h t f a lls o n th e tr a n s is to r b a s e ,
cu rren t can flo w th ro u g h th e tran sisto r. T h e v alu e o f R2 is ch o sen so th at with th at cu rren t
fl o w in g , th e tr a n s is to r c o lle c to r v o lta g e V C f a lls a lm o s t to 0 V. I f n o c u r r e n t fl o w s , th e n V C
ris e s to V S . In g e n e ra l, th e s e n s o r is m a d e m o re s e n s itiv e b y e ith e r d e c re a s in g R 1 o r
in creasin g R2 .
3 .5 .2
C o n n e c t in g a n O p t o -S e n s o r t o t h e m b e d
F ig u re 3 .8 s h o w s h o w a tr a n s m is s iv e o p to - s e n s o r c a n b e c o n n e c te d to a n m b e d . T h e
d ev ice u sed is a KTIR0 6 2 1 DS, mad e b y Kin g b rig h t; similar d ev ices can b e u sed .
Th e p articu lar sen so r u se d h as p in s th at can p lu g d irectly in to th e b read b o ard . Tak e care
Chapter 3
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
1 0 k
2 2 0 R
K T IR
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
m b e d M ic r o c o n tr o lle r
4 4
( a ) C o n n e c tio n d ia g r a m
( b ) B r e a d b o a r d b u ild
F ig u r e 3 .8 :
m b e d c o n n e c t e d t o a t r a n s m is s iv e o p t o - s e n s o r
w h e n c o n n e c tin g th e s e fo u r p in s . C o n n e c tio n s a re in d ic a te d o n its h o u s in g ; a lte rn a tiv e ly ,
y o u can lo o k th ese u p in th e d ata th at K in g b rig h t p ro v id es. Th is can b e o b tain ed fro m th e
K in g b r ig h t w e b s ite , R e f e r e n c e 3 .2 , o r f r o m th e s u p p lie r y o u u s e to b u y th e s e n s o r.
P r o g r a m E x a m p le 3 .4 c o n tr o ls th is c ir c u it. T h e o u tp u t o f th e s e n s o r is c o n n e c te d to p in 1 2 ,
wh ich is co n fig u red in th e p ro g ram as a d ig ital in p u t. Wh en th ere is n o o b ject in th e sen so r, th en
lig h t falls o n th e p h o to -tran sisto r. It co n d u cts, an d th e sen so r o u tp u t is at Lo g ic 0 . Wh en th e
b eam is in terru p ted , th en th e o u tp u t is at Lo g ic 1 . Th e p ro g ram th erefo re switch es th e LED o n
w h e n th e b e a m is in te r r u p te d , i.e . a n o b je c t h a s b e e n s e n s e d . To m a k e th e s e le c tio n , w e u s e th e
if an d e ls e k ey wo rd s, as in th e p rev io u s ex amp le. No w, h o wev er, th ere is ju st o n e lin e o f co d e to
b e ex ecu ted fo r eith er state. It is n o t n ecessary to u se b races to co n tain th ese sin g le lin es.
/ * P r o g r a m
a c c o r d i n g
* /
E x a m p l e 3 . 4 : S i m p l e
t o s t a t e o f s e n s o r
p r o g r a m
t o
t e s t
K T I R
s l o t t e d
o p t o s e n s o r .
# i n c l u d e " m b e d . h "
D i g i t a l O u t r e d l e d ( p 5 ) ;
D i g i t a l I n o p t o _ s w i t c h ( p 1 2 ) ;
i n t
m a i n (
w h i l e ( 1
i f (
r
)
{
)
{
o p t o _ s w i t c h = = 1 )
e d l e d = 1 ;
e l s e
r e d l e d
}
=
0 ;
/ / i n p u t = 1 i f b e a m i n t e r r u p t e d
/ / s w i t c h l e d o n i f b e a m i n t e r r u p t e d
/ / l e d
o f f i f n o i n t e r r u p t i o n
/ / e n d o f w h i l e
}
P r o g r a m E x a m p le 3 .4 A p p ly in g t h e p h o t o - in t e r r u p t e r
S w i t c h e s
a n
L E D
D ig it a l In p u t a n d O u t p u t
3 .5 .3
S e v e n -S e g m e n t D is p la y s
We h av
to g eth e
co n fig u
matrix
e n o
r, to
ratio
an d
4 5
w u sed sin g le LEDs in sev eral p ro g rams. In ad d itio n , LEDs are o ften p ack ag ed
fo rm p attern s, d ig its o r o th er ty p es o f d isp lay . A n u mb er o f stan d ard
n s are v ery wid ely u sed ; th ese in clu d e b ar g rap h , sev en -seg men t d isp lay , d o t
‘star-b u rst’.
Th e sev en -seg men t d isp lay is a p articu larly v ersatile co n fig u ratio n . An ex amp le sin g le d ig it,
m a d e b y K in g b r ig h t ( R e f e r e n c e 3 .2 ) , is s h o w n in F ig u r e 3 .9 . B y lig h tin g d iff e r e n t
co mb in atio n s o f th e sev en seg men ts, all n u merical d ig its can b e d isp lay ed , as well as
a su rp risin g n u mb er o f alp h ab etic ch aracters. A d ecimal p o in t is u su ally in clu d ed , as sh o wn .
Th is mean s th at th ere are eig h t LEDs in th e d isp lay , n eed in g 1 6 co n n ectio n s. To simp lify
matters, eith er all LED an o d es are co n n ected to g eth er, o r all LED cath o d es. Th is is seen in
F ig u r e 3 .9 b ; th e tw o p o s s ib le c o n n e c tio n p a tte r n s a r e c a lle d c o m m o n c a th o d e o r c o m m o n
a n o d e. No w in stead o f 1 6 co n n ectio n s b ein g n eed ed , th ere are o n ly n in e, o n e fo r each LED
an d o n e fo r th e co mmo n co n n ectio n . Th e actu al p in co n n ectio n s in th e ex amp le sh o wn lie in
two ro ws, at th e to p an d b o tto m o f th e d ig it. Th ere are 1 0 p in s in all, with th e co mmo n an o d e
o r cath o d e tak in g two p in s.
F ig u r e 3 .9 :
T h e s e v e n - s e g m e n t d is p la y . ( Im a g e r e p r o d u c e d w it h p e r m is s io n o f K in g b r ig h t E le c . C o . L t d .)
4 6
Chapter 3
A s m a ll s e v e n - s e g m e n t d is p la y a s s e e n in F ig u r e 3 .9 c a n b e d r iv e n d ir e c tly f r o m
a micro co n tro ller. In th e case o f co mmo n cath o d e, th e cath o d e is co n n ected to g ro u n d , an d
each seg men t is co n n ected to a p o rt p in . If th e seg men ts are co n n ected in th is seq u en ce to
fo rm a b y te,
ðMSBÞ DP g f e d c b a ðLSBÞ
th e n th e v a lu e s s h o w n in Ta b le 3 .4 a p p ly . F o r e x a m p
s e g m e n ts , i.e . a b c d e f , m u s t b e lit, w ith th e c o rr e s p o n d
is to b e d isp la y e d , th en o n ly s eg me n ts b a n d c n ee d to
L E D s c o n n e cte d in s e rie s , fo r e a c h s eg me n t. In th is
e ac h s eries c o m b in atio n a n d , d ep en d in g o n th e s u p p
n o t b e p o s s ib le to d riv e th e d is p la y d ir e c tly .
3 .5 .4
le , if 0 is to b e d
in g b its fro m th e
b e lit. N o te th at
c a s e, a h ig h e r v o
ly v o lta g e o f th e
is p lay ed , th e n a ll o u te r
m ic ro c o n tro lle r s e t to 1 . If 1
larg er d is p lay s h av e s ev e ral
lta g e is n e e d e d to d riv e
micro c o n tro lle r, it ma y
C o n n e c t in g a S e v e n -s e g m e n t D is p la y t o t h e m b e d
A s w e k n o w, th e m b e d r u n s f r o m a 3 .3 V s u p p ly . T h e d a ta s h e e t o f o u r d is p la y ( a c c e s s e d
f r o m R e f e r e n c e 3 .2 ) , s h o w s th a t e a c h L E D r e q u ir e s a r o u n d 1 .8 V a c r o s s it in o r d e r to lig h t.
Th is is with in th e mb ed cap ab ility . If th ere were two LEDs in series in each seg men t
h o wev er, th e mb ed wo u ld b arely b e ab le to switch th em in to co n d u ctio n . Bu t can we ju st
co n n ect th e mb ed o u tp u t d irectly to th e seg men t, o r d o we n eed a cu rren t-limitin g resisto r,
a s s e e n in F ig u r e 3 .3 b ? L o o k in g a t th e L P C 1 7 6 8 d a ta s u m m a r iz e d in A p p e n d ix C , w e s e e
th a t th e o u tp u t v o lta g e o f a p o r t p in d r o p s a r o u n d 0 .4 V, w h e n 4 m A is fl o w in g . T h is
imp lies an o u tp u t resistan ce o f 1 0 0 U . Let’s n o t g et in to th e electro n ics o f all o f th is;
su ffice it to say , th is v alu e is ap p ro x imate, an d o n ly ap p lies in th is reg io n o f o p eratio n .
Ap p ly in g Oh m’s law, th e cu rren t flo w in an LED co n n ected d irectly to a p o rt p in o f th is
ty p e is g iv e n b y
I D y ð 3 :3  1 :8 Þ =1 0 0 ¼
1 5 mA
( 3 .1 )
T a b le 3 .4 : E x a m p le s e v e n - s e g m e n t d is p la y c o n t r o l v a lu e s
D is p la y v a lu e
0
1
2
3
4
5
6
7
8
9
S e g m e n t d r iv e ( B )
(M S B )
0 0 1 1
(L S B )
1 1 1 1
0 0 0 0
0 1 1 0
0 1 0 1
1 0 1 1
0 1 0 0
1 1 1 1
0 1 1 0
0 1 1 0
0 1 1 0
1 1 0 1
0 1 1 1
1 1 0 1
0 0 0 0
0 1 1 1
0 1 1 1
1 1 1 1
0 1 1 0
1 1 1 1
B (h e x)
0 x0 6
0 x5 B
0 x4 F
0 x6 6
0 x6 D
0 x7 D
0 x0 7
0 x7 F
0 x6 F
A c t u a l d is p la y
0 x3 F
D ig it a l In p u t a n d O u t p u t
4 7
Th is cu rren t, 1 5 mA, will lig h t th e seg men ts v ery b rig h tly , b u t is accep tab le. It can b e
red u ced , fo r ex amp le fo r p o wer-co n scio u s ap p licatio n s, b y in sertin g a resisto r in series with
each seg men t.
C o n n e c t a s e v e n - s e g m e n t d is p la y to a n m b e d , u s in g th e c ir c u it o f F ig u r e 3 .1 0 . I n th is s im p le
ap p licatio n th e co mmo n cath o d e is co n n ected d irect to g ro u n d , an d each seg men t is
co n n ected to o n e mb ed o u tp u t.
W
to
In
it is in cremen
C c o d e
fe a tu re
b e d riv e n b y P ro g ra m E x a m p le
O u t allo ws y o u to g ro u p a s et o
w o rd d ire c t to it. T h e e q u iv a le n
ly in g B u s O u t simp ly req u ires y
ck e ts th e p in s wh ic h will b e m
ith in th
w h ile ,
th is ex
ted b y
is p ro g
fo r cre
amp le,
1 . Th e
to 7 -s e g m e n t
d is p la y
ram we see
atin g a co n d
th e v ariab le
n ew v alu e o
p in
p in
p in
p in
p in
p in
p in 1
p in 5
p in s 3
7 , s e
6 , s e
4 , s e
2 , s e
1 , s e
9 , s e
0 , s e
, s e g
,8
g . a
g . b
g . c
g . d
g . e
g . f
g . g
. d p
fo r
itio
i is
f i
3 .5 . T h
f d ig ita
t in p u t
o u to sp
emb ers
th e first
n al lo o p
in itially
is ap p lie
G
V
V
n
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
is first o f
l o u tp u ts
is th e B u
ecify a n
o f th at b
time a fo
. Rev iew
set to 0 ,
d with in
n d
in
b
R
0
1
2
3
4
5
6
7
0
9
8
all ap p lies th e B u s O
in to o n e b u s, so th at
s in o b ject, th o u g h th
ame, in th is case d is p
u s.
r lo o
its fo
an d o
th e lo
m b e d M ic r o c o n tr o lle r
Th e circu it can
API class. B u s
write a d ig ital
u sed h ere. Ap p
th en list in b ra
p . Th is
rmat in
n each
o p . Wh
is an a
Sectio
iteratio
en i re
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
F ig u r e 3 .1 0 :
T h e m b e d c o n n e c t e d t o a c o m m o n c a t h o d e s e v e n - s e g m e n t d is p la y
u t
y o
at
la
m
u
is
y ,
b ed
can
n o t
an d
lte rn a tiv e
n B .7 .2 .
n o f th e lo o p
ach es 4 , th e
4 8
Chapter 3
lo o p termin ates. Ho wev er, as th e fo r lo o p is th e o n ly co d e with in th e en d less w h ile lo o p , it
simp ly starts ag ain .
Th e p ro g ram g o es o n to ap p ly th e s w itc h , c a s e an d b r e a k k ey wo rd s. Use
th ese wo rd s p ro v id e a mech an ism to allo w o n e item to b e ch o sen fro m
d e s c r ib e d in S e c tio n B .6 .2 . I n th is c a s e , a s th e v a r ia b le i is in c r e m e n te d ,
u sed to select th e wo rd th at mu st b e sen t to th e d isp lay , in o rd er fo r th e req u ired d
illu min ate. Th is meth o d o f ch o o sin g o n e v alu e fro m a list is o n e way o f ach iev in g
ta b le, wh ich is an imp o rtan t p ro g rammin g tech n iq u e.
C c o d e
fe a tu re
Th ere are n o w q u ite a few n ested b lo ck s o f co d e in th is ex
with in th e fo r b lo ck wh ich lies in s id e th e w h ile b lo c k , wh
b lo ck . Th e clo sin g b race fo r eac h is co mme n ted in th e p ro g
co mp lex C p ro g rams it b eco mes v ery imp o rtan t to en su re
a c lo sin g b ra c e, a t th e rig h t p la ce .
/ * P r o g r a m E x a m p l e
2 , 3 i n t u r n .
* /
3 . 5 :
S i m p l e
d e m o n s t r a t i o n
# i n c l u d e " m b e d . h "
B u s O u t d i s p l a y ( p 5 , p 6 , p 7 , p 8 , p 9 , p 1 0 , p 1 1 , p 1 2 ) ;
i n t
m a i
w h i l e
f o r
s
n (
( 1
( i
w i
)
o f
/ /
7 - s e g m e n t
s e g m e n t s
d to g eth er,
a list, as
its v alu e is
ig it to
a lo o k-u p
a mp le . Th e s w itc h b lo ck lie s
ich fin ally lies with in th e m a in
ram lis tin g . Wh en writin g mo re
th at each b lo ck is en d ed with
d i s p l a y .
D i s p l a y
d i g i t s
0 ,
1 ,
a , b , c , d , e , f , g , d p
{
)
n t
t c
c
c
c
c
{
a
a
a
a
h
i = 0
( i
s e
s e
s e
s e
;
i < 4 ;
) {
0 :
1 :
2 :
3 :
}
d i s
d i s
d i s
d i s
i + + )
p l a
p l a
p l a
p l a
y
=
y
=
y
=
y
=
{
0 x 3
0 x 0
0 x 5
0 x 4
F ;
6 ;
B ;
F ;
b r e
b r e
b r e
b r e
a k ;
a k ;
a k ;
a k ;
/ / d i s p l a y
/ / d i s p l a y
/ / e n d
0
1
o f
s w i t c h
w a i t ( 0 . 2 ) ;
}
/ / e n d
/ / e n d
/ / e n d
}
}
o f
o f
o f
f o r
w h i l e
m a i n
P r o g r a m E x a m p le 3 .5 U s in g f o r t o s e q u e n c e v a lu e s t o a s e v e n - s e g m e n t d is p la y
C o m p ile , d o w n lo a d a n d r u n th e p r o g r a m . T h e d is p la y s h o u ld a p p e a r s im ila r to F ig u r e 3 .1 1 .
No tice carefu lly , h o wev er, b y lo o k in g at wh ere p in 8 is co n n ected , th at th is is
a c o m m o n a n o d e c o n n e c tio n . I t d o e s n o t e x a c tly r e p lic a te F ig u r e 3 .1 0 . P in 3 h a s b e e n
left u n co n n ected .
n
E x e r c is e 3 .6
W r it e a p r o g r a m w h ic h fl a s h e s t h e le t t e r s H E L P in t u r n o n a s e v e n - s e g m e n t d is p la y ,
u s in g t h e c ir c u it o f F ig u r e 3 .1 0 .
n
D ig it a l In p u t a n d O u t p u t
4 9
F ig u r e 3 .1 1 :
m b e d c o n n e c t e d t o a c o m m o n a n o d e s e v e n - s e g m e n t d is p la y
3 .6
3 .6 .1
S w it c h in g L a r g e r D C L o a d s
A p p ly in g T r a n s is t o r S w it c h in g
T h e m b e d c a n d riv e s im p le D C lo a d s d ire c tly w ith its d ig ita l I/O p in s , a s w e s a w w ith th e
LEDs. Th e mb ed su mmary d ata (rep eated in Ap p en d ix C) tells u s th at a p o rt p in can so u rce u p
to aro u n d 4 0 mA. Ho wev er, th is is a sh o rt circu it cu rren t, so we are u n lik ely to b e ab le to
b en efit fro m it with an actu al electrical lo ad co n n ected to th e p o rt.
I f it is n e c e s s a r y to d r iv e a lo a d es a y a m o to r ew h ic h n e e d s m o r e c u r r e n t th a n a n m b e d p o r t
p in can su p p ly , o r wh ich n eed s to ru n fro m a h ig h er v o ltag e, th en an in terface circu it will b e
n e e d e d . T h r e e p o s s ib ilitie s , w h ic h a llo w D C lo a d s to b e s w itc h e d , a r e s h o w n in F ig u r e 3 .1 2 .
E a c h h a s a n in p u t la b e le d V L, w h ic h is th e lo g ic v o lta g e s u p p lie d fro m th e p o rt p in . T h e fi rs t
tw o c irc u its s h o w h o w a re s is tiv e lo a d , s u c h a s a m o to r o r h e a te r, c a n b e s w itc h e d , u s in g
a b ip o lar tran sisto r an d a metal o x id e semico n d u cto r field effect tran sisto r (MOSFET e
a m o u th fu l, b u t a n im p o rta n t d ev ic e in to d a y ’s e le c tro n ic s ), re s p e c tiv e ly . In th e c a s e o f th e
b ip o la r tra n s is to r, th e s im p le fo rm u la e s h o w n c a n b e a p p lie d to c a lc u la te R B, s ta rtin g w ith
k n o wled g e o f th e req u ired lo ad cu rren t an d th e v alu e o f th e cu rren t g ain (b ) o f th e tran sisto r.
In th e case o f th e MOSFET, th ere is a th resh o ld g ate-to -so u rce v o ltag e, ab o v e wh ich th e
tran sisto r switch es o n . Th is is a p articu larly u sefu l co n fig u ratio n , as th e MOSFET g ate can b e
re a d ily d riv e n b y a m ic ro c o n tro lle r p o rt b it o u tp u t.
5 0
Chapter 3
V
V S
R
R
L
S
L
I L
R
V L
G a te
I L
B
V
b
L
I B
G a te
I B >
I L
b
R B <
- 0 .6
V L
V L
S o u rc e
> V GS (th )
I B
( a ) r e s is tiv e lo a d , n p n tr a n s is to r
( b ) r e s is tiv e lo a d , n - c h a n n e l M O S F E T
V
'f r e e w h e e l i n g ' d i o d e
S
L , w ith re s is ta n c e R
d e c a y in g c u rre n t flo w s in
th is p a th , w h e n tra n s is to r
s w itc h e d o ff.
IL
V
L
( c ) In d u c tiv e L o a d , n - c h a n n e l M O S F E T
F ig u r e 3 .1 2 :
T r a n s is t o r s w it c h in g o f D C lo a d s
F ig u r e 3 .1 2 c s h o w s a n in d u c tiv e lo a d , s u c h a s a s o
imp o rtan t a d d itio n h e re is th e freewh eelin g d io d e.
with cu rren t flo win g in it s to re s en erg y in th e mag
cu rren t is in terru p te d , in th is ca se b y th e tra n sisto r
retu rn e d to th e c ircu it. Th is h a p p e n s th ro u g h th e d
co n tin u e to circu late . If th e d io d e is n o t in clu d e d th
can /will d e stro y th e fie ld effe ct tran s isto r.
3 .6 .2
len o id o r DC mo to r, b ein
Th is is n eed ed b ec au s e a
n etic field th at su rro u n d s
b e in g switch e d o ff, th e e
io d e , wh ich allo ws d ec ay
en a h ig h -v o ltag e tran sien
g switch e d .
n y in d u ctan
it. Wh en th
n erg y h as to
in g cu rren t
t o ccu rs, wh
S w it c h in g a M o t o r w it h t h e m b e d
A g o o d switch in g tran sisto r fo r small DC lo ad s is th e ZVN4 2 0 6 A, wh o se main
c h a r a c te r is tic s a r e lis te d in Ta b le 3 .5 . A n im p o r ta n t v a lu e is th e m a x im u m V GS th r e s h o ld
An
ce
at
b e
to
ich
D ig it a l In p u t a n d O u t p u t
5 1
T a b le 3 .5 : C h a r a c t e r is t ic s o f t h e Z V N 4 2 0 6 A n - c h a n n e l M O S F E T
C h a r a c t e r is t ic
Z V N 4 2 0 6 A
M a x im u m d r a in - s o u r c e v o lt a g e V
6 0 V
D S
M a x im u m g a t e - s o u r c e t h r e s h o ld V
3 V
G S (th )
M a x im u m d r a in - s o u r c e r e s is t a n c e w h e n ‘O n ’. R
1 .5 U
D S (o n )
M a x im u m c o n t in u o u s d r a in c u r r e n t ID
6 0 0 m A
M a x im u m p o w e r d is s ip a t io n
0 .7 W
In p u t c a p a c it a n c e
1 0 0 p F
v a lu e , s h o w n a s 3 V. T h is m e a n s th a t th e M O S F E T w ill r e s p o n d , ju s t, to th e 3 .3 V L o g ic 1
o u tp u t lev el o f th e mb ed .
n
E x e r c is e 3 .7
C o n n e c t t h e c ir c u it o f F ig u r e 3 .1 3 , u s in g
c a n b e s u p p lie d fr o m a n e x t e r n a l b a t t e r y p
n o n - c r it ic a l, a n d d e p e n d s o n t h e m o t o r y
m o t o r s w it c h e s o n a n d o ff c o n t in u o u s ly ,
t h e s w it c h in g u n t il y o u c a n n o lo n g e r d e t
th e m o to r s p e e d c o m p a re to w h e n th e m
3 .6 .3
a s m a ll D C m o t o r . T h e 6 V fo r t h e m o t o r
a c k o r b e n c h p o w e r s u p p ly . It s e x a c t v o lt a g e is
o u a r e u s in g . W r it e a p r o g r a m s o t h a t t h e
s a y 1 s o n , 1 s o ff. In c re a s e th e fre q u e n c y o f
e c t t h a t it is s w it c h in g o n a n d o ff. H o w d o e s
o t o r w a s ju s t le ft s w it c h e d o n ?
n
S w it c h in g M u lt ip le S e v e n -S e g m e n t D is p la y s
6 V
M o to r
IN 4 0 0 1
Z V N 4 2 0 6 A
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
m b e d M ic r o c o n tr o lle r
We saw earlier in th e ch ap ter h o w a sin g le sev en -seg men t d isp lay co u ld b e co n n ected to th e
mb e d . Ea ch d is p la y req u ire d e ig h t c o n n e ctio n s , a n d if we wan te d ma n y d is p lay s , th e n w e
F ig u r e 3 .1 3 :
S w it c h in g a D C m o t o r
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
+ 6 V
5 2
Chapter 3
wo u ld q u ick ly ru n o u t o f I/O p in s . Th ere is a v ery u s efu l tech n iq u e to g e t aro u n d th is
p ro b le m , s h o w n in F ig u r e 3 .1 4 . E a c h s e g m e n t ty p e o n e a c h d is p la y is w ir e d to g e th e r, a s
s h o wn , an d co n n ec te d b a ck to a micro c o n tro ller p in co n fig u red as d ig ital o u tp u t. Th e
c o m m o n c a th o d e o f e a c h d ig it is th e n c o n n e c te d to its o w n d riv e M O S F E T. T h e tim in g
d ia g ra m s h o w n th e n a p p lie s . T h e s e g m e n t d riv e s a re c o n fi g u re d fo r D ig it 1 , a n d th a t d ig it’s
d riv e tra n s is to r is a c tiv a te d , illu m in a tin g th e d ig it. A m o m e n t la te r th e s e g m e n t d riv e s a re
c o n fi g u re d fo r D ig it 2 , a n d th a t d ig it’s d riv e tra n s is to r is a c tiv a te d . T h is c o n tin u e s e n d le s s ly
w ith e a c h d ig it in tu rn . If it is d o n e ra p id ly e n o u g h , th e n th e h u m a n e y e p e rc e iv e s a ll d ig its a s
b e in g co n tin u o u s ly illu min a te d ; a u s e fu l ra te is fo r ea c h d ig it to b e illu min a te d in tu rn fo r
aro u n d 5 ms.
g
f
a /c a
b
a /c c
d p
g
f
a /c a
b
a /c c
d p
g
f
a /c a
b
g
a /c c
d p
f
a /c a
b
a /c c
d p
S e g m e n t
D riv e s
e
d
e
d
e
d
e
d
D ig it 1
D ig it 2
D ig it 3
D ig it 4
Z V N 4 2 0 6 A
Z V N 4 2 0 6 A
Z V N 4 2 0 6 A
Z V N 4 2 0 6 A
D ig it
D riv e s
( a ) D is p la y c o n n e c tio n s
s e g m e n t p a tte rn
fo r D ig it 1
s e g m e n t p a tte rn
fo r D ig it 4
s e g m e n t p a tte rn
fo r D ig it 3
s e g m e n t p a tte rn
fo r D ig it 2
S e g m e n t
D riv e s
D ig it 1
D ig it
D riv e s
D ig it 2
D ig it 3
D ig it 4
( b ) T im in g d ia g r a m
F ig u r e 3 .1 4 :
M u lt ip le x in g s e v e n - s e g m e n t d is p la y s
s e g m e n t p a tte rn
fo r D ig it 4
D ig it a l In p u t a n d O u t p u t
3 .7
U
a
th
‘h
5 3
M in i-P r o je c t : L e t t e r C o u n t e r
se th e slo tted
simp le letter
e sen so r. Cle
alf ’ d ig it, so
o p to
co u n
ar th
y o u
-sen so r, p u sh -b u tto n
ter. In cremen t th e n u
e d isp lay wh en th e p
can co u n t fro m 0 to
sw
mb
u sh
1 9
itch an d o n e sev en -seg men t LED d isp lay to create
er o n th e d isp lay b y o n e ev ery time a letter p asses
b u tto n is p ressed . Use an LED to create an ex tra
.
C h a p t e r R e v ie w
•
Lo g ic sig n als, ex p ressed math ematically as 0 o r 1 , are rep resen ted in d ig ital electro n ic
circu its as v o ltag es. On e ran g e o f v o ltag es rep resen ts 0 , an o th er rep resen ts 1 .
Th e mb ed h as 2 6 d ig ital I/O p in s, wh ich can b e co n fig u red as eith er in p u t o r o u tp u t.
L E D s c a n b e d riv e n d ire c tly fro m th e m b e d d ig ita l o u tp u ts . T h e y a re a u s e fu l m e a n s o f
d isp lay in g a lo g ic v alu e, an d o f co n trib u tin g to a simp le h u man in terface.
Electro mech an ical switch es can b e co n n ected to p ro v id e lo g ic v alu es to d ig ital in p u ts.
A ran g e o f simp le o p to -sen so rs h av e almo st d ig ital o u tp u ts, an d can with care b e co n n ected
d irectly to mb ed p in s.
W h e re th e m b e d p in c a n n o t p ro v id e e n o u g h p o w e r to d riv e a n e le c tric a l lo a d d ire c tly ,
in terface circu its mu st b e u sed . Fo r simp le o n /o ff switch in g a tran sisto r is o ften all th at is
n eed ed .
•
•
•
•
•
Q u iz
1 .
2 .
3 .
C o m p le te Ta b le 3 .6 , c o n v e r tin g b e tw e e n th e d iff e r e n t n u m b e r ty
n u mb ers is an ex amp le.
Is it p o ssib le to d isp lay u n amb ig u o u sly all o f th e cap italized alp h
D , E a n d F o n th e s e v e n - s e g m e n t d is p la y s h o w n in F ig u r e 3 .9 ? F o
b e d is p la y e d , d e te rm in e th e s e g m e n t d riv e v a lu e s . G iv e y o u r a n s
h ex ad ecimal fo rmats.
A lo o p in an mb ed p ro g ram is u n tid ily co d ed as fo llo ws:
p es. Th e first ro w o f
ab et ch aracters A, B, C,
r th o se th at can u sefu lly
wers in b o th b in ary an d
w h i l e ( 1 )
{ r e d l e d = 0 ;
w a i t _ m s ( 1 2 ) ;
g r e e n l e d = 1 ;
w a i t ( 0 . 0 0 2 ) ;
g r e e n l e d = 0 ;
w a i t _ u s ( 2 4 0 0 0 ) ;
}
Wh at is th e to tal p erio d o f th e lo o p , ex p ressed in seco n d s, milliseco n d s an d
micro seco n d s?
5 4
Chapter 3
T a b le 3 .6 :
B in a r y
H e x a d e c im a l
D e c im a l
0 1 0 1 1 1 1 0
5 E
9 4
1 1 0 1
7 7
1 2 9
6 F 2
1 1 0 1 1 1 0 0 1 0 0 1
4 0 9 6
4 .
5 .
6 .
7 .
T h e c ir c u it o f F ig u r e 3 .5 b is u s e d e ig h t tim e s o v e r to c o n n e c t e ig h t s w itc h e s to 8 m b e d
d ig ital in p u ts. Th e p u ll-u p resisto rs h av e a v alu e o f 1 0 k U , an d are co n n ected to th e mb ed
s u p p ly o f 3 .3 V. W h a t c u r r e n t is c o n s u m e d d u e to th is c ir c u it c o n fi g u r a tio n w h e n a ll
s w itc h e s a r e c lo s e d s im u lta n e o u s ly ? I f th is c u r r e n t d r a in m u s t b e lim ite d to 0 .5 m A , to
wh at v alu e mu st th e p u ll-u p resisto rs b e in creased ? Reflect o n th e p o ssib le imp act o f p u llu p resisto rs in lo w-p o wer circu its.
W h a t is th e c u r r e n t ta k e n b y th e d is p la y c o n n e c te d in F ig u r e 3 .1 0 , w h e n th e d ig it 3 is
sh o win g ?
I f in F ig u r e 3 .1 0 a s e g m e n t c u r r e n t o f a p p r o x im a te ly 4 m A w a s r e q u ir e d , w h a t v a lu e o f
resisto r wo u ld n eed to b e in tro d u ced in series with each seg men t?
A stu d en t b u ild s an mb ed -b ased sy stem. To o n e p o rt h e co n n ects th e circu it o f
F ig u r e 3 .1 5 a , u s in g L E D s o f th e ty p e u s e d in F ig u r e 3 .4 , b u t is th e n d is a p p o in te d th a t th e
(a )
3 .3 V
(b )
Z V N 4 2 0 6 A
m b e d p o rt o u tp u t
F ig u r e 3 .1 5 :
T r ia l s w it c h in g c ir c u it s
D ig it a l In p u t a n d O u t p u t
8 .
9 .
LEDs d o n o t ap p ear to lig h t wh en ex p ected . Ex p lain wh y th is is so , an d su g g
c h a n g in g th e c irc u it to g iv e th e re q u ire d b e h a v io r.
An o th er stu d en t wan ts to co n tro l a DC mo to r fro m th e mb ed , an d th erefo re
c ir c u it o f F ig u r e 3 .1 5 b , w h e r e V S is a D C s u p p ly o f a p p r o p r ia te v a lu e . A s a
in d icatio n th at th e mo to r is ru n n in g , sh e co n n ects a stan d ard LED, as seen in
d irectly to th e p o rt b it. Sh e th en co mp lain s o f u n reliab le circu it b eh av io r. E
n ecessary ch an g es th at sh o u ld b e mad e to th e circu it.
L o o k a t th e m b e d c ir c u it d ia g r a m , R e f e r e n c e 2 .2 , a n d fi n d th e f o u r o n b o a r d
Estimate th e cu rren t each o n e tak es wh en ‘o n ’, assu min g a fo rward v o ltag e
References
3 .1 .
3 .2 .
Flo y d , T. (2 0 0 8 ). Dig ital Fu n d amen tals. 1 0 th ed itio n . Pearso n Ed u catio n .
T h e K in g b r ig h t h o m e s ite . h ttp ://w w w.k in g b r ig h t.c o m /
5 5
est a way o f
b u ild s th e
fu rth er
F ig u r e 3 .3 ,
x p lain an y
LEDs.
o f 1 .8 V.
C H A P T E R 4
A n a lo g O u t p u t
C h a p t e r O u t lin e
4 .1 In t r o d u c in g D a t a C o n v e r s io n
5 7
4 .1 .1 T h e D ig it a l- t o - A n a lo g C o n v e r t e r
4 .2 A n a lo g O u t p u t s o n t h e m b e d
4 .2
4 .2
4 .2
4 .2
.1 C r e a t in
.2 S a w to o
.3 T e s t in g
.4 G e n e ra
g C o n sta n t
th W a v e fo r
th e D A C R
t in g a S in e
5 8
5 9
O u t p u t V o lt a g e s
m s 6 0
e s o lu t io n
6 3
W a v e 6 4
6 0
4 .3 A n o t h e r F o r m o f A n a lo g O u t p u t : P u ls e W id t h M o d u la t io n
4 .4 P u ls e W id t h M o d u la t io n o n t h e m b e d
6 6
4 .4
4 .4
4 .4
4 .4
4 .4
4 .4
.1 U s in g t h e m b e d P W M S o u r c e
.2 S o m e T r ia l P W M O u t p u t s 6
.3 S p e e d C o n t r o l o f a S m a ll M o
.4 G e n e r a t in g P W M in S o ft w a r e
.5 S e rv o C o n tro l 7 1
.6 O u t p u t t in g t o a P ie z o T r a n s d
C h a p t e r R e v ie w
Q u iz 7 4
R e fe re n c e 7 5
4 .1
s
6 5
6 6
7
to r
6 8
6 8
u c e r
7 2
7 4
In t r o d u c in g D a t a C o n v e r s io n
Micro co n tro llers are d ig ital d ev ices, b u t th ey sp en d mo st o f th eir time d ealin g with a wo rld
w h ic h is a n a lo g , a s illu s tr a te d in F ig u r e 1 .1 . To m a k e s e n s e o f in c o m in g a n a lo g s ig n a ls , f o r
ex amp le fro m a micro p h o n e o r temp eratu re sen so r, th ey mu st b e ab le to co n v ert th em in to
d ig ital fo rm. After p ro cessin g th e d ata, th ey may th en n eed to co n v ert d ig ital d ata b ack to
a n a lo g fo rm , fo r e x a m p le to d riv e a lo u d s p e a k e r o r d ire c t c u rre n t (D C ) m o to r. We g iv e th e s e
p ro cesses th e g lo b al h ead in g ‘d ata co n v ersio n ’. Tech n iq u es ap p lied in d ata co n v ersio n fo rm
a h u g e an d fascin atin g b ran ch o f electro n ics. Th ey are o u tsid e th e su b ject matter o f th is b o o k ;
h o wev er, if y o u wish to learn mo re ab o u t th em, co n sid er an y majo r electro n ics tex t. To g et
d e e p in to th e to p ic , r e a d R e f e r e n c e 4 .1 .
Wh ile co n v ersio n in b o th d irectio n s b etween d ig ital an d an alo g is n ecessary , it is co n v ersio n
fro m an alo g to d ig ital fo rm th at is th e mo re ch allen g in g task ; th erefo re, in th is ch ap ter th e
F a s t a n d E f f e c t iv e E m b e d d e d S y s t e m s D e s ig n . D O I : 1 0 .1 0 1 6 /B 9 7 8 - 0 - 0 8 - 0 9 7 7 6 8 - 3 .0 0 0 0 4 - 0
Co p y rig h t Ó2 0 1 2 Elsev ier Ltd . All rig h ts reserv ed .
5 7
5 8
Chapter 4
e a s ie r o p tio n is c o n s id e re d : c o n v ers io n fro m d ig ita l to a n a lo g . (An a lo g -to -d ig ita l c o n v ers io n
is c o v e r e d in C h a p te r 5 .)
4 .1 .1
T h e D ig it a l-t o -A n a lo g C o n v e r t e r
A d ig ital-to -an
an alo g o u tp u t.
can , h o wev er,
in p u t, rep resen
th e DAC calcu
v o lta g e.
alo g
Th e
rep re
ted b
lates
c
a
s
y
o n v erter (DAC) is a circu it th at co n v erts
c tu a l c irc u itry in s id e a DAC is c o mp lex ,
en t th e DAC as a b lo ck d iag ram, as in F
D, an d an an alo g o u tp u t, rep resen ted b y
its o u tp u t v o ltag e is a vo lta g e referen ce,
a b
an
ig u
Vo
a p
in ary in p u t n u mb er
d n eed n o t co n cern
r e 4 .1 . T h is h a s a d
. Th e ‘y ard stick ’ b y
rec is e, stab le an d k
in to an
u s. We
ig ital
wh ich
n o wn
Mo st DACs h av e a simp le relatio n sh ip b etween th eir d ig ital in p u t an d an alo g o u tp u t, with
m a n y ( in c lu d in g th e o n e in s id e th e L P C 1 7 6 8 ) a p p ly in g E q u a tio n 4 .1 :
V
o
D
¼
2
V
n
4 .1
r
Here, Vr is th e v alu e o f th e v o ltag e referen ce, D is th e v alu e o f th e b in ary in p u t wo rd , n is th e
n u m b e r o f b its in th a t w o r d , a n d Vo is th e o u tp u t v o lta g e . F ig u r e 4 .2 s h o w s th is e q u a tio n
rep resen ted g rap h ically . Fo r each in p u t d ig ital v alu e, th ere is a co rresp o n d in g an alo g o u tp u t. It
is as if we are creatin g a v o ltag e staircase with th e d ig ital in p u ts. Th e n u mb er o f p o ssib le o u tp u t
v a lu e s is g iv e n b y 2 n , a n d th e s te p s iz e b y V r /2 n ; th is is c a lle d th e re s o lu tio n . T h e m a x im u m
p o ssib le o u tp u t v alu e o ccu rs wh en D ¼ (2 n  1 ), so th e v alu e o f Vr as an o u tp u t is n ev er q u ite
reach ed . Th e ra n g e o f th e DAC is th e d ifferen ce b etween its max imu m an d min imu m o u tp u t
v a lu e s . F o r e x a m p le , a 6 - b it DAC w ill h a v e 6 4 p o s s ib le o u tp u t v a lu e s ; if it h a s a 3 .2 V
r e f e r e n c e , it w ill h a v e a r e s o lu tio n ( s te p s iz e ) o f 5 0 m V.
Fig u re
o u tp u t
p o wer
th erefo
2 .3 s h o w s th a t
ch aracteristic,
su p p ly v o ltag e
r e b e 3 .3 /1 0 2 4
th e
i.e .
, i.e
, i.e
LPC1 7 6 8 h as a 1 0 -b it DAC; th ere will th erefo re b e 2 1 0 step s in its
1 0 2 4 . R e f e r e n c e 2 .4 f u r th e r te lls u s th a t it n o r m a lly u s e s its o w n
. 3 .3 V, a s v o lta g e r e f e r e n c e . T h e s te p s iz e , o r r e s o lu tio n , w ill
. 3 .2 2 m V.
+
D
(n -b it Dig ital
in p u t)
Vo ltag e Referen ce
Dig ital to
An alo g
Co n v erter
+
V
-
O
(An alo g
Ou tp u t)
Co n tro l Lin es
F ig u r e 4 .1 :
T h e d ig it a l- t o - a n a lo g c o n v e r t e r
A n a lo g O u t p u t
5 9
Outp ut Vo lta g e
n
(2 - 1 ) V
r
n
(2 - 2 ) V
3 V
V
2
n
2
n
2
r
r
n
2
r
r
2 V
V
r
n
2
n
0 0 0
0 0 1
0 1 0
0 1 1
n
(2 - 2 )
n
(2 - 1 )
2
n
D (In p u t Bin a ry Nu mb er)
F ig u r e 4 .2 :
T h e D A C in p u t / o u t p u t c h a r a c t e r is t ic
4 .2
A n a lo g O u t p u t s o n t h e m b e d
T h e m b e d p in c o n n e c tio n d ia g r a m , F ig u r e 2 .1 c , h a s a lr e a d y s h o w n u s th a t th e m b e d is r ic h in
a n a l o g i n p u t / o u t p u t c a p a b i l i t i e s . P i n s 1 5 e2 0 c a n b e a n a l o g i n p u t , w h i l e p i n 1 8 i s t h e o n l y
an alo g o u tp u t.
T h e a p p lic a tio n p r o g r a m m in g in te r f a c e ( A P I ) s u m m a r y f o r a n a lo g o u tp u t is s h o w n in Ta b le 4 .1 .
It fo llo w s a p a tte rn s im ila r to th e d ig ita l in p u t a n d o u tp u t u tilitie s w e h av e a lre a d y s e e n . H e re,
w ith A n a lo g O u t, w e c a n in itializ e a n d n a m e a n o u tp u t; u s in g w r ite ( ) o r w r ite _ u 1 6 ( ), w e c a n s e t
th e o u tp u t v o lta g e e ith e r w ith a fl o a tin g p o in t n u m b e r o r w ith a h ex a d e c im a l n u m b e r. F in a lly ,
w e ca n simp ly u se th e ¼ s ig n a s a s h o rth a n d fo r w rite, w h ich is w h a t w e w ill mo stly b e d o in g .
N o tic e in Ta b le 4 .1 th e w a y th a t d a ta ty p e s fl o a t, a n d u n s ig n e d s h o r t a re in v o k e d . I n
C /C þ þ a ll d ata e lem en ts h av e to b e d ec lared b efo re u se ; th e s am e is tru e fo r th e retu rn
ty p e o f a fu n c tio n a n d th e p ara m e te rs it ta k e s . A rev iew o f th e c o n ce p t o f fl o a tin g p o in t
n u m b e r r e p r e s e n ta tio n c a n b e f o u n d in A p p e n d ix A . Ta b le B .4 in A p p e n d ix B s u m m a r iz e s th e
d iffe re n t d a ta ty p e s th a t a re av a ila b le . T h e DAC itse lf re q u ire s a n u n sig n e d b in a ry n u mb e r a s
in p u t, s o th e w r ite _ u 1 6 ( ) fu n c tio n re p re se n ts th e m o re d irec t a p p ro a c h o f w ritin g to it.
C c o d e
fe a tu re
T a b le 4 .1 : A P I s u m m a r y f o r m b e d a n a lo g o u t p u t
F u n c t io n
U s a g e
A n a l o g O u t
C r e a t e a n A n a lo g O u t o b je c t c o n n e c t e d t o t h e s p e c ifi e d p in
w r i t e
S e t t h e o u t p u t v o lt a g e , s p e c ifi e d a s a p e r c e n t a g e ( fl o a t )
w r i t e _ u 1 6
S e t t h e o u t p u t v o lt a g e , r e p r e s e n t e d a s a n u n s ig n e d s h o r t in t h e r a n g e [ 0 x 0 , 0 x F F F F ]
r e a d
R e t u r n t h e c u r r e n t o u t p u t v o lt a g e s e t t in g , m e a s u r e d a s a p e r c e n t a g e ( fl o a t )
o p e r a t o r =
A n o p e r a t o r s h o r t h a n d fo r w r it e ( )
6 0
Chapter 4
We try n o w a few simp le p ro g rams wh ich ap p ly th e mb ed DAC, creatin g first fix ed v o ltag es
an d th en wav efo rms.
4 .2 .1
C r e a t in g C o n s t a n t O u t p u t V o lt a g e s
C re a te a n ew
A o u t, b y u s in
b y s ettin g A o
A o u t tak es a
a ctu a l o u tp u t
y o u o u tp u t is
/ *
R e
* /
# i
A n
i n
p ro g ra m u s in g P ro g ra m E x a m p le 4
g th e A n a lo g O u t u tility . It is th e n
u t to a n y p e rm is sib le v a lu e; w e d o
fl o a tin g p o in t n u m b e r b e tw e e n 0 .0
v o ltag e o n p in 1 8 is b e tw ee n 0 V
s ca le d to th is.
P r o g r a m E x a m p l e 4 . 1 : T h r e e
a d t h e o u t p u t o n a D V M .
n c l
a l o
t m
w h i
A o u
w a i
A o u
w a i
A o u
w a i
}
u d e
g O u
a i n
l e (
t = 0
t ( 2
t = 0
t ( 2
t = 0
t ( 2
t
(
)
.
)
.
)
.
1
" m b e d . h "
A o u t ( p 1 8 ) ;
) {
) {
2 5 ;
;
5 ;
;
7 5 ;
;
/ / c r e a t e
v a l u e s
a n
/ /
0 . 2 5 * 3 . 3 V
/ /
0 . 5 * 3 . 3 V
/ /
0 . 7 5 * 3 . 3 V
o f
D A C
a n a l o g
=
.1 . I n th is w e c r e a te a n a n a lo g o u tp u t la b e le d
p o s sib le to s et th e a n alo g o u tp u t s im p ly
th is th ree tim e s in th e p ro g ra m. B y d e fa u lt,
a n d 1 .0 a n d o u tp u ts th is to p in 1 8 . T h e
a n d 3 .3 V, s o th e fl o a ti n g p o in t n u m b e r th a t
a r e
o u t p u t
o u t p u t
i n
o n
1 8
p i n
t u r n
o n
P i n
1 8 .
=
0 . 8 2 5 V
=
1 . 6 5 V
2 . 4 7 5 V
}
P r o g r a m E x a m p le 4 .1 T r ia l D A C o u t p u t
Co mp ile th e p ro g ram in th e u su al way , an d let it ru n . Co n n ect a d ig ital v o ltmeter (DVM)
b e tw e e n p in s 1 a n d 1 8 o f th e m b e d . Yo u s h o u ld s e e th e th r e e o u tp u t v o lta g e s n a m e d in th e
c o m m e n ts o f P r o g r a m E x a m p le 4 .1 b e in g o u tp u t in tu r n .
n
E x e r c is e 4 .1
A d ju s t P r o g r a m E x a m p le 4 .1 s o t h a t t h e w r it e _ u 1 6 ( ) fu n c t io n is u s e d t o s e t t h e a n a lo g
o u t p u t , g iv in g t h e s a m e o u t p u t v o lt a g e s .
n
4 .2 .2
Sawtooth Waveforms
We will n o w mak e a sawto o th wav e an d v iew it o n an o scillo sco p e. Create a n ew p ro g ram an d
e n te r th e c o d e o f P r o g r a m E x a m p le 4 .2 .
As in man y cases p rev io u sly , th e p ro g ram is mad e u p o f an en d less w h ile (1 ) lo o p . With in th is
th ere is a fo r lo o p . In th is ex amp le, th e v ariab le i is in itially set to 0 , an d o n each iteratio n o f
A n a lo g O u t p u t
6 1
th e lo o p it is in c r e m e n te d b y 0 .1 . T h e n e w v a lu e o f i is a p p lie d w ith in th e lo o p . W h e n i r e a c h e s
1 , th e lo o p termin ates. As th e fo r lo o p is th e o n ly co d e with in th e en d less w h ile lo o p , it
rep eats co n tin u o u sly .
/ *
* /
# i
A n
f l
i n
P r o g r a m
n c l
a l o
o a t
t m
w h i
f
u d e
g O u
i ;
a i n
l e
o r
A o
w a
t
E x a m p l e
4 . 2 :
S a w t o o t h
w a v e f o r m
o n
D A C
o u t p u t .
V i e w
o n
o s c i l l o s c o p e
" m b e d . h "
A o u t ( p 1 8 ) ;
( )
( 1
( i
u t
i t
{
) {
= 0 ; i < 1 ; i = i + 0 . 1 ) {
= i ;
( 0 . 0 0 1 ) ;
/ /
i
/ /
i s
w a i t
i n c r e m e n t e d
1
i n
s t e p s
o f
0 . 1
m i l l i s e c o n d
}
}
}
P r o g r a m E x a m p le 4 .2 S a w t o o t h w a v e fo r m
Co n n ect an o scillo sco p e p ro b e to p in 1 8 o f th e mb ed , with its earth co n n ectio n to p in 1 . Ch eck
th a t y o u g e t a s a w to o th w a v e f o r m s im ila r to th a t s h o w n in F ig u r e 4 .3 . E n s u r e th a t th e d u r a tio n
4 .5
V
D C
4 .0
3 .5
3 .0
2 .5
2 .0
1 .5
1 .0
0 .5
0 .0
-0 .5
0 .0
m s
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
P ic o T e c h n o lo g y w w w .p ic o te c h .c o m
F ig u r e 4 .3 :
A s te p p e d s a w to o th w a v e fo rm
3 .5
4 .0
4 .5
5 .0
6 2
Chapter 4
o f each step is th e 1 ms y o u d efin e in th e p ro g ram, an d try v ary in g th is. Th e wav efo rm sh o u ld
s ta r t f r o m 0 V a n d n o t g o h ig h e r th a n 3 .3 V. E x p la in th e m a x im u m v a lu e y o u s e e .
If y o u d o n o t h av e an o scillo sco p e y o u can set th e wait p arameter to b e mu ch lo n g er (say
1 0 0 m s ) a n d u s e th e DV M ; y o u s h o u ld th e n s e e th e v o lta g e s te p u p f r o m 0 V to 3 .3 V a n d th e n
reset b ack to 0 V ag ain .
n
E x e r c is e 4 .2
Im p r o v e t h e r e s o lu t io n o f y o u r s a w t o o t h b y h a v in g m o r e b u t s m a lle r in c r e m e n t s , i.e .
r e d u c e t h e v a lu e b y w h ic h i in c r e m e n t s . T h e r e s u lt s h o u ld b e a s s e e n in F ig u r e 4 .4 .
n
n
E x e r c is e 4 .3
C r e a t e a n e w p r o je c t a n d d e v is e a p r o g r a m w h ic h o u t p u t s a t r ia n g u la r w a v e fo r m ( i.e . o n e
t h a t c o u n t s d o w n a s w e ll a s u p ) . T h e o s c illo s c o p e o u t p u t s h o u ld lo o k lik e F ig u r e 4 .5 .
n
4 .5
V
D C
4 .0
3 .5
3 .0
2 .5
2 .0
1 .5
1 .0
0 .5
0 .0
-0 .5
0 .0
m s
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
P ic o T e c h n o lo g y w w w .p ic o te c h .c o m
F ig u r e 4 .4 :
A s m o o th s a w to o th w a v e fo rm
3 .5
4 .0
4 .5
5 .0
A n a lo g O u t p u t
6 3
4 .5
V
D C
4 .0
3 .5
3 .0
2 .5
2 .0
1 .5
1 .0
0 .5
0 .0
-0 .5
0 .0
m s
2 .0
1 .0
3 .0
4 .0
5 .0
6 .0
P ic o T e c h n o lo g y w w w .p ic o te c h .c o m
7 .0
8 .0
9 .0
1 0 .0
F ig u r e 4 .5 :
A t r ia n g u la r w a v e fo r m
4 .2 .3
T e s t in g t h e D A C R e s o lu t io n
No w let u s retu rn to th e q u estio n o f th e DAC reso lu tio n , wh ich we to u ch ed o n in Sectio n
4 .1 .1 . Tr y c h a n g in g th e fo r lo o p in y o u r v e r s io n o f P r o g r a m E x a m p le 4 .2 to th is :
f o r
(
A o u
w a i
l e d
i =
t =
t (
1 =
0 ; i < 1 ; i = i + 0 . 0 0 0 1 ) {
i ;
1 ) ;
! l e d 1 ;
}
We h av e also in clu d ed a little LED in d icatio n h ere, so ad d th is lin e b efo re m a in ( ) to set it u p :
D i g i t a l O u t
l e d 1 ( L E D 1 ) ;
Th is ad
will ta k
(h en ce
h o u rs!). Ou r p u rp o
C c o d e
fe a tu re
ju ste
e 1 0
th e p
se is
d p ro g
0 0 0 s
erio d
n o t, h
ram
tep s
o f th
o wev
will p ro d
to reach
e wav efo
er, to v ie
u ce
th e
rm
w th
an ex tre
max imu
is 1 0 0 0
e wav ef
mely slo w sawto o th wav efo rm, wh ich
m v alu e, each o n e tak in g a seco n d
0 seco n d s, o r two an d th ree q u arter
o rm, b u t to ex p lo re carefu lly th e DAC
6 4
Chapter 4
c h a r a c te r is tic o f F ig u re 4 .2 . N o tic e th e u s e o f th e N OT o p e r a to r, in th e f o r m o f a n e x c la m a tio n
mark (!), fo r th e first time . Th is cau ses lo g ical in v ersio n , so a Lo g ic 0 is rep laced b y 1 , an d
v ic e v e rs a .
S w itc h y o u r DV M to its fi n e s t v o lta g e ra n g e , s o th a t y o u h a v e m illiv o lt re s o lu tio n o n th e
sca le ; th e 2 0 0 mV scale is u sefu l h ere. C o n n ect th e DVM b etween p in s 1 an d 1 8 , an d
ru n th e p ro g ra m. Th e LED ch an g es state ev ery time a n ew v alu e is o u tp u t to th e DAC.
H o w e v e r, y o u w ill n o tic e a n in te r e s tin g th in g . Yo u r DV M r e a d in g d o e s n o t c h a n g e w ith
ev ery LED ch an g e. In stead it ch an g es s tate in d istin ct step s, with each ch an g e b ein g
a r o u n d 3 m V. Yo u w ill n o tic e a ls o th a t it ta k e s a r o u n d fi v e L E D ‘ b lin k s ’ , i.e . 1 0 u p d a te s
o f th e DAC v a lu e , b e fo re e a c h DAC c h a n g e . A ll th is w a s a n tic ip a te d in S e c tio n 4 .1 .1 ,
w h e r e a s te p s iz e o f 3 .2 2 m V w a s p r e d ic te d ; e a c h s te p s iz e is e q u a l to th e DAC
reso lu tio n . Th e flo at v alu e is ro u n d ed to th e n earest d ig ital in p u t to th e DAC, an d it
ta k e s aro u n d 1 0 in c reme n ts o f th e flo at v alu e fo r th e DAC d ig ital in p u t to b e
in cremen ted b y o n e.
4 .2 .4
G e n e r a t in g a S in e W a v e
It is an easy step fro m h ere to g en erate a sin e wav e. We will ap p ly th e s in ( )
f u n c tio n , w h ic h is p a r t o f th e C s ta n d a r d lib r a r y ( s e e S e c tio n B .9 .2 ) . Ta k e a lo o k a t
P r o g r a m E x a m p le 4 .3 . To p r o d u c e o n e c y c le o f th e s in e w a v e , w e w a n t to ta k e
s i n e v a l u e s o f a n u m b e r w h i c h i n c r e a s e s f r o m 0 t o 2 p$ r a d i a n s . I n t h i s p r o g r a m w e u s e a f o r
lo o p to in cremen t v ariab le i fro m 0 to 2 in small step s, an d th en mu ltip ly th at n u mb er b y
p wh en th e sin e v alu e is calcu lated . Th ere are o th er way s o f g ettin g th is resu lt. Th e fin al
c h a lle n g e is th a t th e DAC c a n n o t o u tp u t n e g a tiv e v a lu e s . T h e re fo re , w e a d d a fi x e d v a lu e (a n
o ffs e t) o f 0 .5 to th e n u m b e r b e in g s e n t to th e DAC ; th is e n s u r e s th a t a ll o u tp u t v a lu e s
lie with in its av ailab le ran g e. No tice th at we are u sin g th e mu ltip ly o p erato r, * , fo r th e
first time.
C c o d e
fe a tu re
/ *
* /
# i
A n
f l
i n
P r o g r a m
n c l
a l o
o a t
t m
w h
u d e
g O u
i ;
a i n
i l e
f o r
A
w
}
t
E x a m p l e
4 . 3 :
S i n e
w a v e
o n
D A C
o u t p u t .
V i e w
o n
o s c i l l o s c o p e
" m b e d . h "
A o u t ( p 1 8 ) ;
( )
( 1
(
o u
a i
{
)
{
i = 0 ; i < 2 ; i = i + 0 . 0 5 ) {
t = 0 . 5 + 0 . 5 * s i n ( i * 3 . 1 4 1 5 9 ) ;
t ( . 0 0 1 ) ;
/ /
/ /
C o m p u t e t h e s i n e v a l u e , + h a l f
C o n t r o l s t h e s i n e w a v e p e r i o d
}
}
P r o g r a m E x a m p le 4 .3 G e n e r a t in g a s in u s o id a l w a v e fo r m
t h e
r a n g e
A n a lo g O u t p u t
n
E x e r c is e 4 .4
O b s e rv e o n th e o s
it s fr e q u e n c y fr o m
a g r e e ? T r y v a r y in g
fre q u e n c y y o u c a n
4 .3
6 5
c illo s c o
in fo r m
th e fre
a c h ie v
p e th e
a t io n
q u e n c
e w it h
s in e
in t h
y b y
t h is
w a v e t h a t P r o g r a m E x a m p le 4 .3 p r o d u c e s . E s t im a t e
e p r o g r a m , a n d t h e n m e a s u r e it . D o t h e t w o v a lu e s
v a r y in g t h e w a it p a r a m e t e r . W h a t is t h e m a x im u m
p ro g ra m ?
n
A n o t h e r F o r m o f A n a lo g O u t p u t : P u ls e W id t h M o d u la t io n
T h e DAC is a fi n e c irc u it a n d it h a s m a n y u s e s . Ye t it a d d s c o m p le x ity to a m ic ro c o n tro lle r,
an d so metimes mo v es u s to th e an alo g d o main b efo re we are read y to g o . Pu lse wid th
m o d u la tio n (P W M ) is a n a lte rn a tiv e ; in d e e d , th e m b e d h a s s ix P W M o u tp u ts b u t o n ly o n e
an alo g o u tp u t. PWM rep resen ts a n eat an d remark ab ly simp le way o f g ettin g a rectan g u lar
d ig ital wav efo rm to co n tro l an an alo g v ariab le, u su ally v o ltag e o r cu rren t. PWM co n tro l is
u sed in a v ariety o f ap p licatio n s, ran g in g fro m teleco mmu n icatio n s to ro b o tic co n tro l.
A P W M s ig n a l is s h o w n in F ig u r e 4 .6 . T h e p e r io d is n o r m a lly k e p t c o n s ta n t, a n d th e p u ls e
wid th , o r ‘o n ’ time, is v aried , h en ce th e n ame. Th e d u ty cycle is th e p ro p o rtio n o f time th at th e
p u ls e is ‘ o n ’ o r ‘ h ig h ’ , a n d is e x p r e s s e d a s a p e r c e n ta g e , i.e .:
d u ty cy cle ¼
p u lse o n time
 1 0 0 %
p u lse p erio d
A 1 0 0 % d u ty cy cle th e re fo re me an s ‘c o n tin u o u sly o n ’ a n d a 0 %
o ff ’. P WM stre ams a re e as ily g en e ra te d b y d ig ita l co u n te rs a n d
b e d e s ig n e d in to a m ic ro co n tro lle r. T h ey c an a ls o b e p ro d u c e d
a s ta n d a rd d ig ita l o u tp u t, w ith n o d e d ic a te d h a rd w a re a t a ll. T h
W h a tev e r d u ty cy c le
in F ig u r e 4 .6 . I f th e o
v a lu e is h ig h . B y c o n
u sin g PWM, it is th is
a P WM s tre am h as , th ere is
n time is s ma ll, th e av erag e
tro llin g th e d u ty cy c le, th e
av e ra g e th a t w e a re u su a lly
'o n ' t im e , t o
an av erag e v a
v a lu e is lo w ;
re fo re , th is av
in te re sted in .
n
A v e ra g e
v a lu e
P e r io d , T
F ig u r e 4 .6 :
A P W M w a v e fo rm
d u ty
co m
s imp
is is
4 .2
cy c le m e a n s ‘c o n tin u o u s ly
p arato rs , w h ich c an re ad ily
ly b y p ro g ra m lo o p s a n d
s ee n la te r in th e c h a p ter.
lu e , a s in
if th e o n
erag e v a
It can b e
d ic ate d b y th e d o tted lin e
time is larg e, th e av e ra g e
lu e is co n tro lle d . Wh en
ex tra cted fro m th e P WM
6 6
Chapter 4
VS
S
R
Vin
R
D
Vo u t
L
C
I
( a ) A r e s is to r - c a p a c ito r lo w - p a s s filte r
( b ) A n in d u c tiv e lo a d
F ig u r e 4 .7 :
S im p le a v e r a g in g c ir c u it s
s tr e a m in a n u m b e r o f w a y s . E le c tr ic a lly , w e c a n u s e a lo w - p a s s fi lte r, e .g . th e r e s is to r - c a p a c ito r
c o m b in a tio n o f F ig u r e 4 .7 a . I n th is c a s e , a n d a s lo n g a s P W M f r e q u e n c y a n d v a lu e s o f R a n d C a r e
a p p ro p ria te ly c h o s e n , Vo u t b e c o me s a n a n a lo g o u tp u t, w ith a b it o f rip p le , a n d th e c o m b in a tio n o f
P W M a n d fi lte r a c ts ju s t lik e a DAC . A lte r n a tiv e ly , if w e s w itc h th e c u r re n t fl o w in g in a n
in d u c tiv e lo a d , a s s e e n in F ig u r e 4 .7 b , th e n th e in d u c ta n c e h a s a n a v e r a g in g e f f e c t o n th e c u r r e n t
fl o w in g th r o u g h it. T h is is v e ry im p o r ta n t, a s th e w in d in g s o f a n y m o to r a re in d u c tiv e , s o th is
te c h n iq u e c a n b e u s e d f o r m o to r c o n tr o l. T h e s w itc h in F ig u r e 4 .7 b is c o n tr o lle d b y th e P W M
s tre a m, a n d c a n b e a tra n s is to r. We h av e in c id e n ta lly to in tro d u c e th e fre ew h e e lin g d io d e , ju s t a s
w e d id in F ig u r e 3 .1 2 c , to p r o v id e a c u r r e n t p a th a s th e s w itc h o p e n s .
In p ractice, th is
in tern al in ertias
a co n v en tio n al fi
av erag e temp era
electric
wh ich ,
lamen t
tu re o f
al filterin g is n o t alwa
in reality , act lik e lo w
lig h t b u lb with PWM.
th e b u lb filamen t, an d
y s req u ired .
-p ass filters.
In th is case,
th e d immin
Man y p
We can
v ary in g
g effect
h y sical sy stems h av e
, fo r ex amp le, d im
th e p u lse wid th v aries th e
is ach iev ed .
As an ex amp le, th e co n tro l o f a DC mo to r is a v ery co mmo n task in ro b o tics; th e sp eed o f a DC
mo to r is p ro p o rtio n al to th e ap p lied DC v o ltag e. We co u ld u se a co n v en tio n al DAC o u tp u t,
d riv e it th ro u g h a n e x p e n s iv e a n d b u lk y p o w e r a m p lifi e r, a n d u s e th e a m p lifi e r o u tp u t to
d riv e th e m o to r. A lte rn a tiv e ly , a P W M s ig n a l c a n b e u s e d to d riv e a p o w e r tra n s is to r d ire c tly ,
w h ic h r e p la c e s th e s w itc h in F ig u r e 4 .7 b ; th e m o to r is th e in d u c to r /r e s is to r c o m b in a tio n in
th e same circu it. Th is tech n iq u e is tak en mu ch farth er in th e field o f p o wer electro n ics, with th e
PWM co n cep t b ein g tak en far b ey o n d th ese simp le b u t u sefu l ap p licatio n s.
4 .4
4 .4 .1
P u ls e W id t h M o d u la t io n o n t h e m b e d
U s in g t h e m b e d P W M S o u r c e s
A s m e n tio n e d , it is e a s y to g e n e ra te P W M p u ls e s tre a m s u s in g s im p le d ig ita l b u ild in g
b lo ck s. We d o n o t ex p lo re h o w th at is d o n e in th is b o o k , b u t y o u can fin d th e in fo rmatio n
A n a lo g O u t p u t
6 7
T a b le 4 .2 : A P I s u m m a r y f o r P W M o u t p u t
F u n c t io n
U s a g e
P w m O u t
C r e a t e a P w m O u t o b je c t c o n n e c t e d t o t h e s p e c ifi e d p in
w r i t e
S e t t h e o u t p u t d u t y c y c l e , s p e c i fi e d a s a n o r m a l i z e d fl o a t ( 0 . 0 e1 . 0 )
r e a d
R e t u r n t h e c u r r e n t o u t p u t d u t y - c y c l e s e t t i n g , m e a s u r e d a s a n o r m a l i z e d fl o a t ( 0 . 0 e1 . 0 )
p e r i o d
S e t t h e P W M p e r io d , s p e c ifi e d in s e c o n d s ( fl o a t ) , k e e p in g t h e d u t y c y c le t h e s a m e
p e r i o d _ m s
S e t t h e P W M p e r io d , s p e c ifi e d in m illis e c o n d s ( in t ) , k e e p in g t h e d u t y c y c le t h e s a m e
p e r i o d _ u s
S e t t h e P W M p e r io d , s p e c ifi e d in m ic r o s e c o n d s ( in t ) , k e e p in g t h e d u t y c y c le t h e s a m e
p u l s e w i d t h
S e t t h e P W M p u ls e w id t h , s p e c ifi e d in s e c o n d s ( fl o a t ) , k e e p in g t h e p e r io d t h e s a m e
p u l s e w i d t h _ m s
S e t t h e P W M p u ls e w id t h , s p e c ifi e d in m illis e c o n d s ( in t ) , k e e p in g t h e p e r io d t h e s a m e
p u l s e w i d t h _ u s
S e t t h e P W M p u ls e w id t h , s p e c ifi e d m ic r o s e c o n d s ( in t ) , k e e p in g t h e p e r io d t h e s a m e
o p e r a t o r =
A n o p e r a t o r s h o r t h a n d fo r w r it e ( )
elsewh ere
th at th ere
LPC1 7 6 8
freq u en cy
if y o u
are six
micro c
; if th e
wish , fo r
PWM o u
o n tro ller,
p erio d is
ex amp le
tp u ts av a
an d h en c
ch an g ed
in R e f e r e n c e 1 .1 . A s f o r th e m b e d , F ig u r e 2 .1 c s h o w s
ila b le , fro m p in 2 1 to 2 6 in c lu s iv e . N o te th a t o n th e
e th e mb ed , th e PWM so u rces all sh are th e same p erio d /
fo r o n e, th en it is ch an g ed fo r all.
A s w ith a ll p e rip h e ra ls , th e m b e d P WM p o rts a re s u p p o rte d b y lib ra ry u tilitie s a n d fu n c tio n s , a s
s h o w n in Ta b le 4 .2 . T h is is r a th e r m o r e c o m p le x th a n th e A P I ta b le s w e h a v e s e e n s o f a r, s o w e c a n
e x p e c t a little m o re c o m p le x ity in its u s e . N o ta b ly , in s te a d o f h av in g ju s t o n e v a riab le to c o n tro l
( e .g . a d ig ita l o r a n a n a lo g o u tp u t) , w e n o w h a v e tw o , p e r io d a n d p u ls e w id th , o r d u ty c y c le
d e r iv e d f ro m th is c o m b in a tio n . S im ila r to p re v io u s e x a m p le s , a P W M o u tp u t c a n b e e s ta b lis h e d ,
n a m e d a n d a llo c ate d to a p in u s in g P w m O u t. S u b s e q u e n tly , it c a n b e v a rie d , b y s e ttin g its p erio d ,
d u ty cy cle o r p u ls e w id th . A s s h o rth an d , th e w r ite ( ) fu n c tio n c an s im p ly b e rep la ce d b y ¼ .
4 .4 .2
S o m e T r ia l P W M O u t p u t s
As a first p ro g ram u sin g an mb ed PWM so u rce, let’s create a sig n al th at can b e seen o n an
o s c illo s c o p e . M a k e a n e w p r o je c t, a n d e n te r th e c o d e o f P r o g r a m E x a m p le 4 .4 . T h is w ill
g e n e r a te a 1 0 0 H z p u ls e w ith 5 0 % d u ty c y c le , i.e . a p e r f e c t s q u a r e w a v e .
/ *
* /
# i
P w
i n
S e t s
P W M
n c l
m O u
t m
P W
P W
"
W M
( )
p e
0 .
u d e
t P
a i n
M 1 .
M 1 =
s o u r c e
t o
m b e d . h "
1 ( p 2 1 ) ;
{
r i o d ( 0 . 0 1 0 ) ;
5 ;
f i x e d
f r e q u e n c y
/ / c r e a t e
/ /
/ /
s e t
s e t
a
a n d
P W M
d u t y
c y c l e .
O b s e r v e
o u t p u t
o u t p u t
c a l l e d
P W M 1
p i n
P W M p e r i o d
d u t y c y c l e
}
P r o g r a m E x a m p le 4 .4 T r ia l P W M o u t p u t
t o
t o
1 0 m s
5 0 %
o n
2 1
o n
o s c i l l o s c o p e .
6 8
Chapter 4
In th is p ro
h av e to u s
a d ecimal
time with
g ram
e th e
n u mb
th e fo
ex amp le we first set th e PWM p erio d . Th ere is n o sh o rth an d fo r th is, so we
f u ll P W M 1 .p e r io d ( 0 .0 1 0 ) ; s ta te m e n t. We th e n d e fi n e th e d u ty c y c le a s
er, b etween th e v alu es o f 0 an d 1 . We co u ld also set th e d u ty cy cle as a p u lse
llo win g :
P W M 1 . p u l s e w i d t h _ m s ( 5 ) ;
/ /
s e t
P W M
p u l s e w i d t h
t o
5
m s
Wh en y o u ru n th e p ro g ram y o u sh o u ld b e ab le to see th e sq u are wav e o n th e o scillo sco p e, an d
v erify th e o u tp u t freq u en cy .
E x e r c is e 4 .5
n
1 . C h a n
(2 0 %
e x a m
2 . C h a n
p u ls e
g
)
p
g
w
e th e d u ty c y c
a n d 0 .8 ( 8 0 %
le a s s h o w n in
e th e p ro g ra m
id t h _ m s ( ) .
le o f P r o g r a m E x a m p le 4 .4 t o s o m e d iffe r e n t v a lu e s , s a y 0 .2
) , a n d c h e c k t h e c o r r e c t d is p la y is s e e n o n t h e o s c illo s c o p e , fo r
F ig u r e 4 .8 .
t o g iv e t h e s a m e o u t p u t w a v e fo r m s , b u t u s in g p e r io d _ m s ( ) a n d
n
4 .4 .3
S p e e d C o n t r o l o f a S m a ll M o t o r
We will n o w ap p ly th e mb ed PWM so u rce to co n tro l mo to r sp eed . Use th e simp le mo to r
c ir c u it w e u s e d in th e p r e v io u s c h a p te r ( F ig u r e 3 .1 3 ) , a n d c r e a te a p r o je c t u s in g P r o g r a m
E x a m p le 4 .5 . T h is r a m p s th e P W M u p f r o m a d u ty c y c le o f 0 % to 1 0 0 % , u s in g p r o g r a m m in g
featu res th at we k n o w.
/ *
* /
# i
P w
f l
i n
P r o g r a m
n c l
m O u
o a t
t m
P W M
w h i
f
u d e
t P
i ;
a i n
1 . p
l e (
o r
P W
w a
E x a m p l e
4 . 5 :
P W M
c o n t r o l
t o
D C
m o t o r
i s
r e p e a t e d l y
p e r i o d
t o
1 0
r a m p e d
" m b e d . h "
W M 1 ( p 2 1 ) ;
( )
e r
1 )
( i
M 1
i t
{
i o d ( 0 . 0 1 0 ) ;
{
= 0 ; i < 1 ; i = i + 0 . 0 1 )
= i ;
( 0 . 2 ) ;
/ / s e t
P W M
m s
{
/ /
u p d a t e
P W M
d u t y
c y c l e
}
}
}
P r o g r a m E x a m p le 4 .5 C o n t r o llin g m o t o r s p e e d w it h m b e d P W M s o u r c e
Co mp ile, d o wn lo ad an d ru n th e p ro g ram. See h o w y o u r mo to r p erfo rms an d o b serv e th e
w a v e f o r m o n th e o s c illo s c o p e . Yo u a r e s e e in g o n e o f th e c la s s ic a p p lic a tio n s o f P W M ,
co n tro llin g mo to r sp eed th ro u g h a simp le d ig ital p u lse stream.
A n a lo g O u t p u t
4 .0
V
D C
3 .5
3 .0
2 .5
2 .0
1 .5
1 .0
0 .5
0 .0
-0 .5
-1 .0
0 .0
m s
0 .5
0 .0
m s
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
3 .5
4 .0
4 .5
5 .0
3 .0
3 .5
2 .0
2 .5
1 .5
P ic o T e c h n o lo g y w w w .p ic o te c h .c o m
4 .0
4 .5
5 .0
P ic o T e c h n o lo g y w w w .p ic o te c h .c o m
4 .0
V
D C
3 .5
3 .0
2 .5
2 .0
1 .5
1 .0
0 .5
0 .0
-0 .5
-1 .0
1 .0
F ig u r e 4 .8 :
P W M o b s e rv e d fro m th e m b e d o u tp u t
6 9
7 0
Chapter 4
4 .4 .4
G e n e r a t in g P W M in S o ft w a r e
Alth o u g h
th at th ese
an d so me
mo to r o n
say 1 ms
we
are
tim
an d
o n ,
h av e ju st u sed PWM so u rces th at are av ailab le o n th e mb ed , it is u sefu l to realize
n o t essen tial fo r creatin g PWM; we can actu ally d o it ju st with a d ig ital o u tp u t
in g . I n E x e r c is e 3 .8 y o u w e r e a s k e d to w r ite a p r o g r a m th a t s w itc h e d a s m a ll D C
o ff co n tin u o u sly , 1 seco n d o n , 1 seco n d o ff. If y o u sp eed th is switch in g u p , to
1 ms o ff, y o u will immed iately h av e a PWM so u rce.
C r e a te a n e w p r o je c t w ith P r o g r a m E x a m p le 4 .6 a s s o u r c e c o d e . N o tic e c a r e f u lly w h a t th e
p ro g ra m d o e s. T h e re a re two fo r lo o p s in th e m a in w h ile lo o p . T h e m o to r is in itia lly s witc h e d o ff
fo r 5 s . T h e fi rs t fo r lo o p th e n s witc h e s th e m o to r o n fo r 4 0 0 m s a n d o ff fo r 6 0 0 u s; it d o e s th is
5 0 0 0 tim e s . T h is re s u lts in a P WM s ig n a l o f p e rio d 1 m s a n d d u ty cy cle 4 0 % . T h e s e c o n d fo r lo o p
s witc h e s th e m o to r o n fo r 8 0 0 m s a n d o ff fo r 2 0 0 u s ; th e p e rio d is s till 1 m s b u t th e d u ty cy cle is
8 0 % . T h e m o to r is th e n s witc h e d fu ll o n fo r 5 s . T h is s e q u e n ce c o n tin u es in d e fin itely .
/ *
o n
* /
# i
D i
i n
i n
w
P r o g r a m E x a m p l e 4 . 6 : S o f t w a r e g e n e r a t e d
a n d o f f i n c l u d e d f o r c o m p a r i s o n .
n c l
g i t
t i
t m
h i l
P W M .
2
P W M
v a l u e s
g e n e r a t e d
i n
t u r n ,
w i t h
f u l l
u d e " m b e d . h "
a l O u t m o t o r ( p 6 ) ;
;
a i
e (
m o
w a
f o
n (
1 )
t o
i t
r
m o
w a
m o
w a
)
{
{
r
(
t
i
t
i
=
( 5
i =
o r
t _
o r
t _
0
) ;
0 ;
=
u s
=
u s
;
i <
1
( 4
0
( 6
/ / m o t o r
5 0 0 0 ; i = i + 1 )
{
/ / 5 0 0 0
s w i t c h e d
P W M
o f f
c y c l e s ,
f o r
l o w
5
s e c s
d u t y
c y c l e
;
0 0 ) ;
;
0 0 ) ;
/ / o u t p u t
h i g h
/ / o u t p u t
l o w
f o r
f o r
4 0 0 u s
6 0 0 u s
}
f o r
(
m o t
w a i
m o t
w a i
i = 0 ; i < 5 0 0 0 ; i = i + 1 )
o r = 1 ;
t _ u s ( 8 0 0 ) ;
o r = 0 ;
t _ u s ( 2 0 0 ) ;
{
/ / 5 0 0 0
P W M
c y c l e s ,
/ / o u t p u t
h i g h
/ / o u t p u t
l o w
f o r
h i g h
d u t y
c y c l e
8 0 0 u s
f o r
2 0 0 u s
s w i t c h e d
f u l l y
}
m o t o r = 1 ;
w a i t ( 5 ) ;
/ / m o t o r
o n
f o r
5
s e c s
}
}
P r o g r a m E x a m p le 4 .6 G e n e r a t in g P W M in s o f t w a r e
Co mp ile an d d o wn lo ad th is p ro g ram, an d ap p ly th
th at th e mo to r ru n s with th e sp eed p ro file in d icate
r e a d ily v e r ifi e d o n th e o s c illo s c o p e . Yo u m a y w o n
d ed icated PWM p o rts, wh en it seems q u ite easy to
h o wev er, th at in th is ex amp le th e cen tral p ro cessin g
e c ir c u it o f F ig u r e 3 .1 2 . Yo u s h o u ld fi n d
d . PWM p erio d an d d u ty cy cle can b e
d er wh eth er it is n ecessary to h av e
g en erate PWM with so ftware. Rememb er,
u n it (CPU) b eco mes to tally co mmitted to
A n a lo g O u t p u t
7 1
th is task an d can d o n o th in g else. With th e h ard ware p o rts, we can set th e PWM ru n n in g an d
th e C P U c a n th e n g e t o n w ith s o m e c o m p le te ly d iffe re n t a c tiv ity .
n
E x e r c is e 4 .6
V a ry th e m o to r
in it ia lly k e e p in g
p r o b a b ly fi n d t h
o w n fr ic t io n . T h
o s c illo s c o p e , a n
a t m u c h h ig h e r
s p e e d s in P r o g r a m E x a m p le 4 .6 b y
t h e fr e q u e n c y c o n s t a n t . D e p e n d in
a t fo r s m a ll v a lu e s o f d u t y c y c le t h e
is is p a r t ic u la r ly t r u e o f g e a r e d m o t
d c o n fi r m t h a t o n a n d o ff t im e s a r e
a n d lo w e r fr e q u e n c ie s .
c h a n g in g t h e d u t y c y c le o f t h e P W
g o n t h e m o t o r y o u u s e , y o u w ill
m o t o r w ill n o t r u n a t a ll, o w in g t o
o rs . O b s e rv e th e P W M o u tp u t o n
a s in d ic a t e d in t h e p r o g r a m . T r y a
M ,
it s
th e
ls o
n
4 .4 .5
Servo Control
A s e rv o is a s m a ll ro ta ry p o s itio n c o n tro l d ev ic e , o fte n u s e d in ra d io -co n tro lle d c a rs a n d a irc ra ft to
c o n tro l th e a n g u la r p o s itio n o f v a ria b le s s u c h a s s te e rin g , e lev a to rs a n d ru d d e rs . S e rv o s a re n o w
p o p u la r in c e rta in ro b o tic a p p lic a tio n s . T h e s e rv o s h a ft c an b e p o s itio n e d a t s p e c ific a n g u la r
p o s itio n s b y s en d in g th e s e rv o a P WM s ig n a l. A s lo n g a s th e m o d u la te d s ig n a l ex is ts o n th e in p u t
lin e , th e s erv o w ill m ain ta in th e a n g u la r p o s itio n o f th e s h a ft. A s th e m o d u la te d s ig n a l c h a n g es ,
th e a n g u la r p o s itio n o f th e s h a f t c h a n g e s . T h is is illu s tr a te d in F ig u r e 4 .9 . M a n y s e r v o s u s e
a P WM s ig n al w ith a 2 0 ms p erio d , a s is s h o w n h ere . In th is ex amp le, th e p u lse w id th is
m o d u la te d f r o m 1 .2 5 m s to 1 .7 5 m s to g iv e th e f u ll 1 8 0 d e g r e e r a n g e o f th e s e r v o .
C o n n e c t a s e r v o to th e m b e d a s in d ic a te d in F ig u r e 4 .1 0 . T h e s e r v o r e q u ir e s a h ig h e r c u r r e n t
th a n th e u n iv e rs a l s e ria l b u s (U S B ) s ta n d a rd c a n p ro v id e , s o it is e s s e n tia l th a t y o u p o w e r
2 0 ms
0 d eg rees
1 .2 5 m s
0
1 .5 m s
1 .7 5 m s
F ig u r e 4 .9 :
S e r v o c h a r a c t e r is t ic s
9 0 d eg rees
1 8 0 d eg rees
Chapter 4
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
+ 6 V
B A T 1
6
m b e d M ic r o c o n tr o lle r
5
S V 1
6 v
G n d
V in
V b
n R
P V V M
6 v
0 v
7 2
+ 6 V
( a ) C ir c u it D ia g r a m
( b ) C o n s tr u c tio n D e ta il
F ig u r e 4 .1 0 :
U s in g P W M t o d r iv e a s e r v o
it u sin g an ex tern al su p p ly . A 4  AA (6 V)
serv o . No te th at th e mb ed itself can still b e s
p ack v o ltag e lies with in th e p ermissib le in p u
th e mb ed co u ld b e su p p lied fro m th e b attery
th e in c o m in g 6 V to th e 3 .3 V th a t it r e q u ir
n
b attery p
u p p lied f
t v o ltag e
p ack , th
es.
ack m
ro m th
ran g e
ro u g h
eets th e su p p ly req u iremen t o f th e
e U S B . A lte rn a tiv e ly , b e c a u s e th e
f o r p in 2 m e n tio n e d in F ig u r e 2 .1 ,
th is p in . Th e mb ed th en reg u lates
E x e r c is e 4 .7
C re a te
P W M
F ig u r e
m o v e s
4 .4 .6
We c
ex am
so n g
Lo n d
a n e w p r
p e r io d t o
4 .9 , a n d
th e s e rv o
o je c t a
2 0 m s
o b s e rv
s h a ft
n d
. T
e t
fro
w r it e a p r o g r a m
ry a n u m b e r o f d
h e s e r v o ’s p o s it io
m o n e lim it t o t h
t h a t s e t s a P W M o u t p u t o n p in 2 1 . S e t t h e
iffe r e n t d u t y p e r io d s , t a k in g v a lu e s fr o m
n . T h e n w r it e a p r o g r a m t h a t c o n t in u a lly
e o th e r.
n
O u t p u t t in g t o a P ie z o T r a n s d u c e r
an u se
p le we
called
o n call
th e PWM so
u se it to so u
‘Oran g es an d
a p articu lar
u rce simp ly
n d a p iezo
Lemo n s’.
messag e. If
Oran -ges an d
as a v ariab le
tran sd u cer an d
Th is so n g ima
y o u can read
freq
p la
g in e
mu s
le-mon s, say th e
u en
y th
s th
ic y
cy
e
at
o u
b ells of
F ig u r e 4 .1 1 :
T h e ‘O ra n g e s a n d L e m o n s ’ tu n e
sig n al g en
start o f an
th e b ells o
may reco
S t.
erato r. In
o ld Lo n d
f each ch
g n ize th e
Cle-men t 's
th is
o n fo lk
u rch in
tu n e in
A n a lo g O u t p u t
7 3
T a b le 4 .3 : F r e q u e n c ie s o f n o t e s u s e d in t u n e
W o r d / s y lla b le
M u s ic a l n o t e
O ra n -
E
F re q u e n c y (H z )
B e a ts
6 5 9
1
g e s
C #
5 5 4
a n d
E
6 5 9
1
1
le -
C #
5 5 4
m o n s,
A
4 4 0
1
sa y
B
4 9 4
½
th e
C #
5 5 4
½
b e lls
D
5 8 7
1
o f
B
4 9 4
S t
E
6 5 9
C le m -
C #
5 5 4
e n t’s
A
4 4 0
1
1
1
1
2
F ig u re 4 .1 1 ; if y o u c a n n o t, d o n ’t w o r r y ! Yo u ju s t n e e d to k n o w th a t a n y n o te th a t is
a min im (‘h alf n o te’ in th e USA) lasts twice as lo n g as a cro tch et (‘q u arter n o te’ in th
USA), wh ich in tu rn lasts twice as lo n g as a q u av er (‘eig h th n o te’ in th e USA). Pu t an o th
way , a cro tch et lasts o n e b eat, a min im two , an d a q u av er a h alf. Th e p attern fo r th e mu s
is a s s h o w n in Ta b le 4 .3 . N o te th a t h e re w e a r e s im p ly u s in g th e P W M a s a v a r ia b le
fre q u e n cy s ig n al s o u rce, a n d n o t actu ally mo d u latin g th e p u ls e wid th as a p ro p o rtio n o
freq u en cy at all.
e
er
ic
f
C r e a te a n e w p r o g r a m a n d e n te r P r o g r a m E x a m p le 4 .7 . T h is in tr o d u c e s a n
im p o rtan t n ew C featu re, th e a rra y. If y o u are u n familiar with th is, th en read th e
r e v ie w in S e c tio n B .8 .1 . T h e p r o g ra m u s e s tw o a r r a y s , o n e d e fi n e d f o r f r e q u e n c y
d ata, th e o th er fo r b eat len g th . Th ere are 1 2 v alu es in each , fo r each o f th e 1 2 n o tes in th e
tu n e. Th e p ro g ram is stru ctu red ro u n d a fo r lo o p , with v ariab le i as co u n te r. As i
in cremen ts, each array eleme n t is selected in tu rn . N o tic e th at i is set ju st to reach th e v alu e
1 1 ; th is is b ecau se th e v alu e 0 ad d re ss es th e first e le men t in each array , an d th e v alu e 1 1
h en ce ad d re ss e s th e twelfth . F ro m th e freq u e n cy a rray th e PWM p erio d is c alcu la ted an d
set, alway s with a 5 0 % d u ty ratio . Th e b eat arra y d etermin es h o w lo n g eac h n o te is h eld ,
u sin g th e w a it fu n ctio n .
C c o d e
fe a tu re
/ * P r o g r a m E x a m p l e 4 . 7 :
* /
# i n c l u d e " m b e d . h "
P w m O u t b u z z e r ( p 2 1 ) ;
P l a y s
t h e
t u n e
" O r a n g e s
a n d
L e m o n s "
o n
a
p i e z o
b u z z e r ,
u s i n g
P W M
7 4
Chapter 4
f l o a t f
f l o a t b
i n t m a i
w h i l e
f o r
b
b u z
w a i
}
}
}
r e
e a
n (
(
(
u z
z e
t (
q u e
t [ ]
) {
1 )
i n t
z e r
r = 0
0 . 4
/ / f r e q u e n c y a r r a y
n c y [ ] = { 6 5 9 , 5 5 4 , 6 5 9 , 5 5 4 , 4 4 0 , 4 9 4 , 5 5 4 , 5 8 7 , 4 9 4 , 6 5 9 , 5 5 4 , 4 4 0 } ;
= { 1 , 1 , 1 , 1 , 1 , 0 . 5 , 0 . 5 , 1 , 1 , 1 , 1 , 2 } ;
/ / b e a t a r r a y
{
i
. p
. 5
* b
= 0 ; i < = 1 1 ; i + + ) {
e r i o d ( 1 / ( 2 * f r e q u e n c y [ i ] ) ) ;
;
/ / s e t d u t y
e a t [ i ] ) ;
/ / h o l d f o r
/ / s e t P W M
c y c l e
b e a t p e r i o d
p e r i o d
P r o g r a m E x a m p le 4 .7 ‘ O r a n g e s a n d L e m o n s ’ p r o g r a m
Co mp ile th e p ro g ram an d d o wn lo ad . Co n n ect th e p iezo tran sd u cer b etween p in s 1 an d 2 1 , an d
th e s e q u e n c e s h o u ld p la y o n r e s e t. T h e tr a n s d u c e r s e e m s v e r y q u ie t w h e n h e ld in a ir. Yo u c a n
in crease th e v o lu me sig n ifican tly b y fix in g o r h o ld in g it to a flat su rface su ch as a tab le to p .
n
E x e r c is e 4 .8
T r y t h e fo llo w in g :
1 . M a k e t h e ‘O r a n g e s a n d L e m o n s ’ s e q u e n c e p la y a n o c t a v e
fre q u e n c y o f e a c h n o te .
2 . C h a n g e t h e t e m p o b y m o d ify in g t h e m u lt ip lie r in t h e w a it
3 . ( F o r t h e m o r e m u s ic a lly in c lin e d .) C h a n g e t h e t u n e , s o t h a
lin e o f ‘T w in k le T w in k le L it t le S t a r ’. T h is u s e s t h e s a m e n o t e
o f fre q u e n c y 6 9 9 H z . T h e tu n e s ta rts o n A . B e c a u s e th e re
c o n s id e r p u t t in g a s m a ll p a u s e b e t w e e n e a c h n o t e .
h ig h e r b y d o u b lin g t h e
c o m m a n d .
t t h e m b e d p la y s t h e fi r s t
s , e x c e p t it a ls o n e e d s F # ,
a re re p e a te d n o te s ,
n
C h a p t e r R e v ie w
•
•
•
•
•
•
A d ig ital-to -an alo g co n v erter (DAC) co n v erts an in p u t b in ary n u mb er to an o u tp u t an alo g
v o ltag e, wh ich is p ro p o rtio n al to th at in p u t n u mb er.
DACs are wid ely u sed to create co n tin u o u sly v ary in g v o ltag es, fo r ex amp le to g en erate
an alo g wav efo rms.
Th e mb ed h as a sin g le DAC, an d an asso ciated set o f lib rary fu n ctio n s.
Pu lse wid th mo d u latio n (PWM) p ro v id es a way o f co n tro llin g certain an alo g q u an tities, b y
v ary in g th e p u lse wid th o f a fix ed freq u en cy rectan g u lar wav efo rm.
PWM is wid ely u sed fo r co n tro llin g flo w o f electrical p o wer, fo r ex amp le LED b rig h tn ess
o r mo to r co n tro l.
T h e m b e d h a s s ix p o s s ib le P W M o u tp u ts . T h e y c a n a ll b e in d iv id u a lly c o n tr o lle d , b u t m u s t
all sh are th e same freq u en cy .
A n a lo g O u t p u t
7 5
Q u iz
1 .
2 .
A 7
(a)
(b )
(c)
(d )
Wh
d ec
Wh
(a)
(b )
(c)
Wh
3 .
4 .
- b it D A C o b e y s E q u a tio n 4 .1 , a n d h a s a v o lta g e r e f e r e n c e r a n g e o f 2 .5 6 V.
Wh at is its reso lu tio n ?
Wh at is its o u tp u t if th e in p u t is 1 0 0 0 1 0 1 ?
Wh at is its o u tp u t if th e in p u t is 0 x 2 A?
W h a t is its d ig ita l in p u t in d e c im a l a n d b in a r y if its o u tp u t r e a d s 0 .4 8 V ?
at is th e mb ed ’s DAC reso lu tio n an d wh at is th e smallest an alo g v o ltag e step in crease o r
rease wh ich can b e o u tp u t fro m th e mb ed ?
at is th e o u tp u t o f th e LPC1 7 6 8 DAC, if its in p u t d ig ital wo rd is:
0 0 0 0 0 0 1 0 0 0
0 x 8 0
1 0 1 0 0 0 1 0 0 0 ?
at o u tp u t v o ltag es will b e read o n a DVM wh ile th is p ro g ram lo o p ru n s o n th e mb ed ?
w h i l e ( 1 ) {
f o r ( i = 0 ; i < 1 ; i = i + 0 . 2 ) {
A o u t = i ;
w a i t ( 0 . 1 ) ;
}
}
5 .
6 .
7 .
8 .
9 .
Th e wav efo rm
Wh at are th e a
actu ato rs?
A PWM d ata s
wid th ?
A PWM d ata
cy cle?
Th e PWM o n
wav efo rm?
in Q u e s tio n 5 g iv e s a c ru d e s a w to o th . W h a t is its p e rio d ?
d v an tag es o f u sin g p u lse wid th mo d u latio n (PWM) fo r co n tro l o f an alo g
tream h as a freq u en cy o f 4 k Hz an d a d u ty cy cle o f 2 5 %. Wh at is its p u lse
stream h as p erio d o f 2 0 ms an d an o n time o f 1 ms . Wh at is its d u ty
an mb ed is set u p with th ese statemen ts. Wh at is th e o n time o f th e
P W M 1 . p e r i o d ( 0 . 0 0 4 ) ;
/ / s e t P W M p e r i o d
P W M 1 = 0 . 7 5 ;
/ / s e t d u t y c y c l e
Reference
4 .1 .
K e s tn e r, W., E d . ( 2 0 0 5 ) . T h e D a ta C o n v e r s io n H a n d b o o k . A n a lo g D e v ic e s I n c . N e w n e s .
C H A P T E R 5
A n a lo g In p u t
C h a p t e r O u t lin e
5 .1 A n a lo g - t o - D ig it a l C o n v e r s io n
5 .1
5 .1
5 .1
5 .1
.1 T
.2 R
.3 S
.4 A
h e A n a lo g - t o - D ig it a
a n g e , R e s o lu t io n a n
a m p lin g F r e q u e n c y
n a lo g In p u t w it h t h
7 7
l C o n v e rte r 7 8
d Q u a n t iz a t io n
8 1
e m b e d 8 2
7 9
5 .2 C o m b in in g A n a lo g In p u t a n d O u t p u t
8 2
5 .2 .1 C o n t r o llin g L E D B r ig h t n e s s b y V a r ia b le V o lt a g e
5 .2 .2 C o n t r o llin g L E D B r ig h t n e s s b y P W M 8 4
5 .2 .3 C o n t r o llin g P W M F r e q u e n c y 8 4
5 .3 P r o c e s s in g D a t a f r o m A n a lo g In p u t s
8 6
5 .3 .1 D is p la y in g V a lu e s o n t h e C o m p u t e r S c r e e n
5 .3 .2 S c a lin g A D C O u t p u t s t o R e c o g n iz e d U n it s
5 .3 .3 A p p ly in g A v e r a g in g t o R e d u c e N o is e 8 7
5 .4 S o m e S im p le A n a lo g S e n s o r s
8 2
8 6
8 7
8 8
5 .4 .1 T h e L ig h t - D e p e n d e n t R e s is t o r 8 8
5 .4 .2 In t e g r a t e d C ir c u it T e m p e r a t u r e S e n s o r
5 .5 E x p lo r in g D a t a C o n v e r s io n T im in g
8 9
9 0
5 .5 .1 E s t im a t in g C o n v e r s io n T im e a n d A p p ly in g N y q u is t
5 .6 M
C h a p t
Q u iz
R e fe re
in i- P r o je c t : T w o - D im e n s io n a l L ig h t T r a c k in g
e r R e v ie w
9 2
9 3
n c e s 9 3
5 .1
A n a lo g -t o -D ig it a l C o n v e r s io n
9 0
9 2
T h e w o r l d a r o u n d t h e e m b e d d e d s y s t e m i s a l a r g e l y a n a l o g o n e , a n d s e n s o r s eo f t e m p e r a t u r e ,
s o u n d , a c c e le ra tio n , a n d s o o n em o s tly h a v e a n a lo g o u tp u ts . Ye t it is e s s e n tia l fo r th e
micro co n tro ller to h av e th ese sig n als av ailab le in d ig ital fo rm. Th is is wh ere th e an alo g to -d ig ital co n v erter (ADC) co mes in . An alo g sig n als can b e rep eated ly co n v erted in to d ig ital
rep resen tatio n s, with a reso lu tio n an d at a rate d etermin ed b y th e ADC. Fo llo win g th is
an alo g -to -d ig ital co n v ersio n , th e micro co n tro ller can b e u sed to p ro cess o r an aly ze th is
in fo rmatio n , b ased o n th e v alu e(s) o f th e an alo g in p u t.
F a s t a n d E f f e c t iv e E m b e d d e d S y s t e m s D e s ig n . D O I : 1 0 .1 0 1 6 /B 9 7 8 - 0 - 0 8 - 0 9 7 7 6 8 - 3 .0 0 0 0 5 - 2
Co p y rig h t Ó2 0 1 2 Elsev ier Ltd . All rig h ts reserv ed .
7 7
7 8
5 .1 .1
Chapter 5
T h e A n a lo g -t o -D ig it a l C o n v e r t e r
An ADC is an electro n ic circu it wh o se d ig ital o u tp u t is p ro p o rtio n al to its an alo g in p u t.
E ffe c tiv e ly , it ‘m e a s u re s ’ th e in p u t v o lta g e a n d g iv e s a b in a ry o u tp u t n u m b e r p ro p o rtio n a l to
its s iz e . T h e lis t o f p o s s ib le a n a lo g in p u t s ig n a ls is e n d le s s , in c lu d in g s u c h d iv e rs e s o u rc e s a s
au d io an d v id eo , med ical o r climatic v ariab les, an d a h o st o f in d u strially g en erated sig n als. Of
th ese, so me, su ch as temp eratu re, h av e a v ery lo w freq u en cy co n ten t. Oth ers, v id eo fo r
ex amp le, are v ery h ig h freq u en cy . Owin g to th is h u g e ran g e o f ap p licatio n s, it is n o t
su rp risin g to d isco v er th at man y ty p es o f ADC h av e b een d ev elo p ed , with ch aracteristics
o p timized fo r th ese d ifferin g ap p licatio n s.
Th e ADC almo st alway s o p erates with in a larg er en v iro n men t, o ften called a d ata acq u isitio n
s y s te m . S o m e f e a tu r e s o f a g e n e r a l p u r p o s e d a ta a c q u is itio n s y s te m a r e s h o w n in F ig u r e 5 .1 . To
th e rig h t o f th e d iag ram is th e ADC itself. Th is h as an an alo g in p u t an d a d ig ital o u tp u t. It is
u n d er co mp u ter co n tro l; th e co mp u ter can start a co n v ersio n . Th e co n v ersio n tak es fin ite time,
may b e so me micro seco n d s, so th e ADC n eed s to sig n al wh en it h as fin ish ed . Th e o u tp u t d ata
can th en b e read . Th e ADC wo rk s with a v o ltag e referen ce, wh ich may b e th o u g h t o f as a ru ler
o r tap e measu re. In o n e way o r an o th er th e ADC co mp ares th e in p u t v o ltag e with th e v o ltag e
referen ce, an d co mes u p with th e o u tp u t n u mb er d ep en d in g o n th is co mp ariso n . As with so
In te rn a l to M ic ro c o n tro lle r
Ex tern al to
Mic ro c o n tro lle r
Clo c k in p u t
ADC
Mu ltip lex er
Tran sd u cer
Gen erates s ig n al
Co n v erts its
An alo g In p u t to
D ig ita l Ou tp u t
Selects o n e in p u t
ch an n el
Vo ltag e
Referen ce
An alo g in p u t
to ADC
Dig ital
Ou tp u t
In p u t S e le c t
Start
Co n v ersio n
Co n v ersio n
Co mp lete
F ig u r e 5 .1 :
A n e x a m p le d a t a a c q u is it io n s y s t e m
CPU
Co n tro l
A n a lo g In p u t
7 9
man y d ig ital o r d ig ital/an alo g su b sy stems, th ere is also a clo ck in p u t, a co n tin u o u sly ru n n in g
sq u are wav e, wh ich seq u en ces th e in tern al o p eratio n o f th e ADC. Th e freq u en cy set h ere
d etermin es h o w rap id ly th e ADC o p erates.
On ce we start wo rk in g with an ADC, we u su ally fin d th at we wan t to wo rk with mo re th an o n
sig n al. We co u ld ju st u se mo re ADCs, b u t th is is co stly an d tak es u p semico n d u cto r sp ace
In stead , th e u su al p ractice is to p u t an an alo g mu ltip lex er in fro n t o f th e ADC. Th is acts a
a selecto r switch . Th e u ser can th en select an y o n e o f sev eral in p u ts to th e ADC. If th is is d o n
q u ick ly en o u g h , it is as if all in p u ts are b ein g co n v erted at th e same time. Man y
micro co n tro llers, in clu d in g th e NXP1 7 6 8 , in clu d e an ADC an d mu ltip lex er o n ch ip . Th e
in p u ts to th e mu ltip lex er are co n n ected to micro co n tro ller p in s an d mu ltip le in p u ts can b e
u s e d . T h is is a s s h o w n in F ig u r e 5 .1 f o r a 4 - b it m u ltip le x e r. We r e tu r n to s o m e o f th e d e ta il o
F ig u r e 5 .1 in C h a p te r 1 4 .
5 .1 .2
e
.
s
e
f
R a n g e , R e s o lu t io n a n d Q u a n t iz a t io n
M a n y A D C s o b e y E q u a tio n 5 .1 , w h e r e Vi is th e in p u t v o lta g e , V r th e r e f e r e n c e v o lta g e , n th e
n u mb er o f b its in th e co n v erter o u tp u t, an d D th e d ig ital o u tp u t v alu e. Th e o u tp u t b in ary
n u mb er D is an in teg er, an d fo r an n -b it n u mb er can tak e an y v alu e fro m 0 to (2 n  1 ). Th e
in te r n a l A D C p r o c e s s e ff e c tiv e ly r o u n d s o r tr u n c a te s th e c a lc u la tio n in E q u a tio n 5 .1 to
p ro d u ce an in teg er o u tp u t. Clearly , th e ADC can n o t ju st co n v ert an y in p u t v o ltag e, b u t h as
max imu m an d min imu m p ermissib le in p u t v alu es. Th e d ifferen ce b etween th is max imu m an d
m in im u m is c a lle d th e r a n g e . O f te n th e m in im u m v a lu e is 0 V, s o th e r a n g e is th e n ju s t th e
max imu m p o ssib le in p u t v alu e. An alo g in p u ts th at ex ceed th e max imu m o r min imu m
p ermissib le in p u t v alu es are lik ely to b e d ig itized as th e max imu m an d min imu m v alu es
r e s p e c tiv e ly , i.e . a lim itin g ( o r ‘ c lip p in g ’ ) a c tio n ta k e s p la c e . T h e in p u t r a n g e o f th e A D C is
d irectly lin k ed to th e v alu e o f th e v o ltag e referen ce; in man y ADC circu its th e ran g e is
actu ally eq u al to th e referen ce v o ltag e.
D ¼
E q u a tio n 5 .1 is r e p r e s e n te d in
v o ltag e is g rad u a lly in crea se d
A D C ’s o u tp u t is in itia lly 0 0 0 . I
w ill ta k e th e v a lu e 0 0 1 . A s th e
s o m e p o in t it r e a c h e s 1 1 1 , i.e .
v a lu e. T h e in p u t m ay b e in c re
Vi
 2
Vr
n
5 .1
g r a p h ic a l f o r m in F ig u r e 5 .2 , f o r a 3 - b it c o n v e r te r. I f th e in p u
fro m 0 V a n d th e c o n v e rter is ru n n in g c o n tin u o u s ly , th e n th e
f th e in p u t s lo w ly in crea se s, th e re co me s a p o in t w h e n th e o u tp u
in p u t in creases fu rth er, th e o u tp u t ch an g es to 0 1 0 , an d so o n . A
7 in d e c ima l, o r (2 3  1 ). T h is is th e m ax im u m p o s s ib le o u tp u
as e d fu rth e r, b u t it c a n n o t fo rc e a n y in c re a s e in o u tp u t v a lu e.
A s F ig u r e 5 .2 d e m o n s tr a te s , b y c o n v e r tin g a n a n a lo g s ig n a l to d ig ita l, w e r u n th e r is k o f
ap p ro x imatin g it. Th is is b ecau se an y o n e d ig ital o u tp u t v alu e h as to rep resen t a small ran g e
o f a n a lo g in p u t v o lta g e s , i.e . th e w id th o f a n y o f th e s te p s o n th e ‘ s ta ir c a s e ’ o f F ig u r e 5 .2 . F o r
t
t
t
t
8 0
Chapter 5
D
(d ig ital
o u tp u t)
1 1 1
1 1 0
1 0 1
1 0 0
0 1 1
0 1 0
0 0 1
0 0 0
V
0
V
r
i
(an alo g in p u t)
F ig u r e 5 .2 :
A 3 - b it A D C c h a r a c t e r is t ic
ex amp le, th e d ig ital o u tp u t 0 0 1 in th e fig u re mu st rep resen t a n y an alo g v o ltag e alo n g th e step
th at it rep resen ts. If th e o u tp u t v alu e o f 0 0 1 is p recisely co rrect fo r th e in p u t v o ltag e at th e
mid d le o f th e step , th en th e g reatest erro r o ccu rs at eith er en d o f th e step . Th is is called
q u a n tiza tio n erro r. Fo llo win g th is lin e o f reaso n in g , th e g reatest q u an tizatio n erro r is o n e h alf
o f th e s te p w id th , o r h a lf o f o n e le a s t s ig n ifi c a n t b it (L S B ) e q u iv a le n t o f th e v o lta g e s c a le .
Clearly , th e mo re step s th ere are rep resen tin g th e ran g e, th e n arro wer th ey will b e, an d h en ce
th e q u an tizatio n erro r is red u ced . Mo re step s are o b tain ed b y in creasin g th e n u mb er o f b its in
th e ADC p ro cess. Th is in ev itab ly in creases th e co mp lex ity an d co st o f th e ADC, an d also
o ften th e time it tak es to co mp lete a co n v ersio n .
As an ex a
d ig ital sig
3 .3 /2 5 6 ¼
is co n cern
o f 3 .3 /2 1 2
mp le, if we wa
n al, th en th ere
1 2 .8 9 m V, a n d
e d , F ig u r e 2 .3
, o r 0 .8 m V; th
n t to co n v ert an an alo g sig n al
a r e 2 5 6 ( i.e . 2 8 ) d is tin c t o u tp
th e wo rst case q u an tizatio n e
sh o ws th at th e LPC1 7 6 8 ADC
e wo rst case q u an tizatio n erro
th at
u t v
rro r
is 1
r is
h a s a r a n g e 0 e3 . 3 V
alu es. Each step h as
is 6 .4 5 m V. A s f a r a
2 b it. Th is lead s to a
th e r e f o r e 0 .4 m V.
to an 8 -b it
a wid th o f
s th e mb ed
step wid th
Fo r man y ap p licatio n s an 8 , 1 0 o r 1 2 -b it ADC allo ws su fficien t reso lu tio n ; it all d ep en d s o n
th e req u ired accu racy . In certain au d io ap p licatio n s listen in g tests h av e d emo n strated th at 1 6 b it reso lu tio n is ad eq u ate; imp ro v ed q u ality can , h o wev er, b e n o ticed u sin g 2 4 -b it co n v ersio n .
All o f th is a
referen ce v o
h av e n o n -lin
b rin g th eir u
ssu mes th at all o th er asp ects o f th e ADC ar
ltag e can b e in accu rate o r can d rift with temp
earities. Fu rth ermo re, d ifferen t meth o d s o f
n iq u e in accu racies to th e eq u atio n . Referen c
e p er
eratu
an alo
e 5 .1
fect, wh ich
re, an d th e s
g -to -d ig ital
d iscu sses in
th ey are
taircase
co n v ers
d etail a
n o t. Th e
p attern can
io n each
n u mb er o f
A n a lo g In p u t
an alo g
slo p e’
Discu s
u n d ers
5 .1 .3
-to -d
an d
sin g
tan d
ig
‘d
th
th
ital co n v ers
elta-sig ma’
e sp ecific d
e co n cep ts
io n d
meth
esig n
o f d a
es
o d
o
ta
ig n s, in clu d in g ,
s, wh ich all h av
f th e ADC sy ste
co n v ersio n an d
‘s u c c e s s iv e a p p ro
e th eir o wn ad v an
m, h o wev er, is n o
imp lemen tin g th e
8 1
x imatio n ’, ‘flash ’, ‘d u al
tag es an d in accu racies.
t n ecessary fo r u s to
mb ed ’s ADC.
S a m p lin g F r e q u e n c y
Wh en co n v ertin g an
th e accu racy d efin ed
accu rate th e d ig ital d
samp lin g freq u en cy .
an alo g sig n al
b y th e reso lu
ata will b e. S
Th is is illu str
to d
tio n
amp
ated
ig ital,
o f th e
lin g is
in Fig
a ‘samp le’ is tak en rep eated ly an d q u an tized to
ADC. Th e mo re samp les tak en , th e mo re
g en erally d o n e at a fix ed freq u en cy , called th e
u r e 5 .3 .
Th e samp lin g freq u en cy d ep en d s o n th e max imu m freq u en cy o f th e sig n al b ein g d ig itized . If
th e samp lin g freq u en cy is to o lo w th en rap id ch an g es in th e an alo g sig n al may n o t b e
rep resen ted in th e resu ltin g d ig ital d ata. Th e Ny q u ist samp lin g criterio n states th at th e
samp lin g freq u en cy mu st b e at least d o u b le th at o f th e h ig h est sig n al freq u en cy . Fo r ex amp le,
th e h u man au d ito ry sy stem is k n o wn to ex ten d u p to ap p ro x imately 2 0 k Hz, so stan d ard au d io
C D s a r e s a m p le d a n d p la y e d b a c k a t 4 4 .1 k H z in o r d e r to a d h e r e to th e N y q u is t s a m p lin g
criterio n . If th e samp lin g criterio n is n o t satisfied , th en a p h en o men o n called a lia sin g o ccu rs
ea n e w lo w e r f r e q u e n c y is g e n e r a te d . T h is is illu s tr a te d in F ig u r e 5 .4 a n d d e m o n s tr a te d la te r
in th e ch ap ter. Aliasin g is v ery d amag in g to a sig n al an d mu st alway s b e av o id ed . A co mmo n
V
time
samp les
F ig u r e 5 .3 :
D ig it iz in g a s in e w a v e
reco n stru cted sig n al
V
time
samp les
o rig in al sig n al
F ig u r e 5 .4 :
T h e e ffe c t o f a lia s in g
8 2
Chapter 5
ap p ro ach is to u se an a n ti-a lia sin g fi lter, wh ich limits all sig n al co mp o n en ts to th o se th at
satisfy th e samp lin g criterio n .
5 .1 .4
Fig
p ro
Tab
(as
u r
g r
le
it
5 .2
A n a lo g In p u t w it h t h e m b e d
e 2 .1 s h o w
ammin g in
5 .1 . I t is u
wo u ld b e
s u s th at th e mb
terface (API) s
sefu l to n o te th
at th e ADC o u
ed h as u
u mmary
at th e A
tp u t) o r
p to six an alo
, fo llo win g a
DC o u tp u t is
as a flo atin g
g i n p u t s , o n p i n s 1 5 e2 0 . T h e a p p l i c a t i o n
p attern th at is q u ite familiar, is sh o wn in
av ailab le eith er in u n sig n ed b in ary fo rm
p o in t n u mb er.
C o m b in in g A n a lo g In p u t a n d O u t p u t
Th e ADC is an in p u t d ev ice, wh ich tran sfers d ata in to th e micro co n tro ller. If we u se it o n its
o wn , we will h av e n o id ea wh at v alu es it h as created . We n o w th erefo re g o o n to d o two th in g s
to mak e th at d ata v isib le. We will first u se th e ADC o u tp u t v alu es to immed iately co n tro l an
o u tp u t v ariab le, fo r ex amp le th e d ig ital-to -an alo g co n v erter (DAC) o r p u lse wid th mo d u latio n
(PWM). We will later tran sfer its o u tp u t v alu es to th e PC screen , an d ex p lo re so me
measu remen t ap p licatio n s.
5 .2 .1
Let u
b rig h
will u
straig
C o n t r o llin g L E D B r ig h t n e s s b y V a r ia b le V o lt a g e
s star
tn ess
se a p
h t to
t with a simp le p
o f a lig h t-emittin
o ten tio meter to g
th e an alo g o u tp u
ro g ram wh ich read s th e an alo g in p u t, an d u ses it to co n tro l th e
g d io d e (L E D ) b y v a ry in g th e v o lta g e d riv e to th e L E D . H e re , w e
en erate th e an alo g in p u t v o ltag e, an d will th en p ass th e v alu e read
t.
Co n n ect u p th e circu it o
p o ten tio meter b etween 0
a n ew p ro g ram an d co p y
u p th e an alo g in p u t an d
f Fig
an d
in to
o u tp
/ *
* /
# i
A n
A n
U s e s
P r o g r a m
E x a m p l e
5 . 1 :
n c l u d e " m b e d . h "
a l o g O u t A o u t ( p 1 8 ) ;
a l o g I n A i n ( p 2 0 )
i n t
m a i n ( ) {
w h i l e ( 1 ) {
A o u t = A i n ;
u r e 5 .5 a . T h is u
3 .3 V. T h e L E D
it th e v ery simp
u t, an d th en co n
a n a l o g
/ / d e f i n e s
/ / d e f i n e s
/ / t r a n s f e r
i n p u t
a n a l o g
a n a l o g
a n a l o g
t o
ses p
is co
le co
tin u o
in
n n
d e
u s
2 0 as th e an alo
ected to p in 1 8
o f Pro g ram Ex
ly tran sfers th e
c o n t r o l
L E D
g in p u t, co
, th e an alo g
a m p le 5 .1 .
in p u t to th
b r i g h t n e s s ,
n n ectin g th e
o u tp u t. Start
Th is ju st sets
e o u tp u t.
t h r o u g h
D A C
o u t p u t
o u t p u t o n P i n 1 8
i n p u t o n P i n 2 0
i n
v a l u e
t o
a n a l o g
o u t ,
b o t h
a r e
}
}
P r o g r a m E x a m p le 5 .1 C o n t r o llin g L E D b r ig h t n e s s b y v a r ia b le v o lt a g e
t y p e
f l o a t
A n a lo g In p u t
8 3
T a b le 5 .1 : A P I s u m m a r y f o r a n a lo g in p u t
F u n c t io n
U s a g e
A n a l o g I n
C r e a t e a n A n a lo g In o b je c t , c o n n e c t e d t o t h e s p e c ifi e d p in
r e a d
R e a d t h e i n p u t v o l t a g e , r e p r e s e n t e d a s a fl o a t i n t h e r a n g e ( 0 . 0 e1 . 0 )
r e a d _ u 1 6
R e a d t h e i n p u t v o l t a g e , r e p r e s e n t e d a s a n u n s i g n e d s h o r t i n t h e r a n g e ( 0 x 0 e0 x F F F F )
Co mp ile th e p ro g ram a
p o ten tio meter sh o u ld co
th ere is a ran g e o f th e p
will b e fo llo win g th e cu
d riv e v o lta g e is lo w.
o wn lo
l th e b
tio me
o f Fig
ad
rig
ter
u re
to th e mb ed . With th e p ro g
h tn e s s o f th e L E D . Yo u w ill
ro tatio n wh ere th e LED is o
3 .3 a , a n d th e r e w ill b e v e r y
ram ru n n
p ro b ab ly
ff. Reme
little illu
in g , th e
fin d , h o wev er, th at
mb er th at th e LED
min atio n wh en th e
E x e r c is e 5 .1
M e a s u r e t h e D A C o u t p u t v o lt a g e a t p in 1 8 , a s y o u a d ju s t t h e p o t e n t io m e t e r . Y o u w ill
fi n d t h a t w h e n t h is is a b o v e a r o u n d 1 .8 V , t h e L E D w ill b e lit , w it h v a r y in g le v e ls o f
b r ig h t n e s s . W h e n it is b e lo w 1 .8 V , t h e L E D n o lo n g e r c o n d u c t s , a n d t h e r e is n o
illu m in a t io n .
n
( a ) c ir c u it d ia g r a m
1 0 k
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
( b ) c o n s tr u c tio n d e ta il
m b e d M ic r o c o n tr o lle r
n
n d d
n tro
o ten
rv e
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
F ig u r e 5 .5 :
A p o t e n t io m e t e r c o n t r o llin g L E D b r ig h t n e s s
8 4
Chapter 5
5 .2 .2
C o n t r o llin g L E D B r ig h t n e s s b y P W M
We c an u se th e p o te n tio
d id fo r th e an alo g o u tp u
b e co n n ected to th e PW
E x a m p le 5 .2 . H e r e , w e
/ *
* /
# i
P w
A n
P r o g r a m
E x a m p l e
meter in p u t to a
t. T h e c irc u it o f
M o u tp u t o n p in
see th e an alo g
5 . 2 :
U s e s
n c l u d e " m b e d . h "
m O u t P W M 1 ( p 2 1 ) ;
a l o g I n A i n ( p 2 0 ) ;
i n t
m
w h i
P
P
w
a i
l e
W M
W M
a i
n (
( 1
1 .
1 =
t (
a n a l o g
i n p u t
/ / d e f i n e s
{
) {
p e r i o d ( 0 . 0 1 0 ) ;
A i n ;
0 . 1 ) ;
lte r th
F ig u r
2 1 .C
in p u t
a n a l o g
e PWM d u ty cy cle, u sin g th e same ap p ro ach a
e 5 .5 is u s e d a g a in , e x c e p t th a t th e L E D s h o u ld
re a te a n ew p ro g ra m , a n d e n te r th e c o d e o f P ro g
v alu e b ein g tran sferred to th e PWM d u ty cy c
t o
c o n t r o l
i n p u t
o n
P W M
P i n
d u t y
c y c l e ,
f i x e d
p e r i o d
t y p e
f l o a t
s we
n o w
ram
le.
2 0
)
/ / s e t P W M p e r i o d
/ / A n a l o g i n v a l u e
t o 1 0 m s
b e c o m e s P W M
d u t y ,
b o t h
a r e
}
}
P r o g r a m E x a m p le 5 .2 C o n t r o llin g P W M p u ls e w id t h w it h a p o t e n t io m e t e r
Th e LED b rig h tn ess sh
v ery similar to th e p rev
we wo u ld n o rmally n o t
we wo u ld b e mo re read
5 .2 .3
o u ld
io u s
wish
y to
ag
p r
to
co
ain b e co n tro lled
o g ram, th e mean
co mmit a wh o le
mmit a (simp ler
b y th e p o
s o f d o in g
DAC to co
an d mo re
ten tio meter. Wh ile th e o
it is q u ite d ifferen t. In p
n tro llin g th e b rig h tn ess o
read ily av ailab le) PWM
u tco me is
ractice,
f an LED;
so u rce.
C o n t r o llin g P W M F r e q u e n c y
In stead o f u sin g th e p o ten tio meter to co n tro l th e PWM d u ty cy cle, it can b e u sed to co n tro l th e
PWM freq u en cy . Leav e th e LED co n n ected to Pin 2 1 . Create a n ew p ro g ram an d en ter th e
c o d e o f P r o g r a m E x a m p le 5 .3 . N o e x tr a c o m p o n e n ts n e e d b e c o n n e c te d to th e m b e d .
No tice th at th e PWM p erio d is calcu lated in th e lin e:
P W M 1 . p e r i o d ( A i n / 1 0 + 0 . 0 0 1 ) ;
/ /
s e t
P W M
p e r i o d
It is first wo rth n o tin g th at a calcu latio n is p laced wh ere o n e mig h t h av e ex p ected
a simp le p arameter to b e p laced . Th is is n o t a p ro b lem fo r C. Th e p ro g ram will first
ev alu ate th e ex p ressio n in sid e th e b rack ets, an d th en call th e p e r io d ( ) fu n ctio n .
T h is c a lc u la tio n in v o k e s fo r th e fi rs t tim e th e d iv id e o p e ra to r, /. It a ls o ra is e s th e th o rn y little
q u estio n , wh ich all ch ild ren face wh en th ey learn arith metic, o f th e o rd er in wh ich o p erato rs
s h o u ld b e e v a lu a te d . I n C th is is v e r y c le a r ly d e fi n e d , a n d c a n b e s e e n b y c h e c k in g Ta b le B .5 in
Ap p en d ix B. Th is sh o ws a p reced en ce fo r each o p erato r, with / h av in g p reced en ce 3 , an d þ
h a v in g p re c e d e n c e 4 . T h e re fo re , th e d iv is io n w ill b e e v a lu a te d b e fo re th e a d d itio n , a n d th e re
will b e n o u n certain ty in ev alu atin g th e ex p ressio n . Th e v alu es u sed mean th at th e min imu m
p e r io d , w h e n A in is z e r o , is 0 .0 0 1 s ( i.e . 1 0 0 0 H z ) . T h e m a x im u m is w h e n A in is 1 , le a d in g to
a p e r io d o f 0 .1 0 1 s , i.e . a r o u n d 1 0 H z .
C c o d e
fe a tu re
A n a lo g In p u t
/ *
* /
# i
P w
A n
P r o g r a m
E x a m p l e
5 . 3 :
U s e s
a n a l o g
i n p u t
t o
c o n t r o l
P W M
8 5
p e r i o d
n c l u d e " m b e d . h "
m O u t P W M 1 ( p 2 1 ) ;
a l o g I n A i n ( p 2 0 ) ;
i n t
m
w h i
P
P
w
a i
l e
W M
W M
a i
n (
( 1
1 .
1 =
t (
)
) {
p e
0 .
0 .
{
r i o d ( A i n / 1 0 + 0 . 0 0 1 ) ;
5 ;
5 ) ;
/ /
/ /
s e t
s e t
P W M p e r i o d
d u t y c y c l e
}
}
P r o g r a m E x a m p le 5 .3 C o n t r o llin g P W M f r e q u e n c y w it h a p o t e n t io m e t e r
O b s e r v e th e w a v e f o r m o n a n o s c illo s c o p e , s e ttin g th e tim e b a s e in itia lly to 5 m s /d iv . Yo u
sh o u ld b e ab le to see th e freq u en cy ch an g e as th e p o ten tio meter is ad ju sted .
n
E x e r c is e 5 .2
1 . A d ju s t t h e v a lu e s in
o u tp u t.
2 . A t w h a t fre q u e n c y
o n ? M e a s u r e t h is a
b e t w e e n d iffe r e n t p
u s to k n o w a t w h a
im a g e is c o n t in u o u
o s c illo s c o p e t r a c e s
t h e P W M p e r io d c a lc u la t io n t o g iv e d iffe r e n t r a n g e s o f fr e q u e n c y
d o e s th e L E D a p p e a r n o t to fl a s h
s c a r e fu lly a s y o u c a n , a n d s e e if
e o p le . T h is is a n im p o r t a n t q u e s t
t fr e q u e n c y w e c a n ‘t r ic k ’ t h e e y e
s , fo r e x a m p le in c o n v e n t io n a l r a
a n d m u lt ip le x e d L E D d is p la y s .
, b u t
th e p
io n , a
in t o
s te r s
s e e m s t o b e c o n t in
e r c e iv e d fr e q u e n c y
s k n o w in g it s v a lu e
t h in k in g t h a t a fl a s
c a n t e le v is io n s c r e e
u o u s ly
v a r ie s
a llo w s
h in g
n s ,
n
T h e 0 .5 s d e la y in P r o g r a m E x a m p le 5 .3 is a d d e d to th e lo o p s o th a t e a c h n e w P W M
p erio d is allo wed to b e imp lemen ted b efo re th e n ex t u p d ate. If it was o mitted th en th e
PWM wo u ld p o te n tially b e u p d ated rep e ated ly with in e a c h cy c le , wh ich wo u ld le ad to
a larg e amo u n t o f jitter in th e o u tp u t (try re mo v in g th e d elay to s ee th e effect). No tice th at
th e re c an b e d is co n tin u itie s in th e PWM o u tp u t a s th e fre q u e n cy v a lu es are u p d ate d . With
care (an d esp ecially if y o u are u sin g a sto rag e o scillo sco p e) y o u can see th is as th e PWM
is u p d a te d e v e r y 0 .5 s . T h is is o n e r e a s o n w h y it is g o o d to fi x th e f r e q u e n c y f o r a P W M
sig n al.
n
E x e r c is e 5 .3
C o n
c o n
s e rv
th e
n e c t th
n e c te d
o p o s it
fu ll r a n
e s e
a s in
io n .
g e o
rv o
F ig
S c a
f s e
to th e m b e d a s
u r e 5 .5 a . W r it e
le v a lu e s s o t h a
r v o p o s it io n c h
in d ic
a p ro
t th e
a n g e s
a t e d in F ig u r e 4 .1 0 a , w it h t h e p o t e n t io m e t e r
g r a m w h ic h a llo w s t h e p o t e n t io m e t e r t o c o n t r o l
fu ll r a n g e o f p o t e n t io m e t e r a d ju s t m e n t le a d s t o
.
n
8 6
Chapter 5
5 .3
P r o c e s s in g D a t a fr o m A n a lo g In p u t s
5 .3 .1
D is p la y in g V a lu e s o n t h e C o m p u t e r S c r e e n
We tu rn n o w to th e seco n d way o f mak in g u se o f th e ADC o u tp u t, p ro mised at th e b eg in n in g
o f S e c tio n 5 .2 . I t is p o s s ib le to r e a d a n a lo g in p u t d a ta th r o u g h th e A D C , a n d th e n p r in t th e
v a lu e to th e P C s c r e e n . T h is is a v e r y im p o r ta n t s te p f o r w a r d , a s it g iv e s th e p o s s ib ility o f
d isp lay in g an y d ata we are wo rk in g with o n th e co mp u ter screen . To d o th is, we n eed to
c o n fi g u re b o th m b e d a n d h o s t c o m p u te r to b e re a d y to s e n d a n d re c e iv e d a ta , a n d w e n e e d th e
h o st co mp u ter to b e ab le to d isp lay th at d ata. Fo r th e co mp u ter a termin al emu lato r is
req u ired . Th e mb ed site reco mmen d s u se o f Tera Term fo r th is p u rp o se; Ap p en d ix E d escrib es
h o w to set th is u p . Th e mb ed can b e mad e to ap p ear to th e co mp u ter as a serial p o rt,
c o m m u n ic a tin g th ro u g h th e u n iv e rs a l s e ria l b u s (U S B ) c o n n e c tio n . It lin k s u p w ith th e
USB th ro u g h o n e o f its o wn asy n ch ro n o u s serial p o rts. Th is is set u p simp ly b y ad d in g th is
p ro g ram lin e:
S e r i a l
p c ( U S B T X ,
U S B R X ) ;
W r itin g to th e c o m p u te r s c r e e n is th e n a c h ie v e d w ith th e p c .p r in t f ( ) f u n c tio n . E x p la n a tio n s
f o r th is a r e g iv e n in S e c tio n 7 .9 .3 .
Start a n ew mb e
p o ten tio meter co
fu n ctio n fo r th e
sp ecifiers. Ch eck Sectio n B
C c o d e
fe a tu re
/ * P r o g r a m E x a m p l e
* /
# i n c l u d e " m b e d . h "
5 . 4 :
d p ro ject an d
n n ected as in
first time, alo
.9 in A p p e n d
R e a d s
i n p u t
e n te r th e c o d e o f P r o g r a m E x a m p le 5 .4 . L e a v e th e
F ig u r e 5 .5 a . N o te th a t th e p r o g r a m u s e s th e p r in t f ( )
n g with so me o f its far-fro m-frien d ly fo rmat
ix B fo r so me b ack g ro u n d o n th is.
v o l t a g e
S e r i a l p c ( U S B T X , U S B R X ) ;
A n a l o g I n A i n ( p 2 0 ) ;
f l o a t A D C d a t a ;
i n t m a i n ( ) {
p c . p r i n t f ( " A D C D a t a V a l u e s . \ n \ r " ) ;
w h i l e ( 1 ) {
A D C d a t a = A i n ;
w a i t ( 0 . 5 ) ;
p c . p r i n t f ( " % 1 . 3 f \ n \ r " , A D C d a t a ) ;
}
}
t h r o u g h
t h e
A D C ,
a n d
t r a n s f e r s
t o
P C
t e r m i n a l
/ / e n a b l e s e r i a l p o r t w h i c h l i n k s t o U S B
/ / s e n d
a n
o p e n i n g
t e x t
m e s s a g e
/ / s e n d
t h e
d a t a
t h e
t e r m i n a l
t o
P r o g r a m E x a m p le 5 .4 L o g g in g d a t a t o t h e P C
Yo u s h o u ld n o w b e a b le to c o m p ile a n d r u n y o u r c o d e to g iv e a n o u tp u t o n Te r a Te r m . I f y o u h a v e
p ro b le m s, c h e c k fro m A p p e n d ix E o r th e m b ed s ite th a t y o u h av e s e t u p Te ra Te rm c o rre c tly .
A n a lo g In p u t
5 .3 .2
S c a lin g A D C O u t p u t s t o R e c o g n iz e d U n it s
Th e d ata d isp lay ed th ro u g h Pro g ram E
v o ltag e in p u t. Th ese v alu es can read ily
3 .3 . S u b s titu te th e c o d e lin e s b e lo w in
th is, an d to p lace a u n it after th e v o lta
A D C
w a i
p c .
p c .
d a t
t ( 0
p r i
p r i
8 7
a = A
. 5 )
n t f
n t f
x a m p le 5 .4 is ju s t a s e t o f n u m b e r s p r o p o r tio n a l to th e
b e s c a le d to g iv e a v o lta g e r e a d in g , b y m u ltip ly in g b y
to th e w h ile lo o p o f P r o g r a m E x a m p le 5 .4 to d o ju s t
g e v alu e.
i n * 3 . 3 ;
;
( " % 1 . 3 f " , A D C d a t a ) ;
( " V \ n \ r " ) ;
R u n th e a d ju s te d p r o g r a m ; its o u tp u t s h o u ld a p p e a r s im ila r to F ig u r e 5 .6 . Vie w th e m e a s u r e d
v o ltag e o n th e PC screen , an d read th e actu al in p u t v o ltag e o n a d ig ital v o ltmeter. Ho w well
d o th ey co mp are?
5 .3 .3
A p p ly in g A v e r a g in g t o R e d u c e N o is e
I f y o u le a v e P r o g r a m E x a m p le 5 .4 r u n n in g , w ith a fi x e d in p u t a n d v a lu e s d is p la y e d o n Te r a
Term, y o u may b e su rp rised to see th at th e measu red v alu e is n o t alway s th e same, b u t v aries
a r o u n d s o m e a v e r a g e v a lu e . Yo u m a y a lr e a d y h a v e n o tic e d th a t th e P W M v a lu e in S e c tio n
5 .2 .2 o r 5 .2 .3 a ls o a p p e a r e d to v a r y , e v e n w h e n th e p o te n tio m e te r w a s n o t b e in g m o v e d .
Sev eral effects may b e at p lay h ere, b u t almo st certain ly y o u are seein g th e effect o f so me
in terferen ce, an d all th e p ro b lems it can b rin g . If y o u lo o k with th e o scillo sco p e at th e ADC
in p u t ( i.e . th e ‘ w ip e r ’ o f th e p o te n tio m e te r ) y o u a r e lik e ly to s e e s o m e h ig h - f r e q u e n c y n o is e
su p erimp o sed o n th is; ex actly h o w mu ch will d ep en d o n wh at eq u ip men t is ru n n in g n earb y ,
h o w lo n g y o u r in terco n n ectin g wires are, an d a n u mb er o f o th er th in g s.
F ig u r e 5 .6 :
L o g g e d d a ta o n T e ra T e rm
8 8
Chapter 5
A v ery simp le first step to imp ro v e th is situ atio n is to av erag e th e in co min g sig n al. Th is
sh o u ld h elp to fin d th e u n d erly in g av erag e v alu e an d remo v e th e h ig h -freq u en cy n o ise
elemen t. Try in sertin g th e fo r lo o p sh o wn b elo w, rep lacin g th e A D C d a t a = A i n ; lin e in Pro g ram
E x a m p le 5 .4 . Yo u w ill s e e th a t th is c o d e f r a g m e n t s u m s 1 0 A D C v a lu e s a n d ta k e s th e ir
av erag e. Try ru n n in g th e rev ised p ro g ram an d see wh eth er a mo re stab le o u tp u t resu lts. No te
th a t w h ile th is s o rt o f a p p ro a c h g iv e s s o m e b e n e fi t, th e a c tu a l m e a s u re m e n t n o w ta k e s
1 0 times as lo n g . Th is is a v ery simp le ex amp le o f d ig ital sig n al p ro cessin g .
f o r
( i n t i = 0 ; i < = 9 ; i + + ) {
A D C d a t a = A D C d a t a + A i n * 3 . 3 ;
/ / s u m
1 0
s a m p l e s
}
A D C d a t a = A D C d a t a / 1 0 ;
/ / d i v i d e
b y
5 .4
S o m e S im p le A n a lo g S e n s o r s
No w
sen so
o u tp u
acro s
th at we are eq
rs. Th ese ten d
t v o ltag e th at
s sen so rs th at
5 .4 .1
u ip
to
can
can
p ed
b e
b e
co
with
th e s
co n n
mmu
1 0
an alo g in p u t, it is ap p ro p
imp ler an d mo re trad itio n
ected to th e mb ed ADC in
n icate with th e mb ed v ia
riate to ex p lo re so me simp le an alo g
al o n es, wh ich h av e an an alo g
p u t. Later in th e b o o k we will co me
a d ig ital in terface.
T h e L ig h t -D e p e n d e n t R e s is t o r
Th e lig h t-d ep en d en t resisto r
Wh en lig h t falls o n it, its en
b rig h ter th e lig h t, th e mo re
co n d u ct electricity , with th e
remo v ed th e electro n s p o p b
effect is th at as illu min atio n
(LDR) is mad e fro m a
erg y flip s so me electro
electro n s are released .
resu lt th at th e resistan
ack in to th eir p lace an d
in creases, th e LDR re
p iece o f ex p o sed semico n d u cto r material.
n s o u t o f th e cry stallin e stru ctu re; th e
Th ese electro n s are th en av ailab le to
ce o f th e material falls. If th e lig h t is
th e resistan ce g o es u p ag ain . Th e o v erall
sistan ce falls.
T h e N O R P 1 2 L D R , m a d e b y S ilo n e x ( R e f e r e n c e 5 .2 ) , is r e a d ily a v a ila b le a n d in e x p e n s iv e . I ts
s u m m a r y d a ta is s h o w n in F ig u r e 5 .7 . T h is s h o w s th a t it h a s a r e s is ta n c e w h e n c o m p le te ly
d a r k o f a t le a s t 1 .0 M U , f a llin g to a f e w h u n d r e d o h m s w h e n v e r y b r ig h tly illu m in a te d . A n
e a s y w a y to c o n n e c t s u c h a s e n s o r is in a s im p le p o te n tia l d iv id e r, g iv in g a v o lta g e o u tp u t; th is
is a ls o s h o w n in F ig u r e 5 .7 . T h e v a lu e o f th e s e r ie s r e s is to r, s h o w n h e r e a s 1 0 k U , is c h o s e n to
g iv e a n o u tp u t v a lu e o f a p p ro x im a te ly m id -ra n g e fo r n o rm a l ro o m lig h t lev e ls . It c a n b e
ad ju sted to mo d ify th e o u tp u t v o ltag e ran g e. Pu ttin g th e LDR at th e b o tto m o f th e p o ten tial
d iv id e r, a s s h o w n h e re , g iv e s a lo w o u tp u t v o lta g e in b rig h t illu m in a tio n a n d a h ig h o n e in lo w
illu m in a tio n . T h is c a n b e r e v e r s e d b y p u ttin g th e L D R a t th e to p o f th e d iv id e r.
T h e L D R is a s im p le , e ffe c tiv e a n d lo w -c o s t lig h t s e n s o r. Its o u tp u t is n o t, h o w ev e r, lin e a r, a n d
e a c h d ev ic e te n d s to g iv e s lig h tly d iffe re n t o u tp u t fro m a n o th e r. H e n c e , it is n o t u s u a lly u s e d
fo r p recisio n measu remen ts.
A n a lo g In p u t
8 9
3 .3 V
1 0 k
Vo
RL D R
0 V
Illu m in a tio n (lu x )
RL
D R
Vo
(W)
D a rk
≥ 1 .0 M
≥ 3 .2 7 V
1 0
9 k
1 .5 6 V
1 ,0 0 0
4 0 0
0 .1 3 V
F ig u r e 5 .7 :
T h e N O R P S 1 2 L D R c o n n e c t e d in a p o t e n t ia l d iv id e r , w it h in d ic a t iv e o u t p u t v a lu e s
n
E x e r c is e 5 .4
U s in g t h e c ir c u it o f F ig u r e 5 .7 , c o n n e c t a N O R P 1 2 L D R t o t h e m b e d . W r it e a p r o g r a m t o
d is p la y lig h t r e a d in g s o n t h e T e r a T e r m s c r e e n . Y o u w ill n o t b e a b le t o s c a le t h e s e in t o a n y
u s e fu l u n it . T r y r e v e r s in g r e s is t o r a n d L D R , a n d n o t e t h e e ffe c t .
n
5 .4 .2
In t e g r a t e d C ir c u it T e m p e r a t u r e S e n s o r
Semico n d u cto r actio n is h ig h ly d e p en d e n t o n temp eratu re, so it is n o t su rp risin g th a t
semico n d u cto r temp eratu re sen so rs are mad e . A v ery u sefu l fo rm o f se n so r is o n e wh ic h is
c o n ta in e d in a n in te g r a te d c ir c u it, s u c h a s th e L M 3 5 ( F ig u r e 5 .8 ) . T h is a llo w s th e o u tp u t to
b e sca led to an imme d ia tely u se fu l v a lu e . Th is d ev ice h as an o u tp u t o f 1 0 mV / C, with
o p eratin g temp eratu re fro m  5 5  C to 1 5 0  C. It is th u s immed iately u sefu l fo r a ra n g e
o f temp e ratu re-s en sin g a p p lica tio n s. Th e simp lest co n n ectio n fo r th e LM3 5 , wh ic h ca n
b e u s e d w ith th e m b e d , is s h o w n in F ig u r e 5 .8 . A r a n g e o f m o r e a d v a n c e d c o n n e c tio n s , f o r
ex amp le to o b tain an o u tp u t fo r te mp era tu re s b elo w 0  C, is s h o wn in th e d a tash ee t
( R e f e r e n c e 5 .3 ) .
n
E x e r c is e 5 .5
D e s ig n , b u ild a n d p r o g r a m a s im p le e m b e d d e d s y s t e m u s in g a n L M 3 5 s e n s o r , w h ic h
d is p la y s t e m p e r a t u r e o n t h e c o m p u t e r s c r e e n a n d fl a s h e s a n L E D w h e n e v e r t h e
9 0
Chapter 5
4 V to 2 0 V
O u tp u t
1 0 .0 m V / ° C
L M 3 5
V s
V o
G ro u n d
F ig u r e 5 .8 :
T h e L M 3 5 in t e g r a t e d c ir c u it t e m p e r a t u r e s e n s o r
te m p e ra tu re e x c e e d s 3 0  C . T
m b e d . W h e n c o n n e c t in g t h e
in t h e m b e d b r e a d b o a r d . E a
t h a t r e m o t e s e n s in g c a n b e
( e .g . w it h s ilic o n e r u b b e r a t
liq u id t e m p e r a t u r e s . H o w w
m b e d A D C ?
h e V S p in o f
s e n s o r , it c a
c h t e r m in a l
u n d e rta k e n .
th e s e n s o r e
e ll d o e s t h is
t h e s e n s o r c a n b e c o n n e c t e d t o p in
n b e p lu g g e d d ir e c t ly in t o a s u it a b le
c a n , h o w e v e r , a ls o b e s o ld e r e d t o a
If t h e s e w ir e s a r e in s u la t e d a p p r o p
n d ), th e n th e s e n s o r c a n b e u s e d to
a r r a n g e m e n t e x p lo it t h e in p u t r a n g
3 9 o f th e
lo c a t io n
w ir e , s o
r ia t e ly
m e a s u re
e o f th e
n
5 .5
E x p lo r in g D a t a C o n v e r s io n T im in g
Ny q u ist’s samp lin g th eo
freq u en cy sig n als. Wh en
to k n o w h o w lo n g each d
mb ed ADC an d DAC co
5 .5 .1
rem s
d esig
ata co
n v ers
u g g
n in
n v e
io n
ests th at a slo w ADC will o n ly b e ab le to co n v ert lo wg a carefu lly sp ecified sy stem it is th erefo re v ery imp o rtan t
rsio n tak es. Fo r th is reaso n , we will mak e a measu remen t o f
times, an d th en p u t Ny q u ist to th e test.
E s t im a t in g C o n v e r s io n T im e a n d A p p ly in g N y q u is t
P r o g r a m E x a m p le 5 .5 p r o v id e s a v e r y s im p le m e c h a n is m f o r m e a s u r in g c o n v e r s io n tim e s a n d
th e n v ie w in g N y q u is t’s s a m p lin g th e o r e m in a c tio n . I t a d a p ts P r o g r a m E x a m p le 5 .1 , b u t
p u lses a d ig ital o u tp u t b etween each stag e. En ter th is as a n ew p ro g ram, co mp ile an d ru n .
/ * P
o s c
A D C
* /
# i n
A n a
A n a
D i g
f l o
r o g r a m E x a m p l e 5 . 5 : I n p u t s s i g n a l t h r o u g h A D C , a n d o u t p u t s t o D A C . V i e w D A C o u t p u t o n
i l l o s c o p e . T o d e m o n s t r a t e N y q u i s t , c o n n e c t v a r i a b l e f r e q u e n c y s i g n a l g e n e r a t o r t o
i n p u t . A l l o w s m e a s u r e m e n t o f c o n v e r s i o n t i m e s , a n d e x p l o r e s N y q u i s t l i m i t .
c l u
l o g
l o g
i t a
a t
d e
" m
O u t A
I n A i
l O u t
A D C d a
b e
o u
n (
t e
t a
;
d .
t (
p 2
s t
h "
p 1 8 ) ;
0 ) ;
( p 5 ) ;
/ / d e f i n e s
/ / d e f i n e s
a n a l o g
a n a l o g
o u t p u t o n P i n 1 8
i n p u t o n P i n 2 0
A n a lo g In p u t
i n t
m a
w h i l
A
t
t
A
t
t
t
t
/
i n
e (
D C
e s
e s
o u
e s
e s
e s
e s
/ w
( )
1 )
d a
t =
t =
t =
t =
t =
t =
t =
a i
9 1
{
{
t a = A i n ;
1 ;
0 ;
A D C d a t a ;
1 ;
0 ;
1 ;
0 ;
t ( 0 . 0 0 1 ) ;
/ / s t a r t s A - D c o n v e r s i o n , a n d a s s i g n s a n a l o g
/ / s w i t c h t e s t o u t p u t , a s t i m e m a r k e r
v a l u e
/ / t r a n s f e r s s t o r e d v a l u e
/ / a d o u b l e p u l s e , t o m a r k
t o D A C ,
t h e e n d
a n d f o r c e s a D - A
o f c o n v e r s i o n
/ / o p t i o n a l
e x p l o r e
d i f f e r e n t
w a i t
s t a t e ,
t o
c y c l e
t o
A D C d a t a
c o n v e r s i o n
t i m e s
}
}
P r o g r a m E x a m p le 5 .5 E s t im a t in g d a t a c o n v e r s io n t im e s
Th is p ro g ram allo ws u s to mak e a n u mb er o f measu remen ts wh ich are o f v ery g reat
imp o rtan ce, an d wh ich req u ire carefu l u se o f th e o scillo sco p e. Th e measu remen ts are
p r e s e n te d a s E x e r c is e s 5 .6 a n d 5 .7 .
n
E x e r c is e 5 .6
R u n n in g P r o g r a m E x a
o u t p u t ( i.e . p in 5 ) o n
r e s u lt s . T h is m a y r e q u
t h e p u ls e s if n e c e s s a r y
y o u d o n o t n e e d a n y t
Y o u w ill b
a n d th e d
t o - d ig it a l
o n th e s e ?
o f th e A D
t h e s e v a lu
m p le 5 .5 , o
a n o s c illo s c
ir e s o m e p a
b y in s e r t in g
h in g c o n n e c
b s e rv e c a r
o p e ea d
t ie n c e , a s
a w a it w h
te d to th e
e fu lly t h e w a v e fo r m
ig it a l s t o r a g e o s c illo
th e y a re v e ry n a rro w
ile t h e o u t p u t is h ig h
A D C in p u t .
d is p la y e
s c o p e w
p u ls e s .
. N o te t
d b y th
ill g iv e
Y o u c a
h a t fo r
e ‘te s t’
th e b e s t
n w id e n
t h is t e s t
e a b le t o d e t e c t t h e s in g le p u ls e a t t h e e n d o f t h e a n a lo g - t o - d ig it a l c o n v e r s io
o u b le o n e a t t h e e n d o f t h e lo o p . M e a s u r e t h e t im e d u r a t io n o f t h e a n a lo g c o n v e r s io n , a n d t h e d ig it a l- t o - a n a lo g c o n v e r s io n . W h a t c o m m e n t c a n y o u m a
N o t e t h a t t h e c o n v e r s io n t im e s y o u m e a s u r e a r e n o t t h e a c t u a l c o n v e r s io n t im
C a n d D A C ; t h e y in c lu d e a ll a s s o c ia t e d p r o g r a m m in g o v e r h e a d s . K e e p a n o t e
e s a s w e a im t o a c c o u n t fo r t h e m la t e r , in E x e r c is e s 5 .7 a n d 1 4 .8 .
n ,
k e
e s
o f
n
n
E x e r c is e 5 .7
A r m e d w it h t h e k n o w le d g e o f t h e c o n v e r s io n t im e s , c o n n e c t a s ig n a l g e n e r a t o r a s a n
in p u t t o t h e A D C . S e t t h e s ig n a l a m p lit u d e s o t h a t it is ju s t u n d e r 3 .3 V p e a k t o p e a k , a n d
a p p ly a D C o ffs e t s o t h a t t h e v o lt a g e v a lu e n e v e r g o e s b e lo w 0 V . T h is fa c ilit y is a v a ila b le
o n m o s t s ig n a l g e n e r a t o r s . In s e r t t h e w a it ( 0 .0 0 1 ) ; lin e a t t h e e n d o f t h e lo o p ( ‘c o m m e n t e d o u t ’ in P r o g r a m E x a m p le 5 .5 ) . T h is w ill g iv e a s a m p lin g fr e q u e n c y o f a lit t le
b e lo w 1 k H z . N y q u is t ’s s a m p lin g t h e o r e m p r e d ic t s t h a t t h e m a x im u m s ig n a l fr e q u e n c y
t h a t c a n b e d ig it iz e d w ill b e 5 0 0 H z , fo r t h is s a m p lin g fr e q u e n c y . L e t ’s t e s t it .
9 2
Chapter 5
S t a r t w it h a n in p u t s ig n a l o f a r o u n
t h e t w o b e a m s o f t h e o s c illo s c o p e .
v e r s io n o f it , s o m e t h in g lik e F ig u r e
m illis e c o n d . N o w g r a d u a lly in c r e a s
a p p r o a c h N y q u is t ’s lim it t h e o u t p u
e q u a ls s a m p lin g fr e q u e n c y , a s t r a ig
p r a c t ic e it m a y b e d iffi c u lt t o fi n d t h
f u r t h e r , a n a lia s s ig n a l ( a s illu s t r a t e
d 2 0 0 H z . O b s e r v e in p u t s ig n a l a n d D A C o u t p u t o n
Y o u s h o u ld s e e t h e in p u t s ig n a l a n d a r e c o n s t r u c t e d
5 .3 , w it h a n e w c o n v e r s io n a p p r o x im a t e ly e v e r y
e t h e s ig n a l fr e q u e n c y t o w a r d s 5 0 0 H z . A s y o u
t b e c o m e s a s q u a r e w a v e . W h e n in p u t fr e q u e n c y
h t lin e o n t h e o s c illo s c o p e s h o u ld o c c u r , a lt h o u g h in
is c o n d it io n e x a c t ly . A s t h e in p u t fr e q u e n c y in c r e a s e s
d in F ig u r e 5 .4 ) a p p e a r s a t t h e o u t p u t .
D e c r e a s e t h e d u r a t io n o f t h e w a it
fr e q u e n c y . F in a lly , r e m o v e t h e w a it
t a k in g p la c e a t t h e fa s t e s t p o s s ib le
e a r lie r m e a s u r e m e n t . T h e N y q u is t
h a r d w a r e / s o ft w a r e c o n fi g u r a t io n .
s ta te ,
s ta te
ra te ,
lim it t
a n d
a lt o g
w it h
h a t y
p re
e th
c o
o u
d ic
e r.
n v e
n o
t a n d
T h e d
r s io n
w fi n d
o b s e
a ta c
t im e
is t h
rv e th e n e w N y
o n v e r s io n s h o u
c o r r e s p o n d in g
e lim it fo r t h is
q u
ld
to
p a
is t
n o w b e
y o u r
r t ic u la r
n
5 .6
M in i-P r o je c t : T w o -D im e n s io n a l L ig h t T r a c k in g
Lig h t-track in g d ev ices are v ery imp o rtan t fo r th e cap tu re o f so lar en erg y . Often th ey o p erate
in th ree d imen sio n s, an d tilt a so lar p an el so th at it is facin g th e su n as accu rately as p o ssib le.
To start rath er mo re simp ly , create a two -d imen sio n al lig h t track er b y fittin g two LDRs,
an g led away fro m each o th er b y aro u n d 9 0  , to a serv o . Co n n ect th e LDRs u sin g th e circu it o f
F ig u r e 5 .7 to tw o A D C in p u ts . W r ite a p r o g r a m th a t r e a d s th e lig h t v a lu e s e n s e d b y th e tw o
L D R s a n d ro ta te s th e s e rv o s o th a t e a c h is re c e iv in g e q u a l lig h t. T h e s e rv o c a n o n ly ro ta te
1 8 0  . Th is is n o t, h o wev er, u n reaso n ab le, as a su n -track in g sy stem will b e lo cated to track th e
s u n f r o m s u n r is e to s u n s e t, i.e . n o t m o r e th a n 1 8 0  . C a n y o u th in k o f a w a y o f m e e tin g th is
n eed u sin g o n ly o n e ADC in p u t?
C h a p t e r R e v ie w
•
•
•
•
•
An ADC is av ailab le in th e mb ed ; it can b e u sed to
It is imp o rtan t to u n d erstan d ADC ch aracteristics, in
co n v ersio n time.
Ny q u ist’s samp lin g th eo rem mu st b e u n d ersto o d , an d
sig n als. Th e samp lin g freq u en cy mu st b e at least tw
co mp o n en t in th e samp led an alo g sig n al.
Aliasin g o ccu rs wh en th e Ny q u ist criterio n is n o t me
to th e d ata. Aliasin g can b e av o id ed b y in tro d u cin g
sig n al b efo re it is samp led .
Data g ath ered b y th e ADC can b e fu rth er p ro cessed
d ig itize an alo g in p u t sig n als.
terms o f in p u t ran g e, reso lu tio n an d
ap p lied with care wh en samp lin g AC
ice th at o f th e h ig h est freq u en cy
t; th is can in tro d u ce false freq u en cies
an an ti-aliasin g filter to th e an alo g
, an d d isp lay ed o r sto red .
A n a lo g In p u t
•
9 3
Th ere are man y sen so rs av ailab le wh ich h av e an an alo g o u tp u t; in man y cases th is o u tp u t
can b e d irectly co n n ected to th e mb ed ADC in p u t.
Q u iz
1 .
2 .
3 .
4 .
5 .
6 .
7 .
8 .
9 .
1 0 .
G iv e th re e ty p e s o f a n a lo g s ig n a l w h ic h m ig h t b e s a m p le d th ro u g h a n A D C .
A n id e a l 8 - b it A D C h a s a n in p u t r a n g e o f 5 .1 2 V. W h a t is its r e s o lu tio n , a n d w h a t is its
g reatest q u an tizatio n erro r?
G iv e a n e x a m p le o f h o w a s in g le A D C c a n b e u s e d to s a m p le fo u r d iffe re n t a n a lo g
sig n als.
A n id e a l 1 0 - b it A D C h a s a r e f e r e n c e v o lta g e o f 2 .0 4 8 V a n d b e h a v e s a c c o r d in g to
E q u a tio n 5 .1 . F o r a p a r tic u la r in p u t its o u tp u t r e a d s 1 0 1 1 1 0 0 0 0 1 . W h a t is th e in p u t
v o ltag e?
W h a t w ill b e th e r e s u lt if a n m b e d is r e q u ir e d to s a m p le a n a n a lo g in p u t v a lu e o f 4 .2 V ?
An u ltraso u n d sig n al o f 4 0 k Hz is to b e d ig itized . Reco mmen d th e min imu m samp lin g
freq u en cy .
T h e c o n v e r s io n tim e o f a n A D C is f o u n d to b e 7 .5 m s . T h e A D C is s e t to c o n v e r t
rep eated ly , with n o o th er p ro g rammin g req u iremen ts. Wh at is th e max imu m freq u en cy
sig n al it can d ig itize?
Th e ADC in Qu estio n 7 is n o w u sed with a mu ltip lex er, so th at fo u r in p u ts are rep eated ly
d ig itized in tu rn . A fu rth er time o f 2 5 0 0 n s p er samp le is req u ired to sav e th e d ata an d
switch th e in p u t. Wh at is th e max imu m freq u en cy sig n al th at can n o w b e d ig itized ?
An LM3 5 temp eratu re sen so r is co n n ected to an mb ed ADC in p u t, an d sen ses
a temp eratu re o f 3 0  C. Wh at is th e b in ary o u tp u t o f th e ADC?
W h a t w ill b e th e v a lu e o f in te g e r x f o r 1 .5 V a n d 2 .5 V s ig n a ls s a m p le d o n th e m b e d u s in g
th e fo llo win g p ro g ram co d e?
# i n c l u d e “ m b e d . h ”
A n a l o g I n A i n ( p 2 0 ) ;
i n t m a i n ( ) {
i n t x = A i n . r e a d _ u 1 6 ( ) ;
}
References
5 .1 .
5 .2 .
5 .3 .
H o r o w itz , P.
Th e Silo n ex
LM3 5 Precis
h ttp ://w w w.n
a n d H ill, W. ( 1 9 8 9 ) . T h e A r t o f E le c tr o n ic s . 2 n d e d itio n . C a m b r id g e U n iv e r s ity P r e s s .
h o m e s ite . h ttp ://w w w.s ilo n e x .c o m /
io n Cen tig rad e Temp eratu re Sen so rs. No v emb er 2 0 0 0 . Natio n al Semico n d u cto r Co rp o ratio n .
a tio n a l.c o m /
C H A P T E R 6
F u r t h e r P r o g r a m m in g T e c h n iq u e s
C h a p t e r O u t lin e
6 .1 T h e B e n e f it s o f C o n s id e r e d P r o g r a m D e s ig n a n d S t r u c t u r e
6 .2 F u n c t io n s 9 6
6 .3 P r o g r a m D e s ig n
9 7
6 .3 .1 U s in g F lo w c h a r t s t o D e fin e C o d e S t r u c t u r e
6 .3 .2 P s e u d o c o d e 9 9
6 .4 W o r k in g w it h F u n c t io n s o n t h e m b e d
9 7
1 0 0
6 .4 .1 Im p le m e n t in g a S e v e n - S e g m e n t D is p la y C o u n t e r
6 .4 .2 F u n c t io n R e u s e 1 0 2
6 .4 .3 A M o r e C o m p le x P r o g r a m U s in g F u n c t io n s 1 0 3
6 .5 U s in g M u lt ip le F ile s in C / C þ þ
6 .5
6 .5
6 .5
6 .5
.1 O v e
.2 T h e
.3 T h e
.4 U s in
rv
C
#
g
ie w
/ C þ
ifn d
m b
o f
þ
e f
e d
th e C / C þ þ
P re p ro c e sso
D ir e c t iv e 1
O b je c t s G lo
6 .6 M o d u la r P r o g r a m E x a m p le
C h a p t e r R e v ie w
1 1 3
Q u iz 1 1 3
6 .1
•
•
•
•
1 0 0
1 0 5
P r o g r a m C o m p ila t io n P r o c e s s
r a n d P r e p r o c e s s o r D ir e c t iv e s
0 8
b a lly 1 0 9
1 0 6
1 0 7
1 0 9
T h e B e n e fi t s o f C o n s id e r e d P r o g r a m D e s ig n a n d S t r u c t u r e
Th ere are n u mero u s ch allen
u su ally wise first to co n sid e
mu lti-fu n ctio n al p ro jects. It
lo o p , so th e ap p ro ach fo r b r
th o u g h t o u t. In p articu lar, it
•
9 5
th
th
th
th
th
a
a
a
a
a
t co d e is
t co d e ca
t d ev elo p
t co d e d e
t fu tu re u
g es wh en tack lin g
r th e so ftware d es
is n o t p o ssib le to
eak in g u p co d e in
h elp s to en su re th
read ab le, stru ctu red
n b e tested fo r p erfo
men t reu ses ex istin g
sig n su p p o rts mu ltip
p g rad es to co d e can
an emb ed d ed sy stem d esig n p ro ject. It is
ig n stru ctu re, p articu larly with larg e an d
p ro g ram all fu n ctio n ality in to a sin g le co n tro l
to u n d erstan d ab le featu res sh o u ld b e well
at th e fo llo win g can b e ach iev ed :
an d d o cu men ted
rman ce in a mo d
co d e u tilities to
le en g in eers wo r
b e imp lemen ted
u lar fo rm
k eep d ev elo p men t time sh o rt
k in g o n a sin g le p ro ject
efficien tly .
Th ere are v ario u s C/Cþ þ p ro g rammin g tech n iq u es th at en ab le th ese d esig n req u iremen ts to
b e co n sid ered , as d iscu ssed in th is ch ap ter.
F a s t a n d E f f e c t iv e E m b e d d e d S y s t e m s D e s ig n . D O I : 1 0 .1 0 1 6 /B 9 7 8 - 0 - 0 8 - 0 9 7 7 6 8 - 3 .0 0 0 0 6 - 4
Co p y rig h t Ó2 0 1 2 Elsev ier Ltd . All rig h ts reserv ed .
9 5
9 6
Chapter 6
6 .2
F u n c t io n s
A fu n c tio n is a p o rtio n o f c o d e w ith in a la rg er p ro g ram . T h e fu n c tio n p e rfo rm s a s p ec ifi c tas k
a n d is re la tiv e ly in d e p e n d e n t o f th e m a in c o d e . F u n c tio n s c a n b e u s e d to m a n ip u la te d a ta ; th is is
p a rticu la rly u s efu l if s ev e ral s im ilar d ata m a n ip u la tio n s a re re q u ired in th e p ro g ra m. D a ta v a lu e s
c a n b e in p u t to th e fu n ctio n a n d th e fu n c tio n c a n retu rn th e re su lt to th e m a in p ro g ram .
F u n c tio n s, th erefo re , a re p artic u larly u s efu l fo r c o d in g m a th em a tica l a lg o rith ms , lo o k -u p tab le s
a n d d ata c o n v e rs io n s , a s w e ll a s c o n tro l fe atu res th at m a y o p erate o n a n u mb er o f d iffe re n t
p a ra lle l d ata s trea ms . It is a lso p o ss ib le to u se fu n c tio n s w ith n o in p u t o r o u tp u t d ata, s imp ly to
r e d u c e c o d e s iz e a n d to im p r o v e r e a d a b ility o f c o d e . F ig u r e 6 .1 illu s tr a te s a f u n c tio n c a ll.
T h e re a re s ev e ral a d v a n tag e s w h en u s in g fu n ctio n s . F irst, a fu n ctio n is w ritte n o n c e a n d
c o m p ile d in to o n e a r e a o f m e m o r y , ir re s p e c tiv e o f th e n u m b e r o f tim e s th a t it is c a lle d f r o m th e
main p ro g ram, so p ro g ram memo ry is red u ced . Fu n ctio n s also allo w clean an d man ag eab le
c o d e to b e d e s ig n e d , a llo win g s o ftwa re to b e w e ll s tru c tu re d a n d re a d a b le a t a n u mb er o f lev e ls
o f a b s tra c tio n . T h e u s e o f fu n c tio n s a ls o e n a b le s th e p ra c tic e o f m o d u la r c o d in g , w h e re
te a ms o f s o ftwa re e n g in e e rs a re o ften re q u ired to d ev e lo p la rg e a n d a d v a n c ed a p p lic atio n s .
W ritin g c o d e w ith fu n c tio n s th e re fo re a llo w s o n e e n g in e e r to d ev e lo p a p a rtic u la r s o ftw a re
fe a tu re , w h ile a n o th e r e n g in e e r m a y ta k e re s p o n sib ility fo r s o me th in g e ls e .
Usin g fu n ctio n s is n o t
o v erh ead in sto rin g p r
th is sh o u ld o n ly b e an
it is p o ssib le to ‘n est’
ch allen g in g to fo llo w.
fro m th e fu n ctio n , an d
alway s co mp letely b en eficial, h o wev er. Th ere is a small ex ecu tio n time
o g ram p o sitio n d ata an d ju mp in g an d retu rn in g fro m th e fu n ctio n , b u t
issu e fo r co n sid eratio n in th e mo st time-critical sy stems. Fu rth ermo re,
fu n ctio n s with in fu n ctio n s, wh ich can so metimes mak e so ftware
A limitatio n o f C fu n ctio n s is th at o n ly a sin g le v alu e can b e retu rn ed
array s o f d ata can n o t b e p assed to o r fro m a fu n ctio n (o n ly sin g le-v alu e
F ig u r e 6 .1 :
A fu n c t io n c a ll
F u r t h e r P r o g r a m m in g T e c h n iq u e s
9 7
v ariab les can b e u sed ). Wo rk in g with fu n ctio n s an d mo d u lar tech n iq u es th erefo re req u ires
a co n sid ered so ftware stru ctu re to b e d esig n ed an d ev alu ated b efo re p ro g rammin g is started .
6 .3
P r o g r a m D e s ig n
6 .3 .1
U s in g F lo w c h a r t s t o D e fi n e C o d e S t r u c t u r e
It is o ften u sefu l to u se a flo wch art to in d icate th e o p eratio n o f p ro g ram flo w an d th e u se o f
f u n c tio n s . C o d e fl o w c a n b e d e s ig n e d u s in g a fl o w c h a r t p r io r to c o d in g . F ig u r e 6 .2 s h o w s
so me o f th e flo wch art sy mb o ls th at are u sed .
Fo r ex amp le, tak e th e fo llo win g so ftware d esig n sp ecificatio n :
Desig n a p ro g ram to
lig h t-emittin g d io d e
u s e d in S e c tio n 3 .5 )
co u n tin g . Th is in clu
•
•
•
•
in cremen t co n tin u o u sly th e o u tp u t o f a sev en -seg men t n u merical
( L E D ) d is p la y ( a s s h o w n in F ig u r e 6 .3 , a n d s im ila r to th e o n e
th ro u g h th e n u mb ers 0 to 9 , th en reset b ack to 0 to co n tin u e
d es:
Use a fu n ctio n to co n v ert a h ex ad ecimal c
seg men t LED o u tp u t b y te B.
Ou tp u t th e LED o u tp u t b y te to lig h t th e co r
If th e co u n t v alu e is g reater th an 9 , th en res
D elay fo r 5 0 0 ms to e n su re th a t th e L ED o u tp u
o u n ter b y te A to th e relev an t sev en rect seg men t LEDs.
et to zero .
t c o u n ts u p a t a ra te th a t is e a s ily v is ib le .
Th e o u tp u t o f th e sev en -seg men t d isp lay h as b een d iscu ssed p rev io u sly in Ch ap ter 3 an d in
p a r tic u la r in Ta b le 3 .4 . A f e a s ib le s o f tw a r e d e s ig n is s h o w n in F ig u r e 6 .4 .
Flo wch arts allo w u s to v isu alize th e o rd er o f o p eratio n s o f co d e an d to mak e ju d g men ts o n
wh ich sectio n s o f a p ro g ram may req u ire th e mo st atten tio n o r tak e th e mo st effo rt to d ev elo p .
F ig u r e 6 .2 :
E x a m p le fl o w c h a r t s y m b o ls
9 8
Chapter 6
A
F
B
G
E
C
D
D P
F ig u r e 6 .3 :
S e v e n - s e g m e n t d is p la y
F ig u r e 6 .4 :
E x a m p le fl o w c h a r t d e s ig n fo r a s e v e n - s e g m e n t d is p la y c o u n t e r
F u r t h e r P r o g r a m m in g T e c h n iq u e s
9 9
Th ey also h elp with co mmu n icatin g a p o ten tial d esig n with n o n -en g in eers, wh ich may h o ld
th e k ey to d esig n in g a sy stem th at meets a v ery d etailed sp ecificatio n .
6 .3 .2
Pseudocode
Pseu d o co
Id eally , p
Pseu d o co
sh o w lo o
p seu d o co
Writin
d ev elo
So me
a co m
d e c
seu d
d e s
p in g
d e.
o n sists o f
o co d e sh
h o u ld b e
p ro cesse
g p seu d o co d
p men t an d a
p ro jects may
b in atio n o f b
sh o rt, En g lish p h rases
o u ld n o t in clu d e k ey w
written as a list o f co n
s. In d en tatio n can b e u
u sed
o rd s
secu
sed
to
in
tiv
to
ex p lain sp ecific ta
an y sp ecific co mp
e p h rases; we can
sh o w th e lo g ical p
sk s with
u ter lan
ev en d r
ro g ram
in a p ro g ram.
g u ag e.
aw arro ws to
flo w in
e sav es time later d u rin g th e co d in g an d testin g stag e o f a p ro g ram’s
lso h elp s co mmu n icatio n amo n g d esig n ers, co d ers an d p ro ject man ag ers.
u se p seu d o co d e fo r d esig n , o th ers may u se flo wch arts, an d so me
o th .
T h e s o f tw a r e d e s ig n s h o w n b y th e fl o w c h a r t in F ig u r e 6 .4 c o u ld a ls o b e d e s c r ib e d in
p s e u d o c o d e a s s h o w n in F ig u r e 6 .5 .
N o te th a t th e fu n c tio n s S e g C o n v e r t( ) a n d D e la y ( ) a re d efi n e d e lsew h e re, fo r ex a mp le in
a s ep a ra te ‘u tilitie s ’ fi le, a u th o re d b y a d iffe re n t e n g in e e r. F u n ctio n S e g C o n v e r t( ) c o u ld
im p le me n t a s imp le lo o k -u p ta b le o r n u m b er o f if s tate me n ts th a t a ss ig n s th e s u itab le v a lu e to B .
F ig u r e 6 .5 :
E x a m p le p s e u d o c o d e fo r s e v e n - s e g m e n t d is p la y c o u n t e r
1 0 0
6 .4
Chapter 6
W o r k in g w it h F u n c t io n s o n t h e m b e d
Th e imp le me n tatio n an d sy n tax o f fu n ctio n s in C/Cþ þ are d iscu s sed in d eta il in
S e c tio n B .4 o f A p p e n d ix B w ith s o m e s im p le e x a m p le s . I t is im p o r ta n t to
rememb er th at, as with v ariab les, all fu n ctio n s (ex cep t th e m a in () fu n ctio n )
mu st b e d eclared at th e start o f a p ro g ram. T h e d ec laratio n stateme n ts fo r fu n ctio n s are
ca lled p ro to typ es. So each fu n ctio n in th e co d e mu st h av e an asso cia ted p ro to ty p e fo r it to
c o mp ile a n d ru n . Th e a c tu a l fu n c tio n co d e n e e d s d e fi n in g with in a C/Cþ þ file a ls o , in o rd e r
fo r it to b e calle d fro m with in th e ma in co d e. Th is is d o n e b y s p e cify in g th e fu n ctio n in th e
same way as th e p ro to ty p e, fo llo wed b y th e ac tu a l fu n ctio n co d e. A n u mb er o f ex a mp les
will b e p resen ted d u rin g th is ch ap ter, so in each case ma k e a p o in t o f id en tify in g th e
fu n ctio n p ro to ty p e an d th e actu al fu n ctio n d efin itio n . A fu rth er p o in t to n o te is th at if
a fu n ctio n is d efin e d in co d e p rio r to th e m a in ( ) C fu n ctio n , th en its d efin itio n serv es as its
p ro to ty p e als o .
C c o d e
fe a tu re
6 .4 .1
Im p le m e n t in g a S e v e n -S e g m e n t D is p la y C o u n t e r
P r o g r a m E x a m p le 6 .1 s h o w s a p r o g r a m w h ic h im p le m e n ts th e d e s ig n s d e s c r ib e d b y th e
fl o w c h a r t in F ig u r e 6 .4 a n d th e p s e u d o c o d e s h o w n in F ig u r e 6 .5 . I t a p p lie s s o m e o f th e
te c h n iq u e s fi r s t u s e d in P r o g r a m E x a m p le 3 .5 , b u t g o e s b e y o n d th e s e . T h e m a in d e s ig n
req u iremen t is th at a sev en -seg men t d isp lay is u sed to co u n t co n tin u o u sly fro m 0 to 9 an d
lo o p b ack to 0 . Declaratio n s fo r th e B u s O u t o b ject an d th e A an d B v ariab les, as well as th e
S e g C o n v e r t( ) fu n ctio n p ro to ty p e, ap p ear early in th e p ro g ram. It can b e seen th at th e m a in ( )
p ro g ram fu n ctio n is fo llo wed b y th e S e g C o n v e r t( ) fu n ctio n , wh ich is called reg u larly fro m
with in th e main co d e. No tice in th e lin e
B = S e g C o n v e r t ( A ) ;
/ /
C a l l
f u n c t i o n
t o
r e t u r n
B
th at B can immed iately tak e o n th e retu rn v alu e o f th e S e g C o n v e r t( ) fu n ctio n .
C c o d e
fe a tu re
r e t u r n
No tice in th e S e g C o n v e r t( ) fu n ctio n th e fin al lin e immed iately b elo w, wh ich
ap p lies th e r e tu r n k ey wo rd :
S e g B y t e ;
Th is lin e cau ses p ro g ram
carry in g th e v alu e S e g B y
y o u start writin g fu n ctio n
as p art o f th e fu n ctio n p r
ex ecu tio
te as its
s th at p r
o to ty p e
n to retu rn to
retu rn v alu e.
o v id e retu rn v
early in th e p
th e p o in t fro m wh ich th e fu n ctio n was called ,
It is an imp o rtan t tech n iq u e to u se o n ce
alu es. No tice th at S e g B y te h as b een d eclared
ro g ram listin g .
F u r t h e r P r o g r a m m in g T e c h n iq u e s
/ * P r o g r a m E x a m p l e
# i n c l u d e " m b e d . h "
B u
c h
c h
c h
i n
s O u t
a r S e
a r A =
a r B ;
t m a i
w h i l
B = S e
S e g 1
A + + ;
i f (
A = 0 ;
6 . 1 :
s e v e n - s e g m e n t
d i s p l a y
c o u n t e r
* /
S e g 1 ( p 5 , p 6 , p 7 , p 8 , p 9 , p 1 0 , p 1 1 , p 1 2 ) ;
g C o n v e r t ( c h a r S e g V a l u e ) ;
0 ;
/ /
/ /
/ /
A , B , C , D , E , F , G , D P
f u n c t i o n p r o t o t y p e
d e c l a r e v a r i a b l e s A
n (
e
g C
= B
/ /
/ /
/ /
/ /
/ /
m a i
i n f
C a l
O u t
i n c
i f
/ /
d e l a y
/ /
f u n c t i o n
) {
( 1 ) {
o n v e r t ( A ) ;
;
/ /
A > 0 x 0 9 ) {
}
w a i t ( 0 . 5 ) ;
}
1 0 1
n
p
i n i
l f
p u t
r e m
A >
r o g r a
t e l o
u n c t i
B
e n t A
9 r e
5 0 0
m
o p
o n
t o
s e t
a n d
r e t u r n
t o
B
B
z e r o
m i l l i s e c o n d s
}
c h a r S e g
c h a r S
s w i t c h
c
c
c
c
c
c
c
c
c
c
}
r e t u r n
C o
e g
(
a s
a s
a s
a s
a s
a s
a s
a s
a s
a s
n v
B y
S e
e
e
e
e
e
e
e
e
e
e
e r t (
t e = 0
g V a l
0 :
1 :
2 :
3 :
4 :
5 :
6 :
7 :
8 :
9 :
c h
x 0
u e
S e
S e
S e
S e
S e
S e
S e
S e
S e
S e
a r
0 ;
)
S e g V a l u e )
{
{
g B
g B
g B
g B
g B
g B
g B
g B
g B
g B
y t
y t
y t
y t
y t
y t
y t
y t
y t
y t
e
=
e
=
e
=
e
=
e
=
e
=
e
=
e
e
=
e
=
=
0 x 3
0 x 0
0 x 5
0 x 4
0 x 6
0 x 6
0 x 7
0 x 0
0 x 7
0 x 6
F ; b
6 ; b
B ; b
F ; b
6 ; b
D ; b
D ; b
7 ; b
F ; b
F ; b
r e a
r e a
r e a
r e a
r e a
r e a
r e a
r e a
r e a
r e a
/ / D
/ /
/ /
/ /
/ /
/ /
/ /
/ /
/ /
/ /
/ /
k ;
k ;
k ;
k ;
k ;
k ;
k ;
k ;
k ;
k ;
P
0
0
0
F
E
D
C
B
0
1
1
1
1
1
0
1
0
1
0
1
1
0
1
1
1
0
1
1
0
1
1
1
1
1
1
0
1
0
1
1
1
1
0
1
0
1
1
1
1
0
1
1
0
1
0
1
1
1
A
1
0
1
0
0
1
1
0
1
0
0
1
0
1
0
0
0
1
0
0
G
0
0
’ S e g C o n v e r t ’
1
1
1
1
1
b i
b i
b i
b i
b i
b i
b i
b i
b i
b i
n a
n a
n a
n a
n a
n a
n a
n a
n a
n a
r y
r y
r y
r y
r y
r y
r y
r y
r y
r y
S e g B y t e ;
}
P r o g r a m E x a m p le 6 .1 S e v e n - s e g m e n t d is p la y c o u n t e r
Co n n ect a sev en d iag ram fo r a sev
th e d isp lay o u tp u
y o u u n d erstan d h
p seu d o co d e d esig
n
seg men t
en -seg m
t co n tin u
o w th e p
n s sh o w
d isp lay to th
en t LED d isp
o u sly co u n ts
ro g ram wo rk
n p rev io u sly .
e mb ed
lay wa
fro m 0
s b y cr
an d
s sh o
to 9
o ss-r
im
wn
an
efe
p lemen
p rev io
d th en
ren cin g
t P r o g r a m 6 .1 . T h e
u s ly in F ig u r e 3 .1 0 .
resets b ack to 0 . En
with th e flo wch art
wirin g
Ve r if y th a t
su re th at
an d
E x e r c is e 6 .1
C h a n g e P r o g r a m E x a m p le 6 .1 s o t h a t t h e d is p la y c o u n t s u p in h e x a d e c im a l fr o m 0 t o F .
Y o u w ill n e e d t o w o r k o u t t h e d is p la y p a t t e r n s fo r A t o F u s in g t h e s e v e n - s e g m e n t d is p la y .
F o r a fe w y o u w ill n e e d lo w e r c a s e , fo r o t h e r s u p p e r c a s e .
n
1 0 2
Chapter 6
6 .4 .2
F u n c t io n R e u s e
No
can
we
mb
w th at we h av e a
b u ild p ro jects u s
can imp lemen t a
ed B u s O u t d ecla
It is p o ssib le to imp
th e main p ro g ram c
fu n ctio n p rev io u sly
th is ex amp le. No te
two fo r lo o p s to co
/ *
P r o g r a m
E x a m p l e
fu n ctio n to co n v ert a d
in g mu ltip le sev en -seg
seco n d sev en -seg men
ratio n an d callin g th e
lemen t a co u n ter p ro
o d e to th at sh o wn in
d efin ed in Pro g ram
also th at a slig h tly d
u n t each o f th e ten s
6 . 2 :
D i s p l a y
ecimal v
men t d is
t d isp lay
same S e
alu e
p lay s
(see
g C o n
g ram th at co
Pro g ram Ex
E x a m p le 6 .1
ifferen t p ro g
an d u n its v a
c o u n t e r
f o r
to a sev en with little
F ig u r e 6 .6
v e r t( ) fu n
seg men t d isp lay b y te, we
ex tra effo rt. Fo r ex amp le,
) b y simp ly d efin in g its
ctio n as b efo re.
u n ts fro m 0 0 to 9 9
a m p le 6 .2 . N o te th
is also req u ired to
rammin g ap p ro ach
lu es.
b y s
at th
b e c
is u
imp ly
e S e g
o p ied
sed ; h
mo d ify
C o n v e r
(reu sed
ere we
in g
t( )
) in
u se
0 - 9 9
* /
# i n c l u d e " m b e d . h "
B u s O u t S e g 1 ( p 5 , p 6 , p 7 , p 8 , p 9 , p 1 0 , p 1 1 , p 1 2 ) ;
B u s O u t S e g 2 ( p 1 3 , p 1 4 , p 1 5 , p 1 6 , p 1 7 , p 1 8 , p 1 9 , p 2 0 ) ;
c h a r
i n t m
w h
f o
S e
a i
i l
r
g C
n (
e
( c
o n v e r t
{
( 1 ) {
h a r j =
S e g 2 = S
f o r ( c
S e
w a
}
( c h a r
S e g V a l u e ) ;
0 ;
e g
h a
g 1
i t
; j
e r
0 ;
C o
) ;
)
j <
C o
r
= S
( 0
1 0
n v
i =
e g
. 2
+ +
t (
i <
n v
)
j )
1 0
e r
{
;
; i + + )
t ( i ) ;
{
/ /
A , B , C , D , E , F , G , D P
/ /
/ /
/ /
/ /
/ /
/ /
/ /
f u
m a
i n
c o
t e
c o
u n
n c
i n
f i
u n
n s
u n
i t
t i o
p r
n i t
t e r
c o
t e r
s c
n
p
o g r
e l
l o
l u m
l o
o l u
r o t o t y p e
a m
o o p
o p 1
n
o p 2
m n
}
}
}
/ /
a d d
S e g C o n v e r t
f u n c t i o n
h e r e .
P r o g r a m E x a m p le 6 .2 T w o d ig it s e v e n - s e g m e n t d is p la y c o u n t e r
Usin g tw
Pro g ram
an d th en
meth o d u
n
o sev en -seg men t
E x a m p le 6 .2 a n d
resets b ack to 0 .
sed to co u n t th e
d isp
v er
Rev
ten s
lay s, w
ify th at
iew th e
an d u n
ith p in c o n n e c tio n s s h o w n in F ig u r e 6 .6 , im p le m e n t
th e d isp lay o u tp u t co u n ts co n tin u o u sly fro m 0 0 to 9 9
p ro g ram d esig n an d familiarize y o u rself with th e
its d ig its each fro m 0 to 9 .
E x e r c is e 6 .2
W r
s e g
1 .
2 .
it e a n d t e s t m b e d p r o g r a m s t o p e
m e n t d is p la y s e t u p :
C o u n t 0 t o F F , in h e x a d e c im a l fo
C r e a t e a o n e - m in u t e t im e r , c o u n t
L E D e v e r y t im e t h e c o u n t o v e r fl o
r fo r m t h e fo llo w in g a c t io n w it h a d u a l s e v e n rm a t
0 e5 9 , w i t h a p r e c i s e i n c r e m e n t r a t e o f 1 s . F l a s h a n
w s b a c k t o z e r o ( i.e . e v e r y m in u t e )
F u r t h e r P r o g r a m m in g T e c h n iq u e s
1 0 3
3 . In t r o d u c e t w o L E D s t o m a k e a s im p le ‘1 ’ d ig it , a n d c o u n t 0 t o 1 9 9 . T h is is c a lle d
a t w o a n d a h a lf d ig it d is p la y .
In e a c h c a s e s e t t h e c o u n t r a t e t o a d iffe r e n t s p e e d a n d e n s u r e t h a t t h e p r o g r a m lo o p s
b a c k t o 0 x 0 0 , s o t h a t t h e c o u n t e r o p e r a t e s c o n t in u o u s ly .
n
6 .4 .3
A M o r e C o m p le x P r o g r a m U s in g F u n c t io n s
A mo re ad v an ced p ro g ram co u ld read two n u merical v alu es fro m a h o st termin al ap p licatio n
an d d isp lay th ese o n two sev en -seg men t d isp lay s co n n ected to th e mb ed . Th e p ro g ram can
th erefo re d isp lay an y in teg er n u mb er b etween 0 0 an d 9 9 , as req u ired b y u ser k ey p resses.
An ex amp le p ro g ram d esig n u ses fo u r fu n ctio n s to imp lemen t th e h o st termin al o u tp u t o n
sev en -seg men t d isp lay s. Th e fo u r fu n ctio n s are as fo llo ws:
S e g I n it( )
H o s tI n it(
G e tK e y In
S e g C o n v e
b y te.
eto set u p
) eto set u
p u t( ) eto
r t( ) efu n
an d
p an
g et
ctio n
in itialize th e sev en -seg men t d
d in itialize th e h o st termin al c
k ey b o ard d ata fro m th e termin
to co n v ert a d ecimal in teg er
to 7 -s e g m e n t
d is p la y
p
p
to 7 -s e g m e n t
d is p la y
p
p
p in
p in
p in
p in
p in
p in
in 1
in 5
p in
p in
p in
p in
p in
p in
in 1
in 5
isp la
o mm
al ap
to a
y s
u n icatio n
p licatio n
sev en -seg men t d isp lay d ata
G n d
V in
V b
n R
p in s 3 ,8
7 , s e g . a
6 , s e g . b
4 , s e g . c
2 , s e g . d
1 , s e g . e
9 , s e g . f
0 , s e g . g
, s e g . d p
7 , s e g . a
6 , s e g . b
4 , s e g . c
2 , s e g . d
1 , s e g . e
9 , s e g . f
0 , s e g . g
, s e g . d p
p in s 3 ,8
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
m b e d M ic r o c o n tr o lle r
•
•
•
•
1 7
1 8
1 9
2 0
G n d
F ig u r e 6 .6 :
T w o s e v e n - s e g m e n t d is p la y c o n t r o l w it h t h e m b e d
1 0 4
Chapter 6
We w ill u s e th e m b e d u n iv e rs a l s e ria l b u s (U S B ) in te rfa c e to c o m m u n ic a te w ith th e h o s t P C ,
a s w e d id in S e c tio n 5 .3 , a n d tw o s e v e n - s e g m e n t d is p la y s , a s in th e p r e v io u s e x e r c is e .
Fo r th e first time n o w we co me acro ss a meth o d fo r co mmu n icatin g k ey b o ard d ata an d d isp lay
ch aracters, u sin g ASCII co d es. Th e term ASCII refers to th e American Stan d ard Co d e fo r
In fo rmatio n In terch an g e meth o d fo r d efin in g alp h an u meric ch aracters as 8 -b it v alu es. Each
a l p h a b e t c h a r a c t e r ( l o w e r a n d u p p e r c a s e ) , n u m b e r ( 0 e9 ) a n d a s e l e c t i o n o f p u n c t u a t i o n
c h a r a c te r s a r e a ll d e s c r ib e d b y a u n iq u e id e n tifi c a tio n b y te , i.e . th e ‘A S C I I v a lu e ’ . T h e r e f o r e ,
fo r ex amp le, wh en a k ey is p ressed o n a co mp u ter k ey b o ard , its ASCII b y te is co mmu n icated
to th e PC. Th e same ap p lies wh en co mmu n icatin g with d isp lay s. Th e u se o f ASCII ch aracters
will b e d ev elo p ed fu rth er in Ch ap ter 8 .
Th e ASCII b y te fo r n u merical ch aracters h as th e h ig h er fo u r b its set to v alu e 0 x 3
an d th e lo wer fo u r b its rep resen t th e v alu e o f th e n u merical k ey wh ich is p ressed
( 0 x 0 t o 0 x 9 ) . N u m b e r s 0 e9 a r e t h e r e f o r e r e p r e s e n t e d i n A S C I I a s 0 x 3 0 t o 0 x 3 9 .
To co n v ert th e ASCII b y te retu rn ed b y th e k ey b o ard to a reg u lar d ecimal d ig it, th e h ig h er fo u r
b its n eed to b e remo v ed . We d o th is b y lo g ically ANDin g th e ASCII co d e with a b itmask ,
a n u mb er with b its set to 1 wh ere we wan t to k eep a b it in th e ASCII, an d set to 0 wh ere we wan t
to fo rce th e b it to 0 . In th is case, we ap p ly a b itmask o f 0 x 0 F. Th e lo g ical AND ap p lies th e
o p e r a to r ‘ & ’ f r o m Ta b le B .5 , a n d a p p e a r s in th e lin e :
C c o d e
fe a tu re
r e t u r n
( c & 0 x 0 F ) ;
/ /
a p p l y
b i t
m a s k
t o
c o n v e r t
t o
d e c i m a l ,
a n d
r e t u r n
E x a m p le f u n c tio n s a n d p r o g r a m c o d e a r e s h o w n in P r o g r a m E x a m p le 6 .3 . O n c e a g a in , th e
f u n c tio n S e g C o n v e r t ( ) , a s s h o w n in P r o g r a m E x a m p le 6 .1 , s h o u ld b e a d d e d to c o m p ile th e
p ro g ram.
/ *
P r o g r a m
E x a m p l e
# i n
S e r
B u s
B u s
c l u d e
i a l p c
O u t S e
O u t S e
" m
( U
g 1
g 2
b e
S B
( p
( p
d .
T X
5 ,
1 3
v o
v o
c h
c h
c h
S e
H o
G e
S e
d a
g I
s t
t K
g C
t a
n i
I n
e y
o n
1 ,
t (
i t
I n
v e
d
v o
( v
p u
r t
a t
a i
g I
s t
i l
t a
g 2
t a
g 1
n (
n i
I n
e
2
= S
1
= S
)
t (
i t
( 1
=
e g
=
e g
{
) ;
( )
)
G e
C o
G e
C o
6 . 3 :
H o s t
k e y p r e s s
t o
7 - s e g
d i s p l a y
* /
i n t
i d
i d
a r
a r
a r
m
S e
H o
w h
d a
S e
d a
S e
h "
,
U S B R X ) ;
p 6 , p 7 , p 8 , p 9 , p 1 0 , p 1 1 , p 1 2 ) ;
, p 1 4 , p 1 5 , p 1 6 , p 1 7 , p 1 8 , p 1 9 , p 2 0 ) ;
/ /
/ /
/ /
c o m m s t o h o s t P C
A , B , C , D , E , F , G , D P
A , B , C , D , E , F , G , D P
i d
o i
t (
( c
a 2
/ /
/ /
/ /
/ /
/ /
f u
f u
f u
f u
v a
) ;
d ) ;
v o i d ) ;
h a r S e g V a l u e ) ;
;
;
{
t K
n v
t K
n v
e y
e r
e y
e r
I n
t (
I n
t (
p u t (
d a t a
p u t (
d a t a
) ;
2 ) ;
) ;
1 ) ;
/ / m a i n p r o
/ / c a l l f u n
/ / c a l l f u
/
/
/
/
/
g r
c t
n c
/
/
/
/
/
n c
n c
n c
n c
r i
a m
i o
t i
i n
c a
c a
c a
c a
t i
t i
t i
t i
a b
o n
o n
o n
o n
l e
p r
p r
p r
p r
d e
o t
o t
o t
o t
c l
o t
o t
o t
o t
a r
n t o i n i t i a l
o n t o i n i t i a
f i n i t e l o o p
l l f u n c t i o n
l l f u n c t i o n
l l f u n c t i o n
l l f u n c t i o n
y p e
y p e
y p e
y p e
a t i o n s
i z e t h e 7 - s e g
l i z e t h e h o s t
t o
t o
t o
t o
g e
c o
g e
c o
t
1
n v e
t 2
n v e
s t
r t
n d
r t
k e
a n
k e
a n
d i s p l a y s
t e r m i n a l
y
d
y
d
p r
o u
p r
o u
e s
t p
e s
t p
s
u t
s
u t
F u r t h e r P r o g r a m m in g T e c h n iq u e s
p c . p r i n t f ( "
" ) ;
/ /
d i s p l a y
s p a c e s
b e t w e e n
1 0 5
n u m b e r s
}
}
/ /
f u n
v o i d S
S e g 1 =
S e g 2 =
c t
e g
S e
S e
i o
I n
g C
g C
n s
i t ( v o i d ) {
o n v e r t ( 0 ) ;
o n v e r t ( 0 ) ;
/ /
/ /
i n i t i a l i z e
i n i t i a l i z e
t o
t o
z e r o
z e r o
}
v o i d H o s t I n i t ( v o i d ) {
p c . p r i n t f ( " \ n \ r T y p e t w o
d i g i t
n u m b e r s
t o
b e
d i s p l a y e d \ n \ r " ) ;
}
c h a
c h
p c
r e
r
G e t K e y
a r c = p
. p r i n t f (
t u r n ( c &
/ /
c o p y
I n
c .
" %
0 x
p u
g e
c "
0 F
t ( v o i d )
t c ( ) ;
, c ) ;
) ;
{
/ /
/ /
/ /
g e t k e y b o a r d d a t a ( a s c i i 0 x 3 0 - 0 x 3 9 )
p r i n t a s c i i v a l u e t o h o s t P C t e r m i n a l
a p p l y b i t m a s k t o c o n v e r t t o d e c i m a l , a n d
r e t u r n
}
S e g C o n v e r t
f u n c t i o n
h e r e
t o o .
P r o g r a m E x a m p le 6 .3 T w o d ig it s e v e n - s e g m e n t d is p la y b a s e d o n h o s t k e y p r e s s e s
I m p le m e n t P r o g r a m E x a m p le 6 .3 a n d v e r if y th a t n u m e r ic a l k e y b o a r d p r e s s e s a r e d is p la y e d o n
th e sev en -seg men t d isp lay s. Familiarize y o u rself with th e p ro g ram d esig n an d u n d erstan d th e
in p u t an d o u tp u t featu res o f each p ro g ram fu n ctio n .
6 .5
U s in g M u lt ip le F ile s in C / C þ þ
Larg e emb ed d ed p ro jects in C/Cþ þ b en efit fro m b ein g sp lit in to a n u mb er o f d ifferen t files,
u su ally so th at a n u mb er o f en g in eers can tak e resp o n sib ility fo r d ifferen t p arts o f th e co d e.
Th is ap p ro ach also imp ro v es read ab ility an d main ten an ce. Fo r ex amp le, th e co d e fo r
a p ro cesso r in a v en d in g mach in e mig h t h av e o n e C/Cþ þ file fo r th e co n tro l o f th e actu ato rs
d e liv e rin g th e ite m s a n d a d iffe re n t fi le fo r c o n tro llin g th e u s e r in p u t a n d liq u id c ry s ta l
d isp lay . It d o es n o t mak e sen se to co mb in e th ese two co d e featu res in th e same so u rce file as
th ey each relate to d ifferen t p erip h eral h ard ware. Fu rth ermo re, if a n ew b atch o f v en d in g
mach in es were to b e b u ilt with an u p d ated k ey p ad an d d isp lay , o n ly th at p iece o f th e co d e
wo u ld n eed to b e mo d ified . All th e o th er so u rce files co u ld b e carried o v er with o u t ch an g e.
Mo d u lar co d in g u ses h ead er files to jo in mu ltip le files to g eth
( m a in .c o r m a in .c p p ) is u s e d to c o n ta in th e h ig h - le v e l c o d
v ariab les (v ariab les th at are av ailab le to all fu n ctio n s) are d e
is g o o d p ractice th erefo re fo r each C/Cþ þ featu re file to h a
a .h e x te n s io n ) . H e a d e r fi le s ty p ic a lly in c lu d e d e c la r a tio n s
d ire c tiv e s , v a ria b le d e c la ra tio n s a n d fu n c tio n p ro to ty p e s .
er. In
e, b u
fin ed
v e an
o n ly ,
g en eral, a m
t all fu n ctio n
in featu re-sp
asso ciated h
fo r ex amp le
ain C/Cþ þ file
s an d g lo b al
ecific C files. It
ead er file (with
co mp iler
A n u mb er o f h ead er files also ex ist fro m with in C/Cþ þ , wh ich can b e u sed fo r mo re
a d v a n c e d d a ta m a n ip u la tio n o r a rith m e tic . M a n y o f th e b u ilt-in C /Cþ þ h e a d e r fi le s a re
1 0 6
Chapter 6
a lr e a d y lin k e d to th r o u g h th e m b e d .h h e a d e r, s o w e d o n o t n e e d to w o r r y to o m u c h a b o u t
th o s e h e r e . T h e y a r e d is c u s s e d in m o r e d e ta il in S e c tio n B .9 o f A p p e n d ix B .
6 .5 .1
O v e r v ie w o f t h e C / C þ þ
P r o g r a m C o m p ila t io n P r o c e s s
To u n d erstan d fu rth er th e d esig n ap p ro ach to mo d u lar co d in g , it h elp s to u n d erstan d th e way
p ro g rams are p rep ro cessed , co mp iled an d lin k ed to create a b in ary ex ecu tio n file fo r th e
m ic r o p r o c e s s o r. A s im p lifi e d v e r s io n o f th is p r o c e s s is s h o w n in F ig u r e 6 .7 a n d d e s c r ib e d in
d e ta il th r o u g h S e c tio n 6 .6 .
In su mmary , first a p rep ro cesso r lo o k s at a p articu lar so u rce file an d imp lemen ts an y
p re p ro c e s s o r d ire c tiv e s a n d a s s o c ia te d h e a d e r fi le s . T h e c o m p ile r th e n g e n e ra te s a n o b je c t fi le
fo r th e p articu lar so u rce co d e. In d o in g so , th e co mp iler en su res th at th e so u rce files d o n o t
F ig u r e 6 .7 :
C p r o g r a m c o m p ile a n d lin k p r o c e s s
F u r t h e r P r o g r a m m in g T e c h n iq u e s
1 0 7
c o n tain an y s y n tax e rro rs en o te th a t a p ro g ra m ca n h av e n o s y n tax e rro rs , b u t s till b e q u ite
u seless. Th e co mp iler th en g en erates co rrect o b ject co d e fo r each file an d en su res th at th e
o b ject an d lib rary files are fo rmatted co rrectly fo r th e lin k er.
Ob jec
( .b in )
allo ca
lo cate
may u
t files an d b u ilt-in lib rary files are th en lin k ed to g eth e
file, wh ich can b e d o wn lo ad ed to th e micro p ro cesso
tio n o f memo ry fo r th e micro p ro cesso r ap p licatio n an
d in to memo ry an d lin k ed to each o th er co rrectly . In
n co v er p ro g rammin g fau lts asso ciated with memo ry
6 .5 .2
The C/Cþ þ
r to g en erate an ex ecu tab le b in ary
r. Th e lin k er also man ag es th e
d it en su res th at all o b ject files are
u n d ertak in g th e task , th e lin k er
allo catio n an d cap acity .
P r e p r o c e s s o r a n d P r e p r o c e s s o r D ir e c t iv e s
Th e C/Cþ þ p rep ro cesso r mo d ifies co d e b efo re th e p ro g ram is co mp iled .
P r e p r o c e s s o r d ir e c tiv e s a r e d e n o te d w ith a ‘ # ’ s y m b o l, a s d e s c r ib e d in S e c tio n B .2
o f A p p e n d ix B . T h e s e m a y a ls o b e c a lle d ‘c o m p ile r d ire c tiv e s ’ a s th e p re p ro c e s s o r
is essen tially a su b p ro cess o f th e co mp iler. Th e # in c lu d e (u su ally referred to as ‘h ash -in clu d e’)
d ire c tiv e is c o m m o n ly u s e d to te ll th e p re p ro c e s s o r to in c lu d e a n y c o d e o r s ta te m e n ts c o n ta in e d
with in an ex tern al h ead er file. In d eed , we h av e seen th is u b iq u ito u s # in c lu d e statemen t in ev ery
co mp lete p ro g ram ex amp le so far in th is b o o k , as th is is u sed to co n n ect o u r p ro g rams with th e
co re mb ed lib raries.
C c o d e
fe a tu re
F ig u r e 6 .8 s h o w s th r e e h e a d e r fi le s to e x p la in h o w th e # in c lu d e s ta te m e n t w o r k s . I t is
imp o rtan t to n o te th at # in c lu d e essen tially ju st acts as a co p y -an d -p aste featu re. If we co mp ile
F ig u r e 6 .8 :
C p r e p r o c e s s o r e x a m p le w it h m u lt ip le d e c la r a t io n e r r o r fo r v a r ia b le ‘p e a r s ’
1 0 8
Chapter 6
a fi le .h a n d b fi le .h w h e r e b o th fi le s a ls o # in c lu d e c fi le .h , w e w ill h a v e tw o c o p ie s o f th e
c o n te n ts o f c fi le .h ( h e n c e v a r ia b le p e a r s w ill b e d e fi n e d tw ic e ) . T h e c o m p ile r w ill th e r e f o r e
h ig h lig h t an erro r, as mu ltip le d eclaratio n s o f th e same v ariab les o r fu n ctio n p ro to ty p es are
n o t allo wed .
We can also u se # d e fi n e statemen ts (u su ally referred to as ‘h ash -d efin e’), wh ich allo w u s to
u se mean in g fu l n ames fo r sp ecific n u merical v alu es th at n ev er ch an g e. Here are so me
ex amp les:
# d e f i n e
# d e f i n e
# d e f i n e
S A M P L E F R E Q U E N C Y
P I
M A X _ S I Z E
4 4 1 0 0
3 . 1 4 1 5 9 2
2 5 5
Th e p rep ro cesso r rep laces # d e fi n e with th e actu al v alu e asso ciated with th at n ame, so u sin g
# d e fi n e statemen ts d o es n o t actu ally in crease th e memo ry lo ad o n th e p ro cesso r.
6 .5 .3
T h e # if n d e f D ir e c t iv e
We h
n o t a
featu
th erefo re n ecess
o n e o ccasio n .
C c o d e
fe a tu re
av e ju st seen th at mu ltip le d ecla
llo wed . Ho wev er, it is imp o rtan t
res th at are req u ired fo r th e lin k
ary to en su re th at all v ariab les an
ratio
fo r h
er to
d fu n
n s o f a v ariab le o r
ead er files to in clu
b u ild th e p ro ject
ctio n p ro to ty p es a
fu n ctio n p ro to ty p e are
d e all th e v ariab les an d
su ccessfu lly . It is
re p rep ro cessed o n ju st
Wh en u sin g h ead er files it is th erefo re g o o d p ractice to u se a co n d itio n al statemen t to d efin e
v ariab les an d p ro to ty p es o n ly if th ey h av e n o t p rev io u sly b een d efin ed . Fo r th is th e # ifn d e f
d ire c tiv e , w h ic h m e a n s ‘if n o t d e fi n e d ’, c a n b e u s e d . In o rd e r to u s e th e # ifn d e f d ire c tiv e
e ffe c tiv e ly , a c o n d itio n a l s ta te m e n t b a s e d o n th e e x is te n c e o f a # d e fi n e v a lu e is re q u ire d . If th e
# d e fi n e v alu e h as n o t p rev io u sly b een d efin ed th en th at v alu e an d all o f th e h ead er file’s
v ariab les an d p ro to ty p es are d efin ed . If th e # d e fi n e v alu e h as p rev io u sly b een d eclared th en
th e h ead er file’s co n ten ts are n o t imp lemen ted b y th e p rep ro cesso r (as th ey mu st certain ly
h av e alread y b een imp lemen ted ). Th is en su res th at all h ead er file d eclaratio n s are ad d ed o n ly
o n c e to th e p r o je c t. T h e e x a m p le c o d e s h o w n in P r o g r a m E x a m p le 6 .4 r e p r e s e n ts a te m p la te
h e a d e r fi le s tr u c tu r e u s in g th e # if n d e f c o n d itio n , a v o id in g th e e r r o r h ig h lig h te d in F ig u r e 6 .8 .
/ *
* /
P r o g r a m
# i f n d e f
# d e f i n e
/ /
6 . 4 :
T e m p l a t e
V A R I A B L E _ H
V A R I A B L E _ H
h e a d e r
# e n d i f
E x a m p l e
d e c l a r a t i o n s
f o r
. h
h e a d e r
f i l e
/ /
/ /
i f V A R I A B L E _ H
d e f i n e i t n o w
h a s
n o t
p r e v i o u s l y
/ /
e n d
d i r e c t i v e
h e r e .
o f
t h e
i f
P r o g r a m E x a m p le 6 .4 E x a m p le h e a d e r fi le t e m p la t e
b e e n
d e f i n e d
F u r t h e r P r o g r a m m in g T e c h n iq u e s
6 .5 .4
1 0 9
U s in g m b e d O b je c t s G lo b a lly
All mb ed o b jects mu st b e d efin ed in an ‘o wn er’ so u rce file. Ho wev er, we m
th o s e o b je c ts f r o m w ith in o th e r s o u r c e fi le s in th e p r o je c t, i.e . ‘ g lo b a lly ’ . T h
b y also d efin in g th e mb ed o b ject in th e asso ciated o wn er’s h ead er file. Wh en
is d efin ed fo r g lo b al u se, th e e x te r n sp ecifier sh o u ld b e u sed . Fo r ex amp le,
c a lle d m y _ f u n c t io n s .c p p m a y d e fi n e a n d u s e a D ig it a lO u t o b je c t c a lle d ‘ R
fo llo ws:
D i g i t a l O u t
ay wish to u se
is can b e d o n e
an mb ed o b ject
a b esp o k e file
e d L e d ’ as
R e d L e d ( p 5 ) ;
If an y o th er so u rce files n eed to man ip u late R e d L e d , th e o b ject mu st also b e d eclared in th e
m y _ f u n c t io n s .h h e a d e r fi le u s in g th e e x t e r n s p e c ifi e r, a s f o llo w s :
e x t e r n
D i g i t a l O u t
R e d L e d ;
No te th at th e sp ecific mb ed p in s d o n o t n eed to b e red efin ed in th e h ead er file, as th ese will
h a v e a lr e a d y b e e n s p e c ifi e d in th e o b je c t d e c la r a tio n in m y _ f u n c t io n s .c p p .
6 .6
M o d u la r P r o g r a m E x a m p le
A mo d u lar
Ex amp le 6
Th erefo re,
created as
•
•
•
p ro g ra m e x a m p le c a n n o w b e b u ilt fro m th e n o n -m o d u la r c o d e g iv e n in P ro g ra m
.3 . H e r e , th e f u n c tio n a l f e a tu r e s a r e s e p a r a te d to d iff e r e n t s o u r c e a n d h e a d e r fi le s .
a k ey b o ard -co n tro lled sev en -seg men t d isp lay p ro ject with mu ltip le so u rce files is
fo llo ws:
m a in .c p p ec o n ta in s th e m a in p r o g r a m f u n c tio n
H o s t I O .c p p ec o n ta in s f u n c tio n s a n d o b je c ts f o r h o s t te r m in a l c o n tr o l
S e g D is p la y .c p p ec o n ta in s f u n c tio n s a n d o b je c ts f o r s e v e n - s e g m e n t d is p la y o u tp u t.
Th e fo llo win g asso ciated h ead er files are also req u ired :
•
•
H o s t I O .h
S e g D is p la y .h
N o te th a t, b y c o n v e n tio n , th e m a in .c p p fi le d o e s n o t n e e d a h e a d e r fi le . T h e p r o g r a m fi le
s tr u c tu r e in th e m b e d c o m p ile r s h o u ld b e s im ila r to th a t s h o w n in F ig u r e 6 .9 . N o te th a t
mo d u lar files can b e created b y rig h t-click in g o n th e p ro ject n ame an d selectin g ‘New File’.
T h e m a in .c p p fi le h o ld s th e s a m e m a in f u n c tio n c o d e a s b e f o r e , b u t w ith # in c lu d e d ir e c tiv e s
to th e n e w h e a d e r fi le s . P r o g r a m E x a m p le 6 .5 d e ta ils th e s o u r c e c o d e f o r m a in .c p p .
/ *
P r o g r a m
E x a m p l e
* /
# i n c l u d e
# i n c l u d e
" m b e d . h "
" H o s t I O . h "
6 . 5 :
m a i n . c p p
f i l e
f o r
m o d u l a r
7 - s e g
k e y b o a r d
c o n t r o l l e r
1 1 0
Chapter 6
# i n c l u
c h a r d
i n t m a
S e g I
H o s t
w h i l
d a
S e
d a
S e
p c
}
}
d e
a t
i n
n i
I n
e
t a
g 2
t a
g 1
. p
"
a 1
( )
t (
i t
( 1
2
=
1
=
r i
S e
,
{
) ;
( )
)
=
S
=
S
n t
g D i s p l a y . h "
d a t a 2 ;
;
{
G e
e g
G e
e g
f (
t K
C o
t K
C o
"
e y
n v
e y
n v
" )
I n
e r
I n
e r
p u t (
t ( d a
p u t (
t ( d a
) ;
t a 2 ) ;
) ;
t a 1 ) ;
;
/ / v
/ / m
/ / c
/ / c
/ / i
/ /
/ /
/ /
/ /
/ /
a r
a i
a l
a l
n f
c
c
c
c
d
i a b l
n p r
l i n
l i n
i n i t
a l l
a l l
a l l
a l l
i s p l
e
d e c
o g r a m
i t f u
i t f u
e l o o
t o g e
t o c o
t o g e
t o c o
a y s p
l a r a t i o n s
n c
n c
p
t
n v
t
n v
a c
t i o n
t i o n
1 s
e r
2 n
e r
e s
t
k e
a n
d k e
t a n
o n
t
y
p r
o u
y p r
d o u
h o s t
d
e s
t p
e s
t p
s
u t
s
u t
P r o g r a m E x a m p le 6 .5 S o u r c e c o d e fo r m a in .c p p
F ig u r e 6 .9 :
F ile s t r u c t u r e fo r m o d u la r s e v e n - s e g m e n t d is p la y p r o g r a m
T h e S e g I n it ( ) a n d S e g C o n v e r t ( ) f u n c tio n s a r e to b e ‘ o w n e d ’ b y th e S e g D is p la y .c p p s o u r c e
fi le , a s a r e th e B u s O u t o b je c ts n a m e d ‘ S e g 1 ’ a n d ‘ S e g 2 ’ . T h e r e s u ltin g S e g D is p la y .c p p fi le is
s h o w n in P r o g r a m E x a m p le 6 .6 .
/ * P
* /
# i n c
B u s O
B u s O
r o g r a m
E x a m p l e
6 . 6 :
S e g D i s p l a y . c p p
f i l e
f o r
l u d e " S e g D i s p l a y . h "
u t S e g 1 ( p 5 , p 6 , p 7 , p 8 , p 9 , p 1 0 , p 1 1 , p 1 2 ) ;
u t S e g 2 ( p 1 3 , p 1 4 , p 1 5 , p 1 6 , p 1 7 , p 1 8 , p 1 9 , p 2 0 ) ;
v o i d S e g I n i t ( v o i d ) {
S e g 1 = S e g C o n v e r t ( 0 ) ;
S e g 2 = S e g C o n v e r t ( 0 ) ;
/ /
/ /
i n i t i a l i z e
i n i t i a l i z e
t o
t o
m o d u l a r
/ /
/ /
7 - s e g
k e y b o a r d
A , B , C , D , E , F , G , D P
A , B , C , D , E , F , G , D P
z e r o
z e r o
}
c h a r
c h a
s w i
c
c
S e g C o n v e r
S e g B y t e
t c h ( S e g V
a s e 0 : S
a s e 1 : S
r
t (
= 0
a l
e g
e g
c h
x 0
u e
B y
B y
a r S e g V a l u e ) {
0 ;
) {
t e = 0 x 3 F ; b r e a k ;
t e = 0 x 0 6 ; b r e a k ;
/ /
f u n c t i o n
/ / D P
/ / 0
/ / 0
G
F
0
E
1
0
’ S e g C o n v e r t ’
0
1
D
0
1
C
B
0
1
A
1
1
1
1
0
b i n a r y
b i n a r y
c o n t r o l l e r
F u r t h e r P r o g r a m m in g T e c h n iq u e s
c a s
c a s
c a s
c a s
c a s
c a s
c a s
c a s
e
2
:
e
3
:
S e g
S e g
e 4 : S e g
e 5 : S e g
e 6 : S e g
e 7 : S e g
e 8 : S e g
e 9 : S e g
B y t
B y t
B y t
B y t
B y t
B y t
B y t
B y t
e
=
e
=
0 x 5
0 x 4
e = 0 x 6
e = 0 x 6
e = 0 x 7
e = 0 x 0
e = 0 x 7
e = 0 x 6
B ;
b r e
b r e
6 ; b r e
D ; b r e
D ; b r e
7 ; b r e
F ; b r e
F ; b r e
F ;
a k ;
a k ;
a k ;
a k ;
a k ;
a k ;
a k ;
a k ;
/ /
0
1
0
1
1
0
1
1
/ /
0
1
0
0
1
1
1
1
b i n
b i n
/ / 0 1 1 0 0 1 1 0 b i n
/ / 0 1 1 0 1 1 0 1 b i n
/ / 0 1 1 1 1 1 0 1 b i n
/ / 0 0 0 0 0 1 1 1 b i n
/ / 0 1 1 1 1 1 1 1 b i n
/ / 0 1 1 0 1 1 1 1 b i n
1 1 1
a r y
a r y
a r y
a r y
a r y
a r y
a r y
a r y
}
r e t u r n
S e g B y t e ;
}
P r o g r a m E x a m p le 6 .6 S o u r c e c o d e fo r S e g D is p la y .c p p
N o te th a t S e g D is p la y .c p p fi le h a s a n # in c lu d e d ir e c tiv e to th e S e g D is p la y .h h e a d e r fi le . T h is
is g iv e n in P r o g r a m E x a m p le 6 .7 .
/ * P r o g r a m E x a m p l e 6 . 7 :
* /
# i f n d e f S E G D I S P L A Y _ H
# d e f i n e S E G D I S P L A Y _ H
# i n c l u d e
e x t e r n
e x t e r n
v o i d
c h a r
S e g D i s p l a y . h
f i l e
f o r
m o d u l a r
7 - s e g
k e y b o a r d
c o n t r o l l e r
" m b e d . h "
B u s O u t
B u s O u t
S e g 1 ;
S e g 2 ;
S e g I n i t ( v o i d ) ;
S e g C o n v e r t ( c h a r
/ /
/ /
a l l o w
a l l o w
S e g V a l u e ) ;
S e g 1
S e g 2
/ /
/ /
t o
b e
t o
b e
m a n i p u l a t e d
m a n i p u l a t e d
f u n c t i o n
f u n c t i o n
b y
o t h e r
o t h e r
b y
f i l e s
f i l e s
p r o t o t y p e
p r o t o t y p e
# e n d i f
P r o g r a m E x a m p le 6 .7 S o u r c e c o d e fo r S e g D is p la y .h
T h e D is p la y S e t a n d G e t K e y I n p u t f u n c tio n s a r e to b e ‘ o w n e d ’ b y th e H o s t I O .c p p s o u r c e
fi le , a s is th e s e r ia l U S B in te r f a c e o b je c t n a m e d ‘ p c ’ . T h e H o s t I O .c p p fi le s h o u ld th e r e f o r e b e
a s s h o w n in P r o g r a m E x a m p le 6 .8 .
/ * P r o g r a m E x a m p l e 6 . 8 : H o s t I O . c p p c o d e f o r m o d u l a r 7 - s e g
* /
# i n c l u d e " H o s t I O . h "
S e r i a l p c ( U S B T X , U S B R X ) ;
/ / c o m m u n i c a t i o n t o h o s t P C
v o i d H o s t I n i t ( v o i d ) {
p c . p r i n t f ( " \ n \ r T y p e
t w o
d i g i t
n u m b e r s
t o
b e
k e y b o a r d
\ n \ r " ) ;
}
c h a r G e t K e y
c h a r c =
p c . p r i n t f
r e t u r n ( c
I n
p c
( "
& 0
p u
. g
% c
x 0
t (
e t
" ,
F )
v o i d )
c ( ) ;
c ) ;
;
{
/ /
/ /
/ /
g e t k e y b o a r d a s c i i d a t a
p r i n t a s c i i v a l u e t o h o s t
r e t u r n v a l u e a s n o n - a s c i i
}
P r o g r a m E x a m p le 6 .8 S o u r c e c o d e fo r H o s t I O .c p p
P C
t e r m i n a l
c o n t r o l l e r
1 1 2
Chapter 6
/ * P r o g r a m E x a m p l e
* /
# i f n d e f H O S T I O _ H
6 . 9 :
H o s t I O . h
c o d e
f o r
m o d u l a r
7 - s e g
k e y b o a r d
c o n t r o l l e r
# d e f i n e H O S T I O _ H
# i n c l u d e " m b e d . h "
e x t e r n S e r i a l p c ;
v o i d H o s t I n i t ( v o i d ) ;
c h a r G e t K e y I n p u t ( v o i d ) ;
/ /
/ /
/ /
a l l o w p c
f u n c t i o n
f u n c t i o n
t o b e m a n i p u l a t e d
p r o t o t y p e
p r o t o t y p e
b y
o t h e r
f i l e s
# e n d i f
P r o g r a m E x a m p le 6 .9 S o u r c e c o d e fo r H o s t I O .h
T h e H o s tI O h e a d e r fi le , H o s t I O .h , is s h o w n in P r o g r a m E x a m p le 6 .9 .
Create th e mo
will n eed to c
rig h t click in g
rep licates Fig
d u lar s
reate a
o n th e
u r e 6 .9
e v e n - s e g m e n t d i s p l a y p r o j e c t g i v e n b y P r o g r a m E x a m p l e s 6 . 5 e6 . 9 . Y o u
n ew p ro ject in th e mb ed co mp iler an d ad d th e req u ired mo d u lar files b y
p ro ject an d selectin g ‘New File’. Hen ce, create a file stru ctu re wh ich
.
Yo u s h o u ld n o w b e a b le to c o m p ile a n d r u n y o u r m o d u la r p r o g r a m . U s e th e c ir c u it o f
F ig u r e 6 .6 .
n
E x e r c is e 6 .3
C r e a t e a n a d v a n c e d m o d u la r p r o je c t w h ic h u s e s a h o s t t e r m in a l a p p lic a t io n a n d a s e r v o .
T h e u s e
a s p e c ifi
m o v e s t
a r e la t iv
r in
e d
h e
e p
p u ts a
p o s it io
s e rv o t
o s it io n
v a lu e b e
n . A n in p
o 9 0 d e g
, fo r e x a
tw e e n 1
u t o f 1 m
r e e s r ig h
m p le t h e
a n d 9 f
o v e s th
t. N u m
v a lu e 5
ro m th e
e s e rv o t
b e rs b e t
p o in t s
k e y b
o 9 0
w e e n
th e s
o a rd w
d e g re e s
1 a n d
e rv o to
h ic h
le ft
9 m
th e
m o
a n d
o v e
c e n
v e s th e s e rv o to
a u s e r in p u t o f 9
th e s e rv o to
te r.
Y o u c a n r e u s e t h e G e t K e y In p u t ( ) fu n c t io n fr o m t h e p r e v io u s e x a m p le s .
Y o u m a y a ls o n e e d t o c r e a t e a lo o k - u p t a b le fu n c t io n t o c o n v e r t t h e n u m e r ic a l in p u t
v a lu e t o a s u it a b le p u ls e w id t h m o d u la t io n ( P W M ) d u t y c y c le v a lu e a s s o c ia t e d w it h t h e
d e s ir e d s e r v o p o s it io n .
n
Th is ch ap ter h as sh o
co d e wh ile allo win g
p ro g rams wh ich allo
d ev elo p men t can als
en ab les reu se o f co d
wn th at
man ip u
w larg e
o b e ma
e an d a
fu n ctio n
latio n o f
mu lti-fu
n ag ed th
simp le a
s are u se
d ata. Th
n ctio n al
ro u g h a
p p ro ach
fu l fo
is, in
p ro je
team
to u p
r allo win g u s to write
tu rn , h as en ab led u s
cts to b e p ro g rammed
o f en g in eers an d with
d atin g an d u p g rad in g
clean an d read
to create mo d u
. Th e p ro g ram
a mech an ism
so ftware featu
ab le
lar
th at
res.
F u r t h e r P r o g r a m m in g T e c h n iq u e s
1 1 3
C h a p t e r R e v ie w
•
We u se fu n ctio n s to allo w co d e to b e reu sab le an d easier to read .
Fu n ctio n s can tak e in p u t d ata v alu es an d retu rn a sin g le d ata v alu e as o u tp
n o t p o ssib le to p ass array s o f d ata to o r fro m a fu n ctio n .
Flo wch arts an d p seu d o co d e can b e u sed to assist p ro g ram d esig n .
Th e tech n iq u e o f mo d u lar p ro g rammin g in v o lv es d esig n in g a co mp
a n u mb er o f so u rce files an d asso ciated h ead er files. So u rce files h o ld th
n itio n s, wh ereas h ead er files h o ld fu n ctio n an d v ariab le d eclaratio n s.
Th e C/Cþ þ co mp ilatio n p ro cess co mp iles all so u rce an d h ead er files
to g eth er with p red efin ed lib rary files to g en erate an ex ecu tab le p ro g ram
P re p ro c e s s o r d ire c tiv e s a re re q u ire d to e n s u re th a t c o m p ila tio n e rro rs
v ariab le d eclaratio n s are av o id ed .
Mo d u lar p ro g rammin g en ab les a n u mb er o f en g in eers to wo rk o n a sin
tak in g resp o n sib ility fo r a p articu lar co d e featu re.
•
•
•
•
•
•
u t; h o wev er, it is
lete p ro g ram as
e fu n ctio n d efian d lin k s th o se
b in ary file.
d u e to mu ltip le
g le p ro ject, each
Q u iz
1 .
2 .
3 .
4 .
5 .
6 .
7 .
8 .
9 .
1 0 .
List th e ad v an tag es o f u sin g fu n ctio n s in a C p ro g ram.
Wh at are th e limitatio n s asso ciated with u sin g fu n ctio n s in a C p ro g ram?
Wh at is p seu d o co d e an d h o w is it u sefu l at th e so ftware d esig n stag e?
Wh at is a fu n ctio n ‘p ro to ty p e’ an d wh ere can it b e fo u n d in a C p ro g ram?
Ho w mu ch d ata can b e in p u t to an d o u tp u t fro m a fu n ctio n ?
Wh at is th e p u rp o se o f th e p rep ro cesso r in th e C p ro g ram co mp ilatio n p ro cess?
At wh at stag e in th e p ro g ram co mp ilatio n p ro cess are p red efin ed lib rary files
imp lemen ted ?
Wh en wo u ld it b e n ecessary to u se th e e x te r n sto rag e class sp ecifier in an mb ed C
p ro g ram?
W h y is th e # ifn d e f p re p ro c e s s o r d ire c tiv e c o m m o n ly u s e d in m o d u la r p ro g ra m h e a d e r
files?
D raw a p ro g ra m fl o w c h a rt w h ic h d e s c rib e s a p ro g ra m th a t c o n tin u o u s ly re a d s a n a n a lo g
temp eratu re sen so r o u tp u t o n ce p er seco n d an d d isp lay s th e temp eratu re in d eg rees
Celsiu s o n a th ree-d ig it sev en -seg men t d isp lay .
C H A P T E R 7
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
C h a p t e r O u t lin e
7 .1 In t r o d u c in g S y n c h r o n o u s S e r ia l C o m m u n ic a t io n
7 .2 S e r ia l P e r ip h e r a l In t e r f a c e 1 1 7
7 .2
7 .2
7 .2
7 .2
.1 In tro d u
.2 S P I o n
.3 S e t t in g
.4 C r e a t in
c in
th e
u p
g a
g S
m
a n
S P
P I 1 1
b e d 1
m b e d
I D a ta
1 1 5
7
1 9
S P I M a ste r
L in k 1 2 1
1 1 9
7 .3 In t e llig e n t In s t r u m e n t a t io n a n d a S P I A c c e le r o m e t e r
7 .3 .1 In t r o d u c in g t h e A D X L 3 4 5 A c c e le r o m e t e r
7 .3 .2 D e v e lo p in g a S im p le A D X L 3 4 5 P r o g r a m
7 .4 E v a lu a t in g S P I 1 2 9
7 .5 T h e In t e r - In t e g r a t e d C ir c u it B u s
1 2 5
1 2 5
1 2 5
1 2 9
2
7 .5 .1 In t r o d u c in g t h e I C B u s 1 2 9
7 .5 .2 I2 C o n th e m b e d 1 3 1
7 .5 .3 S e t t in g u p a n I2 C D a t a L in k 1 3 1
7 .6
7 .7
7 .8
7 .9
C
U
E
A
o m m u n ic
s in g t h e
v a lu a t in g
s y n c h ro n
a t
S R
I2
o u
in g w it h a n I2 C T e m p e r a t u r e S e n s o r
F 0 8 U lt r a s o n ic R a n g e F in d e r 1 3 8
C
1 4 0
s S e r ia l D a t a C o m m u n ic a t io n
1 4 0
1 3 5
7 .9 .1 In t r o d u c in g A s y n c h r o n o u s S e r ia l D a t a 1 4 0
7 .9 .2 A p p ly in g A s y n c h r o n o u s C o m m u n ic a t io n o n t h e m b e d 1 4 1
7 .9 .3 A p p ly in g S y n c h r o n o u s C o m m u n ic a t io n w it h t h e H o s t C o m p u t e r
7 .1 0 M in i- P r o je c t : M u lt i- N o d e I2 C B u s
C h a p t e r R e v ie w
1 4 4
Q u iz 1 4 5
R e fe re n c e s 1 4 5
7 .1
1 4 4
1 4 4
In t r o d u c in g S y n c h r o n o u s S e r ia l C o m m u n ic a t io n
T h e r e is a n u n e n d in g n e e d in c o m p u te r s y s te m s to m o v e d a ta a r o u n d elo ts o f it. I n C h a p te r s
1 an d 2 we came acro ss th e id ea o f d ata b u ses, o n wh ich d ata flies b ack ward s an d fo rward s
b etween d ifferen t p arts o f a co mp u ter. In th ese b u ses, d ata is tran sferred in p a ra llel. Th ere is
F a s t a n d E f f e c t iv e E m b e d d e d S y s t e m s D e s ig n . D O I : 1 0 .1 0 1 6 /B 9 7 8 - 0 - 0 8 - 0 9 7 7 6 8 - 3 .0 0 0 0 7 - 6
Co p y rig h t Ó2 0 1 2 Elsev ier Ltd . All rig h ts reserv ed .
1 1 5
1 1 6
Chapter 7
o n e wire fo r each b it o f d ata, an d o n e o r two mo re to p ro v id e sy n ch ro n izatio n a
d ata is tran sferred a wh o le wo rd at a time. Th is wo rk s well, b u t it req u ires a lo t o
a lo t o f co n n ectio n s o n each d ev ice th at is b ein g in terco n n ected . It is b ad en o u g h
d ev ic e , b u t fo r 1 6 o r 3 2 b its th e s itu a tio n is fa r w o rs e . A n a lte rn a tiv e to p a ra lle l
c o m m u n ic a tio n is s e r ia l. H e re , a s in g le w ire is e ffe c tiv e ly u s e d fo r d a ta tra n s fe r
b ein g sen t in tu rn . A few ex tra co n n ectio n s are almo st in ev itab ly n eed ed , fo r ex
earth retu rn , an d sy n ch ro n izatio n an d co n tro l.
n d co n tro l;
f wires an d
fo r an 8 -b it
, with b its
amp le fo r
On ce we start ap p ly in g th e serial co n cep t, a n u mb er o f ch allen g es arise. Ho w d o es th e
re c e iv e r k n o w w h e n e a c h b it b e g in s a n d e n d s , a n d h o w d o e s it k n o w w h e n e a c h w o rd b e g in s
an d en d s? Th ere are sev eral way s o f resp o n d in g to th ese q u estio n s. A straig h tfo rward
ap p ro ach is to sen d a clo ck sig n al alo n g sid e th e d ata, with o n e clo ck p u lse p er d ata b it. Th e
d ata is syn ch ro n ized to th e clo ck . Th is id ea, called sy n ch ro n o u s serial co mmu n icatio n , is
r e p r e s e n te d in F ig u r e 7 .1 . W h e n n o d a ta is b e in g s e n t, th e r e is n o m o v e m e n t o n th e c lo c k lin e .
Ev ery time th e clo ck p u lses, h o wev er, o n e b it is o u tp u t b y th e tran smitter an d sh o u ld b e read
in b y th e re c e iv e r. In g e n e ra l, th e re c e iv e r s y n c h ro n iz e s its re a d in g o f th e d a ta w ith o n e e d g e
o f th e clo ck . In th is ex amp le it is th e risin g ed g e, h ig h lig h ted b y a d o tted lin e.
A s im p le s e r ia l d a ta lin k is s h o w n in F ig u r e 7 .2 . E a c h d e v ic e th a t c o n n e c ts to th e d a ta lin k is
so metimes called a n o d e. In th is fig u re, No d e 1 is d esig n ated Ma ster; it co n tro ls wh at is g o in g
o n , a s it c o n tro ls th e c lo c k . T h e S la v e is s im ila r to th e m a s te r, b u t re c e iv e s th e c lo c k s ig n a l
fro m th e master.
A n e s s e n tia l f e a tu r e o f th e s e r ia l lin k s h o w n , a n d in d e e d o f m o s t s e r ia l lin k s , is a s h ift re g is te r.
Th is is mad e u p o f a strin g o f d ig ital flip -flo p s, co n n ected so th at th e o u tp u t o f o n e is
co n n ected to th e in p u t o f th e n ex t. Each flip -flo p h o ld s o n e b it o f in fo rmatio n . Ev ery time th e
sh ift reg ister is p u lsed b y th e clo ck sig n al, each flip -flo p p asses its b it o n to its n eig h b o r o n o n e
s id e a n d re c e iv e s a n ew b it fro m its o th e r n e ig h b o r. T h e o n e a t th e in p u t e n d c lo c k s in d a ta
re c e iv e d fro m th e o u ts id e w o rld , a n d th e o n e o f th e o u tp u t e n d o u tp u ts its b it. T h e re fo re , a s th e
clo ck p u lses th e sh ift reg ister can b e b o th feed in g in ex tern al d ata an d o u tp u ttin g d ata. Th e
d ata h eld b y all th e flip -flo p s in th e sh ift reg ister can , mo reo v er, b e read all at th e same time, as
a p arallel wo rd , o r a n ew v alu e can b e lo ad ed in . In su mmary , th e sh ift reg ister is an in cred ib ly
u sefu l su b sy stem: it can co n v ert serial d ata to p arallel d ata, an d v ice v ersa, an d it can act as
a s e ria l tra n s m itte r a n d /o r a s e ria l re c e iv e r.
b it 0
b it 1
b it 2
b it 3
b it 4
S e r ia l D a ta
Clo ck
F ig u r e 7 .1 :
S y n c h r o n o u s s e r ia l d a t a
b it 5
b it 6
b it 7
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
No d e 1
1 1 7
No d e 2
(M a s te r)
(S la v e )
P arallel
read /write
P arallel
read /write
SDO
SDI
Sh ift reg ister
msb
SDI
d ata
d ir e c tio n
SDO
ls b
clo ck
so u rce
Sh ift reg ister
msb
SCLK
ls b
SCLK
F ig u r e 7 .2 :
A s im p le s e r ia l lin k . S D I: s e r ia l d a t a in ; S D O : s e r ia l d a t a o u t ; S C L K : s e r ia l c lo c k
A s a n e x a m p le , s u p p o s e b o th s h if t r e g is te r s in F ig u r e 7 .2 a r e 8 - b it, i.e . e a c h h a s e ig h t fl ip flo p s. Each reg ister is lo ad ed with a n ew wo rd , an d th e master th en g en erates eig h t clo ck
p u lses. Fo r each clo ck cy cle, o n e n ew b it o f d ata ap p ears at th e o u tp u t en d o f each sh ift
reg ister, in d icated b y th e SDO (serial d ata o u tp u t) lab el. Each SDO o u tp u t is, h o wev er,
co n n ected to th e in p u t (SDI: serial d ata in p u t) o f th e o th er reg ister. Th erefo re as each b it is
clo ck ed o u t o f o n e reg ister, it is clo ck ed in to th e o th er. After eig h t clo ck cy cles, th e wo rd th at
was in th e master sh ift reg ister is n o w in th e slav e, an d th e wo rd th at was in th e slav e sh ift
reg ister is n o w h eld in th e master.
Th e circu itry fo r th e ma
an o th er micro co n tro ller
th at allo ws serial d ata to
cen tral p ro cessin g u n it (
7 .2
ster is u su
o r so me o
b e sen t a
CPU) an d
ally
th er
n d r
th e
p laced with in
p erip h eral d ev
e c e iv e d , a n d in
o u tsid e wo rld ,
a micro co
ice. In g e
terfaces b
is called
n tro ller. Th
n eral, th e h
etween th e
a serial p o r
e slav e mig h t b e
ard ware circu itry
micro co n tro lle r
t.
S e r ia l P e r ip h e r a l In t e r fa c e
To en su
p laces,
d ay s is
may g o
in terfac
re th at serial d ata lin k s are reliab
sev eral stan d ard s o r p ro to co ls h
th e u n iv e rs a l s e ria l b u s (U S B ).
o n to d efin e o th er th in g s, su ch
e (SPI) is a simp le p ro to co l th a
7 .2 .1
In t r o d u c in g S P I
le, an d can b e ap p lied b y d ifferen t d ev ices in
av e b een d efin ed . On e th at is v ery wid ely u
A p ro to co l d efin es d etails o f timin g an d sig
as th e ty p e o f co n n ecto r u sed . Serial p erip h
t h as h ad a larg e in flu en ce in th e emb ed d ed
d ifferen t
sed th ese
n als, an d
eral
wo rld .
In th e early d ay s o f micro co n tro llers, b o th Natio n al Semico n d u cto rs an d Mo to ro la started
in tr o d u c in g s im p le s e r ia l c o m m u n ic a tio n , b a s e d o n F ig u r e 7 .2 . E a c h f o r m u la te d a s e t o f r u le s
g o v ern in g h o w th eir micro co n tro llers wo rk ed , an d allo wed o th ers to d ev elo p d ev ices th at
1 1 8
co u ld
in itia
acted
Natio
Chapter 7
in
lly
as
n a
terface co rrectly .
fo rmally d esig n e
a fo rmal stan d ard
l Semico n d u cto rs
Th ese b ecame d e fa cto stan d ard
d as stan d ard s, b u t were ad o p ted
. Mo to ro la called its stan d ard S e
called th eirs Micro wire. Th ey a
s, in o th er wo rd s
b y o th ers to th e
ria l Perip h era l In
re v ery similar to
th ey were n ev er
p o in t wh ere th ey
terfa ce (S PI) an d
each o th er.
It was n o t lo n g b efo re b o th SPI an d Micro wire were ad o p ted b y man u factu rers o f o th er
in teg rated circu its (ICs), wh o wan ted th eir d ev ices to b e ab le to wo rk with th e n ew g en eratio n
o f micro co n tro llers. Th e SPI h as b eco me o n e o f th e mo st d u rab le stan d ard s in th e wo rld o f
electro n ics, ap p lied to sh o rt-d istan ce co mmu n icatio n s, ty p ically with in a sin g le p iece o f
eq u ip men t. Th ere is n o fo rmal d o cu men t d efin in g SPI, b u t d atash eets fo r th e Mo to ro la
6 8 H C 1 1 (n o w a n o ld m ic r o c o n tro lle r) e ffe c tiv e ly d e fi n e it in fu ll. G o o d re la te d te x ts a ls o d o ,
f o r e x a m p le R e f e r e n c e 7 .1 .
I n S P I o n e m ic ro c o n tro lle r is d e s ig n a te d th e m a s te r; it c o n tr o ls a ll a c tiv ity o n th e s e r ia l
in te rc o n n e c tio n . T h e m a s te r c o m m u n ic a te s w ith o n e o r m o re s lav e s . A m in im u m S PI lin k u s e s
ju s t o n e m a s te r a n d o n e s la v e , a n d f o llo w s th e p a tte r n o f F ig u r e 7 .2 . T h e m a s te r g e n e r a te s a n d
c o n tro ls th e c lo c k , a n d h e n c e a ll d a ta tra n s fe r. T h e S DO o f o n e d ev ic e is c o n n e c te d to th e S DI
o f th e o th e r, a n d v ic e v e rs a . In ev e ry c lo c k c y c le o n e b it is m o v e d fro m m a s ter to s lav e , a n d o n e
b it fro m s lav e to m a s te r; a fte r e ig h t c lo ck cy c le s a w h o le b y te h a s b e e n tra n s fe rre d . T h u s , d a ta is
a c tu a lly tr a n s f e r r e d in b o th d ir e c tio n s ein o ld te r m in o lo g y th is is c a lle d fu ll d u p le x ea n d it is
u p to th e d ev ic e s to d e c id e w h e th e r a r e c e iv e d b y te is in te n d e d f o r it o r n o t. I f d a ta tr a n s f e r in
o n ly o n e d ire c tio n is w a n te d , th e n th e d a ta tra n s fe r lin e th a t is n o t n e e d e d c a n b e o mitte d .
If mo re th an o n e slav e is n eed ed , th en th e ap p
is a c tiv e a t a n y tim e , d e te rm in e d b y w h ic h S
th a t w ritin g S S in d ic a te s th a t th e lin e is a c tiv e
b e SS. Th e termin o lo g y Ch ip Select (CS) is
c h a p te r. O n ly th e s la v e a c tiv a te d b y its S S in
o n ly o n e s la v e is a c tiv a te d a t a n y o n e tim e . T h
r o a c h o f F ig u r e 7 .3 c a n b e u s e d . O n ly o n e s la v e
la v e S e le c t (S S ) lin e th e m a s te r a c tiv a te s . N o te
w h e n lo w ; if it w e re a c tiv e h ig h it w o u ld s im p ly
also u sed fo r th e same ro le, in clu d in g in th is
p u t resp o n d s to th e clo ck sig n al, an d n o rmally
e m a s te r th e n c o m m u n ic a te s w ith th e o n e a c tiv e
Po rt
B its
SS
M ic ro c o n tro lle r
(SPI Ma s te r)
SS
S la v e 1
SCK
SDO
SS
S la v e 2
SDI
SCK
SDO
SS
S la v e 3
SDI
SCK
SDO
S la v e 4
SDI
SCK
SDI
SDO
F ig u r e 7 .3 :
S P I in t e r c o n n e c t io n s fo r m u lt ip le s la v e s
SCK
SDO
SDI
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 1 9
s la v e ju s t a s in F ig u r e 7 .2 . N o tic e th a t f o r n s la v e s , th e m ic r o c o n tr o lle r n e e d s to c o m m it
(3 þ n ) lin es. On e o f th e ad v an tag es o f serial co mmu n icatio n , th e small n u mb er o f
in terco n n ectio n s, is b eg in n in g to d isap p ear.
7 .2 .2
SPI on the mbed
A s th e m b e d d ia g r a m o f F ig u r e 2 .1 s h o w s , th e m b e d h a s tw o S P I p o r ts , o n e a p p e a r in g o n p in s
5 , 6 an d 7 , an d th e o th er o n p in s 1 1 , 1 2 an d 1 3 . Th e ap p licatio n p ro g rammin g in terface (API)
s u m m a r y a v a ila b le f o r S P I m a s te r is s h o w n in Ta b le 7 .1 .
7 .2 .3
S e t t in g u p a n m b e d S P I M a s t e r
P r o g r a m E x a m p le 7 .1 s h o w s a v e r y s im p le s e tu p f o r a S P I m a s te r. T h e p r o g r a m in itia liz e s th e
SPI p o rt, ch o o sin g fo r it th e n ame s e r _ p o r t, with th e p in s o f o n e o f th e p o ssib le p o rts b ein g
selected . Th e fo r m a t( ) fu n ctio n req u ires two v ariab les: th e first is th e n u mb er o f b its an d th e
s e c o n d is th e m o d e . T h e m o d e is a f e a tu r e o f S P I w h ic h is illu s tr a te d in F ig u r e 7 .4 , w ith
a s s o c ia te d c o d e s in Ta b le 7 .2 . I t a llo w s th e c h o ic e o f w h ic h c lo c k e d g e is u s e d to c lo c k d a ta
in to th e sh ift reg ister (in d icated as ‘Data stro b e’ in th e d iag ram), an d wh eth er th e clo ck id les
h ig h o r lo w. F o r m o s t a p p lic a tio n s th e d e f a u lt m o d e , i.e . M o d e 0 , is a c c e p ta b le . T h is p r o g r a m
s im p ly a p p lie s d e f a u lt v a lu e s , i.e . 8 b its o f d a ta , a n d M o d e 0 f o r m a t. O n th e m b e d , a s w ith
man y SPI d ev ices, th e same p in is u sed fo r SDI if in master mo d e, o r SDO if slav e. Hen ce, th is
p in g ets to b e called MISO (master in , slav e o u t). Its p artn er p in is MOSI.
/ *
P r o
b y t e
* /
# i n c l u
S P I s e
c h a r s
i n t
g r a m
E x a m p l e
7 . 1 :
d e " m b e d . h "
r _ p o r t ( p 1 1 , p 1 2 ,
w i t c h _ w o r d ;
m a i
s e r _ p
s e r _ p
w h i l e
s w i
s e r
w a i
}
n (
o r
o r
(
t c
_ p
t _
)
t .
t .
1 )
h _
o r
u s
{
f o
f r
{
w o
t .
( 5
S e t s
p 1 3 ) ;
u p
t h e
m b e d
a s
/ / m o s i , m i s o ,
/ / w o r d w e w i l l
r m a t ( 8 , 0 ) ;
e q u e n c y ( 1 0 0 0 0 0 0 ) ;
r d = 0 x A 1 ;
w r i t e ( s w i t c h _ w o r d ) ;
0 ) ;
/ /
/ /
S e t u p
C l o c k
S P I
m a s t e r ,
a n d
c o n t i n u o u s l y
s e n d s
a
s i n g l e
s c l k
s e n d
t h e S P I f o r 8 b i t
f r e q u e n c y i s 1 M H z
/ / s e t u p w o r d t o b e
/ / s e n d s w i t c h _ w o r d
d a t a ,
M o d e
0
o p e r a t i o n
t r a n s m i t t e d
}
P r o g r a m E x a m p le 7 .1
Co mp ile
(p in 1 1 )
d ata are
can read
sig n ific a
, d
an
ac
th
n t
M in im a l S P I m a s te r a p p lic a tio n
o wn lo ad an d ru n Pro g ram Ex amp
d clo ck (p in 1 3 ) lin e s simu ltan e o
tiv e a t th e s a m e tim e , a n d v e rif y
e tran smitte d d ata b y te, 0 x A1 . Is
b it (LS B) se n t first?
le
u s
th
th
7 .1 o n
ly o n a
e clo c k
e mo st
a sin g le mb e
n o s cillo sco p
d ata freq u en
sig n ific an t b
d ,
e.
cy
it
an d
See
. Ch
(MS
o b se rv e th e d ata
h o w clo c k an d
e ck th at y o u
B) o r least
1 2 0
Chapter 7
T a b le 7 .1 : m b e d S P I m a s t e r A P I s u m m a r y
F u n c t io n
U s a g e
S P I
C r e a t e a S P I m a s t e r c o n n e c t e d t o t h e s p e c ifi e d p in s
f o r m a t
C o n fi g u r e t h e d a t a t r a n s m is s io n m o d e a n d d a t a le n g t h
f r e q u e n c y
S e t t h e S P I b u s c lo c k fr e q u e n c y
w r i t e
W r it e t o t h e S P I s la v e a n d r e t u r n t h e r e s p o n s e
SS
SCK, Mo d e 0
SCK, Mo d e 1
SCK, Mo d e 2
SCK, Mo d e 3
Data
Data stro b e
F ig u r e 7 .4 :
P o la r it y a n d p h a s e
T a b le 7 .2 : S P I m o d e s
M o d e
P o la r it y
P h a s e
0
0
0
1
0
1
2
3
1
1
0
1
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
n
1 2 1
E x e r c is e 7 .1
1 . In P ro g r
c lo c k a n
F ig u r e 7
2 . S e t th e S
0 x 8 A 1 a
o s c illo s c
a m E x a m p le
d d a ta w a v e
.4 .
P I fo rm a t o f
n d 0 x 8 A A 1 ,
o p e .
7 .1 , t r y in v o k in g e a c h o f t h e S P I m o d e s in t u r n . O b s e r v e b o t h
fo r m s o n t h e o s c illo s c o p e , a n d c h e c k h o w t h e y c o m p a r e t o
P r o g r a m E x a m p le 7 .1 t o 1 2 a n d t h e n 1 6 b it s , s e n d in g t h e w o r d s
r e s p e c t iv e ly ( o r t o y o u r c h o ic e ) . C h e c k e a c h o n a n
n
7 .2 .4
C r e a t in g a S P I D a t a L in k
We will n o w d ev elo p tw o p ro g rams, o n e ma ster an d o n e slav e, an d g et tw o mb ed s to
co mmu n icate. Each will h av e two switch es an d two lig h t-emittin g d io d es (LEDs); th e aim
will b e to g et th e switch es o f th e ma ster to co n tro l th e LED s o n th e slav e, an d v ice v ersa.
sh o wn
p o rt as
o r d , th
re c e iv e
S 1
S 2
c o n n e c t
to s e c o n d
m b e d
m o s i,
m is o ,
s c k ,
s s e l,
to
to
to
to
p in
p in
p in
p in
1 1
1 2
1 3
1 4
as Pro g ram
b efo re, an d
e wo rd th at
d fro m th e s
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
E x a m p le 7 .2 . T h is is w r
d efin es th e switch in p u
will b e sen t to th e slav
lav e. Fo r th is ap p licatio
m b e d M ic r o c o n tr o lle r
Th e p ro g ram fo r th e master is
F ig u r e 7 .5 . I t s e ts u p th e S P I
d eclares a v ariab le s w itc h _ w
r e c d _ v a l, wh ich is th e v alu e
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
F ig u r e 7 .5 :
U s in g S P I t o lin k t w o m b e d s
itten fo
ts o n p
e, an d
n th e d
r th
in s
th e
efa
e circu it o f
5 an d 6 . It
v ariab le
u lt settin g s
1 2 2
Chapter 7
o f th e SPI p o
main lo o p , th
reco g n izab le
in p u ts are th e
is set, b y AN
s w itc h _ w o r d
acco rd in g ly .
rt are ch o sen , so th ere is n o f
e v alu e o f s w itc h _ w o r d is es
o n th e o scillo sco p e, th e u p p er
n tested in tu rn ; if th ey are fo u
Din g with 0 x 0 1 o r 0 x 0 2 . Th e
is mad e. Th e retu rn v alu e o f
/ * P r o g r a m E x a m p l e 7 . 2 .
s e n d i n g i t s o w n s w i t c h
* /
# i n c l u d e " m b e d . h "
S P
D i
D i
D i
D i
D i
c h
c h
I s
g i t
g i t
g i t
g i t
g i t
a r
a r
i n t
m
w h i
/
/
s
i
e r
a l
a l
a l
a l
a l
s w
r e
_ p
O u
O u
O u
I n
I n
i t
c d
o r
t
t
t
s
s
c h
_ v
a i
l e
/ D
/ S
w i
f
n ( )
( 1 )
e f a u
e t u
t c h _
( s w i
s w i t
i f ( s w i
s w i t
c s = 0 ;
r e c d _ v a
c s = 1 ;
w a i t ( 0 .
/ /
r e
g r
r e
i f
s e
d _
e e
c d
(
r
i f (
g
i f (
r
g
t l
l e d
n _ l
_ v a
r e c
e d _
r e c
r e e
r e c
e d _
r e e
t (
r e
g r
c s
w i
w i
_ w
a l
p 1
d _
e e
( p
t c
t c
o r
;
1 ,
l e
n _
1 4
h _
h _
d
s
t h
r d
h _
_ w
h _
_ w
e t
e
= 0
i p
o r
i p
o r
p 1
d ( p
l e d
) ;
i p 1
i p 2
;
u rth er in itializatio n in th e p ro g ram. On ce in th e
ta b lis h e d . To g iv e a p a tte rn to th is th a t w ill b e
fo u r b its are set to h ex ad ecimal A. Th e two switch
n d to b e h ig h , th e ap p ro p riate b it in s w itc h _ w o r d
c s lin e is set lo w, an d th e co mman d to sen d
th is fu n c tio n is th e re c e iv e d w o rd , w h ic h is re a d
S e t s t h e m b e d u p a s M a s t e r , a n d
p o s i t i o n s , a n d d i s p l a y i n g t h o s e
2 , p 1 3 ) ; / / m o s
2 5 ) ;
/ / r e d
( p 2 6 ) ;
/ / g r e e
/ / t h i s
( p 5 ) ;
( p 6 ) ;
/ / w o r d w e w i
/ / v a l u e r e t u
i , m i s o ,
l e d
n l e d
a c t s a s
l l
r n
s e n d
f r o m
e x c h a n g e s d a t a
o f t h e s l a v e .
w i t h
a
s l a v e ,
s c l k
“ s l a v e
s e l e c t ”
s l a v e
{
{
l t
p
w o
t c
c h
t c
c h
t i
w o
x a
1 =
d =
2 =
d =
n g
r d
0 ;
= 1
s w
= 1
s w
s
f o r S P I M a s t e r c h o s e n , n o n e e d f o r f u r t h e r c o n f i g u r a t i o n
t o b e s e n t , b y t e s t i n g s w i t c h i n p u t s
/ / s e t u p a r e c o g n i z a b l e o u t p u t p a t t e r n
)
i t c h _ w o r d j0 x 0 1 ;
/ / O R
i n
l s b
i t c h _ w o r d j0 x 0 2 ;
/ / O R i n n e x t l s b
/ / s e l e c t s l a v e
/ / s e n d s w i t c h _ w o r d
)
l = s e r _ p o r t . w r i t e ( s w i t c h _ w o r d ) ;
a n d
r e c e i v e
d a t a
0 1 ) ;
e d
= 0
e d
l =
d _
l e
d _
n _
d _
l e
n _
s
a c c o r d i n g
t o
;
= 0 ;
r e c
v a l
d = 1
v a l
l e d
v a l
d = 1
l e d
d _
= =
;
= =
= 1
= =
;
= 1
v a l & 0 x 0 3 ;
1 )
i n c o m i n g
/ / p r e s e t
/ / A N D
o u t
w o r d
b o t h
f r o m
t o 0
u n w a n t e d
s l a v e
b i t s
2 )
;
3 ) {
;
}
}
}
P r o g r a m E x a m p le 7 .2
T h e m b e d s e t u p a s a S P I m a s te r, w ith b id ir e c tio n a l d a ta tr a n s fe r
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 2 3
T a b le 7 .3 : m b e d S P I s la v e A P I s u m m a r y
S P I S l a v e
F u n c t io n
U s a g e
C r e a t e a S P I s la v e c o n n e c t e d t o t h e s p e c ifi e d p in s
f o r m a t
C o n fi g u r e t h e d a t a t r a n s m is s io n fo r m a t
f r e q u e n c y
S e t t h e S P I b u s c lo c k fr e q u e n c y
r e c e i v e
P o ll t h e S P I t o s e e if d a t a h a s b e e n r e c e iv e d
r e a d
R e t r ie v e d a t a fr o m r e c e iv e b u ffe r a s s la v e
r e p l y
F ill t h e t r a n s m is s io n b u ffe r w it h t h e v a lu e t o b e w r it t e n o u t a s s la v e o n
t h e n e x t r e c e iv e d m e s s a g e fr o m t h e m a s t e r
T h e s la v e p r o g r a m d r a w s u p o n th e m b e d f u n c tio n s s h o w n in Ta b le 7 .3 , a n d is s h o w n a s
P r o g r a m E x a m p le 7 .3 . I t is a lm o s t th e m ir r o r im a g e o f th e m a s te r p r o g r a m , w ith s m a ll b u t k e y
d ifferen ces. Th is emp h asizes th e v ery clo se similarity b etween th e master an d slav e ro le in SPI.
Let u s ch eck th e d ifferen ces. Th e serial p o rt is in itialized with S P I S la v e . No w fo u r p in s mu st b e
d efin ed , th e ex tra b ein g th e slav e select in p u t, s s e l. As th e slav e will also b e g en eratin g a wo rd
to b e s e n t, a n d re c e iv in g o n e , it a ls o d e c la re s v a ria b le s s w itc h _ w o r d a n d r e c d _ v a l. C h a n g e
th ese n ames if y o u wo u ld rath er h av e so meth in g d ifferen t. Th e slav e p ro g ram co n fig u res its
s w itc h _ w o r d ju st lik e th e master. No w co mes a d ifferen ce. Wh ile th e master in itiates
a tran smissio n wh en it wish es, th e slav e mu st wait. Th e mb ed lib rary d o es th is with th e
r e c e iv e ( ) fu n c tio n . T h is re tu rn s 1 if d a ta h a s b e e n re c e iv e d , a n d 0 o th e rw is e . O f c o u rs e , if d a ta
h a s b e e n re c e iv e d fro m th e m a s te r, th e n d a ta h a s a ls o b e e n s e n t fro m s la v e to m a s te r. If th e re is
d ata, th en th e slav e read s th is an d sets u p th e LEDs acco rd in g ly . It also sets u p th e n ex t wo rd to
b e sen t to th e master, b y tran sferrin g its s w itc h _ w o r d to th e tran smissio n b u ffer, u sin g r e p ly ( ).
/ *
s e
* /
# i
S P
D i
D i
D i
D i
c h
c h
P r o g r a m E x a m p l e 7 . 3 :
n d i n g i t s o w n s w i t c h
n c l
I S l
g i t
g i t
g i t
g i t
a r
a r
i n t
m
/ / d
w h i
/
s
i
u d e
a v e
a l O
a l O
a l I
a l I
s w i
r e c
a i
e f
l e
/ s
w i
f
d
" m b
s e r
t r
t g
s w
s w
c h _
_ v a
n (
a u
( 1
e t
t c
( s
s w
) {
l t f
) {
u p
h _ w o
w i t c
i t c h
u
u
n
n
t
e d .
_ p o
e d _
r e e
i t c
i t c
w o r
l ;
h "
r t
l e
n _
h _
h _
d
( p 1
d ( p
l e d
i p 1
i p 2
1 ,
2 5
( p
( p
( p
p 1
) ;
2 6
5 )
6 )
2 , p 1 3 , p 1 4 ) ;
) ;
;
;
;
i t
= 0
i p
o r
/ / m o s i , m i s o ,
/ / r e d l e d
/ / g r e e n l e d
s c l k ,
s s e l
/ / w o r d w e w i l l s e n d
/ / v a l u e r e c e i v e d f r o m
o r m a t t i n g
s w
r d
h _
_ w
S e t s t h e m b e d u p a s S l a v e , a n d e x c h a n g e s d a t a w i t h a M a s t e r ,
p o s i t i o n s , a n d d i s p l a y i n g t h o s e o f t h e M a s t e r . a s S P I s l a v e .
c h
x a
1 =
d =
m a s t e r
a p p l i e d
_ w o r d f r o m s w i t c h e s
0 ;
/ / s e t u p a
= 1 )
s w i t c h _ w o r d j0 x 0 1 ;
t h a t a r e p r e s s e d
r e c o g n i z a b l e o u t p u t
p a t t e r n
1 2 4
Chapter 7
i f
( s
s w
i f ( s e
r e
s e
}
/ / n o w
.
( c o n t
.
w i
i t
r _
c d
r _
t c
c h
p o
_ v
p o
h _
_ w
r t
a l
r t
i p 2
o r d
. r e
=
. r e
s e t
l e d s
i n u e s
a s
= =
= s
c e
s e
p l
1 )
w i
i v
r _
y (
t c
e (
p o
s w
h _ w o r d j0 x 0
) ) {
/ /
r t . r e a d ( )
i t c h _ w o r d
a c c o r d i n g
i n
P r o g r a m
t o
2 ;
t e s t i f d a t a t r a n s f e r h a s o c c u r r e d
;
/ / R e a d b y t e f r o m m a s t e r
) ; / / M a k e t h i s t h e n e x t r e p l y
r e c e i v e d
E x a m p l e
w o r d
7 . 2 )
}
}
P r o g r a m E x a m p le 7 .3
No w co n n
each is p o
i.e . p in 1 1
y o u d o n o
master, an
T h e m b e d s e t u p a s a S P I s la v e , w ith b id ir e c tio n a l d a ta tr a n s fe r
ect two mb ed s to g eth er, carefu lly ap p ly in g th e circu it o f Fig u re
w e re d in d iv id u a lly th ro u g h its o w n U S B c a b le . C o n n e c tio n s to
g o es to p in 1 1 an d so o n , so it d o es n o t matter wh ich is ch o sen a
t wan t to set u p th e fu ll circu it straig h t away , th en co n n ect ju st
d ju st th e LEDs to th e slav e, o r v ice v ersa.
C o m p ile a n d d o w n lo a d P r o g r a m E x a m p le 7 .2 in
P r o g r a m E x a m p le 7 .3 in to th e o th e r. O n c e y o u
p ressin g th e switch es o f th e master co n tro ls th e
an o th er b ig step fo rward ; we are co mmu n icatin g
fro m o n e sy stem to an o th er.
n
7 .5 . I t is s im p le s t if
b o th are id en tical,
s master o r slav e. If
th e switch es to th e
to o n e mb ed (wh ich will b e th
ru n th e p ro g rams, y o u sh o u ld
LEDs o f th e slav e, an d v ice
d ata fro m o n e micro co n tro lle
e master), an d
fin d th at
v ersa. Th is is
r to an o th er, o r
E x e r c is e 7 .2
T ry
tra n
1 .
2 .
3 .
4 .
th e fo
s m is s
R e m o
R e m o
R e m o
R e m o
llo
io n
v e
v e
v e
v e
N o t ic e t h a t
y o u m ig h t g
a n e x a m p le
in t e r p r e t e d
w in g , a n
fro m m
t h e p in 1
t h e p in 1
t h e p in 1
t h e p in 1
in s
e t o
o f t
b y a
d b e
a s te r
1 lin
2 lin
3 lin
4 lin
o m e c a s e s
d d in t e r m
h e im p a c t
n m b e d in
s u
to
k ,
k ,
k ,
k ,
re y
s la
i.e .
i.e .
i.e .
i.e .
o
v
t
t
t
t
u u n d e rs ta n
e , a n d fro m
h e d a t a lin k
h e d a t a lin k
h e c lo c k .
h e c h ip s e le c
d t
s la
fro
fro
h e
v e
m
m
r e s u lt . In e a c h c a s e t e s t fo r d a t a
to m a s te r.
m a s t e r t o s la v e .
s la v e t o m a s t e r .
t lin e .
if y o u d is c o n n e c t a w ir e , b u t le a v e it d a n g lin g in t h e a ir ,
it t e n t b e h a v io r w h ic h c h a n g e s if y o u t o u c h t h e w ir e . T h is is
o f e le c t r o m a g n e t ic in t e r fe r e n c e , w h e r e in t e r fe r e n c e is b e in g
p u t a s a c lo c k o r d a t a s ig n a l.
n
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
7 .3
1 2 5
In t e llig e n t In s t r u m e n t a t io n a n d a S P I A c c e le r o m e t e r
7 .3 .1
In t r o d u c in g t h e A D X L 3 4 5 A c c e le r o m e t e r
Desp ite (o r b ecau se o f ) its ag e, th e SPI stan d ard is wo n d erfu lly simp le, an d h en ce wid ely
u s e d . It is e m b e d d e d in to a ll s o rts o f e le c tro n ic d ev ic e s , IC s a n d g a d g e ts . G iv e n a n
u n d erstan d in g o f h o w SPI wo rk s, we can n o w co mmu n icate with an y o f th ese.
With th e v ery h ig h lev el o f in teg ratio n fo u n d in mo d ern ICs, it is co mmo n to fin d a sen so r,
sig n al co n d itio n in g , an an alo g -to -d ig ital co n v erter (ADC) an d a d ata in terface, all co mb in ed
o n to a sin g le ch ip . Su ch d ev ices are p art o f th e n ew g en eratio n o f in tellig en t in stru men ta tio n .
In stead o f ju st h av in g a stan d -alo n e sen so r, as we d id with th e lig h t-d ep en d en t resisto r in
Ch ap ter 5 , we can n o w h av e a co mp lete measu remen t su b sy stem in teg rated with th e sen so r,
with a co n v en ien t serial d ata o u tp u t.
Th e ADXL3 4 5 accelero meter, mad e b y An alo g Dev ices, is an ex amp le o f an in teg rated
‘ in te llig e n t’ s e n s o r. I ts d a ta s h e e t a p p e a r s a s R e f e r e n c e 7 .2 . T h is is a ls o a n e x a m p le o f
a micro electro mech a n ica l system (MEMS), in wh ich th e accelero meter mech an ics are
actu ally fab ricated with in th e IC stru ctu re. Th e accelero meter o u tp u t is an alo g in n atu re, an d
measu res acceleratio n in th ree ax es. Th e accelero meter h as an in tern al cap acito r mo u n ted in
th e p lan e o f each ax is. Acceleratio n cau ses th e cap acito r p lates to mo v e, h en ce ch an g in g th e
o u tp u t v o ltag e in p ro p o rtio n to th e acceleratio n o r fo rce. Th e ADXL3 4 5 accelero meter
co n v erts th e an alo g v o ltag e flu ctu atio n s to d ig ital an d can o u tp u t th ese v alu es o v er a SPI serial
lin k (o r I2 C; see later in th is ch ap ter).
Co n
Ex a
mak
reco
mad
9 .8 1
tro l o f th e ADXL3 4 5 is d o n e b y writin g to a se t o f reg is te
m p le s o f th e s e a r e s h o w n in Ta b le 7 .4 . I t is c le a r th a t th e d
in g d irect me asu remen ts. It is p o ssib le to c alib rate it, ch an
g n iz e certa in ev en ts , fo r ex amp le wh e n it is tap p ed o r in f
e in terms o f g (wh ere 1 g is th e v alu e o f acceleratio n d u e
ms 2).
rs th ro u g h th e s eria l lin
ev ic e g o es w e ll b ey o n d
g e its ran g e an d en ab le
ree fall. Measu remen ts
to e a r th ’s g r a v ity , i.e .
k .
ju st
it to
are
Th e ADXL3 4 5 IC is ex tremely small an d d esig n ed fo r su rface mo u n tin g o n a p rin ted circu it
b o a r d ; th e r e f o r e , it is r e a d y - m o u n te d o n a ‘ b r e a k o u t’ b o a r d , a s s h o w n in F ig u r e 7 .6 . I t w o u ld
o th erwise b e d ifficu lt to h an d le.
7 .3 .2
Pro g r
o u tp u
an d 1
Tab le
D e v e lo p in g a S im p le A D X L 3 4 5 P r o g r a m
a m E x a m p le 7 .4 a p p lie s th e A D X L 3 4 5 , r e a d in g a c c e le r a tio n in th r e e a x e s , a n d
ttin g th e d a ta to th e h o s t c o m p u te r s c r e e n . T h e s e c o n d S P I p o r t ( i.e . p in s 1 1 , 1 2
3 ) is u sed fo r co n n ectin g th e accelero meter, ap p ly in g th e co n n ectio n s sh o wn in
7 .5 .
1 2 6
Chapter 7
T a b le 7 .4 : S e le c t e d A D X L 3 4 5 r e g is t e r s
A d d re s s
*
N a m e
D e s c r ip t io n
0 x0 0
D E V ID
D e v ic e ID
0 x1 D
T H R E S H _ T A P
T a p t h r e s h o ld
0 x1 E / 1 F / 2 0
O F S X , O F S Y , O F S Z
x , y , z -a x is o ffs e t s
0 x2 1
D U R
T a p d u r a t io n
0 x2 D
P O W E R _ C T L
P o w e r - s a v in g fe a t u r e s c o n t r o l. D e v ic e p o w e r s u p in s t a n d b y m o d e ;
s e t t in g b it 3 c a u s e s it t o e n t e r M e a s u r e m o d e
0 x3 1
D A T A _ F O R M A T
D a ta fo rm a t c o n tro l
B it s :
7 : fo r c e a s e lf- t e s t b y s e t t in g t o 1
6 : 1 ¼
3 - w ir e S P I m o d e ; 0 ¼
4 - w ir e S P I m o d e
5 : 0 s e t s in t e r r u p t s a c t iv e h ig h , 1 s e t s t h e m a c t iv e lo w
4 : a lw a y s 0
3 : 0 ¼
o u t p u t is 1 0 - b it a lw a y s ; 1 ¼
2 : 1 ¼
le ft a lig n r e s u lt ; 0 ¼
1 -0 : 0 0 ¼

2 g; 0 1 ¼

o u t p u t d e p e n d s o n r a n g e s e t t in g
r ig h t a lig n r e s u lt
4 g; 1 0 ¼

8 g; 1 1 ¼

2 g
0 x 3 3 :0 x 3 2
D A T A X 1 :D A T A X 0
x - a x is d a t a , fo r m a t t e d a c c o r d in g t o D A T A _ F O R M A T , in 2 ’s
c o m p le m e n t .
0 x 3 5 :0 x 3 4
D A T A Y 1 :D A T A Y 0
y - a x is d a t a , a s a b o v e
0 x 3 7 :0 x 3 6
D A T A Z 1 :D A T A Z 0
z -a x is d a t a , a s a b o v e
*
I n a n y d a t a t r a n s f e r t h e r e g i s t e r a d d r e s s i s s e n t fi r s t , a n d f o r m e d : b i t 7 ¼ R =W ( 1 f o r r e a d , 0 f o r w r i t e ) ; b i t 6 : 1 f o r m u l t i p l e
b y t e s , 0 fo r s in g le ; b it s 5 - 0 : t h e lo w e r 6 b it s fo u n d in t h e A d d r e s s c o lu m n .
F ig u r e 7 .6 :
T h e A D X L 3 4 5 a c c e le r o m e t e r o n b r e a k o u t b o a r d . ( Im a g e r e p r o d u c e d w it h p e r m is s io n o f S p a r k F u n
E le c t r o n ic s )
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 2 7
T a b le 7 .5 : A D X L 3 4 5 p in c o n n e c t io n s t o m b e d
A D X L 3 4 5 s ig n a l n a m e
m b e d p in
V c c
V o u t
G n d
G n d
S C L
1 3
M O S I
1 1
M IS O
1 2
C S
1 4
Th e p ro g ram in itializes a master SPI p o rt, wh ich we h av e ch o sen to call a c c , an d
sets u p th e USB lin k to th e h o st co mp u ter. It fu rth er d eclares two array s; o n e is
a b u ffer th at will h o ld d ata read d irect fro m th e accelero meter’s reg isters, two fo r
ax is. Th e seco n d array ap p lies th e in t1 6 _ t sp ecifier. Th is is fro m th e C stan d ard lib rary
t; it tells th e co mp iler th at ex actly 1 6 -b it (sig n ed ) in teg er ty p e d ata is b ein g d eclared . Th is
w ill h o ld th e fu ll a c c e le ro m e te r a x is v a lu e s , c o m b in e d fro m th e tw o b y te s re c e iv e d fro m
eg isters.
C c o d e
fe a tu re
each
s td in
array
th e r
Th e main fu n ctio n in itializes th e SPI p o rt in a man n er with wh ich we are familiar. It th en
lo ad s two o f th e accelero meter reg isters, writin g th e ad d ress first, fo llo wed b y th e d ata b y te. It
sh o u ld b e p o ssib le to wo rk o u t wh at is b ein g written b y lo o k in g at eith er th e d atash eet o r
Ta b le 7 .4 . A c o n tin u o u s w h ile lo o p is th e n in itia te d . F o llo w in g a p a u s e , a m u lti- b y te r e a d is
s e t u p , u s in g a n a d d r e s s w o r d f o r m e d f r o m in f o r m a tio n s h o w n in Ta b le 7 .4 . T h is fi lls th e
b u f f e r a r r a y . T h e d a t a a r r a y is th e n p o p u la te d , c o n c a te n a tin g ( i.e . c o m b in in g to f o r m a s in g le
n u mb er) p airs o f b y tes fro m th e b u ffe r array . Th ese v alu es are th en scaled to actu al g v alu es,
u s in g th e c o n v e r s io n f a c to r f r o m th e d a ta s h e e t, o f 0 .0 0 4 g p e r u n it. R e s u lts a r e th e n d is p la y e d
o n -screen .
/ * P r o g r a m E x a m p l e 7 . 4 : R e a d s v a l u e s
c o n t i n u o u s l y t o t e r m i n a l s c r e e n .
* /
# i n
S P I
D i g
S e r
c h a
i n t
f l o
i n t
c l
a
i t
i a
r
1 6
a t
u d
c c
a l
l
b u
_ t
x
e
( p
O u
p c
f f
d
,
" m
1 1
t
( U
e r
a t
y ,
b e
, p
c s
S B
[ 6
a [
z
d .
1 2
( p
T X
] ;
3 ]
;
h “
, p 1 3 ) ;
1 4 ) ;
, U S B R X ) ;
;
m a i n ( ) {
c s = 1 ;
a c c . f o r m a t ( 8 , 3 ) ;
/ / s
/ / u
/ / s
/ / r a
/ / 1
/ / f
e t
s e
e t
w d
6 - b
l o a
f r o m
a c c e l e r o m e t e r
u p S P I i
p i n 1 4 a
u p U S B i
a t a a r r a
i t t w o s t i n g p o i
n t
s
n t
y
c o
n t
e r
c h
e r
t y
m p
d
f a
i p
f a
p e
l e
a t
/ / i n i t i a l l y A D X L 3 4 5
/ / 8 b i t d a t a , M o d e
t h r o u g h
c e o n p
s e l e c t
c e t o h
c h a r
m e n t i n
a , t o b
i s
3
n o t
S P I ,
a n d
i n s
1 1 , 1 2 , 1 3
o s t
t e r m i n a l
t e g e r d a t a
e d i s p l a y e d
a c t i v a t e d
o u t p u t s
o n - s c r e e n
1 2 8
Chapter 7
a c
c s
a c
a c
c s
c s
a c
a c
c s
w h
c .
= 0
c .
c .
= 1
= 0
c .
c .
= 1
i l
w a
c s
a c
f o
f r e q u e n c y ( 2 0 0 0 0 0 0 ) ;
/ /
/ /
/ /
/ /
/ /
/ /
/ /
/ /
/ /
/ /
;
w r
w r
;
;
w r
w r
;
e
i t
= 0
c .
i t e ( 0 x 3 1 ) ;
i t e ( 0 x 0 B ) ;
i t e ( 0 x 2 D ) ;
i t e ( 0 x 0 8 ) ;
( 1
( 0
;
w r
r ( i
b u f f
)
{
2
s e
d
f
e n
s t
p
m
e n
i
M H
l e
a t
o r
d
a r
o w
e a
d
n f
z
c t
a
m a
o f
t
e r
s u
o f
i n
c l
t
f o
t
t
a
c
r e
t
i t
o c
h e
r m
+ /
r a
n e
t r
m
r a
e
k
a
r
d e
t
1 6
s m
t
r
d e
s m
o o
n
w
l
o
n
l
a t
v i
r e
g ,
i s
r a
e g
e
c e
g i
0
s i
n s
i s
s t
. 0
o n
m i
t e
e r
0 4 g / L S B
s s i o n
r
i s s i o n
p
. 2 ) ;
/ / s t a r t a t r a n s m i s s i o n
i t e ( 0 x 8 0 j0 x 4 0 j0 x 3 2 ) ;
/ / R W b i t h i g h , M B b i t h i g h ,
n t i = 0 ; i < = 5 ; i + + ) {
e r [ i ] = a c c . w r i t e ( 0 x 0 0 ) ;
/ / r e a d b a c k 6 d a t a
p l u s
a d d r e s s
b y t e s
}
c s
d a
d a
d a
x =
= 1
t a
t a
t a
0 .
;
[ 0
[ 1
[ 2
0 0
]
=
b u
] = b u
] = b u
4 * d a t a
f f
f f
f f
[ 0
p c . p r i n t f ( " x
=
e r [ 1
e r [ 3
e r [ 5
] ; y
] <
] <
] <
= 0
< 8
< 8
< 8
. 0 0
j b
j b
j b
4 *
% + 1 . 2 f g \ t
y
u f
u f
u f
d a
0 ]
2 ]
4 ]
] ;
/ / e n d o f t r a n s m i s s i o n
/ / c o m b i n e M S B a n d L S B
f e
f e
f e
t a
r [
r [
r [
[ 1
;
=
% + 1 . 2 f g \ t
;
;
z = 0 . 0 0 4 * d a t a [ 2 ] ;
z
=
/ / c o n v e r t t o f l o a t ,
/ / a c t u a l g v a l u e
% + 1 . 2 f g \ n \ r " , x , y , z ) ; / / p r i n t
}
}
P r o g r a m E x a m p le 7 .4
sc r e e n
A c c e le r o m e te r c o n tin u o u s ly o u tp u ts th r e e -a x is d a ta to te r m in a l
C a r e f u lly m a k e th e c o n n e c tio n s o f Ta b le 7 .5 , a n d c o m p ile , d o w n lo a d a n d r u n th e c o d e o n y o u r
mb ed . Op en a Tera Term screen (as d escrib ed in Ap p en d ix E) o n y o u r co mp u ter.
A c c e le r o m e te r r e a d in g s s h o u ld b e d is p la y e d to th e s c r e e n . Yo u w ill s e e th a t w h e n th e
accelero meter is flat o n a tab le th e z-ax is sh o u ld read ap p ro x imately 1 g , with th e x- an d
y-ax es ap p ro x imately 0 g . As y o u ro tate an d mo v e th e d ev ice th e g read in g s will ch an g e. If th e
accelero meter is sh ak en o r d isp laced at a q u ick rate, g v alu es in ex cess o f 1 g can b e o b serv ed .
No te th at th ere are so me in accu racies in th e accelero meter d ata, wh ich can b e red u ced in
a real ap p licatio n b y d ev elo p in g co n fig u ratio n /calib ratio n ro u tin es an d d ata av erag in g
fu n ctio n s.
Alth o u g h we h av e n o t u sed it h ere, th e mb ed site p ro v id es an ADXL3 4 5 lib rary , lo cated in th e
Co o k b o o k . Th is simp lifies u se o f th e accelero meter, an d sav es wo rry in g ab o u t reg ister
ad d resses an d b it v alu es.
n
E x e r c is e 7 .3
R e w r it e P r o g r a m E x a m p le 7 .4 u s in g t h e lib r a r y fu n c t io n s a v a ila b le o n t h e m b e d s it e
C o o k b o o k .
n
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
7 .4
1 2 9
E v a lu a t in g S P I
T h e S P I s ta n d a r d is e x tre m e ly e ffe c tiv e . T h e e le c tro n ic h a rd w a re is s im p le a n d th e r e fo re
ch eap , an d d ata can b e tran sferred rap id ly . It d o es h av e its d isad v an tag es, h o wev er. Th ere is
n o a c k n o w le d g m e n t fro m th e re c e iv e r, s o in a s im p le s y s te m th e m a s te r c a n n o t b e s u re th a t
d a ta h a s b e e n re c e iv e d . A ls o th e re is n o a d d re s s in g . In a s y s te m w h e re th e re a re m u ltip le
s la v e s , a s e p a r a te S S lin e m u s t b e r u n to e a c h s la v e , a s s h o w n in F ig u r e 7 .3 . T h e r e f o r e , w e
b e g in to lo s e th e a d v a n ta g e th a t s h o u ld c o m e f r o m s e r ia l c o m m u n ic a tio n s , i.e . a lim ite d
n u mb er o f in terco n n ect lin es. Fin ally , th ere is n o erro r ch eck in g . Su p p o se so me
electro mag n etic in terferen ce was ex p erien ced in a lo n g d ata lin k ; th e d ata o r clo ck wo u ld b e
c o r r u p te d , b u t th e s y s te m w o u ld h a v e n o w a y o f d e te c tin g th is o r c o r r e c tin g f o r it. Yo u m a y
h a v e e x p e r ie n c e d th is in a s m a ll w a y in E x e r c is e 7 .2 . O v e r a ll, S P I c o u ld b e g r a d e d a s : s im p le ,
co n v en ien t an d lo w co st, b u t n o t ap p ro p riate fo r co mp lex o r h ig h -reliab ility sy stems.
7 .5
7 .5 .1
T h e In t e r -In t e g r a t e d C ir c u it B u s
In t r o d u c in g t h e I2 C B u s
Th e in ter-in teg rated circu it (I2 C) stan d ard was d ev elo p ed b y Ph ilip s to reso lv e so me o f th e
p e rc e iv e d w e a k n e s s e s o f S P I a n d its e q u iv a le n ts . A s its n a m e s u g g e s ts , it is a ls o in te n d e d fo r
in terco n n ectio n o v er sh o rt d istan ces an d g en erally with in a p iece o f eq u ip men t. It u ses o n ly
P o w e r s u p p ly
P u ll-u p re s is to rs
M ic ro c o n tro lle r 1
's t r a y '
c a p a c ita n c e
S CL
to o th e r p e rip h e ra ls
a n d m ic ro c o n tro lle rs
S DA
P e rip h e ra l 1
P e rip h e ra l 2
F ig u r e 7 .7 :
A n I C -b a se d sy ste m
2
P e rip h e ra l 3
1 3 0
Chapter 7
two in terco n n ect wires, b u t man y d e
(serial clo ck ) an d SDA (serial d ata).
s h o w n in F ig u r e 7 .7 . T h e S D A lin e
o n ly o n e d irectio n at an y o n e time.
a sy n ch ro n o u s serial stan d ard .
v ices
All d
is b id
In th
are co n n ecte
ev ices o n th e
irectio n al, so
e jarg o n th is
d to th e b
b u s are c
d ata can
is called
u s. T
o n n e
trav
h a lf
h ese lin es
cted to th e
el in eith e
d u p lex. L
are called SCL
se two lin es, as
r d irectio n , b u t
ik e SPI, it is
On e o f th e in terestin g featu res o f I2 C, an d o n e th at mak es it v ersatile, is th at an y n o d e co n n ected
to it can o n ly p u ll d o wn th e SCL o r SDA lin e to Lo g ic 0 ; it can n o t fo rce th e lin e u p to Lo g ic 1 .
Th is ro le is p lay ed b y a sin g le p u ll-u p resisto r co n n ected to each lin e. Wh en a n o d e p u lls a lin e
to Lo g ic 0 an d th en releases it, it is retu rn ed to Lo g ic 1 b y th e actio n o f th e p u ll-u p resisto r. Th ere
is, h o wev er, cap acitan ce asso ciated with th e lin e. Alth o u g h th is is lab eled ‘stray ’ cap acitan ce in
F ig u r e 7 .7 , it is in r e a lity m a in ly u n a v o id a b le c a p a c ita n c e th a t e x is ts in th e s e m ic o n d u c to r
stru ctu res co n n ected to th e lin e. Th is cap acitan ce is th u s h ig h er if th ere are man y n o d es
co n n ected , an d lo wer o th erwise. Th e h ig h er th e cap acitan ce an d /o r p u ll-u p resistan ce, th e
lo n g er th e rise time o f th e lo g ic tran sitio n fro m 0 to 1 . Th e I2 C stan d ard req u ires th at th e rise
tim e o f a s ig n a l o n S C L o r S DA m u s t b e le s s th a n 1 0 0 0 n s . G iv e n a k n o w n b u s s e tu p , it is
p o ssib le to p erfo rm reaso n ab ly p recise calcu latio n s o f p u ll-u p resisto r req u ired (see Referen ce
1 .1 ) , p a r tic u la r ly if y o u n e e d to m in im iz e p o w e r c o n s u m p tio n . F o r s im p le a p p lic a tio n s d e f a u lt
p u l l - u p r e s i s t o r v a l u e s , i n t h e r a n g e 2 . 2 e4 . 7 k U , a r e q u i t e a c c e p t a b l e .
Th e I2 C p ro to co l h as b een th ro u g h sev eral rev isio n s,
p o ssib le sp eed s an d reflect tech n o lo g ical ch an g es, fo
o p eratin g v o ltag es. Th e o rig in al v ersio n , stan d ard mo
Ve r s io n 1 .0 , in 1 9 9 2 , in c r e a s e d th e m a x im u m d a ta r a
estab lish ed an d still p ro b ab ly acco u n ts fo r mo st I2 C
in c r e a s e d th e p o s s ib le b it r a te to 3 .4 M b it/s . Ve r s io n
m a n n e r in R e f e r e n c e 7 .3 , a n d f o r m s th e b a s is o f th e
wh ich h av e d ramatically in creased th e
r ex amp le in red u ced min imu m
d e, allo wed d ata rates u p to 1 0 0 k b it/s.
te to 4 0 0 k b it/s. Th is latter is v ery well
im p le m e n ta tio n . Ve r s io n 2 .0 , in 1 9 9 8 ,
3 is d efin ed in a su rp risin g ly read ab le
fo llo win g d escrip tio n .
No d es o n an I2 C b u s can act as master o r slav e. Th e master in itiates an d termin ates tran sfers,
an d g en erates th e clo ck . Th e slav e is an y d ev ice ad d ressed b y th e master. A sy stem may h av e
m o re th a n o n e m a s te r, a lth o u g h o n ly o n e m a y b e a c tiv e a t a n y tim e . T h e re fo re m o re th a n o n e
micro co n tro ller co u ld b e co n n ected to th e b u s, an d th ey can claim th e master ro le at d ifferen t
times, wh en n eed ed . An arb itratio n p ro cess is d efin ed if mo re th an o n e master attemp ts to
co n tro l th e b u s.
A d ata tran sfer is mad e u p o f th e master sig n alin g a sta rt co n d itio n , fo llo wed b y o n e o r two
b y te s c o n ta in in g a d d r e s s a n d c o n tr o l in f o r m a tio n . T h e s ta r t c o n d itio n ( F ig u r e 7 .8 a ) is d e fi n e d
b y a h ig h to lo w tran sitio n o f SDA wh en SCL is h ig h . All su b seq u en t d ata tran smissio n
f o llo w s th e p a tte r n o f F ig u r e 7 .8 b . O n e c lo c k p u ls e is g e n e r a te d f o r e a c h d a ta b it, a n d d a ta
may o n ly ch an g e wh en th e clo ck is lo w. Th e b y te fo llo win g th e start co n d itio n is mad e u p o f
s e v e n a d d r e s s b its a n d o n e d a ta d ir e c tio n b it, a s s h o w n in F ig u r e 7 .8 c . E a c h s la v e h a s
a p red efin ed d ev ice ad d ress; th e slav es are th erefo re resp o n sib le fo r mo n ito rin g th e b u s an d
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
(a )
(b )
start an d stop con d ition s
S D A
S D A
S C L
S C L
clock an d d ata timin g
D a ta V a lid
S ta rt
S to p
(c )
1 3 1
D a ta
m a y c h a n g e
a comp lete tran sfer of on e b yte
S C L
S D A
1 -7
S ta rt
8
A d d re ss
R /W
1 -7
A c k
8
D a ta
A c k
S to p
F ig u r e 7 .8 :
I2 C d a ta tra n s fe r
resp o n d in g o n ly to co mman d s asso ciated with th eir o wn ad d ress. A slav e d ev ice th at
re c o g n iz e s its a d d re s s w ill th e n b e re a d ie d e ith e r to re c e iv e d a ta o r to tra n s m it it o n to th e b u s .
A 1 0 -b it ad d ressin g mo d e is also av ailab le.
All d ata tran sferred
o n e messag e. Each
d u rin g wh ich time
wh ile SCL is h ig h
a sin g le b y te.
7 .5 .2
is in
b y te
th e tr
d efin
u n its o f o
mu st b e
an smitter
es a sto p
n e b y te, with n o limit o n th e n u mb er o f b y tes tran sferred in
fo llo w e d b y a 1 -b it a c k n o w le d g m e n t fro m th e re c e iv e r,
relin q u ish es SDA co n tro l. A lo w to h ig h tran sitio n o f SDA
c o n d itio n . F ig u r e 7 .8 c illu s tr a te s th e c o m p le te tr a n s f e r o f
I2 C o n th e m b e d
F ig u r e 2 .1 s h o w s u s th a t th e m b e d o ff e r s tw o I 2 C p o r ts , o n p in s 9 a n d 1 0 , o r 2 7 a n d 2 8 . T h e ir
u se fo llo ws th e p attern o f o th er mb ed p erip h erals, with av ailab le fu n ctio n s sh o wn in Tab les
7 .6 a n d 7 .7 .
7 .5 .3
S e t t in g u p a n I2 C D a t a L in k
We w ill n o w r e p lic a te th e a c tio n o f S e c tio n 7 .2 .4 , b u t u s in g I 2 C a s th e c o m m u n ic a tio n lin k ,
r a th e r th a n S P I . We w ill u s e th e I 2 C p o r t o n p in s 9 a n d 1 0 . P r o g r a m E x a m p le 7 .5 , th e m a s te r,
1 3 2
Chapter 7
T a b le 7 .6 : m b e d I2 C m a s t e r A P I s u m m a r y
F u n c t io n
U s a g e
2
I 2 C
C r e a t e a n I C m a s t e r in t e r fa c e , c o n n e c t e d t o t h e s p e c ifi e d p in s
f r e q u e n c y
S e t t h e fr e q u e n c y o f t h e I2 C in t e r fa c e
r e a d
R e a d fr o m a n I2 C s la v e
w r i t e
W r it e t o a n I2 C s la v e
s t a r t
C r e a t e s a s t a r t c o n d it io n o n t h e I2 C b u s
s t o p
C r e a t e s a s t o p c o n d it io n o n t h e I2 C b u s
T a b le 7 .7 : m b e d I2 C s la v e A P I s u m m a r y
F u n c t io n
U s a g e
I 2 C S l a v e
C r e a t e a n I2 C s la v e in t e r fa c e , c o n n e c t e d t o t h e s p e c ifi e d p in s
f r e q u e n c y
S e t t h e fr e q u e n c y o f t h e I2 C in t e r fa c e
r e c e i v e
C h e c k s t o s e e if t h is I2 C s la v e h a s b e e n a d d r e s s e d
r e a d
R e a d fro m a n I2 C m a s te r
w r i t e
W r it e t o a n I2 C m a s t e r
a d d r e s s
S e t s t h e I2 C s la v e a d d r e s s
s t o p
R e s e t t h e I2 C s la v e b a c k in t o t h e k n o w n r e a d y r e c e iv in g s t a t e
f o llo w s e x a c tly th e p a tte r n o f P r o g r a m E x a m p le 7 .2 , e x c e p t th a t S P I - r e la te d s e c tio n s a r e
rep laced b y th o se th at relate to I2 C. Early in th e p ro g ram an I2 C serial p o rt is co n fig u red u sin g
th e mb ed u tility I 2 C . Th e n ame i2 c _ p o r t is ch o sen fo r th e p o rt n ame, an d lin k ed to th e p o rt o n
p in s 9 an d 1 0 . An arb itrary slav e ad d ress is ch o sen , 0 x 5 2 . Fo llo win g d etermin atio n o f th e
v ariab le s w itc h _ w o r d , th e I2 C tran smissio n can b e seen . Th is is created fro m th e sep arate
c o m p o n e n ts o f a s in g le - b y te I 2 C tr a n s m is s io n , i.e . s ta r t es e n d a d d r e s s es e n d d a ta es to p , a s
allo wed b y th e mb ed fu n ctio n s. A way o f g ro u p in g th ese to g eth er is p resen ted later in th e
ch ap ter. Fu rth er d o wn th e p ro g ram we see a req u est fo r a b y te o f d ata fro m th e slav e, with
s i m i l a r m e s s a g e s t r u c t u r e . N o w t h e s l a v e a d d r e s s i s O R e d w i t h 0 x 0 1 , w h i c h s e t s t h e R =W b i t
in th e a d d re s s w o rd to in d ic a te R e a d . T h e re c e iv e d w o rd is th e n in te rp re te d in o rd e r to s e t th e
LEDs, ju st as we d id in th e SPI p ro g ram earlier.
/ *
s l
* /
# i
I 2
D i
P r o g r a m E x a m p l e 7 . 5 : I 2 C M a s t e r , t r a n s f e r s s w i t c h s t a t e t o
a v e , a n d d i s p l a y s s t a t e o f s l a v e ’ s s w i t c h e s o n i t s l e d s .
n c l u d e " m b e d . h "
C i 2 c _ p o r t ( p 9 , p 1 0 ) ;
g i t a l O u t r e d _ l e d ( p 2 5 ) ;
/ / C o n f i g u r e
/ / r e d l e d
a
s e r i a l
p o r t ,
p i n s
9
s e c o n d
m b e d
a n d
a r e
1 0
a c t i n g
s d a ,
s c l
a s
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
D i g i t a l O u t g r e e n _ l e d ( p 2 6 ) ;
D i g i t a l I n s w i t c h _ i p 1 ( p 5 ) ;
D i g i t a l I n s w i t c h _ i p 2 ( p 6 ) ;
c h a r s w i t c h _ w o r d
c h a r r e c d _ v a l ;
c o n s t i n t a d d r =
i n t
m
w h i
s
i
i
/
i
i
i
i
w
/
i
i
r
i
/
r
a i
l e
w i
f
s
f
s
/ s
2 c
2 c
2 c
2 c
a i
/ r
2 c
2 c
e c
2 c
/ s
e d
n (
( 1
t c
( s
w i
( s
w i
e n
_ p
_ p
_ p
_ p
t (
e c
_ p
_ p
d _
_ p
e t
_ l
)
)
;
/ / g r e e n
/ / i n p u t
l e d
s w i t c h
/ / w o r d w e w i l l s e n d
/ / v a l u e r e c e i v e d f r o m s l a v e
/ / t h e I 2 C s l a v e a d d r e s s , a n
0 x 5 2 ;
1 3 3
a r b i t r a r y
e v e n
n u m b e r
{
{
h _
w i
t c
w i
t c
d
o r
o r
o r
o r
0 .
e i
o r
o r
v a
o r
l
e d
w o
t c
h _
t c
h _
a
t .
t .
t .
t .
0 0
v e
t .
t .
l =
t .
e d
= 0
r d
h _
w o
h _
w o
s i
s t
w r
w r
s t
2 )
a
s t
w r
i 2
s t
s
;
= 0
i p
r d
i p
r d
n g
a r
i t
i t
o p
;
s
a r
i t
c _
o p
a c
x a
1 =
= s
2 =
= s
l e
t (
e (
e (
( )
0 ;
= 1
w i
= 1
w i
b
) ;
a d
s w
;
/ / s e t
i n
t (
e (
p o
( )
c o
g l e b y t e o f d a t a ,
) ;
a d d r j0 x 0 1 ) ;
r t . r e a d ( a d d r ) ;
u p
a
r e c o g n i z a b l e
o u t p u t
p a t t e r n
)
t c h _ w o r d j0 x 0 1 ;
/ / O R
i n
l s b
)
t c h _ w o r d j0 x 0 2 ;
y t e o f d a t a , i n
d r ) ;
i t c h _ w o r d ) ;
;
r d i n g
t o
w o r d
c o
/ /
/ /
/ /
/ /
/ /
r
f
s
s
f
i n
/ /
/ /
/ /
r e c e i v
/ /
O R
r e
o r
e n
e n
o r
i n
c t
c e
d t
d o
c e
n e
I 2 C
a s
h e
n e
a s
c o r r e c t
s e
R e
f o
e d
p r
n d a
a d a
r c e
f r o
e s e t
x t
p
t a
a d
b y
t o
l s b
a c k a g e
r t c o n d i t i o n
d r e s s
t e o f d a t a , i e
p c o n d i t i o n
I 2 C
d d r e
n d s
a s t
m s l
b o t
s s
a v
o p
a v
h
s w i t c h _ w o r d
p a c k a g e
,
e
w i t h R / W b i t
t h e r e c e i v e d
c o n d i t i o n
s e t t o
b y t e
R e a d
e
t o
0
.
( c o n t i n u e s
a s
i n
P r o g r a m
E x a m p l e
7 . 2 )
.
}
}
P r o g r a m E x a m p le 7 .5
I 2 C d a ta lin k m a s te r
T h e s la v e p r o g r a m is s h o w n in P r o g r a m E x a m p le 7 .6 , a n d is s im ila r to P r o g r a m E x a m p le 7 .3 ,
w ith S PI fe atu res rep la ce d b y I2 C . A s in S PI, th e I2 C s la v e ju st res p o n d s to c alls fro m th e m a s te r.
T h e s la v e p o rt is d efi n e d w ith th e m b ed u tility I 2 C s la v e , w ith s la v e c h o s en a s th e p o rt n am e . J u s t
w ith in th e m a in fu n c tio n th e s lav e a d d res s is d efi n e d , imp o rtan tly th e s a me 0 x 5 2 a s w e s a w in th e
m a ster p ro g ra m. A s b efo re, th e s w itc h _ w o r d v a lu e is s e t u p fro m th e s ta te o f th e s witc h e s; th is is
th e n s av e d u s in g th e w r ite fu n c tio n , in re a d in es s fo r a re q u e st fro m th e m a ster. T h e r e c e iv e ( )
f u n c tio n is u s e d to te s t w h e th e r a n I2 C tra n s m is s io n h a s b e e n re c e iv e d . T h is re tu rn s a 0 if th e s la v e
h as n o t b e en ad d res se d , a 1 if it h a s b ee n a d d re ss ed to rea d , a n d a 3 if ad d res se d to w rite. If a re ad
h as b ee n in itiated , th en th e v a lu e a lre ad y s to re d is a u to m a tic ally s en t. If th e v a lu e is 3 , th e n th e
p ro g r a m s to re s th e r e c e iv e d v a lu e a n d s e ts u p th e L E D s o n th e m a s te r a c c o rd in g ly .
/ * P
M a s
* /
# i n
I 2 C
D i g
r o g r a m E x a m p l e 7 . 6 : I 2 C
t e r , a n d d i s p l a y s s t a t e
c l u d e < m b e d . h >
S l a v e s l a v e ( p 9 , p 1 0 ) ;
i t a l O u t r e d _ l e d ( p 2 5 ) ;
S l a v e , w h e n
o f M a s t e r ’ s
c a l l e d t r a n s f e r s s w i t c h
s w i t c h e s o n i t s l e d s .
/ / C o n f i g u r e
/ / r e d l e d
I 2 C
s l a v e
s t a t e
t o
m b e d
a c t i n g
a s
1 3 4
D i g
D i g
D i g
c h a
c h a
i n t
Chapter 7
i t a l
i t a l
i t a l
r s w
r r e
m
s l a
w h i
/
s
i
i
s
/
i
i
a i
v e
l e
/ s
w i
f
s
f
s
l a
/ t
n t
f
r
O u t
I n
I n
i t c
c d _
g r
s w i
s w i
h _ w
v a l
n (
. a
(
e t
t c
( s
w i
( s
w i
v e
e s
i
( i
e c
{
r e
{
p
w o
t c
h _
t c
h _
r i
f o
s
=
v a
/ n
)
d d
1 )
u
h _
w i
t c
w i
t c
. w
t
=
=
d _
/
e e
t c
t c
o r
n _ l e d ( p 2 6 ) ;
h _ i p 1 ( p 5 ) ;
h _ i p 2 ( p 6 ) ;
d ;
/ / g r e e n
l e d
/ / w o r d w e w i l l s e n d
/ / v a l u e r e c e i v e d f r o m
;
m a s t e r
s s ( 0 x 5 2 ) ;
s w
r d
h _
w o
h _
w o
t e
r
l a
3 )
l =
o w
i t
= 0
i p
r d
i p
r d
( s
I 2
v e
{
s
s
c h
x a
1 =
= s
2 =
= s
w i
C ,
. r
_ w
0 ;
= 1
w i
= 1
w i
t c
a
e c
o r d
/ /
)
t c h
)
t c h
h _ w
n d
e i v
/ /
l a v e . r e
e t l e d s
f r o m s w i t c h e s t h a t a r e p r e s s e d
s e t u p a r e c o g n i z a b l e o u t p u t p a t t e r n
_ w o r d j0 x 0 1 ;
_ w
o r
a c
e (
s l
a d
a
o r
d )
t
) ;
a v
( )
c c
d j0 x 0 2 ;
; / / l o a d u p
a c c o r d i n g l y
e
i s
w o r d
a d d r e s s e d ,
t o
s e n d
M a s t e r
w i l l
w r i t e
;
o r d i n g
t o
r e c e i v e d
w o r d
.
( c o n t i n u e s
.
a s
i n
P r o g r a m
E x a m p l e
7 . 2 )
}
}
P r o g r a m E x a m p le 7 .6
I 2 C d a ta lin k s la v e
C o n n e c t tw o m b e d s to g e th e r w ith a n I 2 C lin k , a p p ly in g th e c ir c u it d ia g r a m o f F ig u r e 7 .9 . T h is is
v e r y s im ila r to F ig u r e 7 .5 , e x c e p t th a t th e S P I c o n n e c tio n is r e m o v e d , a n d r e p la c e d b y th e I 2 C
c o n n e c tio n . I t is e s s e n tia l to in c lu d e th e p u ll- u p r e s is to r s ; v a lu e s o f 4 .7 k U a r e s h o w n , b u t th e y
c a n b e a n y w h e r e i n t h e r a n g e 2 . 2 e4 . 7 k U . N o t e t h a t e a c h m b e d s h o u l d h a v e t w o s w i t c h e s a n d
two LEDs co n n ected , b u t th ere sh o u ld ju st b e o n e p air o f p u ll-u p resisto rs b etween th em.
C o m p ile a n d d o w n lo a d P r o g r a m E x a m p le 7 .5 in to e ith e r m b e d , a n d E x a m p le 7 .6 in to th e o th e r.
Yo u s h o u ld fi n d th a t th e s w itc h e s o f o n e m b e d c o n tr o l th e L E D s o f th e o th e r, a n d v ic e v e r s a .
Mo n ito r SCL an d SDA lin e
trig g erin g . With ap p ro p riate
two messag es b ein g sen t b e
in clu d in g th e id le h ig h co n d
R / W b i t e m b e d d e d .
n
s o n an o scillo sco p e. Th is
settin g o f th e o scillo sco p e
tween th e mb ed s. Id en tify
itio n , th e start an d sto p co
may req u ire carefu l
time b ase y o u sh o u ld
as man y featu res o f
n d itio n s, an d th e ad d
o scillo sco p e
b e ab le to see th e
I2C as y o u can ,
ress b y te with th e
E x e r c is e 7 .4
In P r o g r a m E x a m p le 7 .6 , r e p la c e t h e lin e i f ( i = = 3 ) w it h i f ( i = = 4 ) ; in o t h e r w o r d s , t h e
c o n d it io n c a n n o t b e s a t is fi e d . C o m p ile , d o w n lo a d a n d r u n . N o t ic e t h a t t h e s la v e s t ill
S 1
c o n n e c t
to s e c o n d
m b e d
S 2
4 k 7
s d a , to p in 9
s c l, to p in 1 0
4 k 7
G n d
V in
V b
n R
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
m b e d M ic r o c o n tr o lle r
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 3 5
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
F ig u r e 7 .9 :
L in k in g t w o m b e d s w it h I2 C
c o n t in u e s t o w r it e t o t h e m a s t e r , b u t is n o w u n a b le t o r e s p o n d t o t h e m a s t e r ’s m e s s a g e .
W h y is t h is ?
n
7 .6
C o m m u n ic a t in g w it h a n I2 C T e m p e r a t u r e S e n s o r
Ju st as we d id with th e SPI p o rt an d th e accelero meter, we can u se th e mb ed I2 C p o rt to
co mmu n icate with a v ery wid e ran g e o f p erip h eral d ev ices, in clu d in g man y in tellig en t
s e n s o r s . T h e Te x a s I n s tr u m e n ts T M P 1 0 2 te m p e r a tu r e s e n s o r ( R e f e r e n c e 7 .4 ) h a s a n I 2 C d a ta
lin k . T h is is s im ila r to th e a c c e le r o m e te r in tr o d u c e d in S e c tio n 7 .3 , in th a t a n a n a lo g s e n s in g
d ev ice is in teg rated with an ADC an d a serial p o rt, p ro d u cin g an id eal an d easy -to -u se sy stem
elemen t. No te fro m th e d atash eet th at th e TMP1 0 2 actu ally mak es u se o f th e sy stem
man ag emen t b u s (SMb u s). Th is was d efin ed b y In tel in 1 9 9 5 , an d is b ased o n I2 C. In simp le
ap p licatio n s th e two stan d ard s can b e mix ed ; fo r mo re ad v an ced ap p licatio n s it is wo rth
ch eck in g th e small d ifferen ces b etween th em.
Th e TMP
u sed mo u
sh o wn in
I2C. On c
1 0 2 is
n ted o
Tab le
o n n ec
a tin y d
n a sma
7 .8 . C o
to r JP2
ev ice, as req u ired o f a
ll b reak o u t b o ard , seen
n n ectio n s o n co n n ecto
th e sen so r h as an ad d r
temp eratu re sen
in F ig u r e 7 .1 0 a
r JP1 are th e ess
ess p in , ADD0 ;
so r. Lik e th
. It h as six p
en tial o n es
th is is u sed
e accelero meter, it is
o ssib le co n n ectio n s,
o f p o wer su p p ly an d
to select th e ad d ress
1 3 6
Chapter 7
(a )
(b )
c o n n e c te d to th e m b e d
4
3
2
1
J P
1
2
1
J P
2
s e n s o r o n b re a k o u t b o a rd
F ig u r e 7 .1 0 :
T h e T M P 1 0 2 t e m p e r a t u r e s e n s o r ( Im a g e r e p r o d u c e d w it h p e r m is s io n o f S p a r k F u n E le c t r o n ic s )
o f th e d ev ice, as seen in th e tab le. Th is allo ws fo u r d ifferen t ad d ress o p tio n s, h en ce fo u r o f th e
same sen so r can b e u sed o n th e same I2 C b u s.
P r o g r a m E x a m p le 7 .7 c a n b e a p p lie d to lin k th e m b e d to th e s e n s o r. I t d e fi n e s a n I 2 C p o r
p in s 9 a n d 1 0 , a n d n a m e s th is te m p s e n s o r . T h e s e ria l lin k to b e u s e d to c o m m u n ic a te w ith
P C is s e t u p . A s s e n s o r p in A D D 0 is tie d to g r o u n d , Ta b le 7 .8 s h o w s th a t th e s e n s o r a d d r e s s
b e 0 x 9 0 ; th is is d efi n e d in th e p ro g ram , w ith n a m e a d d r . Tw o s ma ll a rra y s a re a ls o d efi n e d ,
to h o ld th e s e n s o r c o n fi g u ra tio n d a ta, a n d th e o th e r to h o ld th e raw d ata rea d fro m th e s e n so
f u r th e r v a r ia b le t e m p w ill h o ld th e s c a le d d e c im a l e q u iv a le n t o f th e r e a d in g .
t o n
th e
w ill
o n e
r. A
T a b le 7 .8 : C o n n e c t in g t h e T M P 1 0 2 s e n s o r t o t h e m b e d
S ig n a l
T M P 1 0 2 p in
V c c ( 3 .3 V )
JP 1 : 1
S D A
JP 1 : 2
S C L
JP 1 : 3
G n d (0 V )
JP 1 : 4
A le r t
JP 2 : 1
A D D 0
JP 2 : 2
m b e d p in
N o te s
4 0
9
2 .2 k U p u ll- u p t o 3 .3 V
1 0
2 .2 k U p u ll- u p t o 3 .3 V
1
1
1
C o n n e c t to
S la v e a d d r e s s
0 V
0 x9 0
V c c
0 x9 1
S D A
0 x9 2
S C L
0 x9 3
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 3 7
Th e co n fig u ratio n o p tio n s can b e fo u n d fro m th e TMP1 0 2 d atash eet. To set th e co n fig u ratio n
reg ister we first n eed to sen d a d ata b y te o f 0 x 0 1 to sp ecify th at th e p o in ter reg ister is set to
‘Co n fig u ratio n Reg ister’. Th is is fo llo wed b y two co n fig u ratio n b y tes, 0 x 6 0 an d 0 x A0 . Th ese
select a simp le co n fig u ratio n settin g , in itializin g th e sen so r to n o rmal mo d e o p eratio n . Th ese
v alu es are sen t at th e start o f th e m a in fu n ctio n . No te th e fo rmat o f th e write co mman d u sed
h ere, wh ich is ab le to sen d mu lti-b y te messag es. Th is is d ifferen t fro m th e ap p ro ach u sed in
P r o g r a m E x a m p le 7 .5 , w h e r e o n ly a s in g le b y te w a s s e n t in a n y o n e m e s s a g e . U s in g th e I 2 C
w r ite ( ) fu n ctio n , we n eed to sp ecify th e d ev ice ad d ress an d th e array o f d ata, fo llo wed b y th e
n u mb er o f b y tes to sen d .
Th e s e n s o r will n o w o p e ra te a n d a c q u ire temp e ra tu re d ata , s o we s imp ly n e e d to re a d
th e d ata reg ister. To d o th is we n eed first to set th e p o in ter reg ister v alu e to 0 x 0 0 . In
th is co mman d we h av e o n ly sen t o n e d ata b y te to set th e p o in ter reg ister. Th e p ro g ram
n o w s ta rts a n in fin ite lo o p to re ad c o n tin u o u s ly th e 2 -b y te temp e ra tu re d a ta. Th is d ata
is th en co n v erted fro m a 1 6 -b it read in g to an actu al temp eratu re v alu e. Th e co n v ersio n
req u ired (as sp ecified b y th e d atash eet) is to sh ift th e d ata rig h t b y 4 b its (it is
actu ally o n ly 1 2 -b it d ata h eld in two 8 -b it reg is ters) an d to mu ltip ly b y th e sp ecifie d
c o n v e r s io n f a c to r, 0 .0 6 2 5 d e g r e e s C p e r L S B . T h e v a lu e is th e n d is p la y e d o n th e P C
screen .
/ *
d i
* /
# i
I 2
S e
c o
c h
c h
f l
P r o g r a m E x a m p l e
s p l a y s r e a d i n g s
n c l
C t
r i a
n s t
a r
a r
o a t
i n t
c o
c o
c o
t e
c o
t e
w h
u d
e m
l
i
c o
t e
t
m a
n f
n f
n f
m p
n f
m p
i l
w a
t e
t e
p c
e
p s
p c
n t
n f
m p
e m
i n
i g
i g
i g
s e
i g
s e
e (
i t
m p
m p
. p
" m
e n
( U
a
i g
_ r
p ;
( )
_ t
_ t
_ t
n s
_ t
n s
1 )
( 1
s e
=
r i
b e
s o
S B
d d
_ t
e a
{
[ 0
[ 1
[ 2
o r
[ 0
o r
{
) ;
n s
0
n t
]
]
]
.
.
]
d .
r (
T X
r
[ 3
d [
h “
p 9 , p 1 0 ) ;
, U S B R X ) ;
= 0 x 9 0 ;
] ;
2 ] ;
=
0 x
0 x
= 0 x
w r i t e
= 0 x
w r i t e
=
7 . 7 : M b e d c o m m u n i c a t e s
t o s c r e e n .
0 1
6 0
A 0
( a
0 0
( a
T M P 1 0 2
t e m p e r a t u r e
s e n s o r ,
a n d
s c a l e s
a n d
/ / s d a , s c 1
/ / t x , r x
;
/ / s e t p o i n t e r r e g t o
/ / c o n f i g d a t a b y t e 1
/ / c o n f i g d a t a b y t e 2
;
;
d d r ,
;
d d r ,
w i t h
c o n f i g _ t ,
3 ) ;
c o n f i g _ t ,
1 ) ;
/ / s e t p o i n t e r r e g
/ / s e n d t o p o i n t e r
’ c o n fig
r e g i s t e r ’
t o ’ d a t a r e g i s t e r ’
’ r e a d t e m p ’
o r . r e a d ( a d d r , t e m p _ r e a d , 2 ) ;
/ / r e a d t h e t w o - b y t e t e m p d a t a
. 0 6 2 5 * ( ( ( t e m p _ r e a d [ 0 ] < < 8 ) + t e m p _ r e a d [ 1 ] ) > > 4 ) ; / / c o n v e r t
f ( " T e m p = % . 2 f d e g C \ n \ r " , t e m p ) ;
d a t a
}
}
P r o g r a m E x a m p le 7 .7
C o m m u n ic a tin g b y I 2 C w ith th e T M P 1 0 2 te m p e r a tu r e s e n s o r
1 3 8
Chapter 7
C o n n e c t th e s e n s o r a c c o r d in g to th e in f o r m a tio n in Ta b le 7 .8 , le a d in g to a c ir c u it th a t
lo o k s s o m e th in g lik e F ig u r e 7 .1 0 b . O n c e a g a in , th e S DA a n d S C L lin e s e a c h n e e d to b e
p u lle d u p to 3 .3 V th r o u g h a r e s is to r, o f v a lu e b e tw e e n 2 .2 a n d 4 .7 k U . C o m p ile ,
d o w n lo a d a n d r u n th e c o d e o f P r o g r a m E x a m p le 7 .7 . Te s t th a t th e d is p la y e d te m p e r a tu r e
in c r e a s e s w h e n y o u p r e s s y o u r fi n g e r a g a in s t th e s e n s o r. Yo u c a n tr y p la c in g th e s e n s o r
o n so meth in g warm, fo r ex amp le a rad iato r, in o rd er to ch eck th at it resp o n d s to
temp eratu re ch an g es. If y o u h av e a calib rated temp eratu re sen so r, try to co mp are
read in g s fro m b o th so u rces.
n
E x e r c is e 7 .5
R e w r it e P r o g r a m E x a m p le 7 .5 u s in g t h e I2 C m a s t e r r e a d ( ) a n d w r it e ( ) fu n c t io n s , a s s e e n
in P r o g r a m E x a m p le 7 .7 . C o m p ile , d o w n lo a d a n d t e s t t h a t it w o r k s a s e x p e c t e d .
n
U s in g t h e S R F 0 8 U lt r a s o n ic R a n g e F in d e r
T h e S R F 0 8 u ltr a s o n ic r a n g e fi n d e r, a s s h o w n in F ig u r e 7 .1 1 , c a n b e u s e d to m e a s u r e th e
d is ta n c e b e tw e e n th e s e n s o r a n d a n a c o u s tic a lly re fl e c tiv e s u rfa c e o r o b je c t in fro n t o f it.
mak es th e measu remen t b y tran smittin g a p u lse o f u ltraso u n d fro m o n e o f its tran sd u cers, a
th en measu rin g th e time fo r an ech o to retu rn to th e o th er. If th ere is n o ech o it times o u t. T
d is ta n c e to th e re fl e c tin g o b je c t is p ro p o rtio n a l to th e tim e ta k e n fo r th e e c h o to re tu rn . G iv
k n o wled g e o f th e sp eed o f so u n d in air, th e actu al d istan ce can b e calcu lated . Th e SRF0 8 h
(b )
+ 3 ,3 V
c o n n e c tio n s to th e m b e d
+ 5 V
IC 7
+ 5 V
G n d
V in
V b
n R
5
1 k 8
R 1
+ 5 V
6
7
V C
S D
S C
N
G N
C
A
L
C
D
V C
S D
S C
N C
G N
C
A
L
8
9
1 0
1 1
D
1 2
1 3
1 4
1 5
1 6
1 7
U S R F 1
1 8
1 9
2 0
m b e d M ic r o c o n tr o lle r
th e s e n s o r
1 k 8
R 2
(a )
It
n d
h e
en
as
V o u t
V u
IF IF +
R D R D +
T D T D +
D D +
3 0
2 9
2 8
2 7
2 6
2 5
2 4
2 3
2 2
2 1
F ig u r e 7 .1 1 :
T h e S R F 0 8 u lt r a s o n ic r a n g e fi n d e r . ( Im a g e r e p r o d u c e d w it h p e r m is s io n o f S p a r k F u n E le c t r o n ic s )
+ 5 V
7 .7
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 3 9
a n I 2 C in te r f a c e . D a ta is r e a d ily a v a ila b le f o r it, f o r e x a m p le R e f e r e n c e 7 .5 , a lth o u g h a t th e
time o f writin g n o t as a fo rmalized d o cu men t.
T h e S R F 0 8 c a n b e c o n n e c te d to th e m b e d a s s h o w n in F ig u r e 7 .1 1 b . I t m u s t b e p o w e r e d f r o m
5 V, w ith I 2 C p u ll- u p r e s is to r s c o n n e c te d to th a t v o lta g e . T h e m b e d r e m a in s p o w e r e d f r o m
3 .3 V, a n d is a b le to to le r a te th e h ig h e r v o lta g e b e in g p r e s e n te d a t its I 2 C p in s .
Hav in g wo rk ed th ro u g h th e p reced in g p ro g rams, it sh o u ld b e easy to g rasp h o w Pro g ram
E x a m p le 7 .8 w o r k s . N o te th e f o llo w in g in f o r m a tio n , ta k e n f r o m th e d e v ic e d a ta :
•
SRF0 8 I2C ad d ress
p o in ter v alu e fo r th
ata v alu e o f 0 x 5 1 to
rn d ata in cm.
o in ter v alu e o f 0 x 0 2
Th e
Th e
A d
retu
A p
•
•
•
/ *
f i
* /
# i
I 2
S e
c o
c h
c h
f l
is 0 x E0 .
e co mman d reg ister is 0 x 0 0 .
th e co mman d reg ister in itializes th e ran g e fin d er to o p erate an d
p r e p a r e s f o r 1 6 - b it d a ta ( i.e . tw o b y te s ) to b e r e a d .
P r o g r a m E x a m p l e 7 . 8 : C o n f i g u r e s a n d
n d e r , a n d d i s p l a y s t h e m o n s c r e e n .
n c l
C r
r i a
n s t
a r
a r
o a t
i n t
m
w h i
c
c
r
w
c
r
r
r
p
w
u d
a n
l
i
c o
r a
r
e
" m
f i
( U
a
i g
e _
g e
g e
p c
n t
n f
n g
a n
a i
l e
o n
o n
a n
a i
o n
a n
a n
a n
c .
a i
n ( )
( 1 )
f i g _
f i g _
g e f i
t ( 0 .
f i g _
g e f i
g e f i
g e =
p r i n
t ( 0 .
b e
n d
S B
d d
_ r
r e
;
d .
e r
T X
r
[ 2
a d
h "
( p 9
, U
= 0
] ;
[ 2 ]
t a k e s
r e a d i n g s
f r o m
t h e
S R F 0 8
u l t r a s o n i c
r a n g e
, p 1 0 ) ; / / s d a , s c 1
S B R X ) ; / / t x , r x
x E 0 ;
;
{
{
r [ 0
r [ 1
n d e
0 7 )
r [ 0
n d e
n d e
( (
t f (
0 5 )
]
=
0 x 0 0 ;
0 x 5 1 ;
r . w r i t e ( a d d r ,
]
=
c o n f i g _ r ,
/ / s e t p o i n t e r
/ / i n i t i a l i z e ,
r e g t o
r e s u l t
/ /
/
/
[
/
o n
‘ c m d r e g i s t e r ’
i n c m
2 ) ;
;
]
r .
r .
r a
" R
=
w r
r e
n g
a n
0 x
i t
a d
e _
g e
0 2
e (
( a
r e
=
;
a d d r , c
d d r , r a
a d [ 0 ] <
% . 2 f c
o n
n g
<
m \
f i g _ r , 1 ) ;
/
e _ r e a d , 2 ) ;
/
8 ) + r a n g e _ r e a d
n \ r " , r a n g e ) ; /
s e
s e
r e
1 ]
p r
t p o i n t e
n d t o p o
a d t h e t
) ;
i n t r a n g
r r e g t o ’ d a t a r e g i s t e r ’
i n t e r ’ r e a d r a n g e ’
w o - b y t e r a n g e d a t a
e
s c r e e n
;
}
}
P r o g r a m E x a m p le 7 .8
Co n n ect th e circu it
m b e d . Ve r if y c o r r e c
su rface. Th en ex p lo
h an d le) an d d istan t
o f
t o
re
o b
C o m m u n ic a tin g b y I 2 C w ith th e S R F 0 8 r a n g e fi n d e r
F ig u r e 7 .1 1 b . C o m p ile P r o g r a m E x a m p le 7 .8 a n d d o w n lo a d to th e
p eratio n b y p lacin g th e ran g e fin d er a k n o wn d istan ce fro m a h ard , flat
its a b ility to d e te c t ir r e g u la r s u r f a c e s , n a r r o w o b je c ts ( e .g . a b r o o m
jects.
1 4 0
Chapter 7
E v a lu a t in g I2 C
7 .8
As we h av e seen , th e I2 C p ro to co l is well estab lish ed an d v ersatile. Lik e SPI, it is wid ely
ap p lied to sh o rt d istan ce d ata co mmu n icatio n . Ho wev er, it g o es well b ey o n d SPI in its ab ility
to s e t u p m o re c o m p le x n e tw o rk s , a n d to a d d a n d s u b tra c t n o d e s w ith c o m p a ra tiv e e a s e .
Alth o u g h we h av e n o t really ex p lo red it h ere, it p ro v id es fo r a mu ch mo re reliab le sy stem. If
an ad d ressed d ev ice d o es n o t sen d an ack n o wled g men t, th e master can act u p o n th at fau lt.
Do es th is mean th at I2 C is g o in g to meet all o u r n eed s fo r serial co mmu n icatio n , in an y
ap p licatio n ? Th e an swer is a clear No , fo r at least two reaso n s. On e is th at th e b an d wid th is
c o m p a ra tiv e ly lim ite d , ev e n in th e fa s te r v e rs io n s o f I2 C . T h e s e c o n d is th e s e c u rity o f th e
d ata. Wh ile fin e in , say , a d o mestic ap p lian ce, I2 C is still su scep tib le to in terferen ce an d d o es
n o t ch eck fo r erro rs. Th erefo re, o n e wo u ld b e u n lik ely to co n sid er u sin g it in a med ical, mo to r
v eh icle o r o th er h ig h -reliab ility ap p licatio n .
7 .9
A s y n c h r o n o u s S e r ia l D a t a C o m m u n ic a t io n
Th e sy
SPI an
h o wev
allo ws
n ch ro n o u s serial co mmu n icatio n p ro to co ls d escrib ed so far th is ch
d I2 C, are ex tremely u sefu l way s o f mo v in g d ata aro u n d . Th e q u
er: d o we really n eed to sen d th at clo ck sig n al wh erev er th e d ata
an easy way o f sy n ch ro n izin g th e d ata, it d o es h av e th ese d isad v
•
•
•
An ex tra (clo ck )
Th e b an d wid th n
th erefo re, it is th
Ov er lo n g d istan
7 .9 .1
lin e n eed s
eed ed fo r
e d eman d s
ces, clo ck
to
th e
o f
an d
g o
clo
th e
d a
ap ter, in th e fo rm o f
estio n remain s,
g o es? Alth o u g h it
an tag es:
to ev ery d ata n o d e.
ck is alway s twice th e b an d wid th n eed ed fo r th e d ata;
clo ck th at limit th e o v erall d ata rate.
ta th emselv es co u ld lo se sy n ch ro n izatio n .
In t r o d u c in g A s y n c h r o n o u s S e r ia l D a t a
Fo r th e reaso n s ju st stated , sev eral serial stan d ard s h av e b een d ev elo p ed th
a clo ck sig n al to b e sen t with th e d ata. Th is is g en erally called a syn ch ro n o
c o m m u n ic a tio n . It is n o w u p to th e re c e iv e r to e x tra c t a ll tim in g in fo rm a tio
sig n al itself. Th is h as th e effect o f lay in g n ew an d d ifferen t d eman d s o n th e
tra n s m itte r a n d re c e iv e r n o d e s s o m ew h a t m o re c o m p le x th a n c o m p a ra b le s
at d o n o t
u s serial
n d irectly
sig n al, an
y n ch ro n o
req u ire
fro m th e
d mak in g
u s n o d es.
A co mmo n ap p ro ach to ach iev in g asy n ch ro n o u s co mmu n icatio n is b ased o n th is:
•
Data rate
d ata rate.
rate can b
acco mmo
i s p r e d e t e r m i n e d eb o t h t r a n s m i t t e r a n d r e c e iv e r a r e p r e s e t t o r e c o g n i z e t h e s a m e
Hen ce, each n o d e n eed s an accu rate an d stab le clo ck so u rce, fro m wh ich th e d ata
e g en erated . Small v ariatio n s fro m th e th eo retical v alu e can , h o wev er, b e
d ated .
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
Idle State
Start
bit
First
Data bit
Start
synchronization
Last
Data bit
Stop
bit
Extra “parity”
bit could be
inserted here
1 4 1
Idle State
Earliest possible
new Start Bit
F ig u r e 7 .1 2 :
A c o m m o n a s y n c h r o n o u s s e r ia l d a t a fo r m a t
•
Each b y te o r wo rd is fra med with a Start an d Sto p b it. Th ese allo w sy n ch ro n izatio n to b e
in itiated b efo re th e d ata starts to flo w.
An asy n ch ro n o u s d ata fo rmat, o f th e so rt u sed b y su ch stan d ard s as RS-2 3 2 , is sh o wn in
F ig u r e 7 .1 2 . T h e r e is n o w o n ly o n e d a ta lin e . T h is id le s in a p r e d e te r m in e d s ta te , in th is
ex amp le at Lo g ic 1 . Th e start o f a d ata wo rd is in itiated b y a S ta rt b it, wh ich h as p o larity
o p p o site to th at o f th e id le state. Th e lead in g ed g e o f th e Start b it is u sed fo r sy n ch ro n izatio
Eig h t d ata b its are th en clo ck ed in . A n in th b it, fo r p arity ch eck in g , is also so metimes u se
Th e lin e th en retu rn s to th e id le state, wh ich fo rms a Sto p b it. A n ew wo rd o f d ata can b e se
immed iately , fo llo win g th e co mp letio n o f a sin g le Sto p b it, o r th e lin e may remain in th e id
state u n til it is n eed ed ag ain .
n .
d .
n t
le
An a s y n ch ro n o u s s erial p o rt in teg rate d in to a mic ro c o n tro ller o r p e rip h e ra l d ev ice is g e n erally
c a lle d a UA RT, s ta n d in g f o r u n iv e r s a l a s y n c h ro n o u s re c e iv e r /tr a n s m itte r. I n its s im p le s t f o r m ,
a UA RT h a s o n e c o n n e c tio n fo r tra n s m itte d d a ta , u s u a lly c a lle d T X , a n d a n o th e r fo r re c e iv e d
d ata, called RX. Th e p o rt sh o u ld sen se wh en a start b it h as b een in itiated , an d au to matically
clo ck in an d sto re th e n ew wo rd . It can in itiate a tran smissio n at an y time. Th e d ata rate th at
re c e iv e r a n d tra n s m itte r w ill o p e ra te a t m u s t b e p re d e te rm in e d ; th is is s p e c ifi e d b y its b a u d r a te .
F o r th e p r e s e n t p u r p o s e s b a u d r a te c a n b e v ie w e d a s b e in g e q u iv a le n t to b it r a te ; f o r m o r e
ad v an ced ap p licatio n s o n e sh o u ld ch eck th e d istin ctio n s b etween th ese two terms.
7 .9 .2
A p p ly in g A s y n c h r o n o u s C o m m u n ic a t io n o n t h e m b e d
I f w e lo o k b a c k a t F ig u r e 2 .3 . w e s e e th a t th e L P C 1 7 6 8 h a s f o u r UA RT s . T h r e e o f th e s e
ap p ear o n th e p in o u t o f th e mb ed , simp ly lab eled ‘Serial’, o n p in s 9 an d 1 0 , 1 3 an d 1 4 , an d 2 7
a n d 2 8 . T h e ir n o t in s ig n ifi c a n t A P I s u m m a r y is g iv e n in Ta b le 7 .9 .
We
mb
Fig
slig
will n o w rep eat w
ed s to d emo n strate
u r e 7 .1 3 , th e b u ild
h tly simp ler th an e
h at we h av e alrea
a serial lin k , b u t
we will u se, as a
ith er o f th ese. We
d y d o n e with SPI an d I2 C,
th is time an asy n ch ro n o u s
v a r ia tio n o n F ig u r e s 7 .5 a n
will ap p ly Pro g ram Ex amp
wh ich is
o n e . Yo u
d 7 .9 ; n o
le 7 .9 . I t
to c
can
tice
will
o n n ect two
v iew
th at it is
b e th e same
1 4 2
Chapter 7
T a b le 7 .9 : S e r ia l ( a s y n c h r o n o u s ) A P I s u m m a r y
F u n c t io n
U s a g e
S e r i a l
C r e a t e a s e r ia l p o r t , c o n n e c t e d t o t h e s p e c ifi e d t r a n s m it a n d r e c e iv e p in s
b a u d
S e t t h e b a u d r a t e o f t h e s e r ia l p o r t
f o r m a t
S e t t h e t r a n s m is s io n fo r m a t u s e d b y t h e s e r ia l p o r t
p u t c
W r it e a c h a r a c t e r
g e t c
R e a d a c h a ra c te r
p r i n t f
W r it e a fo r m a t t e d s t r in g
s c a n f
R e a d a fo r m a t t e d s t r in g
r e a d a b l e
D e t e r m in e if t h e r e is a c h a r a c t e r a v a ila b le t o r e a d
w r i t e a b l e
D e t e r m in e if t h e r e is s p a c e a v a ila b le t o w r it e a c h a r a c t e r
a t t a c h
A t t a c h a fu n c t io n t o c a ll w h e n e v e r a s e r ia l in t e r r u p t is g e n e r a t e d
p r o g r a m w e lo a d in to b o th m b e d s . I t f o llo w s a s im ila r p a tte r n to P r o g r a m E x a m p le 7 .2 , b u t
rep laces all SPI co d e with UART co d e. Th e co d e itself ap p lies a n u mb er o f fu n ctio n s fro m
Ta b le 7 .9 , a n d er e a d in g th e c o m m e n ts es h o u ld n o t b e to o d if fi c u lt to f o llo w.
F ig u r e 7 .1 3 :
L in k in g t w o m b e d U A R T s
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
/ * P r
a s i
* /
# i n c
S e r i
D i g i
D i g i
D i g i
D i g i
D i g i
c h a r
c h a r
i n t
o g r a m
m i l a r
l u d
a l
t a l
t a l
t a l
t a l
t a l
s w
r e
m a
a s y n
/ / a c
w h i l
/ /
s w
i f
i f
s t
w a
s t
a s
i f
e
a s
O u
O u
O u
I n
I n
i t
c d
i n
c _
c e
e
S e
i t
(
s w
(
s w
r o
i t
r o
y n
(
r e
E x a m p l e 7 . 9 : S e t s t h e m b e d u p
n o d e , s e n d i n g i t s o w n s w i t c h
" m b
y n c
t r
t g
t s
s w
s w
c h _
_ v a
( )
p o
p t
( 1
t
c h
s w
i t
s w
i t
b e
_ u
b e
c _
a s
c d
e d
_ p
e d
r e
t r
i t
i t
w o
l ;
{
r t .
d e
) {
u p
_ w o
i t c
c h _
i t c
c h _
= 1
s ( 1
= 0 ;
p o r
y n c
_ v a
. h
o r
_ l
e n
o b
c h
c h
r d
"
t (
e d
_ l
e (
_ i
_ i
;
p 9
( p
e d
p 7
p 1
p 2
,
2 5
( p
) ;
( p
( p
p 1 0 ) ;
) ;
2 6 ) ;
e
= 0
i p
r d
i p
r d
w o
x a
1 =
= s
2 =
= s
f o r a s y n c c o m m u n i c a t i o n , a n d e x c h a n g e s d a t a w i t h
p o s i t i o n s , a n d d i s p l a y i n g t h o s e o f t h e o t h e r .
/ / s e t u p T X a n d R X o n
/ / r e d l e d
/ / g r e e n l e d
/ / a s t r o b e t o t r i g g e r
/ / t h e
/ / t h e
r d
0 ;
= 1
w i
= 1
w i
p i n s
t h e
9
a n d
1 0
s c o p e
5 ) ;
6 ) ;
b a u d ( 9 6 0 0 ) ;
f a u l t f o r m a t ,
t h
r d
h _
w o
h _
w o
;
0 )
1 4 3
t o
o f
b e
8
b i t s ,
s e n t ,
b y
w o r d w e w i l l s e n d
r e c e i v e d v a l u e
/ / s e t b a u d
n o p a r i t y
r a t e
t o
9 6 0 0
( i e
t e s t i n g s w i t c h i n p u t s
/ / s e t u p a r e c o g n i z a b l e
d e f a u l t )
o u t p u t
p a t t e r n
)
t c h _ w o r d j0 x 0 1 ;
/ / O R
i n
t c h _ w o r d j0 x 0 2 ;
/ / O R i n
/ / s h o r t
l s b
)
n e x t l s b
s t r o b e p u l s e
;
t . p u t c ( s w i t c h _ w o r d ) ;
_ p o r t . r e a d a b l e ( ) = = 1 )
l = a s y n c _ p o r t . g e t c ( ) ;
/ / t r a n s m i t s w i t c h _ w o r d
/ / i s t h e r e a c h a r a c t e r
/ / i f y e s , t h e n r e a d i t
t o
b e
r e a d ?
.
( c o n t i n u e s
a s
i n
P r o g r a m
E x a m p l e
7 . 2 )
.
}
}
P r o g r a m E x a m p le 7 .9
B id ir e c tio n a l d a ta tr a n s fe r b e tw e e n tw o m b e d U A R T s
C o n n e c t tw o m b e d s a s s h o w n in F ig u r e 7 .1 3 , a n d c o m p ile a n d d o w n lo a d P r o g r a m E x a m p le
7 .9 in to e a c h . Yo u s h o u ld fi n d th a t th e s w itc h e s f r o m o n e m b e d c o n tr o l th e L E D s f r o m th e
o th er, an d v ice v ersa.
n
E x e r c is e 7 .6
O n a n o s c
t h e o s c illo
s w it c h e s a
1 . W h a t
C h a n g
illo s c o p e , o
s c o p e fro m
re p re s s e d .
is t h e t im e
e th e b a u d
b s e r v e t h e d a t a w a v e fo r m o f o n e o f t h e T X lin e s . It h e lp s t o t r ig g e r
t h e s t r o b e p u ls e ( p in 7 ) . S e e h o w t h e p a t t e r n c h a n g e s a s t h e
d u r a t io n o f e a c h d a t a b it ? H o w d o e s t h is r e la t e t o t h e b a u d r a t e ?
r a t e a n d m e a s u r e t h e n e w d u r a t io n .
1 4 4
Chapter 7
2 . Is t h e d a t a b y t e t r a n s m it t e d M S B fi r s t o r L S B ? H o w d o e s t h is c o m p a r e t o t h e s e r ia l
p r o t o c o ls s e e n e a r lie r in t h is c h a p t e r ?
3 . W h a t is t h e e ffe c t o f r e m o v in g o n e o f t h e d a t a lin k s in F ig u r e 7 .1 3 ?
n
7 .9 .3
A p p ly in g S y n c h r o n o u s C o m m u n ic a t io n w it h t h e H o s t C o m p u t e r
Wh ile th e mb ed h as th ree UARTs co n n ectin g to th e ex tern al p in s,
Th is is reserv ed fo r co mmu n icatio n b ack to th e USB lin k , an d can
d ia g r a m in F ig u r e 2 .2 . T h is UA RT a c ts ju s t lik e a n y o f th e o th e r s , in
( Ta b le 7 .8 ) . We h a v e b e e n u s in g it a lr e a d y , f o r th e fi r s t tim e in P r o
saw th ere, th e mb ed co mp iler will reco g n ize p c , U S B T X an d U S B
th is co n n ectio n , as in th e lin e:
S e r i a l
p c ( U S B T X ,
th e LPC1 7 6 8 h as a fo u rth .
b e seen in th e mb ed b lo ck
terms o f its u se o f th e API
g r a m E x a m p le 5 .4 . A s w e
R X as id en tifiers to set u p
U S B R X ) ;
With p c th u s c rea te d , th e A P I m e mb er fu n c tio n s c a n b e ex p lo ite d . T h e re q u ire m e n t to s et u p
th e h o s t c o m p u te r c o rr e c tly in o rd e r to c o m m u n ic a te w ith th is lin k w a s o u tlin e d in S e c tio n 5 .3 .
7 .1 0
M in i-P r o je c t : M u lt i-N o d e I2 C B u s
Desig n a simp le circu it wh ich h as a temp eratu re sen so r, ran g e fin d er an d mb ed co n n ected to
th e s a m e I 2 C b u s . M e rg e P r o g r a m E x a m p le s 7 .8 a n d 7 .9 , s o th a t m e a s u r e m e n ts f r o m e a c h
s e n s o r a r e d is p la y e d in tu r n o n th e s c r e e n . Ve r if y th a t th e I 2 C p r o to c o l w o r k s a s e x p e c te d .
C h a p t e r R e v ie w
•
•
•
•
•
•
Serial d ata lin k s p ro v id e a read y mean s o f co mmu n icatio n b etween micro co n tro ller an d
p e rip h e ra ls , a n d /o r b e twee n micro co n tro llers .
SPI is a simp le sy n ch ro n o u s stan d ard , wh ich is still v ery wid ely ap p lied . Th e mb ed h as two
SPI p o rts an d a su p p o rtin g lib rary .
Wh ile a v ery u sefu l stan d ard , SPI h as certain v ery clear limitatio n s, relatin g to a lack o f
flex ib ility an d ro b u stn ess.
T h e I2 C p ro to c o l is a m o re s o p h is tic a te d s e ria l a lte rn a tiv e to S P I; it ru n s o n a tw o -w ire b u s ,
an d in clu d es ad d ressin g an d ack n o wled g men t.
I2 C is a flex ib le an d v ersatile stan d ard . Dev ices can b e read ily ad d ed to o r remo v ed fro m an
ex istin g b u s, mu lti-master co n fig u ratio n s are p o ssib le, an d a master can d etect if a slav e
fails to resp o n d , an d can tak e ap p ro p riate actio n . Nev erth eless, I2 C h as limitatio n s wh ich
mean th at it can n o t b e u sed fo r h ig h -reliab ility ap p licatio n s.
A v ery wid e ran g e o f p erip h eral d ev ices is av ailab le, in clu d in g in tellig en t sen so rs, wh ich
co mmu n icate th ro u g h SPI an d I2 C.
S t a r t in g w it h S e r ia l C o m m u n ic a t io n
1 4 5
A u s e fu l a s y n c h ro n o u s a lte rn a tiv e to I 2 C a n d S P I is p ro v id e d b y th e UA RT. T h e m b e d h a s
fo u r o f th ese, o n e o f wh ich p ro v id es a co mmu n icatio n lin k b ack to th e h o st co mp u ter.
•
Q u iz
1 .
2 .
3 .
4 .
5 .
6 .
7 .
8 .
9 .
1 0 .
Wh at d o th e ab b rev iatio n s SPI, I2 C an d UART stan d fo r?
Draw u p a tab le co mp arin g th e ad v an tag es an d d isad v an tag es o f u sin g SPI v ersu s I2 C fo r
serial co mmu n icatio n s.
Wh at are th e limitatio n s fo r th e n u mb er o f d ev ices th at can b e co n n ected to a sin g le SPI,
I2C o r UART b u s?
A SPI lin k is ru n n in g with a 5 0 0 k Hz clo ck . Ho w lo n g d o es it tak e fo r a sin g le messag e
co n tain in g o n e d ata b y te to b e tran smitted ?
An mb ed co n fig u red as SPI master is to b e co n n ected to th ree o th er mb ed s, each
co n fig u red as slav e. Sk etch a circu it sh o win g h o w th is in terco n n ectio n co u ld b e mad e.
Ex p lain y o u r sk etch .
An mb ed is to b e set u p as SPI master, u sin g p in s 1 1 , 1 2 an d 1 3 , ru n n in g at a freq u en cy o f
4 MHz, with 1 2 -b it wo rd len g th . Th e clo ck sh o u ld id le at Lo g ic 1 , an d d ata sh o u ld b e
la tc h e d o n its n e g a tiv e e d g e . W rite th e n e c e s s a ry c o d e to s e t th is u p .
Rep eat Qu estio n 4 , b u t fo r I2 C, en su rin g th at y o u calcu late time fo r th e co mp lete
messag e.
Rep eat Qu estio n 5 , b u t fo r I2 C. Id en tify carefu lly th e ad v an tag es an d d isad v an tag es o f
each co n n ectio n .
Yo u n e e d to s e t u p a s e r ia l n e tw o r k , w h ic h w ill h a v e o n e m a s te r a n d f o u r s la v e s . E ith e r
SPI o r I2 C can b e u sed . Ev ery seco n d , d ata h as to b e d istrib u ted , su ch th at o n e b y te is sen t
to Slav e 1 , fo u r to Slav e 2 , th ree to Slav e 3 an d fo u r to Slav e 4 . If th e co mp lete d ata
tran sfer mu st tak e n o t mo re th an 2 0 0 m s, estimate th e min imu m clo ck freq u en cy th at is
allo wab le fo r SPI an d I2 C. Assu me th ere are n o o th er timin g o v erh ead s.
Rep eat Qu estio n 4 , b u t fo r asy n ch ro n o u s co mmu n icatio n th ro u g h a UART, assu min g
a b au d rate o f 5 0 0 k Hz. En su re th at y o u calcu late time fo r th e co mp lete messag e.
References
7 .1 .
7 .2 .
7 .3 .
7 .4 .
7 .5 .
S p a s o v , P. ( 1 9 9 6 ) . M ic r o c o n tr o lle r Te c h n o lo g y , T h e 6 8 H C 1 1 . 2 n d e d itio n . P r e n tic e H a ll.
A D X L 3 4 5 D a ta s h e e t, R e v . C . w w w.a n a lo g .c o m
NXP Semico n d u cto rs. Th e I2 C Bu s Sp ecificatio n an d User Man u al. Rev . 0 3 . 2 0 0 7 . Do cu men t n u mb er
UM1 0 2 0 4 .
Tex as In stru men ts. TMP1 0 2 . Lo w Po wer Dig ital Temp eratu re Sen so r. Au g u st 2 0 0 7 . Rev . Octo b er 2 0 0 8 .
Do cu men t n u mb er SBOS3 9 7 B.
S R F 0 8 d a ta . h ttp ://w w w.r o b o t- e le c tr o n ic s .c o .u k /h tm /s r f 0 8 te c h .s h tm l. A c c e s s e d 3 0 J u ly 2 0 1 1 .
CHAPTER 8
Liquid Crystal Displays
Chapter Outline
8.1 Display Technologies 147
8.1.1 Introducing Liquid Crystal Technology 148
8.1.2 Liquid Crystal Character Displays 149
8.2 Using the PC1602F LCD 150
8.2.1 Introducing the PC1602F Display 151
8.2.2 Connecting the PC1602F to the mbed 152
8.2.3 Using Modular Coding to Interface the LCD 153
8.2.4 Initializing the Display 154
Function Mode 154
DisplayMode 154
Clear Display 155
8.2.5 Sending Display Data to the LCD 155
8.2.6 The Complete LCD.cpp Definition 156
8.2.7 Utilizing the LCD Functions 157
8.2.8 Adding Data to a Specified Location 158
8.3 Using the mbed TextLCD Library 160
8.4 Displaying Analog Input Data on the LCD 162
8.5 More Advanced LCDs 163
8.5.1 Color LCDs 163
8.5.2 Controlling a SPI LCD Mobile Phone Display 163
8.6 Mini-Project: Digital Spirit Level 166
Chapter Review 167
Quiz 167
References 167
8.1 Display Technologies
Light-emitting diodes (LEDs), particularly individual LEDs and seven-segment displays,
havealready been encountered in previous chapters. On their own, LEDs can only informus
of a few different states, for example to indicate whether something is on or off, or whether
a variable is cleared or set. It is possible to be innovative in the way LEDs are used, for
example, differentflashingspeedscanbeusedto representdifferentstates, butclearly thereis
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00008-8
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
147
148 Chapter 8
a limit to how much information a single LED can communicate. With seven-segment
displays it is possibleto display numbers and afew characters fromthe alphabet, but there is
a problem with seven-segment displays in that in simple use they require a microcontroller
output for each LED segment. The multiplexing technique described in Section 3.6.3 can be
used, but a limitation is still seen. Equally, LEDs are relatively power hungry, which is an
issueforpower-conscious designs. Embeddedsystems designersneedto becleverintheway
they usetheavailableinputandoutputcapabilitiesof amicrocontroller, butobviously theuse
of LEDs as a display has its limitations. If text-based messages with a number of characters
are required to be communicated with a user, then a more advanced display type is needed,
such as the liquid crystal displays (LCDs) that are commonly used in consumer electronics.
8.1.1 Introducing Liquid Crystal Technology
Liquid crystal displays are of great importance these days, both in the electronic world in
general, andintheembeddedsystemenvironment. Theirmainadvantagesaretheirextremely
lowpowerrequirements, light weight andhighflexibility of application. They havebeenone
of the enabling technologies for battery-powered products such as the digital watch, the
laptopcomputerandthemobiletelephone, andareavailableinahugerangeof indicatorsand
displays. However, LCDs do have some disadvantages; these include limited viewing angle
andcontrastinsomeimplementations, sensitivity totemperatureextremes, andvery highcost
for the more sophisticated graphical displays.
Although LCDs do not emit light, they can reflect incident light, or transmit or block
backlight. Theprincipleof anLCD isillustratedinFigure8.1. Theliquidcrystal isanorganic
compound which responds to an applied electric field by changing the alignment of its
molecules, and hence the light polarization that it introduces. A small quantity of liquid
crystal is contained between two parallel glass plates. A suitable field can be applied if
transparent electrodes are located on the glass surface. In conjunction with the external
polarizing light filters, light is either blocked or transmitted by the display cell.
The electrodes can be made in any pattern desired. These may include single digits or
symbols, orthestandardpatternsof bargraph, seven-segment, dotmatrix, starburst andso on.
Alternatively, they may be extended to complex graphical displays, with addressable pixels.
Transparent Conductive Coating
Polarizing Filter
Electrical
Contacts
Glass
Liquid Crystal
Figure 8.1:
A simple liquid crystal structure
Liquid Crystal Displays 149
8.1.2 Liquid Crystal Character Displays
A popular, indeed ubiquitous, form of LCD is the character display, as seen in Figure 8.2.
These are widely available from one line of characters to four or more, and are commonly
seen on many domestic and office items, such as photocopiers, burglar alarms and DVD
players. Driving this complex array of tiny LCD dots is far from simple, so such displays
always contain a hidden microcontroller, customized to drive the display. The first such
controller to gain widespread acceptance was the Hitachi HD44780. While this has been
supersededby others, they havekepttheinterfaceandinternal structureof theHitachi device.
It is important to know its main features, in order to design with it.
The HD44780 contains an 80-byte random access memory (RAM) to hold the display data,
and aread only memory (ROM) forgenerating the characters. It has asimple instruction set,
including instructions for initialization, cursor control (moving, blanking, blinking) and
clearing the display. Communication with the controller is made via an 8-bit data bus, three
control lines and an enable/strobe line (E). These are itemized in Figure 8.3a.
Data written to the controller is interpreted either as instruction or as display data
(Figure 8.3b), depending on the state of the RS (Register Select) line. An important use of
reading data back fromthe LCD is to check the controller status via the Busy flag. As some
instructions take a finite time to implement (e.g. a minimum of 40 ms is required to receive
one character code), it is sometimes useful to be able to read the Busy flag and wait until the
LCD controller is ready to receive further data.
Thecontrollercanbeset upto operatein8-bit or4-bit mode. Inthelattermodeonly thefour
most significant bits of the bus are used, and two write cycles are required to send a single
byte. In both cases the most significant bit doubles as the Busy flag when a Read is
undertaken.
Figure 8.2:
The mbed driving an LCD
150 Chapter 8
(a)
(b)
(c)
RS
R/W
0
0
Write instruction code
0
1
Read busy flag and address counter
1
0
Writedata
1
1
Read data
higher order
bits transmitted
E
Action
lower order
bits transmitted
RS
R/W
E
Instruction Write
Busy Flag Check
Busy Flag Check
Data Write
Figure 8.3:
(a) HD44780 user interfacelines; (b) HD44780 data and instruction transfers; (c) HD44780 timing
for 4-bit interface
8.2 Using the PC1602F LCD
Thembedprocessorcanbeinterfacedto anexternal LCD, inordertodisplay messagesonthe
screen. Interfacing an LCD requires a few involved steps to prepare the device and achieve
thedesireddisplay. Thefollowingtasks must beconsideredinorderto successfully interface
the LCD:
Liquid Crystal Displays 151
• Hardware integration: the LCD will need to be connected to the correct mbed pins.
• Modular coding: as there are many processes that need to be completed, it makes sense
to define LCD functions in modular files.
• Initializing the LCD: a specific sequence of control signals must be sent to the LCD
to initialize it.
• Outputtingdata: wewill needtounderstandhowtheLCD convertscontrol dataintolegible
display data.
Here, wewill usethe2 16characterPowertipPC1602FLCD, althoughanumberof similar
LCDs can be found with the same hardware configuration and functionality.
8.2.1 Introducing the PC1602F Display
ThePC1602Fdisplay isa2 16characterdisplay withanonboarddatacontrollerchipandan
integratedbacklight, asseeninFigure8.2. TheLCD has 16connections, definedinTable8.1.
Inthisexamplewewill usetheLCD in4-bitmode. Thismeansthatonly theupper4bitsof the
data bus (DB4eDB7) are connected. The two halves of any byte are sent in turn on these
Table 8.1: PC1602F pin descriptions
Pin number
Pin name
Function
1
VSS
Power supply (GND)
2
VDD
Power supply (5 V)
3
V0
Contrast adjust
4
RS
Register select signal
5
R=W
Data read / write
6
E
Enable signal
7
DB0
Data bus line bit 0
8
DB1
Data bus line bit 1
9
DB2
Data bus line bit 2
10
DB3
Data bus line bit 3
11
DB4
Data bus line bit 4
12
DB5
Data bus line bit 5
13
DB6
Data bus line bit 6
14
DB7
Data bus line bit 7
15
A
Power supply for LED backlight (5 V)
16
K
Power supplyfor LED backlight (GND)
152 Chapter 8
lines. As a result, the LCD can be controlled with only seven lines, rather than the 11 lines
requiredfor8-bitmode. Every timeanibble(a4-bitwordissometimescalledanibble) issent,
the E line must be pulsed, as seen in Figure 8.3. The display is initialized by sending
control instructions to the configuration registers in the LCD. This is done by setting RS and
R=W low; once the LCD has been initialized, display data can be sent by setting the RS bit
high. As before, the E bit must be pulsed for every nibble of display data sent.
8.2.2 Connecting the PC1602F to the mbed
An mbed pin should be attached to each of the LCD data pins that are used, configured as
digital output. Fouroutputsarerequiredto sendthe4-bitinstructionanddisplay dataandtwo
outputs are required to manipulate the RS and E control lines.
Thesuggestedinterfaceconfigurationforconnectingthembedto thePC1602Fisasshownin
Table8.2. Notethat, insimpleapplications, theLCD canbeusedonly inwritemode, so R=W
canbetiedpermanently to ground(mbedpin1). If rapidcontrol of theLCD isrequired, i.e. if
it needs updating faster than approximately every 1 ms, then the R=W input can be activated
to enable reading from the LCD’s Busy flag. The Busy flag changes state when a display
request has been completed, so by testing it the LCD can be programmed to operate as
quickly aspossible. If theR=W lineistiedtoground, thena1 msdelay betweendatatransfers
is adequate to ensure that all internal processes can complete before the next transfer.
Table 8.2: Wiring table for connecting the PC1602F to the mbed
mbed pin number
LCD pin number
LCD pin name
Power connection
1
1
VSS
0V
39
2
VDD
5V
1
3
V0
0V
19
4
RS
1
5
R=W
20
6
E
21
11
DB4
22
12
DB5
23
13
DB6
24
14
DB7
39
15
A
5V
1
16
K
0V
A: anode; K: cathode of the backlight LED.
0V
Liquid Crystal Displays 153
Figure 8.4:
PC1602F physical pin layout
TheLCD pins DB0, DB1, DB2 andDB3areleft unconnectedas thesearenot requiredwhen
operating in 4-bit data mode. The PC1602F has the pin layout shown in Figure 8.4.
8.2.3 Using Modular Coding to Interface the LCD
Initializing and interfacing a peripheral device, such as an LCD, can be done effectively by
usingmodularfilestodefinevariouspartsof thecode. Wewill thereforeusethreefilesforthis
application. The files required are:
• a main code file (main.cpp), which can call functions defined in the LCD feature file
• an LCD definition file (LCD.cpp), which will include all the functions for initializing
and sending data to the LCD
• an LCD header file (LCD.h), which will be used to declare data and function prototypes.
We will declare the following functions in the LCD header file:
• toggle_enable( ): a function to toggle/pulse the enable bit
• LCD_init( ): a function to initialize the LCD
• display_to_LCD( ): a function to display characters on the LCD.
The LCD.h header file should therefore define the function prototypes as shown in Program
Example 8.1.
/* Program Example 8.1: LCD.h header file
*/
#ifndef LCD_H
#define LCD_H
#include "mbed.h"
void toggle_enable(void);
//function to toggle/pulse the enable bit
void LCD_init(void);
//function to initialize the LCD
void display_to_LCD(char value); //function to display characters
#endif
Program Example 8.1 LCD header file
IntheLCD definitionfile(LCD.cpp), weneedto definethembedobjects requiredto control
the LCD. Here, one digital output each can be used for RS and E, and an mbed BusOut
154 Chapter 8
object for the 4-bit data. The suggested mbed object definitions need to conform to the pin
connections listed in Table 8.2.
To control and initialize the LCD, we send a number of control bytes, each sent as two
4-bit nibbles. Aseachnibbleis asserted, theLCD requirestheE bit to bepulsed. Thisisdone
by the toggle_enable( ) function, detailed in Program Example 8.2.
8.2.4 Initializing the Display
A specific initialization procedure must be programmed in order for the PC1602F display to
operate correctly. Full details are provided in the Powertip PC1602F datasheet (Reference
8.1) and also in Reference 8.2.
Readingthisdata, weseethatwefirstneedtowaitashortperiod(approximately 20 ms), then
set theRSandElinesto zero, andthensendanumberof configurationmessagesto set upthe
LCD. WethenneedtosendconfigurationdatatotheFunctionMode, Display ModeandClear
Display registers in order to initialize the display. These are now introduced.
Function Mode
To set the LCD function mode, the RS, R=W and DB0-7 bits should be set as shown in
Figure 8.5. Data bus values are sent as two nibbles.
If,forexample,wesendabinaryvalueof 00101000(0x28hex)totheLCDdatapins,thisdefines
4-bitmode, 2linedisplay and5x7dotcharacters. Inthegivenexamplewewouldthereforesend
thevalue0x2, pulseE, thensend0x8, thenpulseE again. TheC/Cþ þ codeFunctionMode
registercommands areshownintheLCD_init( ) functiondetailedin ProgramExample8.2.
Display Mode
TheDisplay Modecontrol registermust also beset upduringinitialization. Here, weneedto
send a command to switch the display on, and to determine the cursor function. The Display
Mode register is defined as shown in Figure 8.6.
Figure 8.5:
Function Mode control register
Liquid Crystal Displays 155
Figure 8.6:
Display Mode control register
Inorderto switchthedisplay onwithablinking cursor, thevalue0x0F(intwo 4-bit nibbles)
isrequired. TheC/Cþ þ codeDisplay ModeregistercommandsareshownintheLCD_init( )
function detailed in Program Example 8.2.
Clear Display
Before data can be written to the display, the display must be cleared, and the cursor reset
to the first character in the first row (or any other location that you wish to write data to).
The Clear Display command is shown in Figure 8.7.
Figure 8.7:
Clear Display command
8.2.5 Sending Display Data to the LCD
A function called (in this example) display_to_LCD( ) displays characters on the LCD
screen. Characters aredisplayedby settingtheRSflagto 1(datasetting), thensendingadata
byte describing the ASCII character to be displayed.
ThetermASCII (AmericanStandardCodeforInformationInterchange) has beenintroduced
previously in Chapter 6; it is a method for defining alphanumeric characters as 8-bit values.
Whencommunicatingwithdisplays, by sendingasingleASCII byte, thedisplay is informed
which particular character should be shown. The complete ASCII table is included with the
LCD datasheet, but for interest some common ASCII values for display on the LCD are
shown in Table 8.3.
The display_to_LCD( ) function needs to accept an 8-bit value as adata input, so that it can
display thedesiredcharacterontheLCD screen. InC/Cþ þ thechar datatypecanbeusedto
define an 8-bit number. It can be seen in the LCD.h header file (Program Example 8.1) that
156 Chapter 8
Table 8.3: Common ASCII values
Less significant bits (lower nibble)
0x0 0x1 0x2 0x3 0x4 0x5 0x6 0x7 0x8 0x9 0xA 0xB 0xC 0xD 0xE 0xF
More significant bits 0x0
(upper nibble)
0x1
0x2
!
"
#
$
%
&
’
(
)
*
þ
,
-
.
/
0x3 0
1
2
3
4
5
6
7
8
9
:
;
<
¼
>
?
0x4 @
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
0x5 P
Q
R
S
T
U
V
W
X
Y
Z
[
\
]
^
_
0x6
‘
a
b
C
d
e
f
g
h
i
j
k
l
m
n
o
0x7 p
q
r
S
t
u
v
w
x
y
z
{
j
}
~
display_to_LCD( ) has already been defined as a function with a char input. Referring to
Table 8.3, it can be seen, for example, that if we send the data value 0x48 to the display, the
character ‘H’ will be displayed.
The display_to_LCD( ) function is shown in Program Example 8.2. Note that, as we are
using4-bit mode, themost significant bits of theASCII bytemust beshiftedright inorderto
be output on the 4-bit bus created through BusOut. The lower 4 bits can then be output
directly.
8.2.6 The Complete LCD.cpp Definition
The LCD definition file (LCD.cpp) contains the three C functions e toggle_enable( ),
LCD_init( ) anddisplay_to_LCD( ) e asdescribedabove. Thecompletelistingof LCD.cpp
is shown in Program Example 8.2. Note that the toggle_enable( ) function has two 1 ms
delays, whichremove the need to monitor theBusy flag; the downsideto this is that wehave
introduced a timing delay into our program, the consequences of which will be discussed in
more detail in Chapter 9.
/* Program Example 8.2: Declaration of objects and functions in LCD.cpp file
*/
#include "LCD.h"
DigitalOut RS(p19);
DigitalOut E(p20);
BusOut data(p21, p22, p23, p24);
void toggle_enable(void){
E=1;
wait(0.001);
E=0;
Liquid Crystal Displays 157
wait(0.001);
}
//initialize LCD function
void LCD_init(void){
wait(0.02);
// pause for 20 ms
RS=0;
// set low to write control data
E=0;
// set low
//function mode
data=0x2;
toggle_enable();
data=0x8;
toggle_enable();
//display mode
data=0x0;
toggle_enable();
data=0xF;
toggle_enable();
//clear display
data=0x0;
toggle_enable();
data=0x1;
toggle_enable();
// 4 bit mode (data packet 1, DB4-DB7)
// 2-line, 7 dot char (data packet 2, DB0-DB3)
// 4 bit mode (data packet 1, DB4-DB7)
// display on, cursor on, blink on
//
// clear
}
//display function
void display_to_LCD(char value){
RS=1;
// set high to write character data
data=value>>4;
// value shifted right 4 = upper nibble
toggle_enable();
data=value;
// value bitmask with 0x0F = lower nibble
toggle_enable();
}
Program Example 8.2 Declaration of objects and functions in LCD.cpp
8.2.7 Utilizing the LCD Functions
We can now develop a main control file (main.cpp) to utilize the LCD functions described
above. Program Example 8.3 initializes the LCD, displays the word ‘HELLO’ and then
displays the numerical characters from 0 to 9.
/* Program Example 8.3 Utilizing LCD functions in the main.cpp file
*/
#include "LCD.h"
int main() {
LCD_init();
// call the initialize function
display_to_LCD(0x48);
// ‘ H’
display_to_LCD(0x45);
// ‘ E’
display_to_LCD(0x4C);
// ‘ L’
display_to_LCD(0x4C);
// ‘ L’
display_to_LCD(0x4F);
// ‘ O’
158 Chapter 8
for(char x=0x30;x<=0x39;x++){
display_to_LCD(x);
// display numbers 0-9
}
}
Program Example 8.3 File main.cpp utilizing LCD functions
n
Exercise 8.1
Applying Table8.2, connect a Powertip PC1602F LCD to an mbed and construct a new
program with the files main.cpp, LCD.cpp and LCD.h as described in the program
examples above. Compile and run the program.
1. Verify that the word ‘HELLO’ and the numerical characters 0e9 are correctly
displayed with a flashing cursor.
2. Change the program so that your name appears on the display, after the word
‘HELLO’, instead of the numerical characters 0e9.
n
n
Exercise 8.2
If you look at the toggle_enable( ) function in Program Example 8.2, you will notice
that it sets theE linehigh and low, waiting 1 ms in each case. This saves us testing the
Busy flag, as seen in Figure 8.3c, as the display will have exited from its Busy state
during the 1 ms delay. For many embedded applications, however, this loss of 2 ms
would be quite unacceptable.
Tryapplyingtheinformation so far supplied to writearevised program, so that thedelays
areremoved, and theBusyflagistested. You will nowneed to activateR=W and set it to 1
in order to read theBusyflag, which is indicated bydata bit DB7. OnceBusyis clear, the
program should proceed. Estimate how much time is saved by your new program. You
can test this with an oscilloscope by making your program write a digit continuously to
the display, and measuring on the oscilloscope the time between the E pulses.
n
8.2.8 Adding Data to a Specified Location
The display is mapped out with memory address locations so that each display unit has
auniqueaddress. Thedisplay pointercanthereforebeset beforeoutputtingdata, andthedata
will then appear in the position specified. The display address layout is shown in Figure 8.8.
If theprogramcounterisset to address0x40, datawill bedisplayedatthefirstpositiononthe
second line. To change the pointer address, the desired 6-bit address value must be sent in
a control byte with bit 7 also set, as shown in Figure 8.9.
Liquid Crystal Displays 159
Figure 8.8:
Screen display pointer address values
A new function can be created to set the location of the display pointer prior to writing. We
will call this function set_location( ), as shown in Program Example 8.4.
/* Program Example 8.4 function to set the display location. Parameter “ location”
address of display unit to be selected
*/
void set_location(char location){
RS=0;
data=(locationj0x80)>>4;
// upper nibble
toggle_enable(); data=location&0x0F;
// lower nibble
toggle_enable();
}
holds
Program Example 8.4 Function to change the display pointer position
Notice that bit DB7 is set by ORing the location value with 0x80.
n
Exercise 8.3
Addtheset_location( ) functionshowninProgramExample8.4totheLCD.cppdefinition.
You will also need to declarethefunction prototypein LCD.h. Now add set_location( )
function callsto main.cpp so that theword ‘HELLO’ appearsin thecenter of thefirst line
and thenumerical characters 0e9 appear in thecenter of thesecond lineof thedisplay.
n
n
Exercise 8.4
Modify the LCD_init( ) function to disable the flashing cursor. Here, you will need to
modify the value sent to the Display Mode register.
n
Figure 8.9:
Display pointer control
160 Chapter 8
8.3 Using the mbed TextLCD Library
Thereis anmbedlibrary whichmakes theuseof analphanumeric LCD muchsimplerand
quickertoprogram.ThembedTextLCDlibraryisalsomoreadvancedthanthesimplefunctions
wehavecreated;inparticular,theTextLCDlibraryperformsthelaboriousLCDsetuproutinefor
us. TheTextLCD definitionalso tellstheLCD objectwhichpinsareusedforwhichfunctions.
The object definition is defined in the following manner:
TextLCD lcd(int rs, int e, int d0, int d1, int d2, int d3);
We need to ensure that our pins are defined in the same order. For our particular hardware
setup (described in Table 8.2) this will be:
TextLCD lcd(p19, p20, p21, p22, p23, p24);
Simple printf( ) statements are used to display characters on the LCD screen; this
statement is also discussed in Section B.9 of Appendix B. The printf( ) function
allows formatted print statements to be used. This means that text strings and
formatted data can be sent to a display, meaning that a function call is not required for each
individual character (as was the case in Program Example 8.3).
C code
feature
Whenusingtheprintf( ) functionwiththembedTextLCD library, thedisplay object’s name
isalso required, so if theTextLCD objectisdefinedasintheexamplesabove(withtheobject
name lcd), then a ‘Hello World’ string can be written as follows:
lcd.printf("Hello World!");
Whenusingpredefinedmbedlibraries, suchasTextLCD, thelibrary fileneedstobeimported
to the mbed program. The mbed TextLCD library can (at the time of writing) be accessed
from the following location:
http://mbed.org/users/simon/libraries/TextLCD/livod0
Thelibrary headerfilemustalso beincludedwiththe#includestatementinthemain.cpp file
orrelevant project headerfiles. ProgramExample8.5 is asimpleHello Worldexampleusing
the TextLCD library.
/*Program Example 8.5: TextLCD library example
*/
#include "mbed.h"
#include "TextLCD.h"
TextLCD lcd(p19, p20, p21, p22, p23, p24); //rs,e,d0,d1,d2,d3
int main() {
lcd.printf("Hello World!");
}
Program Example 8.5 TextLCD Hello World
The cursor can be moved to a chosen position to allow you to choose where to display data.
This uses the locate( ) function as follows. The display is laid out as two rows (0e1) of 16
Liquid Crystal Displays 161
columns (0e15). The locate function defines the column first followed by the row. For
example, add the following statement to move the cursor to the second line and fourth
column:
lcd.locate(3,1);
Any printf( ) statements after this will be printed at the new cursor location.
n
Exercise 8.5
1. Create a new program and import the TextLCD library file (right click on the
project and select ‘import library’).
2. Add Program Example 8.5 to the main.cpp file and compile and run. Verify that
the program correctly displays the Hello World characters.
3. Experiment further with thelocatefunction and verifythat theHello World stringcan
be positioned to a desired location on the display.
n
The screen can also be cleared with the following command:
lcd.cls();
Program Example 8.6 displays a count variable on the LCD. The count variable increments
every second.
/* Program Example 8.6: LCD Counter example
*/
#include "mbed.h"
#include "TextLCD.h"
TextLCD lcd(p19, p20, p21, p22, p23, p24); // rs, e, d0, d1, d2, d3
int x=0;
int main() {
lcd.printf("LCD Counter");
while (1) {
lcd.locate(5,1);
lcd.printf("%i",x);
wait(1);
x++;
}
}
Program Example 8.6 LCD counter
n
Exercise 8.6
1. Implement Program Example 8.6 as a new program. Do not forget to import the
TextLCD library!
2. Increasethespeed of thecounter and investigatehowthecursor position changes as
the count value increases.
n
162 Chapter 8
8.4 Displaying Analog Input Data on the LCD
An analog input can be defined e before the main( ) function e on pin 18 as follows:
AnalogIn Ain(p18);
We can now connect a potentiometer between 3.3 V (mbed pin 40) and 0 V (mbed pin
1) with the wiper connected to pin 18. The analog input variable has a floating point
value between 0 and 1, where 0 is 0 V and 1 represents 3.3 V. We will multiply the
analog input value by 100 to display a percentage between 0 and 100%, as shown in
Program Example 8.7. An infinite loop can be used so that the screen updates
automatically. To do this it is necessary to clear the screen and add a delay to set the
update frequency.
/*Program Example 8.7: Display analog input data
*/
#include "mbed.h"
#include "TextLCD.h"
TextLCD lcd(p19, p20, p21, p22, p23, p24); //rs,e,d0, d1,d2,d3
AnalogIn Ain(p17);
float percentage;
int main() {
while(1){
percentage=Ain*100;
lcd.printf("%1.2f",percentage);
wait(0.002);
lcd.cls();
}
}
Program Example 8.7 Display analog input data
n
Exercise 8.7
1. Implement ProgramExample8.7 and verifythat thepotentiometer modifiesreadings
between 0 and 100%.
2. Modify the wait( ) statement to be a larger value and evaluate the change in
performance. It is interesting to make a mental note that a certain range of update
ratesappearsirritatingto view(too fast), whileothersmaybeperceived astoo slow.
n
n
Exercise 8.8
Create a program to make the mbed and display act like a standard voltmeter, as
shown in Figure8.10. Potential differenceshould bemeasured between 0 and 3.3 V and
displayed to the screen. Note the following:
Liquid Crystal Displays 163
Figure 8.10:
Voltmeter display
• You will need to convert the 0.0e1.0 analog input value to a value that represents
0 e3.3 V.
• An infinite loop is required to allow the voltage value to update continuously as
the potentiometer position changes.
• Check the display with the reading from an actual voltmeter: is it accurate?
• Increase the number of decimal places that the voltmeter reads. Evaluate the noise
and accuracyof thevoltmeter readings with respect to thembed’s ADC resolution.
n
8.5 More Advanced LCDs
8.5.1 Color LCDs
Nowadays, LCD technology is used in advanced displays for mobile phones, PC monitors
and televisions. For color displays, each pixel is made up of three subpixels for red, green
and blue. Each subpixel can be set to 256 different shades of its color, so it is therefore
possible for a single LCD pixel to display 256 * 256 * 256 ¼16.8 million different colors.
The pixel color is therefore usually referred to by a 24-bit value, where the highest 8 bits
definetheredshade, themiddle8bitsthegreenshadeandthelower8bitstheblueshade, as
shown in Table 8.4.
Given that each pixel needs to be assigned a 24-bit value and a 1280 1024 LCD computer
display has over1 millionpixels (1280 * 1024 ¼1 310 720), alot of dataneeds to besent to
a color LCD. Standard color LCDs are set to refresh the display at a frequency of 60 Hz,
so the digital input requirements are much greater than those associated with the
alphanumeric LCD discussed previously in this chapter.
8.5.2 Controlling a SPI LCD Mobile Phone Display
A mobile phone screen, like the Nokia 6610, represents state-of-the-art, low-cost, highvolume display technology. Yet many have an interface based on the good old serial
peripheral interface(SPI) standard, introducedinChapter7. Itishardtofindformal published
164 Chapter 8
Table 8.4: 24-bit color values
Color
24-bit value
Red
0xFF0000
Green
0x00FF00
Blue
0x0000FF
Yellow
0xFFFF00
Orange
0xFF8000
Purple
0x800080
Black
0x000000
White
0xFFFFFF
data on the Nokia display; Reference 8.3, however, gives some useful background. We can
use the Nokia 6610 display with the mbed. Here, we will see how to connect it and how to
type data on the screen fromthe PC keyboard. Instead of using the basic SPI library, wewill
use the predesigned mbed MobileLCD library. You will need to import this library from:
http://mbed.co.uk/projects/cookbook/svn/MobileLCD/tests/MobileLCD
Themobilescreenhas10pins, asshowninFigure8.11. Itcanbeconnectedtothembedusing
the connections shown in Table 8.5.
It is possible to clear the screen, set the background color, fill blocks, draw pixel images,
movethepointerandperformmany otheroperations. ProgramExample8.8writescharacters
to the display from the computer keyboard, and uses the #key to clear the screen.
/*Program Example 8.8: Program which reads character from computer screen, and displays
on Nokia LCD display.
*/
#include "mbed.h"
#include "MobileLCD.h"
// include the Nokia display library
MobileLCD lcd(p11, p12, p13, p15, p16); // mosi,miso,clk,cs,rst
Serial pc(USBTX, USBRX);
char c;
void screen_setup(void);
// host terminal comms setup
// char variable for keyboard input
// function prototype
int main() {
pc.printf("\n\rType something to be displayed:\n\r");
screen_setup();
// call the screen setup function
while(1){
c = pc.getc();
// c = character input from computer keyboard
wait(0.001);
if (c==’ #’ ){
// perform the following if "#" is pressed
screen_setup();
// call the screen setup function
Liquid Crystal Displays 165
lcd.locate(0,0);
}
else{
lcd.printf("%c",c);
pc.printf("%c",c);
}
}
// move the cursor back to row 0 column 0
// print character on the LCD screen
// print character on the terminal screen
}
//function definition for screen_setup
void screen_setup(void) {
lcd.background(0x0000FF);
lcd.cls();
}
// set the background color
// clear the screen
+3.3V
BL+(5-7V)
RST
GND
CS
DIO / MOSI
CLK
Nokia 6610
Figure 8.11:
Nokia 6610 display connections
Table 8.5: Connections between Nokia display and mbed
Nokia 6610 pin
mbed pin
þ 3.3 V
40
GND
1
BLþ
39
RST
9
CS
8
MOSI
5
CLK
7
166 Chapter 8
Program Example 8.8 Writing characters to the Nokia display
Having connected the circuit, create a new project, and enter and compile the example code.
Onceit is runningonthembed, andwiththeterminal applicationopen, youshouldbeableto
type to the LCD screen.
n
Exercise 8.9
Add the following fill functions to the screen_setup( ) function (after the lcd.cls
command) in order to fill some areas of the LCD:
lcd.fill(2, 51, 128, 10, 0x00FF00);
lcd.fill(50, 1, 10, 128, 0xFF0000);
It is also possibleto draw on thescreen pixel bypixel. Thefollowing loop will createthe
function for a sine wave and print this to the screen. Add this to the setup function.
for(int i=0; i<130; i++) {
lcd.pixel(i, 80 + sin((float)i / 5.0)*10, 0x000000);
}
n
We have seen that LCDs allow greater flexibility than simple LED-based displays, and that
simple alphanumeric and advanced color LCD devices can be utilized with microcontrollers
and embedded systems.
8.6 Mini-Project: Digital Spirit Level
Design, buildandtestadigital spiritlevel basedonthembed. UseanADXL345(Section7.3)
accelerometer to measure the angle of orientation in two planes, a digital push-to-make
switch to allow calibration and zeroing of the orientation, and a color LCD to output the
measured orientation data, in degrees.
To help you proceed, consider the following:
1. Design your display to show a pixel or an image moving around the LCD screen with
respect to the orientation of the accelerometer.
2. Add the digital switch to allow simple calibration and zeroing of the data.
3. Improve your display output to give accurate measurements of the two-plane orientation
angles in degrees from the horizontal (i.e. horizontal ¼0 ).
Liquid Crystal Displays 167
Chapter Review
• Liquid crystal displays (LCDs) use an organic crystal which can polarize and block
light when an electric field is introduced.
• Many types of LCDs areavailableand, wheninterfacedwithamicrocontroller, they allow
digital control of alphanumeric character displays and high-resolution color displays.
• The PC1602F is a 16-column by 2-row character display which can be controlled by
the mbed.
• DatacanbesenttotheLCDregisterstoinitializethedeviceandtodisplaycharactermessages.
• Character data is defined using the 8-bit ASCII table.
• The mbed TextLCD library can be used to simplify working with LCDs and allow
the display of formatted data using the printf( ) function.
• Color LCDs use a serial interface to display data, with each pixel given a 24-bit
color setting.
• The mbed MobileLCD library can be used to interface a Nokia 6610 mobile
phone-type LCD.
Quiz
1. What are the advantages and disadvantages associated with using an alphanumeric LCD
in an embedded system?
2. What types of cursor control are commonly available on alphanumeric LCDs?
3. How does the mbed BusOut object help to simplify interfacing an alphanumeric display?
4. What is the function of the E input on an alphanumeric display such as the PC1602F?
5. What does ASCII stand for?
6. What are the ASCII values associated with the numerical characters from 0 to 9?
7. What is the correct printf( ) C/Cþ þ code required to display the value of a floating
point variable called ‘ratio’ to two decimal places ?
8. List five practical examples of a color LCD used in an embedded system.
9. If a color LCD is filled to a single background color, what colors will the following
24-bit codes give:
(a) 0x00FFFF
(b) 0x00007F
(c) 0x7F7F7F?
References
8.1. Rapid Electronics. PC1602F datasheet. http://www.rapidonline.com/pdf/57-0913.pdf
8.2. HD44780 LCDisplays. http://www.a-netz.de/lcd.en.php
8.3. Nokia 6100 LCD Display Driver. http://www.sparkfun.com/tutorial/Nokia%206100%20LCD%20Display%
20Driver.pdf
CHAPTER 9
Interrupts, Timers and Tasks
Chapter Outline
9.1 Time and Tasks in Embedded Systems 170
9.1.1 Timers and Interrupts 170
9.1.2 Tasks 170
9.1.3 Event-Triggered and Time-Triggered Tasks 171
9.2 Responding to Event-Triggered Events 171
9.2.1 Polling 171
9.2.2 Introducing Interrupts 172
9.3 Simple Interrupts on the mbed 173
9.4 Getting Deeper into Interrupts 175
9.4.1 Interrupts on the LPC1768 178
9.4.2 Testing Interrupt Latency 178
9.4.3 Disabling Interrupts 179
9.4.4 Interrupts from Analog Inputs 180
9.4.5 Conclusion on Interrupts 181
9.5 An Introduction to Timers 181
9.5.1 The Digital Counter 181
9.5.2 Using the Counter as a Timer 182
9.5.3 Timers on the mbed 183
9.6 Using the mbed Timer 183
9.6.1 Using Multiple mbed Timers 184
9.6.2 Testing the Timer Duration 185
9.7 Using the mbed Timeout 187
9.7.1 A Simple Timeout Application 187
9.7.2 Further Use of Timeout 189
9.7.3 Timeout used to Test Reaction Time 190
9.8 Using the mbed Ticker 191
9.8.1 Using Ticker for a Metronome 192
9.8.2 Reflecting on Multi-Tasking in the Metronome Program 194
9.9 The Real Time Clock 195
9.10 Switch Debouncing 196
9.11 Mini-Projects 198
9.11.1 A Self-Contained Metronome 198
9.11.2 Accelerometer Threshold Interrupt 198
Chapter Review 199
Quiz 199
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00009-X
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
169
170 Chapter 9
9.1 Time and Tasks in Embedded Systems
9.1.1 Timers and Interrupts
The very first diagram in this book, Figure 1.1, shows the key features of an embedded
system. Amongtheseistime. Embeddedsystemshavetorespondinatimely mannertoevents
as they happen. Usually, this means they have to be able to do the following:
• measure time durations
• generate time-based events, which may be single or repetitive
• respond with appropriate speed to external events, which may occur at unpredictable
times.
Indoingall of thesethesystemmayfindthatithasaconflictofinterest,withtwoactionsneeding
attentionat thesametime. Forexample, anexternal event may demand attentionjust when
aperiodic event needs to takeplace. Therefore, thesystemmay needto distinguishbetween
events that haveahighlevel of urgency and thosethat do not, andtake actionaccordingly.
It follows that we need a set of tools and techniques to allow effective time-based activity to
occur. Key featuresof thistoolkitareinterruptsandtimers, thesubjectof thischapter. Inbrief,
atimerisjustwhatitsnameimplies, adigital circuitthatallowsustomeasuretime, andhence
make things happen when a certain time has elapsed. An interrupt is a mechanism whereby
a running program can be interrupted, with the central processing unit (CPU) then being
requiredto jumpto someotheractivity. Inourstudy of interrupts andtimers, wemakemajor
stepsforwardinourunderstandingof howprogramscanbestructured, andhowwecanmove
towards more sophisticated programdesign. That is where the third topic in the chapter title
comes in e the concept of program tasks.
9.1.2 Tasks
Inalmostall embeddedprograms,theprogramhastoundertakeanumberof differentactivities.
Enteringbriefly theworldof small-scalefarming, considertherequirementsforatemperature
controllerforamushroom-growingshed, workinginacoldenvironment. Thesefriendly little
fungi growbestundertightly controlledconditionsof temperatureandhumidity. Theirgrowth
goes throughseparate phases, at whichtime different preferred temperatures may apply. The
systemwill need to control, display and log thetemperature, keep track of time, respond to
changesinsettingby theuser, andcontrol theheaterandfan. Eachisafairly distinct activity,
and each will requireablock of code. Inprogramming terminology wecall these distinct
activities tasks. Our ability to partitionany programinto tasks becomes an important skill in
moreadvancedprogramdesign. Onceaprogramhasmorethanonetask, weenterthedomain
of multi-tasking. As tasks increasein number, the challengeof responding to theneeds of all
tasks becomes greater, and many techniques aredeveloped to do this.
Interrupts, Timers and Tasks 171
9.1.3 Event-Triggered and Time-Triggered Tasks
Tasks performed by embedded systems tend to fall into two categories: event triggered and
timetriggered. Tasks that areevent triggeredoccurwhenaparticularexternal event happens,
atatimewhichisusually notpredictable. Tasksthataretimetriggeredhappenperiodically, at
a time determined by the microcontroller. To continue with our example of the mushroom
shed, Table9.1listssomepossibletasksandsuggestswhethereachisevent ortimetriggered.
For those that are time triggered, it becomes necessary to indicate how frequently the tasks
occur; suggestions for this are also shown.
9.2 Responding to Event-Triggered Events
9.2.1 Polling
A simple example of an event-triggered activity is when a user pushes a button. This can
happen at any time, without warning, but when it does the user expects a response. Oneway
of programming for this is to continuously test that external input. This is illustrated in
Figure9.1, whereaprogramisstructuredasacontinuousloop. Withinthis, itteststhestateof
two input buttons, and responds to themif activated. This way of checking external events is
called polling; the program ensures that it periodically checks input states and responds if
thereisaneed. Thisisthesortof approachwehaveusedsofarinthisbook, wheneverneeded.
It works well for simple systems; however, it is not adequate for more complex programs.
Supposetheprogramof Figure9.1isextended, sothatamicrocontrollerhas20inputsignalsto
testineachloop. Onmostloopiterations, theinputdatamay notevenchange, sowearerunning
thepollingfornoapparentbenefit. Worse, theprogrammightspendtimecheckingthevalueof
unimportantinputs, whilenotrecognizingvery quickly whenamajorfaultconditionhasarisen.
There are two main problems with polling:
• The processor cannot perform any other operations during a polling routine.
• All inputsaretreatedasequal; theurgent changehastowaititsturnbeforeit isrecognized
by the computer.
Table 9.1: Example tasks: temperature controller for a mushroom shed
Task
Event or time triggered
Measure temperature
Time (every minute)
Compute and implement heater and fan settings
Time (every minute)
Respond to user control
Event
Record and display temperature
Time (every minute)
Orderly switch to battery backup in case of power loss
Event
172 Chapter 9
Respond to
button 1
Y
Is button 1
pressed?
N
Respond to
button 2
Y
Is button 2
pressed?
N
Do other
things
Figure 9.1:
A simple program using polling
A better solutionis for input changes to announce themselves; time is not wasted finding out
that thereisno change. Thedifficulty liesinknowingwhentheinput valuehaschanged. This
is the purpose of the interrupt system.
9.2.2 Introducing Interrupts
The interrupt represents a radical alternative to the polling approach just described. With an
interrupt, thehardwareisdesignedso that theexternal variablecanstoptheCPUinitstracks,
anddemandattention. Supposeyoulivedinahouse, andwereworriedthatathief mightcome
induringthenight. Youcouldarrangeanalarmclocktowakeyouupevery half hourto check
there was no thief, but you would not get much sleep. In this case you would be polling the
possible thief ‘event’. Alternatively, you could fit a burglar alarm. You would then sleep
peacefully, unless the alarm went off, interrupted your sleep, and you would jump up and
chase the burglar. In very simple terms, this is the basis of the computer interrupt.
Interrupts have become a hugely important part of the structure of any microprocessor or
microcontroller, allowing external events and devices to force a change in CPU activity. In
early processors interrupts were mainly used to respond to really major external events;
designs allowed for just one, or a small number of interrupt sources. The interrupt concept
was, however, found to be so useful that more and more possible interrupt sources were
introduced, sometimes for dealing with rather routine matters.
In responding to interrupts, most microprocessors follow the general pattern of the flow
diagramshown in Figure9.2. The CPU completes the current instructionit is executing. It is
Interrupts, Timers and Tasks 173
Interrupt Detected
main programis running
Complete Current Instruction
Save 'Context' on Stack,
including Program Counter
(PC)
Find Interrupt Vector, and
Reload PC
Interrupt Service Routine
(ISR) execution starts
Continue Program Execution
No
End of ISR ?
Yes
Load Context, including PC,
from Stack
Continue Program Execution
main programresumes
Figure 9.2:
A typical microprocessor interrupt response
about to go off and find a completely different piece of code to execute, so it must save key
informationabout what it has just beendoing; this is called thecontext. Thecontext includes
at least the value of the Program Counter (this tells the CPU where it should come back to
when the interrupt has completed) and generally a set of key registers, for example those
holding current data values. All this is saved on a small block of memory local to the CPU,
called the Stack. The CPU then runs a section of code called an Interrupt Service Routine
(ISR); this has been specifically written to respond to the interrupt that has occurred. The
address of the ISR is found through a memory location called the Interrupt Vector. On
completing the ISR, the CPU returns to the point in the main code immediately after the
interruptoccurred, findingthisby retrievingtheProgramCounterfromtheStack whereitleft
it. Itthencontinuesprogramexecutionasif nothinghappened. Later, inSection9.4, weaddto
this explanation, and set it more in the context of the mbed.
9.3 Simple Interrupts on the mbed
Thembedapplicationprogramminginterface(API)exploitsonlyasmall subsetof theinterrupt
capability of the LPC1768 microcontroller, mainly focusing on theexternal interrupts. Their
174 Chapter 9
useisveryflexible,asanyof pins5e30canbeusedasaninterruptinput,exceptingonlypins19
and 20. The availableAPI functions are shown in Table9.2. Usingthesewecan create an
interrupt input, write thecorresponding ISR andlink the ISR withthe interrupt input.
ProgramExample9.1isavery simpleinterruptprogram, adaptedfromthembedwebsite. Itis
madeupof acontinuousloop, whichswitches onandoff LED4(labeledflash). Theinterrupt
input is specifiedas pin5, andlabeledbutton. A tiny ISR is writtenforit, called ISR1, which
isstructuredexactly asafunction. Theaddressof thisfunctionisattachedtotherisingedgeof
the interrupt input, in the line
button.rise(&ISR1);
When the interrupt is activated, by this rising edge, the ISR executes, and LED1 is toggled.
This can occur at any time in program execution. The program has effectively one timetriggered task, the switching of LED4, and one event-triggered task, the switching of LED1.
/* Program Example 9.1: Simple interrupt example. External input causes interrupt,
while led flashes
*/
#include "mbed.h"
InterruptIn button(p5); //define and name the interrupt input
DigitalOut led(LED1);
DigitalOut flash(LED4);
void ISR1() {
led = !led;
}
int main() {
button.rise(&ISR1);
while(1) {
flash = !flash;
wait(0.25);
}
}
//this is the response to interrupt, i.e. the ISR
// attach the address of the ISR function to the
// interrupt rising edge
// continuous loop, ready to be interrupted
Program Example 9.1 Introductory use of an interrupt
Table 9.2: API interrupts summary
Function
Usage
InterruptIn
Create an InterruptIn connected to the specified pin
rise
Attach a function to call when a rising edge occurs on the input
rise
Attach amember function to call when arisingedgeoccurson theinput
fall
Attach a function to call when a falling edge occurs on the input
fall
Attach a member function to call when a falling edge occurs on the
input
mode
Set the input pin mode
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
mbed Microcontroller
Interrupts, Timers and Tasks 175
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
Figure 9.3:
Circuit build for Program Example 9.1
Compile and run this program, applying the very simple build shown in Figure 9.3. Notice
that when you push the button the interrupt is taken high, and LED1 changes state; LED4
meanwhile continues its flashing, almost unperturbed. The program depends on the internal
pull-down resistor, which is enabled by default during setup. If you experience erratic
behavior with this program, you may be experiencing switch bounce. In this case, fast
forward to Section 9.10 for an introduction to this important topic.
n
Exercise 9.1
Change Program Example 9.1 so that:
1. The interrupt is triggered by a falling edge on the input.
2. Therearetwo ISRs, fromthesamepush-button input. Onetoggles LED1 on a rising
interrupt edge, and the other toggles LED2 on a falling edge.
n
9.4 Getting Deeper into Interrupts
We can take our first generalized understanding of interrupts further, and try to understand
a little morewhat goes on inside the mbed. It should be pointed out straight away that this is
not essential as far as using the mbed API is concerned. In fact, although the LPC1768
176 Chapter 9
microcontroller, inside the mbed, has a very sophisticated interrupt structure, the mbed only
uses a small part of this, and that only in a modest way. Therefore, go straight to the next
section if you do not want to get into any deeper interrupt detail.
Okay, so you’re still there! Let’s extend our ideas of interrupts. Whilewe said earlier that an
interrupt was possibly like a thief coming in the night, imagine now a different scenario.
Supposeyouareateacherof abigclassmadeupof enthusiastic butpoorly behavedkids; you
have set them a task to do but they need your help. Tom calls you over, but while you are
helping him Jane starts clamoring for attention.
Do you:
• tell Jane to be quiet and wait until you’ve finished with Tom?
OR
• tell Tom you’ll come back to him, and go over to sort Jane out?
To makematters worse, yourschool principal has askedyouto let themembers of theschool
band out of class half anhourearly, but youreally want themto finishtheirwork beforethey
go. This influences the above decision. Suppose Jane is in the band, and Tom is not.
Therefore, in this situation you decide you must leave Tom to help Jane. This school
classroomsituation is reflected in almost any embedded system. There could be a number of
interrupt sources, all possibly needing attention. Some will be of great importance, others
much less. Therefore, most processors contain four important mechanisms:
• Interrupts can be prioritized, in other words some are defined as more important than
others. If two occur at the same time, then the higher priority one executes first.
• Interruptscanbemasked, i.e. switchedoff, if they arenotneeded, orarelikely to get inthe
way of moreimportant activity. This maskingcouldbejust forashort period, forexample
while a critical program section completes.
• Interrupts can be nested. This means that a higher priority interrupt can interrupt one of
lower priority, just like the teacher leaving Tom to help Jane. Working with nested
interrupts increases the demands on the programmer and is strictly for advanced players
only. Not all processors permit nested interrupts, and some allow youto switchnesting on
or off.
• The location of the ISR in memory can be selected, to suit the memory map and
programmer’s wishes.
Let us take on just a couple more important interrupt concepts, these ones from the
point of view of the interrupt source. Go to the moment in the above scenario when Jane
suddenly realizes she needs help, and puts her hand up. Some short time later the
teacher comes over. The delay between her putting up her hand and the teacher actually
arriving is called the interrupt latency. Latency may be due to a number of things; in
Interrupts, Timers and Tasks 177
this case the teacher has to notice Jane’s hand in the air, may need to finish with another
pupil, and then has to walk over. Once the teacher arrives, Jane puts her hand down.
While Jane is waiting with her hand in the air, patiently we hope, her interrupt is said to
be pending.
These concepts and capabilities hint at some of the deep magic that can be achieved with
advanced interrupt structures.
We can put all of this into more technical terms, and hence refine our understanding of
interrupt action. This was first illustrated in the flow diagram of Figure 9.2. Further detail
on part of this figure is now shown in Figure 9.4. The interrupt being asserted is like Jane
putting up her hand. In a microprocessor, the interrupt input will be a logic signal;
depending on its input configuration it may be active high or low, or triggered by a rising
or falling edge. This input will cause an internal ‘flag’ to be set. This is normally just
a single bit in a register, which records the fact that an interrupt has occurred. This does
not necessarily mean that the interrupt automatically gets the attention it seeks. If it is not
enabled (i.e. it is masked), then therewill be no response. The flag is left high, however, as
the program may later enable that interrupt or the program may just poll the interrupt flag.
Back to the flow diagram: if another ISR is already running then again the incoming
interrupt may not get a response, at least not immediately. If it is higher priority and nested
interrupts are allowed, then it will be allowed to run. If it is lower priority, it will have to
wait for the other ISR to complete. The subsequent actions in the flow diagram, as already
Interrupt Asserted
Interrupt Flag is set
Interrupt Detected
This
Interrupt is
enabled?
Complete Current Instruction
No No immediate response,
but flag stays set
Yes
Higher
Priority Interrupt is
Running?
Yes
No action till other
ISR completes
No
Complete Current Instruction
Figure 9.4:
A typical microprocessor interrupt response e some greater detail
178 Chapter 9
seen in Figure 9.2, then follow. Note that the figure is potentially misleading, as it implies
these actions happen in turn. To get low latency, they should happen as rapidly as possible;
a good interrupt management system will allow some of the actions to take place in
parallel. For example, the interrupt vector could be accessed while the current instruction
is completing.
9.4.1 Interrupts on the LPC1768
Now let’s get back to microprocessor hardware. Recall that the mbed contains the LPC1768
microprocessor, andthat theLPC1768containstheARMCortex core. It isto theCortex core
that wemust turnfirst, as it provides theinterrupt structureusedby themicrocontroller. Back
in Figure2.3, wesaw the Cortex core, set withintheLPC1768 microprocessor. Management
of all interrupts in the Cortex is undertaken by the formidable-sounding Nested Vectored
Interrupt Controller (NVIC). You could think of this as managing the processes overviewed
inFigures9.2and9.4. TheNVIC isalso abitlikeadigital electronic control box withalotof
unconnectedwireshangingout. WhentheCortex isembeddedinto amicrocontroller, suchas
the LPC1768, the chip designer assigns and configures those features (through the ‘loose
wires’) of the NVIC that are needed for that application. For example, the Cortex allows for
240 possible interrupts, both external and from the peripherals, and 256 possible priority
levels. The LPC1768, however, has ‘only’ 33 interrupt sources, with 32 possible
programmable priority levels.
9.4.2 Testing Interrupt Latency
Now that we have met the concept of interrupt latency, we can test it in the mbed. Program
Example 9.2 adapts Program Example 9.1, but the interrupt is now generated by an
external square wave, instead of an external button push. This makes it easier to see on
an oscilloscope. When an interrupt occurs, the external LED is pulsed high for a fixed
duration.
/* Program Example 9.2: Tests interrupt latency. External input causes interrupt, which
pulses external LED while LED4 flashes continuously.
*/
#include "mbed.h"
InterruptIn squarewave(p5);
DigitalOut led(p6);
DigitalOut flash(LED4);
void pulse() {
led = 1;
wait(0.01);
led = 0;
}
int main() {
//Connect input square wave here
//ISR sets external led high for fixed duration
Interrupts, Timers and Tasks 179
squarewave.rise(&pulse);
while(1) {
flash = !flash;
wait(0.25);
}
// attach the address of the pulse function to
// the rising edge
// interrupt will occur within this endless loop
}
Program Example 9.2 Testing interrupt latency
CreateanewprojectfromProgramExample9.2. Connectanexternal LED betweenpin6and
ground, ensuring correct polarity. Connect to pin5 asignal generatorset to logic-compatible
square wave output (probably labeled ‘TTL compatible’), running initially at around 10 Hz.
Once connected, with the program running, the external LED should flash at this rate, i.e.
around 10 times a second.
n
Exercise 9.2
Connect two inputs of an oscilloscope to the interrupt input, and the LED output,
triggering from the interrupt. Increase the input frequency to around 50Hz. Set the
oscilloscope time base to 5 ms per division. You should be able to see the rising edge
of the interrupt input, and a few microseconds later the LED output rising. The time
delay between the two is an indication of latency. The rise of the LED will be flickering
a little, as the delay will depend on what the CPU is doing at the moment the interrupt
occurs. It is important to note that the latency as measured here depends on both
hardware and software factors.
n
9.4.3 Disabling Interrupts
Interrupts are an essential tool in embedded design. But because they can occur at any time,
they can have unexpected or undesirable side effects. In some situations it is essential to
disable (mask) the interrupt. These include when you are undertaking a time-sensitive
activity, or a complex calculation that must be completed in one go. As any incoming
interrupt will have left its flag set, it can be responded to once interrupts are enabled
again. Of course, a delay in response has been introduced and the latency greatly
compromised.
You can simply disable interrupts, as shown here:
__disable_irq();
//activity which can’t be interrupted
__enable_irq();
//disable interrupts
//enable interrupts
Note that each line starts with two underscores.
180 Chapter 9
n
Exercise 9.3
Using Program Example 9.2, disable the interrupt for the duration of the wait(0.25)
delay. Connect the oscilloscope as in Exercise 9.2 and observe the output again.
Comment on the outcome.
Experiment with different values for this wait function. Try then splitting it into two
waits, runningimmediatelyafter each other, with onehavinginterrupts disabled and the
other enabled.
n
9.4.4 Interrupts fromAnalog Inputs
Asidefromdigital inputs, it isuseful to generateinterruptswhenanalogsignalschange, for
exampleif ananalogtemperaturesensorexceeds acertainthreshold. Oneway to do thisisby
applyingacomparator. A comparatorisjust that, it comparestwo input voltages. If oneinput is
higherthantheotherthentheoutputswitchestoahighstate;ifitislower,theoutputswitcheslow.A
comparatorcaneasily beconfiguredfromanoperational amplifier(opamp), asshownin
Figure9.5. Here, aninput voltage, labeledVin, iscomparedto athresholdvoltagederivedfrom
apotential divider, madefromthetwo resistorsR1 andR2. Theseareconnectedto thesupply
voltage, labeledVsup. Fortheright choiceof opamporcomparator, andwithsuitablesupply
voltages,theoutputisaLogic1whentheinputisabovethethresholdvalue,andLogic0otherwise.
Thethresholdvoltagejustmentioned, labeledVe inthediagram, iscalculatedusingEquation9.1:
V ¼VsupR2=ðR1 þ R2Þ
9.1
As an example, we might want to generate an interrupt from a temperature input, using an
LM35temperaturesensor(Figure5.8), withtheinterrupttriggeredif thetemperatureexceeds
30 C. As we know, this sensor has an output of 10 mV/ C, which would lead to an output
Vin
3
+
7
6
Vsup
R1
V–
2
-
4
R2
Figure 9.5:
A comparator circuit
Vo
(a logic value
connected to an
mbed input)
Interrupts, Timers and Tasks 181
voltageof 300 mV at thetriggerpoint proposed. To set Ve to 300 mV inFigure9.5, weapply
Equation 9.1, with a Vsup value of 3.3 V; we find that R1 ¼0.1R2. Values of R1 ¼10k, and
R2 ¼1k could therefore be chosen.
n
Exercise 9.4
Unlike many op amps, the ICL7611 can be run from very low supply voltages, even the
3.3 V of the mbed. Using an LM35 IC temperature sensor, and an ICL7611 op amp
connected as a comparator, design and build a circuit that causes an interrupt when the
temperature exceeds 30 C. Write a program that lights an LED when the interrupt
occurs. The pin connections shown in Figure 9.5 can be applied. Pin 7 is the positive
supply, and can be connected to the mbed 3.3 V; pin 4 is the negative supply, and is
connected to 0 V. For thisop amp, also connect pin 8to thepositivesupplyrail. (Thispin
controls the output drive capability; check the datasheet for more information.)
n
9.4.5 Conclusion on Interrupts
This section has provided a good introduction to interrupts and the main concepts associated
with them. They become an essential part of the toolkit of any embedded designer. We
havesofarlimitedourselvestosingle-interruptexamples. Wheremultipleinterruptsareused,
the design challenges become considerably greater e interrupts can have a very destructive
effect if not used well. The design of advanced multiple interrupt programs is, however,
beyond the scope of this book.
9.5 An Introduction to Timers
A simpleprogram, forexamplethevery first ProgramExample2.1, uses wait( ) functions to
performtimingoperations, forexampleto introduceadelay of 200 ms. This is easy andvery
convenient, but during this delay loop the microcontroller cannot performany other activity;
thetimespent waitingis just wastedtime. As wetry to writemoredemandingprograms, this
simpletimingtechniquebecomes inadequate. Weneed away of letting timingactivity go on
in the background, while the program continues to do useful things. We turn to the digital
hardware for this.
9.5.1 The Digital Counter
It is an easy task in digital electronics to make electronic counters; you simply connect
togetheraseriesof bistablesorflip-flops. Eachoneholdsonebitof information, andall those
bits together form a digital word. This is illustrated in very simple form in Figure 9.6, with
182 Chapter 9
We can preload
Input
an interrupt at the
moment of overflow
0
0
1
0
1
0
0
1
We can read
Figure 9.6:
A simple 8-bit counter
each little block representing one flip-flop, each holding one bit of the overall number. If the
input of thisarrangementis connectedto aclock signal thenthecounterwill count, inbinary,
thenumberof clockpulsesappliedtoit. Itiseasy toreadtheoverall digital numberheldinthe
counter, and it is not difficult to arrange the necessary logic to preload it with a certain
number, or to clear it to zero.
Thenumberthat acountercancount upto isdeterminedby thenumberof bits inthecounter.
In general, an n-bit counter can count from 0 to (2n 1). For example, an 8-bit counter can
count from 0000 0000 to 1111 1111, or 0 to 255 in decimal. Similarly, a 16-bit counter can
count from 0 to 65 535. If a counter reaches its maximum value, and the input clock pulses
keeponcoming, thenitoverflowsbacktozero, andstartscountingupall overagain. All isnot
lost if this happens e in fact we have to be ready to deal with it. Many microcontroller
counters cause an interrupt as the counter overflows; this interrupt can be used to record the
overflow, and the count can continue in a useful way.
9.5.2 Using the Counter as a Timer
The input signal to a counter can be a series of pulses coming from an external source, for
example counting people going through a door. Alternatively, it can be a fixed-frequency
logic signal, suchas theclock sourcewithinamicrocontroller. Very importantly, if that clock
source is a known and stable frequency, then the counter becomes a timer. As an example, if
the clock frequency is 1.000 MHz (hence with period of 1 ms), then the count will update
every microsecond. If thecounterisclearedto zero, andthenstartscounting, thevalueheldin
the counter will give the elapsed time since the counting started, with a resolution of 1 ms.
This can be used to measure time, or trigger an event when a certain time has elapsed. It can
also beusedto control time-basedactivity, forexampleserial datacommunication, orapulse
width modulation (PWM) stream.
Interrupts, Timers and Tasks 183
Alternatively, if the counter is just free-running with a continuous clock signal, then the
‘interrupt on overflow’ occurs repeatedly. This becomes very useful where a periodic
interrupt is needed. For example, if an 8-bit counter is clocked with a clock frequency of
1 MHz, it will reach its maximumvalue and overflow back to zero in 256ms (it is the 256th
pulsethat causes theoverflowfrom255 to 0). If it is left runningcontinuously, thenthis train
of interrupt pulses can be used to synchronize timed activity, for example it could define the
baud rate of a serial communication link.
Timers based on these principles are an incredibly important feature of any microcontroller.
Indeed, most microcontrollers have many more than one timer, applied to a variety of
different tasks. These include generating timing in PWM or serial links, measuring the
duration of external events and producing other timed activity.
9.5.3 Timers on the mbed
To findoutwhathardwaretimersthembedhas, weturnback to Figure2.3andReference2.4,
the LPC1768 usermanual. We find that themicrocontrollerhas fourgeneral-purposetimers,
aRepetitiveInterrupt Timer andaSystemTickTimer. All are basedon theprinciples just
described. The mbed makes useof these inthree distinct applications, described inthe
followingsections. ThesearetheTimer, usedforsimpletimingapplications, Timeout, which
calls afunctionafterapredetermined delay, and Ticker, whichrepeatedly calls afunctionat
apredeterminedrate. ItalsoappliesaReal TimeClocktokeeptrackof timeof day andthedate.
9.6 Using the mbed Timer
The mbed Timer allows basic timing activities to take place, for comparatively short
durations. A Timercanbecreated, started, stoppedandread. Thereis no limit onthenumber
of Timersthatcanbesetup. TheAPI summary isshowninTable9.3. Thembedsitenotesthat
Table 9.3: API summary for Timer
Function
Usage
start
Start the timer
stop
Stop the timer
reset
Reset the timer to 0
read
Get the time passed in seconds
read_ms
Get the time passed in milliseconds
read_us
Get the time passed in microseconds
184 Chapter 9
the Timer is based on the 32-bit counters, and can time up to a maximum of (231 1)
microseconds, i.e. something over 30 minutes. Based on the theory above, one might expect
theTimerto countupto (232 1). However, onebitisreservedintheAPI objectasasignbit,
so only 31 bits are available for counting.
Program Example 9.3 gives a simple but interesting timing example, and is taken
from the mbed site. It measures the time taken to write a message to the screen, and
displays that message. Compile and run the program, with Tera Term activated
(Appendix E). Then carry out Exercise 9.5 to make some further measurements and
calculations.
/* Program Example 9.3: A simple Timer example, from mbed website.
Activate Tera Term terminal to test.
*/
#include "mbed.h"
Timer t;
Serial pc(USBTX, USBRX);
// define Timer with name “t”
int main() {
t.start();
//start the timer
pc.printf("Hello World!\n");
t.stop();
//stop the timer
pc.printf("The time taken was %f seconds\n", t.read()); //print to pc
}
Program Example 9.3 A simple Timer application
n
Exercise 9.5
Run Program Example 9.3, and note from the computer screen readout the time taken
for themessageto bewritten. Then writesomeother messages, of differinglengths, and
record in each case the number of characters and the time taken. Can you relate the
times taken to the baud rate used? Can you make any other deductions? If necessary,
check Section 7.9 to recall someof thetiming issues relating to theasynchronous serial
data link used.
n
9.6.1 Using Multiple mbed Timers
We are now going to apply the Timer in a different way, to run one function at one rate and
anotherfunctionat anotherrate. Two LEDs will beusedto show this; you will quickly realize
that theprincipleis powerful, and can beextended to more tasks and moreactivities. Program
Example 9.4 shows the program listing. The program creates two Timers, named timer_fast
and timer_slow. The main program starts these running, and tests when each exceeds
a certain number. When the time value is exceeded, a function is called, which flips the
associated LED.
Interrupts, Timers and Tasks 185
/*Program Example 9.4: Program which runs two time-based tasks
*/
#include "mbed.h"
Timer timer_fast;
// define Timer with name "timer_fast"
Timer timer_slow;
// define Timer with name "timer_slow"
DigitalOut ledA(LED1);
DigitalOut ledB(LED4);
void task_fast(void);
void task_slow(void);
int main() {
timer_fast.start();
timer_slow.start();
while (1){
if (timer_fast.read()>0.2){
task_fast();
timer_fast.reset();
}
if (timer_slow.read()>1){
task_slow();
timer_slow.reset();
}
}
}
//function prototypes
//start the Timers
//test Timer value
//call the task if trigger time is reached
//and reset the Timer
//test Timer value
void task_fast(void){
ledA = !ledA;
}
//”Fast” Task
void task_slow(void){
ledB = !ledB;
}
//”Slow” Task
Program Example 9.4 Running two timed tasks
CreateaprojectaroundProgramExample9.4, andrunitonthembedalone. Checkthetiming
with a stopwatch or an oscilloscope.
n
Exercise 9.6
Experiment with different repetition ratesin ProgramExample9.4, includingonesthat are
not multiplesofeachother. Addathirdandthenafourthtimertoit, flashingall mbedLEDs
at different rates.
n
9.6.2 Testing the Timer Duration
Wequotedabovethemaximumvaluethat theTimercanreach. Aswithmany microcontroller
features, it is very important to understand the operating limits and to be certain to remain
within them. Program Example 9.5 measures the Timer limit in a simple way. It clears the
Timerandthensetsitrunning, displayingatimeupdateto theTeraTermscreenevery second.
186 Chapter 9
Indoingthisitkeepsitsownrecordof secondselapsedandcomparesthiswiththeelapsedtime
valuegivenby theTimer. Fromtheprogramstructureweexpect theTimervaluealways to be
just ahead of the recorded time value. At some point, however, the Timer overflows back to
zero andthis conditionis no longersatisfied. Theprogramdetectsthis andsends amessageto
the screen. The highest value reached by the Timer is recorded on the screen.
/* Program Example 9.5: Tests Timer duration, displaying current time values to terminal
*/
#include "mbed.h"
Timer t;
float s=0;
float m=0;
DigitalOut diag (LED1);
Serial pc(USBTX, USBRX);
//seconds cumulative count
//minutes cumulative count
int main() {
pc.printf("\r\nTimer Duration Test\n\r");
pc.printf("-------------------\n\n\r");
t.reset();
//reset Timer
t.start();
// start Timer
while(1){
if (t.read()>=(s+1)){ //has Timer passed next whole second?
diag = 1;
//If yes, flash LED and print a message
wait (0.05);
diag = 0;
s++ ;
//print the number of seconds exceeding whole minutes
pc.printf("%1.0f seconds\r\n",(s-60*(m-1)));
}
if (t.read()>=60*m){
printf("%1.0f minutes \n\r",m);
m++ ;
}
if (t.read()<s){
//test for overflow
pc.printf("\r\nTimer has overflowed!\n\r");
for(;;){}
//lock into an endless loop doing nothing
}
}
//end of while
}
Program Example 9.5 Testing Timer duration
n
Exercise 9.7
Create a project around Program Example 9.5 and run it on an mbed, enabling also
a connection to Tera Term (see AppendixE). No further hardware is needed. Calculate
precisely the time duration you expect from the Timer, i.e. (231 1) microseconds. You
can leavetheprogramrunning whiledoing other things. Comeback, however, just after
half an hour, and watch intently to see whether it overflows! Do your predicted and
measured times agree?
n
Interrupts, Timers and Tasks 187
In some embedded programs there comes a moment when we just want the
programto stop; thereissimply nothingmoreforit to do. ProgramExample9.3is
onesuch. Yet wehaveno instructionavailableto us that just says stop; theCPU is
designed to go on and on running until it is switched off. Notice at the end of this program
examplehowthestopis implemented, by trapping programexecutioninanendless loopthat
does nothing.
C code
feature
9.7 Using the mbed Timeout
Program Example 9.4 showed the mbed Timer being used to trigger time-based events in
an effective way. However, we needed to poll the Timer value to know when the event
should be triggered. The Timeout allows an event to be triggered by an interrupt, with no
polling needed. Timeout sets up an interrupt to call a function after a specified delay.
There is no limit on the number of Timeouts created. The API summary is shown in
Table 9.4.
9.7.1 A Simple Timeout Application
A simple first example of Timeout is shown in Program Example 9.6. This causes an
action to be triggered a fixed period after an external event. This simple program is made
up of the main( ) function and a blink( ) function. A Timeout is created, named
Response, along with some familiar digital input and output. Looking in the main( )
function, we see an if declaration, which tests if the button is pressed. If it is, the blink( )
function gets attached to the Response Timeout. We can expect that two seconds after
this attachment is made, the blink( ) function will be called. To aid in the diagnostics, the
button also switches on LED3. As a continuous task, the state of LED1 is reversed every
0.2 s. This program is thus a microcosm of many embedded systems programs. A timetriggered task needs to keep going, while an event-triggered task needs to take place at
unpredictable times.
Table 9.4: API summary for Timeout
Function
Usage
attach
Attach a function to be called by the Timeout, specifying the delay in seconds
attach
Attach a member function to be called by the Timeout, specifying the delay in seconds
attach_us
Attach a function to be called by the Timeout, specifying the delay in microseconds
attach_us
Attach amember function to becalled bytheTimeout, specifyingthedelayin microseconds
detach
Detach the function
188 Chapter 9
/*Program Example 9.6: Demonstrates Timeout, by triggering an event a fixed duration
after a button press.
*/
#include "mbed.h"
Timeout Response;
//create a Timeout, and name it "Response"
DigitalIn button (p5);
DigitalOut led1(LED1);
DigitalOut led2(LED2);
DigitalOut led3(LED3);
void blink() {
led2 = 1;
wait(0.5);
led2=0;
}
//this function is called at the end of the Timeout
int main() {
while(1) {
if(button==1){
Response.attach(&blink,2.0); // attach blink function to Response Timeout,
//to occur after 2 seconds
led3=1;
//shows button has been pressed
}
else {
led3=0;
}
led1=!led1;
wait(0.2);
}
}
Program Example 9.6 Simple Timeout application
Compile Program Example 9.6 and download to an mbed, with the build of Figure 9.3.
Observe the response as the push-button is pressed.
n
Exercise 9.8
With Program Example 9.6 running, answer the following questions:
1. Is the2s Timeout timed fromwhen thebutton is pressed or when it is released? Why
is this?
2. When theevent-triggered task occurs(i.e. theblinkingof LED2), what impact doesit
have on the time-triggered task (i.e. the flashing of LED1)?
3. If you tap the button very quickly, you will see that it is possible for the program to
miss it entirely (even though electrically we can prove that the button has been
pressed). Why is this?
n
Interrupts, Timers and Tasks 189
9.7.2 Further Use of Timeout
Program Example 9.6 has demonstrated use of the Timeout nicely, but the questions in
Exercise 9.8 throw up some of the classic problems of task timing, notably that execution of
the event-triggered task can interfere with the timing of the time-triggered task.
ProgramExample9.7doesthesamethingasthepreviousexample, but inamuchbetterway.
Glancingthroughit, weseetwo Timeoutscreated, andaninterrupt. Thelatterconnectsto the
push-button, and replaces the digital input which previously took that role. There are now
three functions, in addition to main( ). This has become extremely short and simple, and is
concerned primarily with keeping the time-triggered task going. Now response to the pushbutton is by interrupt, and it is within the interrupt function that the first Timeout is set up.
Whenitisspent, theblink( ) functioniscalled. ThissetstheLED output, butthenenablesthe
second Timeout, which will trigger the end of the LED blink.
/*Program Example 9.7: Demonstrates the use of Timeout and interrupts, to allow response
to an event-driven task while a time-driven task continues.
*/
#include "mbed.h"
void blink_end (void);
void blink (void);
void ISR1 (void);
DigitalOut led1(LED1);
DigitalOut led2(LED2);
DigitalOut led3(LED3);
Timeout Response;
//create a Timeout, and name it Response
Timeout Response_duration; //create a Timeout, and name it Response_duration
InterruptIn button(p5);
//create an interrupt input, named button
void blink() {
//This function is called when Timeout is complete
led2=1;
// set the duration of the led blink, with another timeout, duration 0.1 s
Response_duration.attach(&blink_end, 1);
}
void blink_end() {
led2=0;
}
//A function called at the end of Timeout Response_duration
void ISR1(){
led3=1;
//shows button is pressed; diagnostic and not central to program
//attach blink1 function to Response Timeout, to occur after 2 seconds
Response.attach(&blink, 2.0);
}
int main() {
button.rise(&ISR1);
while(1) {
led3=0;
led1=!led1;
wait(0.2);
}
}
//attach the address of ISR1 function to the rising edge
//clear LED3
190 Chapter 9
Program Example 9.7 Improved use of Timeout
Compile and run the code of Program Example 9.7, again using the build of Figure 9.3.
n
Exercise 9.9
Repeat all thequestionsof Exercise9.8 for themost recent programexample, notingand
explaining the differences you find.
n
9.7.3 Timeout used to Test Reaction Time
ProgramExample9.8 shows aninterestingrecreational applicationof Timeout, in
whichthetimeout durationis itself avariable. It tests reactiontimeby blinkingan
LED, andtiminghowlongit takes fortheplayerto hit aswitchinresponse. To add
challenge, a‘random’ delay is generatedbeforetheLED is lit. This uses theC library function
rand( ). Theprogramshouldbeunderstandablefromthecomments it contains.
C code
feature
The circuit build is the same as seen in Figure 9.3. Now the push-button is the switch the
player must hit to show a reaction. A Tera Term terminal should be enabled.
/*Program Example 9.8: Tests reaction time, and demos use of Timer and Timeout functions
*/
#include "mbed.h"
#include <stdio.h>
#include <stdlib.h>
//contains rand() function
void measure ();
Serial pc(USBTX, USBRX);
DigitalOut led1(LED1);
DigitalOut led4(LED4);
DigitalIn responseinput(p5); //the player hits the switch connected here to respond
Timer t;
//used to measure the response time
Timeout action;
//the Timeout used to initiate the response speed test
int main (){
pc.printf("Reaction Time Test\n\r");
pc.printf("------------------\n\r");
while (1) {
int r_delay;
//this will be the “random” delay before the led is blinked
pc.printf("New Test\n\r");
led4=1;
//warn that test will start
wait(0.2);
led4=0;
r_delay = rand() % 10 + 1; // generates a pseudorandom number range 1-10
pc.printf("random number is %i\n\r", r_delay); // allows test randomness;
//removed for normal play
action.attach(&measure,r_delay); // set up Timeout to call measure()
// after random time
wait(10);
//test will start within this time, and we then return to it
Interrupts, Timers and Tasks 191
}
}
void measure (){
// called when the led blinks, and measures response time
if (responseinput ==1){
//detect cheating!
pc.printf("Don’t hold button down!");
}
else{
t.start();
//start the timer
led1=1;
//blink the led
wait(0.05);
led1=0;
while (responseinput==0) {
//wait here for response
}
t.stop();
//stop the timer once response detected
pc.printf("Your reaction time was %f seconds\n\r", t.read());
t.reset();
}
}
Program Example 9.8 Reaction time test: applying Timer and Timeout
n
Exercise 9.10
Run Program Example 9.8 for a period of time and note the sequence of ‘random’
numbers. Run it again. Do you recognizea pattern in thesequenceof ‘random’ numbers?
In fact, it is difficult for a computer to generate true random numbers, although various
tricks and algorithms are used to create pseudorandomsequences of numbers.
n
9.8 Using the mbed Ticker
The mbed Ticker feature sets up a recurring interrupt, which can be used to call a function
periodically, at aratedecidedby theprogrammer. Thereis no limit onthenumberof Tickers
created. The API summary is shown in Table 9.5.
Table 9.5: API summary for Ticker
Function
Usage
attach
Attach a function to be called by the Ticker, specifying the interval in seconds
attach
Attach a member function to be called by the Ticker, specifying the interval in seconds
attach_us
Attach a function to be called by the Ticker, specifying the interval in microseconds
attach_us
Attach amember function to becalled bytheTicker, specifyingtheinterval in microseconds
detach
Detach the function
192 Chapter 9
WecandemonstrateTickerby returningto thevery firstprogramexampleinthisbook, number
2.1. This simply flashes anLED every 200ms. Creatingaperiodic event is oneof themost
natural andcommonrequirementsinanembeddedsystem, soitisnotsurprisingthatitappeared
inourfirstprogram. Wecreatedthe200msperiodby usingadelay function; theseareuseful in
alimitedway, but whenthey arerunningthey tieuptheCPU, so that it cando nothingelse
productive.
Program Example 9.9 simply replaces the Delay functions with the Ticker.
/* Program Example 9.9: Simple demo of "Ticker". Replicates behavior of first
led flashing program.
*/
#include "mbed.h"
void led_switch(void);
Ticker time_up;
//define a Ticker, with name “time_up”
DigitalOut myled(LED1);
void led_switch(){
myled=!myled;
}
//the function that Ticker will call
int main(){
time_up.attach(&led_switch, 0.2);
//initializes the ticker
while(1){
//sit in a loop doing nothing, waiting for Ticker interrupt
}
}
Program Example 9.9 Applying Ticker to our very first program
Itshouldbeeasy tofollowwhatisgoingoninthisprogram. Themajorstepforwardisthatthe
CPUisnowfreedto do anythingthatisneeded, whilethetask of measuringthetimebetween
LED changes is handed over to the Timer hardware, running in the background.
We have already called functions periodically with the Timer feature, so at first Ticker does
not seem to add anything really new. Remember, however, that we have to poll the Timer
value in Program Example 9.4 to test its value, and instigate the related function. Using
Ticker, it is an interrupt that calls the associated function when time is up. As already
discussed, this is a more effective approach to programming.
9.8.1 Using Ticker for a Metronome
ProgramExample9.10uses thembedto createametronome, usingtheTickerfacility. If you
have not met one before, a metronome is an aid to musicians, setting a steady beat, against
which they can play their music. The musician generally selects a beat rate in the range
40e208 beats per second. Old metronomes were based on elegant clockwork mechanisms,
with a swinging pendulum arm. Most are made these days electronically. Normally the
indicationgiven to the musicianis aloud audible‘tick’, sometimes accompanied by anLED
flash. Here we just restrict ourselves to the LED.
Interrupts, Timers and Tasks 193
The main program while loop checks the up and down buttons, adjusts the beat rate
accordingly, anddisplaysthecurrentratetoscreen. Thisloopscontinuously, butlurkinginthe
background is the Ticker. This has been initialized before the while loop, in the line
beat_rate.attach(&beat, period); //initializes the beat rate
Oncethetimeindicatedby period haselapsed, thebeat functioniscalled. Atthismomentthe
Ticker is updated, possibly with a new value of period, and the LED is flashed to indicate
a beat. Program execution then returns to the main while loop, until the next Ticker
occurrence.
/*Program Example 9.10: Metronome. Uses Ticker to set beat rate
*/
#include "mbed.h"
#include <stdio.h>
Serial pc(USBTX, USBRX);
DigitalIn up_button(p5);
DigitalIn down_button(p6);
DigitalOut redled(p19);
//displays the metronome beat
Ticker beat_rate;
//define a Ticker, with name “beat_rate”
void beat(void);
float period (0.5);
//metronome period in seconds, initial value 0.5
int rate (120);
//metronome rate, initial value 120
int main() {
pc.printf("\r\n");
pc.printf("mbed metronome!\r\n");
pc.printf("_______________\r\n");
period = 1;
redled = 1;
//diagnostic
wait(.1);
redled = 0;
beat_rate.attach(&beat, period);
//initializes the beat rate
//main loop checks buttons, updates rates and displays
while(1){
if (up_button = =1)
//increase rate by 4
rate = rate + 4;
if (down_button = =1)
//decrease rate by 4
rate = rate - 4;
if (rate > 208)
//limit the maximum beat rate to 208
rate = 208;
if (rate < 40)
//limit the minimum beat rate to 40
rate = 40;
period = 60/rate;
//calculate the beat period
pc.printf("metronome rate is %i\r", rate);
//pc.printf("metronome period is %f\r\n", period); //optional check
wait (0.5);
}
}
void beat() {
//this is the metronome beat
beat_rate.attach(&beat, period); //update beat rate at this moment
194 Chapter 9
redled = 1;
wait(.1);
redled = 0;
}
Program Example 9.10 Metronome, applying Ticker
The hardware build for the metronome is simple, and is shown in Figure 9.7. The Tera Term
terminal should be enabled. Build the hardware, and compile and download the program.
With a stopwatch or another timepiece check that the beat rates are accurate.
n
Exercise 9.11
Wehaveleft a bit of polling in ProgramExample9.10. Rewriteit so that responseto the
external pins is done with interrupts.
n
9.8.2 Reflecting on Multi-Tasking in the Metronome Program
“up”
“down”
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
mbed Microcontroller
Having picked up the idea of program tasks at the beginning of this chapter, we have seen
a sequence of program examples that could be described as multi-tasking. This started with
Figure 9.7:
Metronome build
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
Interrupts, Timers and Tasks 195
ProgramExample 9.1, which runs one time-triggered and one event-triggered task. Many of
the subsequent programs have clear multi-tasking features. In the metronome example, the
program has to keep a regular beat going. While doing this, it must also respond to inputs
from the user, calculate beat rates and write to the display. There is, therefore, one
time-triggered task, the beat, and at least one event-triggered task, the userinput. Sometimes
itisdifficulttodecidewhichprogramactivitiesbelongtogetherinonetask. Inthemetronome
example the user response seems to contain several activities, but they all relate closely to
each other, so it is sensible to view them as the same task.
We have developed here a useful program structure, whereby time-triggered tasks can be
linked to a Timer or Ticker function, and event-triggered tasks to an external interrupt. This
will be useful as long as there are not too many tasks, and as long as they are not too
demanding of CPU time. When that happens, we will need to look at more sophisticated
solutions, either with a real-time operating systemor with a multi-processor hardware. Both
of these are beyond the scope of this book, but topics to look out for as your knowledge and
confidence increase.
9.9 The Real Time Clock
The Real Time Clock (RTC) is an ultra-low-power peripheral on the LPC1768, which is
implemented by the mbed. The RTC is a timing/counting system that maintains a calendar
and time-of-day clock, with registers for seconds, minutes, hours, day, month, year, day of
monthandday of year. It canalso generateanalarmforaspecific dateandtime. It runs from
its own 32 kHz crystal oscillator, and can have its own independent battery power supply. It
can thus be powered, and continue in operation, even if the rest of the microcontroller is
powereddown. ThembedAPI doesnotcreateany Cþ þ objects, butjustimplementsstandard
functions fromthestandardC library, as shownin Table9.6. Simpleexamples forusecanbe
found on the mbed website.
Table 9.6: API summary for Real Time Clock
Function
Usage
Time
Get the current time
set_time
Set the current time
mktime
Converts a tm structure to a timestamp
localtime
Converts a timestamp to a tm structure
ctime
Converts a timestamp to a human-readable string
strftime
Converts a tm structure to a custom format human-readable string
196 Chapter 9
Ideal switch
response
Actual switch response
Figure 9.8:
Demonstrating switch bounce
9.10 Switch Debouncing
With the introduction of interrupts, we now have some choices to make when writing
a program to a particular design specification. For example, Program Example 3.3 uses
a digital input to determine which of two LEDs to flash. The digital input value is
continuously polled within an infinite loop. However, we could equally have designed this
programwithanevent-drivenapproach, to flipacontrol variableevery timethedigital input
changes. Importantly, there are some inherent timing constraints within Program Example
3.3 which have not previously been discussed. One is that the frequency of polling is
actually quite slow, because once the switch input has been tested a 0.4 s flash sequence is
activated. This means that the system has a response time of at worst 0.4 s, because it only
tests the switch input once for every program loop. When the switch changes position it
could take up to 0.4s for the LED to change, which is very slow in terms of embedded
systems.
Interrupt-driven systems can have much quicker response rates to switch presses, because
theresponseto thedigital inputcantakeplacewhileothertasksarerunning. However, when
a system can respond very rapidly to a switch change, a new issue arises which needs
addressing, called switch bounce. This is due to the fact that the mechanical contacts of
a switch literally bounce together as the switch closes. This can cause a digital input to
swing wildly between Logic 0 and Logic 1 for a short time after a switch closes, as
illustrated in Figure 9.8. The solution to switch bounce is a technique called switch
debouncing.
First, the problem with switch bounce can be identified by evaluating a simple eventdriven program. Program Example 9.11 attaches a function to a digital interrupt on
pin 5, so the circuit build shown in Figure 9.3 can again be used. The function
simply toggles (flips) the state of the mbed’s onboard LED1 for every raising edge on
pin 18.
Interrupts, Timers and Tasks 197
/* Program Example 9.11: Toggles LED1 every time p18 goes high. Uses hardware build
shown in Figure 9.3.
*/
#include "mbed.h"
InterruptIn button(p5);
// Interrupt on digital pushbutton input p18
DigitalOut led1(LED1);
// mbed LED1
void toggle(void);
// function prototype
int main() {
button.rise(&toggle);
}
// attach the address of the toggle
// function to the rising edge
void toggle() {
led1=!led1;
}
Program Example 9.11 Toggles LED1 every time mbed pin 5 goes high
Implement program18 with a push-button or SPDT (single-pole, double-throw) type switch
onpin18. Dependingalittleonthetypeof switchyouuse, youwill seethattheprogramdoes
not work very well. It can become unresponsive or the button presses can become out of
synch with the LED. This demonstrates the problem with switch bounce.
From Figure 9.8 it is easy to see how a single button press or change of switch position can
causemultipleinterruptsandhencetheLED canget out of synchwiththebutton. Theswitch
can be ‘debounced’ with a timer feature. The debounce feature needs to ensure that once the
raising edge has been seen, no further raising edge interrupts should be implemented until
a calibrated time period has elapsed. In reality, some switches move positions more cleanly
than others, so the exact timing required needs some tuning. To assist, switch manufacturers
often provide dataon switch bounce duration. The downside to including debouncing of this
type is that the implemented timing period also reduces the response of the switch, although
not as much as that discussed with reference to polling.
Switch debouncing can be implemented in a number of ways. In hardware, there are little
configurations of logic gates that can be used (see Reference 5.1). In software, timers,
bespokecounters orother programmingmethods can beused. ProgramExample9.12 solves
the switch bounce issue by starting a timer on a switch event and ensuring that 10ms has
elapsed before allowing a second event to be processed.
/* Program Example 9.12: Event driven LED switching with switch debounce
*/
#include "mbed.h"
InterruptIn button(p18);
// Interrupt on digital pushbutton input p18
DigitalOut led1(LED1);
// digital out to LED1
Timer debounce;
// define debounce timer
void toggle(void);
// function prototype
int main() {
debounce.start();
button.rise(&toggle);
// attach the address of the toggle
198 Chapter 9
}
void toggle() {
if (debounce.read_ms()>10)
led1=!led1;
debounce.reset();
}
// function to the rising edge
// only allow toggle if debounce timer
// has passed 10 ms
// restart timer when the toggle is performed
Program Example 9.12 Event-driven LED switching with switch debounce
n
Exercise 9.12
1. Experiment with modifying thedebouncetimeto beshorter or greater values. There
comes a point wherethetimer does not effectively solvethedebouncing, and at the
other end of thescaleresponsiveness can bereduced too. What is thebest debounce
time for the switch you are using?
2. Implement Program Example 9.12 using an mbed Timeout object instead of the
Timer object. Are there any advantages or disadvantages to each approach?
3. Rewrite Program Example 3.3 to do the same thing but with an event-driven
approach (i.e. using interrupts). How much improvement can be made to
the system’s responsiveness to switch changes?
n
9.11 Mini-Projects
9.11.1 A Self-Contained Metronome
The metronome described in Section 9.8.1 is interesting, but it does not result in something
thatamusicianwouldreally wantto use. So try revisingtheprogram, anditsassociatedbuild,
to make aself-contained battery-powered unit, using an LCD instead of the computer screen
to display beat rate. Experiment also withgettingaloudspeakerto ‘tick’ alongwiththeLED.
If you succeed in this, then try including the facility to play ‘concert A’ (440 Hz), or another
pitch, to allow the musicians to tune their instruments.
9.11.2 Accelerometer Threshold Interrupt
WemettheADXL345accelerometer,withitsSPIserial interface,inSection7.3.Althoughwedidnot
usethematthetime, itisinterestingto notethatthedevicehastwo interruptoutputs, asseenin
Figure7.6. Thesecanbeconnectedto anmbeddigital inputto runaninterruptroutinewheneveran
accelerationthresholdisexceeded.Forexample,theaccelerometermightbeacrashdetectionsensorin
avehiclewhich, whenaspecifiedaccelerationvalueisexceeded, activatesanairbag.
Use an accelerometer on a cantilever armto provide the acceleration data. Figure 9.9 shows
thegeneral construction. A plastic 30 cmor1foot ruler, clampedatoneendto atable, canbe
Interrupts, Timers and Tasks 199
Accelerometer
SPI wires to mbed
Cantilever beam
held or fixed at one end
Figure 9.9:
Cantilever beam held or fixed one end
used. Set the accelerometer to generate an interrupt whenever a threshold in the z-axis is
exceeded. Connect this as an interrupt input to the mbed, and program it so a warning LED
lightsforonesecondwheneverthethresholdisexceeded. Youcanexperimentwiththeactual
threshold value to alter the sensitivity of the detection system.
Chapter Review
• Signal inputs can be repeatedly tested in a loop, a process known as polling.
• An interrupt allows an external signal to interrupt the action of the CPU, and start code
execution from somewhere else in the program.
• Interrupts are a powerful addition to the structure of the microprocessor. Generally,
multiple interrupt inputs are possible, which adds considerably to the complexity of both
hardware and software.
• It is easy to make a digital counter circuit, which counts the number of logic pulses
presented at its input. Such a counter can be integrated into a microcontroller structure.
• Given a clock signal of known and reliable frequency, a counter can readily be used as
a timer.
• Timers can be structured in different ways so that interrupts can be generated from their
output, for example to give a continuous sequence of interrupt pulses.
• Switchdebounceis required inmany cases to avoidmultipleresponses being triggeredby
a single switch press.
Quiz
1. Explainthedifferencesbetweenusingpollingandevent-driventechniques fortestingthe
state of one of the digital input pins on a microcontroller.
2. List the most significant actions that a CPU takes when it responds to an enabled
interrupt.
200 Chapter 9
3. Explain the following terms with respect to interrupts:
(a) priority
(b) latency
(c) nesting.
4. A comparator circuit and LM35 are to be used to create an interrupt source, using the
circuit of Figure 9.5. The comparator is supplied from 5.0 V and the temperature
threshold is to be approximately 38 C. Suggest values for R1 and R2. Resistor values of
470, 680, 820, 1k, 1k2, 1k5 and 10k are available.
5. Describe in overview how a timer circuit can be implemented in hardware as part of
a microprocessor’s architecture.
6. What is the maximum value, in decimal, that a 12-bit and a 24-bit counter can count
up to?
7. A 4.0 MHz clock signal is connected to the inputs of a 12-bit and a 16-bit counter. Each
startscountingfromzero. Howlongdoesit takebeforeeachreachesitsmaximumvalue?
8. A 10-bit counter, clocked with an input frequency of 512 kHz, runs continuously. Every
timeitoverflows, itgeneratesaninterrupt. Whatisthefrequency of thatinterruptstream?
9. Whatisthepurposeof thembed’sReal TimeClock? Giveanexampleof whenitmightbe
used.
10. Describetheissueof switchbounceandexplainhowtimerscanbeusedto overcomethis.
CHAPTER 10
Memory and Data Management
Chapter Outline
10.1 A Memory Review 201
10.1.1 Memory Function Types 201
10.1.2 Essential Electronic Memory Types 202
10.2 Using Data Files with the mbed 205
10.2.1 Reviewing some Useful C/C++Library Functions 205
10.2.2 Defining the mbed Local File System 205
10.2.3 Opening and Closing Files 206
10.2.4 Writing and Reading File Data 206
10.3 Example mbed Data File Access 207
10.3.1 File Access 207
10.3.2 String File Access 208
10.3.3 Using Formatted Data 209
10.4 Using External Memory with the mbed 210
10.5 Introducing Pointers 212
10.6 Mini-Project: Accelerometer Data Logging on Exceeding Threshold 214
Chapter Review 215
Quiz 216
References 216
10.1 A Memory Review
10.1.1 Memory Function Types
Broadly speaking, amicroprocessorneeds memory for two reasons: to hold its program, and
to hold the data that it is working with; these are often called programmemory and data
memory.
To meet these needs a number of different semiconductor memory technologies is available,
which can be embedded on the microcontroller chip. Memory technology is divided broadly
into two types: volatile and non-volatile. Non-volatile memory retains its data when
power is removed, but tends to be more complex to write to in the first place. For historical
reasons it is still often called ROM (read only memory). Non-volatile memory is generally
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00010-6
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
201
202 Chapter 10
requiredforprogrammemory, so thattheprogramdataisthereandready whentheprocessor
is poweredup. Volatilememory loses all datawhenpoweris removed, but is easy to writeto.
Volatile memory is traditionally used for data memory; it is essential to be able to write to
memory easily, and there is little expectation for data to be retained when the product is
switchedoff. Forhistorical reasonsitisoftencalledRAM(randomaccessmemory), although
this terminology tells us little that is useful. These categorizations of memory, however, give
an over-simplified picture. It can be useful to change the contents of program memory, and
there are times when we want to save data long term. Moreover, new memory technologies
now provide non-volatile memory that is easy to write to.
10.1.2 Essential Electronic Memory Types
Inany electronic memory wewant to beableto storeall the1s and0s that makeupourdata.
There are several ways that this can be done, and a few essential ones are outlined here.
A simpleone-bit memory isacoin. It isstableintwo positions, witheither‘heads’ facingup,
or ‘tails’. We can try to balance the coin on its edge, but it would pretty soon fall over. The
coin is stable in two states, and we call this bistable. It could be said that ‘heads’ represents
Logic 1 and ‘tails’ Logic 0. With eight coins, an8-bit number can be represented and stored.
If we had 10 million coins, we could store the data that makes up one photograph of good
resolution, but that would take up a lot of space indeed!
Therearevariouselectronic alternativesto thecoin, whichtakeupmuchlessspace. Oneisto
use an electronic bistable (or ‘flip-flop’) circuit, as shown in Figure 10.1. The two circuits of
(a)
acoin
(b)
two transistors and two resistors
(c)
VCC
Figure 10.1:
Three ways of implementing a one-bit memory unit
two NAND gates
Memory and Data Management 203
Figure10.1bandc arestableinonly two states, andeachcanbeusedto storeonebit of data.
Circuits like these have been the bedrock of volatile memory.
Looking at the bigger picture, there are several different types of volatile and non-volatile
memory, as shown in Figure 10.2 and described in some detail in References 1.1 and 10.1.
Static randomaccess memory (SRAM) consists of avast array of memory cells based on the
circuit of Figure 10.1b. These have to be addressable, so that just the right group of cells is
writtento, orreadfrom, atany onetime. Tomakethispossible, twoextratransistorsareadded
to the two outputs. To reduce the power consumption, the two resistors are usually replaced
by two transistors also. That means six transistors per memory cell. Each transistor takes up
a certain area on the integrated circuit (IC), so when the circuit is replicated thousands or
millions of times, it can be seen that this memory technology is not actually very space
efficient. Despite that, it is of great importance; it is low power, can be written to and read
fromwithease, canbeembeddedontoamicrocontroller, andhenceformsthestandardway of
implementing data memory in most embedded systems. All data is lost when power is
removed.
Dynamic random access memory (DRAM) is intended to do the same thing as SRAM with
a reduced silicon area. Instead of using a number of transistors, one bit of information is
storedinatiny capacitor, likeasmall rechargeablebattery. Suchcapacitors canbefabricated
in large numbers on an IC. In order to select the capacitor for reading or writing, a simple
transistor switch is required. Unfortunately, owing to the small capacitors and leakage
currents on the chip, the memory loses its charge over a short period of time (around
10 to 100ms). So the DRAM needs to be accessed every few milliseconds to refresh the
charges, otherwisetheinformationislost. DRAMhasaboutfourtimeslargerstoragecapacity
than SRAM at about the same cost and chip size, with a compromise of the extra work
involved in regular refreshing. It is, moreover, power hungry, so inappropriate for any
Figure 10.2:
Electronic memory types. SRAM: static random access memory; DRAM: dynamic random access
memory; ROM: read only memory; PROM: programmable ROM; EPROM: electrically
programmable ROM; EEPROM: electrically erasable and programmable ROM
204 Chapter 10
battery-powered device. DRAM has found wide application as data memory in
mains-powered computers, such as the PC.
The original ROMs and programmable read only memories (PROMs) could only ever be
programmed once, and have now completely disappeared from normal usage. The first type
of non-volatile reprogrammable semiconductor memory, electrically programmable read
only memory (EPROM), represented a huge step forward e a non-volatile memory could
now be reprogrammed. With a process called hot electron injection (HEI), electrons can be
forced through a very thin layer of insulator, onto a tiny conductor embedded within the
insulator, and can be trapped there almost indefinitely. This conductor is placed so that it
interferes withtheactionof afieldeffect transistor(FET). Whenit ischarged/discharged, the
action of the FET is disabled/enabled. This modified FET effectively becomes a single
memory cell, far more dense than the SRAM discussed previously. Moreover, the memory
effect is non-volatile; trapped charge is trapped charge! This programming does require
a comparatively high voltage (around 25V), so generally needs a specialist piece of
equipment. The memory is erased by exposure to intense ultraviolet light; EPROMs can
always be recognized by the quartz window on the IC, which allows this to happen.
Thenext stepbeyondHEI was NordheimFowler Tunneling. Thisrequiresevenfinermemory
cell dimensions, and gives a mechanism for the trapped charge to be retrieved electrically,
which had not previously been possible. With electrically erasable and programmable read
only memory (EEPROM), words of data are individually writeable, readable and erasable,
and are non-volatile. The downside of this is that more transistors are needed to select each
word. Inmany casesthisflexibility isnotrequired. A revisedinternal memory structureledto
flashmemory; inthis, theability toeraseindividual wordsisnotavailable. Wholeblockshave
to be erased at any one time, ‘in a flash’. This compromise leads to a huge advantage: flash
memory isvery highdensity indeed, moreorlessthehighestavailable. Thismemory typehas
been a key feature of many recent products that we have now become used to, like digital
cameras, memory sticks, solid-state hard drives and so on. A curious feature of flash and
EEPROM, unlike most electronics, is that they exhibit awear-out mechanism. Electrons can
get trappedintheinsulatorthroughwhichthey areforcedwhenawriteoperationtakesplace.
Therefore this limitation is often mentioned in datasheets, for example a maximum of
100000write/erasecycles. This is avery highnumber, andis not experiencedinnormal use.
Although EPROM had become very widely used, and had been integrated onto
microcontrollers, itwasrapidly andcompletely replacedby flashmemory. Nowinembedded
systems, the two dominant memory technologies are flash and SRAM. A glance back at
Figures 2.2 and 2.3 shows how important they are to the mbed. Program memory on the
LPC1768isflash, anddatamemory isSRAM. Onthembedcard, the‘USB disk’ isaflashIC.
The other technology that we are likely to meet at times is EEPROM; this is still used where
the ability to rewrite single words of data remains essential.
Memory and Data Management 205
10.2 Using Data Files with the mbed
Armed with a little knowledge about memory technologies, let’s explore how to access
andusethembedmemory. TheLocalFileSystemlibrary allowsustosetupalocal filesystem
for accessing the mbed flash universal serial bus (USB) disk drive. This allows programs
to read and write files on the same disk drive that is used to hold the mbed programs,
andwhichareaccessedfromthehostcomputer. Oncethesystemhasbeensetup, thestandard
C/Cþ þ file access functions can be used to open, read and write files.
10.2.1 Reviewing some Useful C/C++Library Functions
In C/Cþ þ we can open files, read and write data and also scan through files to
specific locations, even searching for particular types of data. The functions for
input and output operations are all defined by the C Standard Input and Output
Library (stdio.h), introduced in Section B.9 of Appendix B. Here, we will use the functions
summarized in Table 10.1 (this is effectively Table B.6, repeated here for convenience).
C code
feature
Data can be stored in files (as chars) or as words and strings (as character arrays). The mbed
provides a mechanism to allow storage in its USB disk, to save data that can be recalled at
a later date.
10.2.2 Defining the mbed Local File System
The compiler needs to know where to store and retrieve files; this is done using the mbed
LocalFileSystem declaration. This sets up the mbed as an accessible flash memory storage
unit and defines a directory for storing local files. To implement, simply add the line:
Table 10.1: Useful stdio library functions
Function
Format
Summary action
fopen
FILE * fopen ( const char * filename, const char * mode);
Opens the file of name
filename
fclose
int fclose ( FILE * stream );
Closes a file
fgetc
int fgetc ( FILE * stream );
Getsa character froma stream
fgets
char * fgets ( char * str, int num, FILE * stream );
Gets a string from a stream
fputc
int fputc ( int character, FILE * stream );
Writes a character to a stream
fputs
int fputs ( const char * str, FILE * stream );
Writes a string to a stream
fseek
int fseek ( FILE * stream, long int offset, int origin );
Moves file pointer to specified
location
str: An array containing the null-terminated sequence of characters to be written.
stream: Pointer to a FILE object that identifies the stream where the string is to be written.
206 Chapter 10
LocalFileSystem local("local"); //Create local file system named "local"
to the declarations section of a program.
10.2.3 Opening and Closing Files
A filestoredonthembed(inthis examplecalled‘datafile.txt’) canthereforebeopenedwith
the following command:
FILE* pFile = fopen("/local/datafile.txt","w");
C code
feature
Thefopen( ) functioncall usesthe* operatorto assignapointerwithnamepFileto
the file at the specific location given. Fromhere onwards we can access the file by
referring to its pointer (pFile) rather than having to use the specific filename.
We also need to specify whether wewant read or write access to the file. This is done by the
‘w’ specifier, whichisreferredto asanaccessmode(inthiscasedenotingwriteaccess). If the
file does not already exist the fopen( ) function will automatically create it in the specified
location. Several other file open access modes, and their specific meanings, are shown in
TableB.7andareelaboratedfurtherinReference10.2. Thethreemostcommonaccessmodes
are given also in Table 10.2. The append access mode (denoted by ‘a’) is useful for opening
an existing file and writing additional data to the end of that file.
When you have finished using a file for reading or writing it is essential to close it, for
example using:
fclose(pFile);
If you fail to do this you might lose all access to the mbed (see Section 10.3.1)!
10.2.4 Writing and Reading File Data
If the intention is to store numerical data, this can be done in a simple way by storing
individual 8-bit data values. The fputc function allows this, as follows:
char write_var=0x0F;
fputc(write_var, pFile);
Table 10.2: Common access modes for fopen
Access mode
Meaning
Action
‘r’
Read
Opens an existing file for reading
‘w’
Write
Creates a new empty file for writing. If a file of the same name already
exists it will be deleted and replaced with a blank file
‘a’
Append
Appends to a file. Writeoperations result in data being appended to the
end of the file. If the file does not exist a new blank file will be created
Memory and Data Management 207
Thisstoresthe8-bitvariablewrite_var tothedatafile. Thedatacanalsobereadfromafileto
a variable as follows:
read_var = fgetc(pFile);
Using the stdio.h functions, it is also possibleto read and writewords andstrings andsearch
ormovethroughfiles lookingforparticulardataelements. TheC/Cþ þ fseek( ) functioncan
be used to search through text files.
10.3 Example mbed Data File Access
10.3.1 File Access
ProgramExample10.1 creates adata file and writes the arbitrary value 0x23 to that file. The
fileissavedonthembedUSB disk. Theprogramthenopensandreadsbackthedatavalueand
displays it to the screen in a host terminal application.
/* Program Example 10.1: read and
*/
#include "mbed.h"
Serial pc(USBTX,USBRX);
LocalFileSystem local("local");
int write_var;
int read_var;
write char data bytes
// setup terminal link
// define local file system
// create data variables
int main () {
FILE* File1 = fopen("/local/datafile.txt","w");
write_var=0x23;
fputc(write_var, File1);
fclose(File1);
FILE* File2 = fopen ("/local/datafile.txt","r");
read_var = fgetc(File2);
fclose(File2);
pc.printf("input value = %i \n",read_var);
//
//
//
//
open file
example data
put char (data value) into file
close file
//
//
//
//
open file for reading
read first data value
close file
display read data value
}
Program Example 10.1 Saving data to a file
CreateanewprojectandaddthecodeinProgramExample10.1. Runtheprogram, andverify
that the data file is created on the mbed and read back correctly. If you navigate to and open
thefiledatafile.txt inastandardtexteditorprogram(suchasMicrosoftWordpad), youshould
seeahashcharacter(#) inthetopleft corner. This is becausetheASCII characterfor0x23is
the hash character (recall Table 8.3).
n
Exercise 10.1
ChangeProgramExample10.1 to experiment with other datavalues; check theseagainst
their associated ASCII codes. Write the numbers 1e10 and view them on the screen.
n
208 Chapter 10
Note that when the microcontroller programopens a file on the local drive, the mbed will be
marked as ‘removed’ on a host PC. This means the PC will often display a message such as
‘insert a disk into drive’ if you try to access the mbed at this time; this is normal, and stops
both the mbed and the PC trying to access the USB disk at the same time. Note also that the
USB drive will only reappear when all file pointers are closed in your program, or the
microcontroller programexits. If a running programon the mbed does not correctly close an
open file, you will no longer be able to see the USB drive when you plug the mbed into
yourPC. Itisthereforeimportantforaprogrammertotakecarewhenusingfilestoensurethat
all files are closed when they are not being used.
If a running programon the mbed does not exit correctly, to allow you to see the drive again
(and load a new program), use the following procedure:
1. Unplug the mbed.
2. Hold the mbed reset button down.
3. Whilestill holdingthebutton, pluginthembed. ThembedUSB driveshouldappearonthe
host computer screen.
4. Keep holding the button until the new program is saved onto the USB drive.
10.3.2 String File Access
ProgramExample 10.2 creates a file and writes text data to that file. The file is saved on the
mbed. The program then opens and reads back the text data and displays it to the screen in
a host terminal application.
/* Program Example 10.2: Read and write text string data
*/
#include "mbed.h"
Serial pc(USBTX,USBRX);
// setup terminal link
LocalFileSystem local("local");
// define local file system
char write_string[64];
// character array up to 64 characters
char read_string[64];
// create character arrays (strings)
int main () {
FILE* File1 = fopen("/local/textfile.txt","w");
fputs("lots and lots of words and letters", File1);
fclose(File1);
FILE* File2 = fopen ("/local/textfile.txt","r");
fgets(read_string,256,File2);
fclose(File2);
pc.printf("text data: %s \n",read_string);
// open file access
// put text into file
// close file
//
//
//
//
open file for reading
read first data value
close file
display read data string
}
Program Example 10.2 Saving a string to a file
Compileandruntheprogram,andverify thatthetextfileiscreatedandreadbackcorrectly e if
youopenthefiletextfile.txt, foundonthembed, thecorrecttextdatashouldbefoundwithin.
Memory and Data Management 209
When reading data from a file, the file pointer can be moved with the fseek( ) function. For
example, thefollowingcommandwill repositionthefilepointerto the8thbyteinthetextfile:
fseek (File2 , 8 , SEEK_SET ); // move file pointer to byte 8 from the start
Thefseek( ) functionneeds threeinput terms; first, thenameof thefilepointer; secondly, the
value to offset the file pointer to; and thirdly, an ‘origin’ term which tells the function
where exactly to apply the offset. The termSEEK_SET is a predefined origin term(defined
in the stdio library) which ensures that the 8-byte offset is applied from the start of the file.
n
Exercise 10.2
Add the following fseek( ) statement to Program Example 10.2 just prior to the data
being read back:
fseek (File2 , 8 , SEEK_SET ); // move file pointer to byte 8
Verifythat Tera Termonlydisplays thedata after byte8 of thedata file. Remember byte
values increment from zero, so it will actually be after the 9th character in the file.
n
10.3.3 Using Formatted Data
It is possible to format data stored in a file. This can be done with the fprintf( )
function, whichhasvery similarsyntax to printf( ), exceptthatthefilenamepointer
is also required. Wemay want, forexample, to logspecific events to adatafileand
include variable data values such as time, sensor input data and output control settings.
Program Example 10.3 shows use of the fprintf( ) function in an interrupt controlled pushbutton project. Each time the push-button is pressed, the light-emitting diode (LED) toggles
and changes state. Also on each button press, the file log.txt is updated to include the time
elapsedsincethepreviousbuttonpress, andthecurrent LED state. ProgramExample10.3also
implementsasimpledebouncetimer(asdescribedinSection9.10) to avoidmultipleinterrupts
and file write operations.
C code
feature
/* Program Example 10.3: Interrupt toggle switch with formatted data logging to text
file
*/
#include "mbed.h"
InterruptIn button(p30);
DigitalOut led1(LED1);
Timer debounce;
LocalFileSystem local("local");
void toggle(void);
int main() {
//
//
//
//
//
Interrupt on digital input p30
digital out to onboard LED1
define debounce timer
define local file system
function prototype
210 Chapter 10
debounce.start();
// start debounce timer
button.rise(&toggle);
// attach the toggle function to the rising edge
}
void toggle() {
// perform toggle if debounce time has elapsed
if (debounce.read_ms()>200)
led1=!led1;
// toggle LED
FILE* Logfile = fopen ("/local/log.txt","a"); // open file for appending
fprintf(Logfile,"time=%.3fs: setting led=%d\n\r",debounce.read(),led1.read());
fclose(Logfile);
// close file
debounce.reset();
// reset debounce timer
}
}
Program Example 10.3 Push-button LED toggle with formatted data logging
Note that the text file log.txt may not display full formatting in a simple text viewer such as
Microsoft Notepad. If line breaks are not displaying correctly then try a more advanced text
file viewer such as Microsoft Wordpad.
n
Exercise 10.3
Create a program which prompts the user to type some text data into a terminal
application. When the user presses return the text is captured and stored in a file on
the mbed.
Ensure that the data is correctly written to the data file by opening it with a standard
text viewer program.
n
10.4 Using External Memory with the mbed
A flash SD (secure digital) card can be used with the mbed via the serial peripheral interface
(SPI) protocol, as described in Reference 10.3. Using a micro SD card with a card holder
cradle (as shown in Figure 10.3), it is possible to access the SD card as an external memory.
Figure 10.3:
An SD card with holder. (Image reproduced with permission of SparkFun Electronics)
Memory and Data Management 211
Table10.3: Connections for SPI access to theSD card
MicroSD breakout
mbed pin
CS
8 (DigitalOut)
DI
5 (SPI mosi)
Vcc
40 (Vout)
SCK
7 (SPI sclk)
GND
1 (GND)
DO
6 (SPI miso)
CD
No connection
Table 10.4: Library files and import paths for implementing the SD card interface
Library
Import path
SDFileSystem
http://mbed.org/users/simon/programs/SDFileSystem/5yj8f
FATFileSystem
http://mbed.org/projects/libraries/svn/FATFileSystem/trunk?rev¼ 29
The SD card can be configured into SPI communication mode, which requires the serial
connections described in Table10.3. This example uses the mbedSPI port onpins 5, 6 and 7
and an arbitrary digital output on pin 8 to act as the SPI chip select signal.
To implement the SD card interface, it is necessary to import the mbed libraries shown in
Table 10.4.
Having imported the described files and libraries, and connected the SD card as suggested,
Program Example 10.4 writes a test text file to the card.
/* Program Example 10.4: writing data to an SD card
*/
#include "mbed.h"
#include "SDFileSystem.h"
SDFileSystem sd(p5, p6, p7, p8, "sd");
Serial pc(USBTX, USBRX);
// MOSI, MISO, SCLK, CS
int main() {
FILE *File = fopen("/sd/sdfile.txt", "w");
// open file
if(File == NULL) {
// check for file pointer
pc.printf("Could not open file for write\n");
// error if no pointer
}
else{
pc.printf("SD card file successfully opened\n"); // if pointer ok
}
fprintf(File, "Here’ s some sample text on the SD card"); // write data
fclose(File);
// close file
}
Program Example 10.4 Writing data to an SD card
212 Chapter 10
CompileProgramExample10.4 andverify that theSD cardis correctly accessedby viewing
the created text file sdfile.txt in a standard text editor program.
Program Example 10.4 requires an enhanced definition statement for the SDFileSystem
object, which in this case is named sd. Within the interface definition, SDFileSystem sd(p5,
p6, p7, p8, "sd"), wesee that not only arethe SPI interfaceconnectionpins defined, but the
name of the SDFileSystem object is also defined as an input variable in quotation marks.
Notice that the line:
if(File == NULL) {
effectively performs an error check to ensure that the file was opened correctly by the
previous fopen( ) call. If thefilepointerFilehas aNULL valuethenit means it has not been
created and the fopen( ) call was not successfully implemented.
n
Exercise 10.4
Createa programwhich records 5seconds of analog data to thescreen and to a text file
on an SD card. Use a potentiometer to generate the analog input data with sample
period of 100 ms. Ensure that your data file records the elapsed time and voltage data.
You can then open your datafilefromwithin astandard spreadsheet application, such as
Microsoft Excel. This will enable you to plot a chart and to visualize the analog data.
n
10.5 Introducing Pointers
C code
feature
Pointers areusedinC/Cþ þ to indicatewhereaparticularelement orblock of data
isstoredinmemory. Pointersarealso discussedinSectionB.8.2. Wewill look here
in a little more detail at a specific mbed example using functions and pointers.
When a pointer is defined it can be set to a particular memory address and C/Cþ þ syntax
allows the data at that address to be accessed. Pointers are required for a number of reasons;
one is because the C/Cþ þ standard does not allow us to pass arrays of data to and from
functions, sowemustusepointersinstead. Insomeprogramminglanguages(suchasMatlab),
it is possible to develop a mean average calculation function which reads in a data array,
calculates the average from the sum of the data divided by the number of data values and
returns the calculated mean. Using the mbed, however, we cannot pass the array of data into
the function, so instead we need to pass the pointer value which points to the data array.
Pointers are defined similarly to variables but by additionally using the * operator. For
example, the following declaration defines a pointer called ptr which points to data of
type int:
Memory and Data Management 213
int *ptr;
// define a pointer which points to data of type int
The specific address of a data variable can also be assigned to a pointer by using the &
operator, for example:
int datavariable=7;
int *ptr;
ptr = &datavariable;
// define a variable called datavariable with value 7
// define a pointer which points to data of type int
// assign the pointer to the address of datavariable
Inprogramcodethe* operatorcanalso beusedtogetthedatafromthegivenpointeraddress,
for example:
int x = *ptr;
// get the contents of location pointed to by ptr and
// assign to x (in this case x will equal 7)
Pointers can also be used with arrays, because an array is really just a number of datavalues
stored at consecutive memory locations. So if the following is defined:
int dataarray[]={3,4,6,2,8,9,1,4,6}; // define an array of arbitrary values
int *ptr;
// define a pointer
ptr = &dataarray[0];
// assign pointer to the address of
// the first element of the data array
the following statements will therefore be true:
*ptr == 3;
*(ptr+1) == 4;
*(ptr+2) == 6;
// the first element of the array pointed to
// the second element of the array pointed to
// the third element of the array pointed to
So array searching can be done by moving the pointer value to the correct array offset.
ProgramExample 10.5 implements a function for analyzing an array of data and returns the
average of that data.
/* Program Example 10.5: Pointers example for an array average function
*/
#include "mbed.h"
Serial pc(USBTX, USBRX);
// setup serial comms
char data[]={5,7,5,8,9,1,7,8,2,5,1,4,6,2,1,4,3,8,7,9}; //define some input data
char *dataptr;
// define a pointer for the input data
float average;
// floating point average variable
float CalculateAverage(char *ptr, char size);
// function prototype
int main() {
dataptr=&data[0];
// point pointer to address of the first array element
average = CalculateAverage(dataptr, sizeof(data));
// call function
pc.printf("\n\rdata = ");
for (char i=0; i<sizeof(data); i++) {
// loop for each data value
pc.printf("%d ",data[i]);
// display all the data values
}
pc.printf("\n\raverage = %.3f",average);
// display average value
}
// CalculateAverage function definition and code
float CalculateAverage(char *ptr, char size) {
214 Chapter 10
int sum=0;
// define variable
float mean;
// define variable
for (char i=0; i<size; i++) {
sum=sum + *(ptr+i);
// add all
}
mean=(float)sum/size;
// divide
return mean;
for calculating the sum of the data
for floating point mean value
data elements together
by size and cast to floating point
}
Program Example 10.5 Averaging function using pointers
Lookingatsomekey elementsof ProgramExample10.5, wecanseethatthepointer
dataptr is assignedto theaddress of thefirst element of thedata array. The
CalculateAverage( ) functiontakesinapointervaluethatpointsto thefirstvalueof
the data array and a second value that defines the size of the array. The function returns the
floatingpoint meanvalue. Thereis also anadditional C/Cþ þ keywordusedhere, sizeof, which
deduces thesizeof aparticulararray. This gets thesize(i.e. thenumberof dataelements) inthe
array data. Alsonotethatthecalculationof themeanvalueisintheformof anintegerdividedby
a char, yet we want the answer to be a floating point value. To implement this, we cast the
equation as floating point by using (float), which ensures that the resultant mean value is to
floating point precision.
C code
feature
Pointers are used for a number of reasons, especially in Cþ þ programs which rely
heavily on functions, methods and passing data between these. Pointers can also be used
to improve programming efficiency and speed, by directly accessing memory locations
and data. In general, the use of pointers in the programming seen in this book is required
because of a deficiency in the C/Cþ þ capabilities, i.e. not being able to pass arrays to
functions. Where possible, we try to avoid the use of pointers in order to keep code
simple and readable, but sometimes they offer the best solution to a programming
challenge.
10.6 Mini-Project: Accelerometer Data Logging on Exceeding Threshold
In this mini-project you are challenged to create a program that records the acceleration
profileencounteredinasimplevibratingcantilever. It isthenpossibleto plot theacceleration
data as shown in Figure 10.4. This project develops from the mini-project in Section 9.11.
Design and implement a new project to the following specification:
1. Enable the mbed to access external SD card memory.
2. Attach a SPI accelerometer to a simple plastic cantilever with a flying lead to the mbed.
3. Program the accelerometer to cause an interrupt trigger when excessive acceleration is
encountered.
4. Create an interrupt routine to log 100 data samples to a file on the SD card. You may
wish to use the fprintf( ) function to format accelerometer data in the text file.
Memory and Data Management 215
4
acceleration, g
Accelerometer
2
0
-2
0
0.5
1
time, s
1.5
2
Figure 10.4:
Accelerometer data-logging mini-project
5. When a certain acceleration threshold is exceeded, the data should be logged. The text
file can then be opened in a spreadsheet software program, such as Microsoft Excel, to
plot the recorded acceleration waveform.
Note: to achieveasuitablesampleperiodyoumay want to apply anmbedTickerorTimerto
ensure that the accelerometer logs data regularly. A sampling frequency of around 50Hz
should be sufficient to record a detailed acceleration waveform.
Chapter Review
• Microprocessors use memory for holding the program code (program memory)
and the working data (data memory) in an embedded system.
• A coinoralogic flip-flop/bistablecanbethoughtof asasingle1-bitmemory devicewhich
retains its state until the state is actively changed.
• Volatile memory loses its data once power is removed, whereas non-volatile can retain
memory with no power. Different technologies are used to realize these memory types,
including SRAM and DRAM (volatile) and EEPROM and flash (non-volatile).
• The mbed has 512kb of flash and 64kb of SRAM in the LPC1768 IC, and a further
16-megabit USB memory area.
• Files can be created on the mbed for storing and retrieving data and formatted text.
• An external SD memory card can be interfaced with the mbed to allow larger memory.
216 Chapter 10
• Pointers point to memory address locations to allow direct access to the data stored at
the pointed location.
• Pointers aregenerally requiredowingto thefact that C/Cþ þ does not allowarrays of data
to be passed into functions, so a pointer to the array data must be passed instead.
Quiz
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
What does the term bistable mean?
How many bistables would you expect to find in the mbed’s SRAM?
What are the fundamental differences between SRAM and DRAM type memory?
What are the fundamental differences between EEPROM and flash type memory?
Which C/Cþ þ command would open a text file for adding additional text to the end
of the current file.
Which C/Cþ þ command should be used to open a text file called ‘data.txt’ and read
the 12th character?
Give a practical example where data logging is required and explain the practical
requirements with regard to timing, memory type and size.
Give one reason why pointers are used for direct manipulation of memory data.
Give the C/Cþ þ code that defines an empty five-element array called dataarray and
a pointer called datapointer which is assigned to the first memory address of the data
array.
How is a pointer used to access and manipulate the different elements of a data array?
References
10.1. Grindling, G. and Weiss, B. (2007). Introduction to Microcontrollers. https://ti.tuwien.ac.at/ecs/teaching/
courses/mclu/theory-material/Microcontroller.pdf
10.2. Cþ þ stdio.h fopen reference. http://www.cplusplus.com/reference/clibrary/cstdio/fopen/
10.3. SD Group (Matsushita Electric Industrial Co. and SanDisk Corporation) (2006). SD Physical Layer
Simplified Specification, Version 2.00. http://www.sdcard.org/developers/tech/sdcard/pls/Simplified_
Physical_Layer_Spec.pdf
CHAPTER 11
An Introduction to Digital Signal Processing
Chapter Outline
11.1 What is a Digital Signal Processor? 219
11.2 Digital Filtering Example 220
11.3 An mbed DSP Example 222
11.3.1 Input and Output of Digital Data 222
11.3.2 Signal Reconstruction 224
11.3.3 Adding a Digital Low-Pass Filter 224
11.3.4 Adding a Push-Button Activation 228
11.3.5 Digital High-Pass Filter 229
11.4 Delay/ Echo Effect 229
11.5 Working with Wave Audio Files 232
11.5.1 The Wave Information Header 233
11.5.2 Reading the Wave File Header with the mbed 235
11.5.3 Reading and Outputting Mono Wave Data 236
11.6 Summary on DSP 239
11.7 Mini-Project: Stereo Wave Player 240
11.7.1 Basic Stereo Wave Player 240
11.7.2 Stereo Wave Player with PC Interface 240
11.7.3 Portable Stereo Wave Player with Mobile Phone Display Interface 241
Chapter Review 241
Quiz 241
References 242
11.1 What is a Digital Signal Processor?
Digital signal processing (DSP) refers to the computation of mathematically intensive
algorithms applied to data signals, such as audio signal manipulation, video compression,
datacoding/decodinganddigital communications. A digital signal processor, also informally
calledaDSPchip, isaspecial typeof microprocessorusedforDSPapplications. A DSP chip
providesrapidinstructionsequences, suchasshift-and-addandmultiply-and-add(sometimes
called multiply-and-accumulate or MAC), which are commonly used in signal processing
algorithms. Digital filteringandfrequency analysis withtheFouriertransformrequires many
numbers to be multiplied and added together, so a DSP chip provides specific internal
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00011-8
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
219
220 Chapter 11
hardwareandassociatedinstructionstomaketheseoperationsrapidinoperationandeasierto
code in software.
A DSP chip is therefore particularly suitable for number crunching and mathematical
algorithm implementations. It is possible to perform DSP applications with any
microprocessor or microcontroller, although specific DSP chips will outperform a standard
microprocessor with respect to execution time and code size efficiency.
11.2 Digital Filtering Example
Filters are used to remove chosen frequencies from a signal, as shown in Figure 11.1, for
example. Here, thereisasignal withbothlow-frequency andhigh-frequency components. We
may wishto removeeitherthelow-frequency component, by implementingahigh-passfilter
(HPF), or the high-frequency component, by implementing a low-pass filter (LPF). A filter
1
HP filter
0
0.5
voltage
1
voltage
0.5
-0.5
-1
0
-0.5
0
20
40 60
1
80 100 120 140 160 180 200
time, ms
-1
0
LP filter
20
40
60
voltage
0.5
0
-0.5
-1
0
20
40
60
80 100 120 140 160 180 200
time, ms
Figure 11.1:
High-pass and low-pass filtered signals
80 100 120 140 160 180 200
time, ms
An Introduction to Digital Signal Processing 221
has acut-off frequency, whichdetermines whichsignal frequencies areinthepass-band(and
are still evident afterthe filtering) and whicharein thestop-band (whichare removed by the
filteringoperation). Thecut-off effect isnot perfect, however, as frequenciesinthestop-band
are attenuated more the further away from the cut-off frequency they are. Filters can be
designed to have different steepness of cut-off attenuation and so adjust the filter’s roll-off
rate. Ingeneral, themorecomplex thefilterdesign, thesteeperitsroll-off canbe. Thefiltering
can be performed with an active or a passive analog filter, but the same process can also be
performed in software with a DSP operation.
Wewill not go deeply into themathematicsof digital filtering, but thesoftwareprocessrelies
heavily on addition and multiplication. Figure 11.2 shows an example block diagram for
a simple digital filtering operation.
Figure11.2showsthatcoefficienta1 ismultipliedby themostrecentsamplevalue. Coefficient
a2 is multiplied by the previous sample and a3 is multiplied by the sample before that. The
valuesof thefilter tapsdeterminewhetherthefilterishighpassorlowpassandwhatthecut-off
frequency is. The order of the filter is given by the number of delays that are used, so the
example shown in Figure 11.2 is a third order filter. The order of the filter determines the
steepnessof thefilterroll-off curve; atoneextremeafirstorderfiltergivesavery gentleroll-off,
while at the other an eighth order is used for demanding anti-aliasing applications.
This filter is an example of a finite impulse response (FIR) filter, because it uses a finite
numberof input values to calculatetheoutput value. Findingtherequiredvalues forthefilter
taps is a complex process, but many design packages exist to simplify this process. As an
Figure 11.2:
Third order FIR filter
222 Chapter 11
example, if thefiltertapsinFigure11.2all haveavalueof 0.25, thenthisimplementsasimple
meanaveragefilter(acrudeLPF) whichcontinuously averages thelast fourconsecutivedata
values.
It can be seen that the filter has a number of multiply and addition processes, which can be
grouped into a single MAC operation. A DSP chip has a special area of hardware for
processing MAC operations. This is specifically designed so that MAC commands can be
processed in a single clock cycle, i.e. much faster than on conventional processors. It should
be noted that although the mbed is not a dedicated DSP processor, it is still powerful enough
to perform many DSP operations.
11.3 An mbed DSP Example
11.3.1 Input and Output of Digital Data
We can develop an mbed program which reads signal data in via the analog-to-digital
converter (ADC), processes the data digitally and then outputs the signal via the digital-toanalog converter (DAC). Since DSP algorithms need to sample and process data at regular
fixedintervals, wewill usethembedTimerinterruptsto enableascheduledprogram. Wewill
now write a programthat reads audio data and implements low- and high-pass digital filters.
In these examples, the data will be output from the mbed’s DAC and through some simple
electronic circuitry. The resulting audio output can then be routed to a loudspeaker amplifier
(such as a set of portable PC speakers) so that you can listen to the processed signal.
First, weneedasignal sourcetoinputtothembed. A simpleway tocreateasignal sourceisto
use ahost PC’s audio output while playinganaudio file of thedesired signal data. A number
of audio packages, such as Steinberg Wavelab, can be used to create wave audio files (with
a .wav file extension); here, we will use three audio files as follows:
• 200hz.wav e an audio file of a 200Hz sine wave
• 1000hz.wav e an audio file of a 1000Hz sine wave
• 200hz1000hz.wav e an audio file with the 200Hz and 1000Hz audio mixed.
Each audio signal should be mono and around 60 seconds in duration. These files are
available for download from the book website.
The audio files can be played directly from the host PC into a set of headphones, and the
differentsinewavesignalscanbeheard. Nowwecanconnecttheaudio signal to thembedby
connecting the host PC’s audio cable to an mbed analog input pin. The signal can also be
viewed on an oscilloscope. You will see that the signal oscillates positiveand negativeabout
0V. Thisisnotmuchuseforthembed, asitcanonly readanalogdatabetween0and3.3V, so
all negative data will be interpreted as 0V.
An Introduction to Digital Signal Processing 223
3.3V
47k
audio
signal
in
to mbed
analog
input pin
+
10uF
47k
GND
Figure 11.3:
Input circuit with coupling capacitor and bias resistors
Because the mbed can only accept positive voltage inputs, it is necessary to add a small
coupling and biasing circuit to offset the signal to a midpoint of approximately 1.65V. The
offsethereisoftenreferredto asadirectcurrent(DC) offset. ThecircuitshowninFigure11.3
effectively couplesthehostPC’saudio outputto thembed. Createanewprojectandenterthe
code of Program Example 11.1.
/* Program Example 11.1 DSP input and Output
*/
#include "mbed.h"
//mbed objects
AnalogIn Ain(p15);
AnalogOut Aout(p18);
Ticker s20khz_tick;
//function prototypes
void s20khz_task(void);
//variables and data
float data_in, data_out;
//main program start here
int main() {
s20khz_tick.attach_us(&s20khz_task,50); // attach task to 50us tick
}
// function 20khz_task
void s20khz_task(void){
data_in=Ain;
data_out=data_in;
Aout=data_out;
}
Program Example 11.1 DSP input and output
ProgramExample 11.1 first defines analog input and output objects (data_in and data_out)
andasingleTickerobjectcalleds20khz_tick. Thereisalso afunctioncalleds20khz_task( ).
224 Chapter 11
The main( ) function simply assigns the 20kHz Ticker to the 20kHz task, with a Ticker
interval of 50ms (which sets up the 20kHz rate). The input is now sampled and processed at
this regular rate, within s20khz_task( ).
n
Exercise 11.1
Compile Program Example 11.1 and use a two-channel oscilloscope to check that the
analog input signal and the DAC output signal are similar. Use the oscilloscope to see
how accurate the DAC output is with respect to the analog input signal for all three
audio files. Consider amplitude, phase and the waveform profile.
You will also need to implement the input circuit shown in Figure 11.3.
n
11.3.2 Signal Reconstruction
If you look closely at the audio signals, particularly the 1000Hz signal or the mixed signal,
you will see that the DAC output has discrete steps. This is more obvious in the highfrequency signal as it is closer to the sampling frequency chosen, as shown in Figure 11.4a.
With many audio DSP systems, the analog output from the DAC is converted to
a reconstructed signal by implementing an analog reconstruction filter, which removes all
steps from the signal, leaving a smooth output. In audio applications, a reconstruction filter
is usually designed to be an LPF with a cut-off frequency at around 20kHz, because the
humanhearingrange does not exceed 20kHz. Thereconstructionfiltershownin Figure11.5
can be implemented with the current project (which gives the complete DSP input/output
circuitasshowninFigure11.6). NotethataftertheLPF, adecouplingcapacitor isalso added
to remove the 1.65V DC offset from the signal. Once the DC offset has been removed,
the signal can be routed to a loudspeaker amplifier to monitor the processed DAC output.
The mathematical theory associated with digital sampling and reconstruction is complex and
beyondthescopeof thisbook. AsdiscussedinChapter4, inaudio samplingsystemsit isoften
necessary to add an anti-aliasing filter prior to the analog-to-digital conversion. For simplicity
this extra filter has not been implemented here. For those interested, the theory of sampling,
aliasing and reconstruction is described well and in detail by a number of authors including
Marven and Ewers (Reference 11.1) and Proakis and Manolakis (Reference 11.2).
11.3.3 Adding a Digital Low-Pass Filter
We will add a digital low-pass filter routine to filter out the 1000Hz frequency component.
This can be assigned to a switch input so that the filter is implemented in real time when
a push-button is pressed.
An Introduction to Digital Signal Processing 225
(a)
without reconstuction filter
5.0
V
4.0
5.0
V
4.0
3.0
3.0
2.0
2.0
1.0
1.0
0.0
0.0
-1.0
-1.0
-2.0
-2.0
(b)
with reconstuction filter implemented
5.0
V
4.0
5.0
V
4.0
3.0
3.0
2.0
2.0
1.0
1.0
0.0
0.0
-1.0
-1.0
-2.0
-2.0
Figure 11.4:
Signal output
DAC
output
1.5 k
+0.1 uF
reconstructed
audio signal
4.7 nF
GND
Figure 11.5:
Analog reconstruction filter and decoupling capacitor
This example will use a third order infinite impulse response (IIR) filter, as shown in
Figure 11.7. The IIR filter uses recursive output data (i.e. data fed back from the output), as
well as input data, to calculate the filtered output.
47k
audio
signal in
+
10uF
47k
GND
reconstructed
signal out
0.1uF
+
1.5k
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
4.7nF
GND
Figure 11.6:
DSP input/output circuit
Figure 11.7:
Third order digital IIR filter
mbed Microcontroller
226 Chapter 11
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
An Introduction to Digital Signal Processing 227
This filterresults inthefollowingequationforcalculatingthefilteredvaluegiventhecurrent
input, the previous three input values and the previous three output values:
yðnÞ¼b0xðnÞþ b1xðn 1Þþ b2xðn 2Þþ b3xðn
þ a1yðn 1Þþ a2yðn 2Þþ a3yðn 3Þ
3Þ
11.1
where x(n) is the current data input value; x(n 1) is the previous data input value, x(n 2)
is the data value before that, and so on; y(n) is the calculated current output value; y(n 1) is
the previous data output value y(n 2) is the data value before that, and so on; and a0e3 and
b0e3 are coefficients (filter taps) that define the filter’s performance.
Wecanimplement this equationto achievefiltered datafromtheinput data. Thechallenging
task is determining the values required for the filter coefficients to give the desired filter
performance. Filter coefficients can, however, be calculated by a number of software
packages, such as the Matlab Filter Design and Analysis Tool (Reference 11.3), and those
provided in Reference 11.4.
The LPF designed for a 600Hz cut-off frequency (third order with 20kHz sampling
frequency) can be implemented as a C function as shown in Program Example 11.2. We
chose a 600Hz LPF because the cut-off is exactly half way between the two signal
frequencies.
/*Program Example 11.2 Low-pass filter function
*/
float LPF(float LPF_in){
float a[4]={1,2.6235518066,-2.3146825811,0.6855359773};
float b[4]={0.0006993496,0.0020980489,0.0020980489,0.0006993496};
static float LPF_out;
static float x[4], y[4];
x[3] = x[2]; x[2] = x[1]; x[1] = x[0];
y[3] = y[2]; y[2] = y[1]; y[1] = y[0];
// move x values by one sample
// move y values by one sample
x[0] = LPF_in;
// new value for x[0]
y[0] = (b[0]*x[0]) + (b[1]*x[1]) + (b[2]*x[2]) + (b[3]*x[3])
+ (a[1]*y[1]) + (a[2]*y[2]) + (a[3]*y[3]);
LPF_out = y[0];
return LPF_out;
// output filtered value
}
Program Example 11.2 Low-pass filter function
Here, wecanseethecalculatedfiltervaluesforthea andb coefficients. Thesecoefficientsare
deduced by the online calculator provided by Reference 11.4. Note also that we have used
the static variable type for the internally calculated datavalues. The static definition ensures
that data calculated are held even after the function is complete, so the recursive data values
for previous samples are held within the function and not lost during program execution.
228 Chapter 11
n
Exercise 11.2
Create a new mbed program based on Program Example 11.1, only this time add the
function for the LPF seen in Program Example 11.2. Now in the 20 kHz task process
the input data by feeding it through the LPF function, for example:
data_out=LPF(data_in);
Compile and run the code to check that high-frequency components are filtered from
the mbed’s analog output.
Your system should use the mbed with the input and output circuitry shown in
Figure 11.6.
n
11.3.4 Adding a Push-Button Activation
Wecannowassignaconditional statementto adigital input, allowingthefilterto beswitched
in and out in real-time. The following conditional statement implemented in the 20kHz task
will allow real-time activation of the digital filter:
data_in=Ain-0.5;
if (LPFswitch==1){
data_out=LPF(data_in);
}
else {
data_out=data_in;
}
Aout=data_out+0.5;
You will notice that before performing the calculation, the mean value of the signal is
subtracted, in the line:
data_in=Ain-0.5;
Thisisto normalizethesignal to anaveragevalueof zero, so thesignal oscillatespositiveand
negative and allows the filter algorithm to perform DSP with no DC offset in the data. As
the DAC anticipates floating point data in the range 0.0e1.0, we must also add the mean
offset back to the data before we output, in the line:
Aout=data_out+0.5;
n
Exercise 11.3
Implement the real-time push-button activation in your LPF program. You can now use
theoscilloscopeor headphonesto listen to thesignal which includesboth the200Hzand
1000Hz signal played simultaneously. When the switch is pressed, the high-frequency
An Introduction to Digital Signal Processing 229
component should be removed, leaving just the low-frequency component audible, or
visible on the oscilloscope.
n
11.3.5 Digital High-Pass Filter
The implementation of an HPF function is identical to the low-pass function, but with
different filter coefficient values. The filter coefficients for a 600Hz HPF (third order with
a 20kHz sample frequency) are as follows:
float a[4]={1,2.6235518066,-2.3146825811,0.6855359773 };
float b[4]={0.8279712953,-2.4839138860,2.4839138860,-0.8279712953};
n
Exercise 11.4
Add a second switch to the circuit and a filter function to the program, to act as an
HPF. This switch will enable filtering of the low-frequency component and leave the
high-frequency signal. Implement the second function with a second conditional
statement in the 20 kHz task, so that either the LPF or HPF can be activated in real
time. You should now beableto listen to thesimultaneous 200 Hz and 1000 Hz audio
signal and filter either thelow frequency or thehigh frequency, or both, dependent on
which switch is pressed.
n
11.4 Delay/Echo Effect
Many audio effects use DSP systems for manipulating and enhancing audio. These can be
useful forlive audio processing(e.g. guitar effects) orforpost-production. Audio production
DSP effects includeartificial reverb, pitchcorrection, dynamic rangemanipulationandmany
other techniques to enhance captured audio.
A feedback delay can be used to make an echo effect, which sees a single sound
repeated a number of times. Each time the signal is repeated it is attenuated until it
eventually decays away. Therefore, the speed of repetition and the amount of feedback
attenuation can be manipulated. This effect is used commonly as a guitar effect, and for
vocal processing and enhancement. Figure 11.8 shows a block diagram design for
a simple delay effect unit.
To implement the systemdesign shown in Figure 11.8, historical sample data must be stored
so that it canbemixedback inwithimmediatedata. Therefore, sampleddigital dataneeds to
be copied into a buffer (a large array) so that the feedback data is always available. The
feedback gain determines how much buffer data is mixed with the sampled data.
230 Chapter 11
Figure 11.8:
Delay effect block diagram
For each sample coming in, an earlier value is mixed in also, but the length of time between
the immediate and historical value can be varied by the delay time potentiometer.
Effectively, thischangesthesizeof thedatabufferby resettinganarray counterbasedonthe
value of the delay time input. If the delay time input is large, a large number of array
datawill befedback beforeresetting, resultinginalongdelay time. Forsmallerdelay values
the array counter will reset sooner, giving a more rapid delay effect.
/* Program Example 11.3 Delay / Echo Effect
*/
#include "mbed.h"
AnalogIn Ain(p15);
//object definitions
AnalogOut Aout(p18);
AnalogIn delay_pot(p16);
AnalogIn feedback_pot(p17);
Ticker s20khz_tick;
void s20khz_task(void);
// function prototypes
#define MAX_BUFFER 14000
// max data samples
signed short data_in;
// signed allows positive and negative
unsigned short data_out;
// unsigned just allows positive values
float delay=0;
float feedback=0;
signed short buffer[MAX_BUFFER]={0};
// define buffer and set values to 0
int i=0;
//main program start here
int main() {
s20khz_tick.attach_us(&s20khz_task,50);
}
// function 20khz_task
void s20khz_task(void){
data_in=Ain.read_u16()-0x7FFF;
buffer[i]=data_in+(buffer[i]*feedback);
data_out=buffer[i]+0x7FFF;
// read data and normalize
// add data to buffer data
// output buffer data value
An Introduction to Digital Signal Processing 231
Aout.write_u16(data_out);
if (i>(delay)){
i=0;
delay=delay_pot*MAX_BUFFER;
feedback=(1-feedback_pot)*0.9;
}else{
i=i+1;
}
//
//
//
//
//
write output
if delay loop has completed
reset counter
calculate new delay buffer size
calculate feedback gain value
// otherwise increment delay counter
}
Program Example 11.3 Delay/echo effect
Notice that the data_in and data_out values are defined as follows:
signed short data_in;
unsigned short data_out;
// signed allows positive and negative
// unsigned just allows positive values
The short data type (see Table B.4) ensures that the data is constrained to 16bit values; this can be specified as signed (i.e. to use the range 32768 to
32767 decimal) or unsigned (to use the range 0 to 65535). We need the
computation to take place with signed numbers. When outputting to the mbed’s DAC,
however, this needs an offset adding owing to the fact that DAC is configured to work
with unsigned data.
C code
feature
The initial mbed hardware setup shown in Figure 11.6 can be used here. This demonstrates
the advantage of a single hardware design that can operate multiple software features in
adigital system. Wewill needto addpotentiometersto mbedanaloginputsinorderto control
the feedback speed and gain in real time, as shown in Figure 11.8. Program Example 11.3
definesthedelay andfeedback potentiometersbeingconnectedto mbedanaloginputsonpins
16 and 17, respectively.
n
Exercise 11.5
Implement a new project with the code shown in Program Example 11.3. Initially, you
will need to useatest signal which givesashort pulsefollowed byaperiod of inactivityto
evaluate the echo effect performance. You should see a similar echo response to that
shown in Figure 11.9. Verify that the delay and feedback gain potentiometers alter
the output signal as expected. For testing purposes, a pulse signal called pulse.wav can
be downloaded from the book website.
n
Figure 11.9 shows input and output waveforms for the delay/echo effect. It can be seen that
for a single input pulse, the output is a combination of the pulse plus repeated echoes of that
pulse with slowly diminishing amplitude. The rate of echo and the rate of attenuation are
alteredby adjustingthepotentiometersasdescribed. Thisprojectcanbedevelopedasaguitar
232 Chapter 11
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1.5
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2.5
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Pico Technology www.picotech.com
500.0
mV
DC
400.0
300.0
200.0
100.0
0.0
-100.0
-200.0
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-400.0
-500.0
0.0
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0.25
0.5
0.75
1.5
1.0
1.25
Pico Technology www.picotech.com
Figure 11.9:
Input signal (top) and output signal (bottom) for the digital echo effect system
effect unit by adding extra signal conditioning, variable amplification stages and enhanced
output conditioning. Some good examples are given in Reference 11.5.
11.5 Working with Wave Audio Files
Thischapter, sofar, hasanalyzedaDSPapplicationwhichcapturesdata, manipulatesthedata
andthenoutputsit. However, anumberof signal processingapplicationsrely ondatathat has
previously been captured and stored in data memory. Continuing with examples relating to
An Introduction to Digital Signal Processing 233
digital audio, this can be explored by analyzing wave audio files and evaluating the
requirements in order to output a continuous and controlled stream of audio data.
11.5.1 The Wave Information Header
There are many types of audio data file, of which the wave (.wav) type is one of the most
widely used. Wave files contain a number of details about the audio data followed by the
audio data itself. Wave files contain uncompressed (i.e. raw) data, mostly in a format known
as linear pulse code modulation (PCM). Since PCM specifically refers to the coding of
amplitude signal data at a fixed sample rate, each sample value is given to a specified
resolution (often16-bit) on alinear scale. Timedataforeachsampleis not recorded because
the sample rate is known, so only amplitude data is stored. The wave header information
details theactual resolutionandsamplefrequency of theaudio, so by readingthis headerit is
possibleto accurately decodeandprocess thecontainedaudio data. Thefull waveheaderfile
description is shown in Table 11.1.
It is possible to identify much of thewave header information simply by opening a .wav file
withatext editorapplication, asshowninFigure11.10. HereweseetheASCII charactersfor
Table 11.1: Wave file information header structure
Data name
Offset
(bytes)
Size
(bytes)
Details
ChunkID
0
4
The characters ‘RIFF’ in ASCII
ChunkSize
4
4
Details the size of the file from byte 8 onwards
Format
8
4
The characters ‘WAVE’ in ASCII
Subchunk1ID
12
4
The characters ‘fmt’ in ASCII
Subchunk1Size
16
4
16 for PCM
AudioFormat
20
2
PCM ¼1. Any other value indicates data compressed format
NumChannels
22
2
Mono ¼1; stereo ¼2
SampleRate
24
4
Sample rate of the audio data in Hz
ByteRate
28
4
ByteRate¼SampleRate* NumChannels* BitsPerSample/8
BlockAlign
32
2
BlockAlign ¼NumChannels* BitsPerSample/8. The number of
bytes per sample block
BitsPerSample
34
2
Resolution of audio data
SubChunk2ID
36
4
The characters ‘data’ in ASCII
Subchunk2Size
40
4
Subchunk2Size¼Number of samples* BlockAlign. Thetotal size
of the audio data in bytes
Data
44
e
The actual data of size given by Subchunk2Size
234 Chapter 11
Figure 11.10:
Wave file opened in text editor
SubChunk1Size
=16 bytes
ChunkID
“RIFF”
SampleRate
=16000 Hz
ChunkSize
=27124
bytes
ByteRate
=32000
bits per
second
Format
“WAVE”
BlockAlign
=2 bytes
BitsPerSample
=16 bits
Subchunk1ID
“fmt “
NumChannels
=1 (mono)
AudioFormat
=1 (PCM)
Subchunk2ID
“data”
First packets of
audio data ...
Subchunk2Size
=27100 bytes
Figure 11.11:
Structure of the wave file header data
ChunkID (‘RIFF’), Format (‘WAVE’), Subchunk1ID (‘fmt’) and Subchunk2ID (‘data’).
These are followed by the ASCII data which makes up the raw audio.
Looking more closely at the header information for this file in hexadecimal format, the
specific values of the header information can be identified, as shown in Figure 11.11. Note
also that for each value made up of multiple bytes, the least significant byte is always given
first andthemost significantbytelast. Forexample, thefourbytesdenotingthesamplerateis
given as 0x80, 0x3E, 0x00 and 0x00, which gives a 32-bit value of 0x00003E80¼16000
decimal.
An Introduction to Digital Signal Processing 235
11.5.2 Reading the Wave File Header with the mbed
In order to accurately read and reproduce wave data via a DAC, we need to interpret the
information given in the header data, and configure the read and playback software features
with the correct audio format (from AudioFormat), number of channels (NumChannels),
sample rate (SampleRate) and the data resolution (BitsPerSample).
Toimplementwavefilemanipulationonthembed,thewavedatafileshouldbestoredonaSecure
Digital (SD) memory cardandtheprogramshouldbeconfiguredwiththecorrect SD reader
libraries,asdiscussedinChapter10. TheSDcardisrequiredpredominantlybecausewaveaudio
filesaregenerallyquitelarge,muchtoolargetobestoredinthembed’sinternal memory,butalso
becausethefileaccessthroughtheserial peripheral interface(SPI)/SDcardinterfaceisveryfast.
ProgramExample 11.4 reads a wave audio file called test.wav, utilizes the fseek( ) function
used to move the file pointer to the relevant position, and then reads the header data and
displays this to a host terminal. Notealso that to read .wav files onthe mbed, youshould use
the ‘8.3 filename’ convention (sometimes referred to as short file name or SFN) for audio
files. TheSFNconventionstipulatesthat filenamesshouldnot belongerthaneight characters
with a three-character extension.
/* Program Example 11.4 Wave file header reader
*/
#include "mbed.h"
#include "SDFileSystem.h"
SDFileSystem sd(p5, p6, p7, p8, "sd");
Serial pc(USBTX,USBRX);
// set up terminal link
char c1, c2, c3, c4;
// chars for reading data in
int AudioFormat, NumChannels, SampleRate, BitsPerSample ;
int main() {
pc.printf("\n\rWave file header reader\n\r");
FILE *fp = fopen("/sd/sinewave.wav", "rb");
fseek(fp, 20, SEEK_SET);
fread(&AudioFormat, 2,
if (AudioFormat==0x01)
pc.printf("Wav file is
}
else {
pc.printf("Wav file is
}
// set pointer to byte 20
1, fp);
// check file is PCM
{
PCM data\n\r");
not PCM data\n\r");
fread(&NumChannels, 2, 1, fp);
// find number of channels
pc.printf("Number of channels: %d\n\r",NumChannels);
fread(&SampleRate, 4, 1, fp);
// find sample rate
pc.printf("Sample rate: %d\n\r",SampleRate);
fread(&BitsPerSample, 2, 1, fp); // find resolution
pc.printf("Bits per sample: %d\n\r",BitsPerSample);
fclose(fp);
}
Program Example 11.4 Wave header reader
236 Chapter 11
Try reading the header of a number of different wave files and verify that the correct
information is always read to a host PC. A wave audio file can be created in many simple
audio packages, such as Steinberg Wavelab, or can be extracted from a standard music
compact disc with music player software such as iTunes or Windows Media Player. When
reading different wavefiles remember to update the fopen( ) call to use the correct filename.
In Program Example 11.4 the fread( ) function is used to read a number of data
bytesinasinglecommand. Examplesof fread( ) showthatthememory addressfor
the data destination is specified, along with the size of each data packet (in bytes)
andthetotal numberof datapackets to beread. Thefilepointeris also given. Forexample, the
command
C code
feature
fread(&SampleRate, 4, 1, fp);
// find sample rate
reads asingle4-bytedatavaluefromthedatafilepointedto by fp andplaces thereaddataat
the internal memory address location of variable SampleRate.
n
Exercise 11.6
Extend Program Example 11.4 to display ByteRate and Subchunk2Size, which
indicates the size of the raw audio data within the file.
n
11.5.3 Reading and Outputting Mono Wave Data
Having accessed the wave file and gathered important information about its characteristics,
it is possible to read the raw audio data and output that from the mbed’s DAC. To do this,
we need to understand the format of the audio data. Initially, we will use an oscilloscope to
verify that the data outputs correctly, but it is possible to output the audio data to
a loudspeaker amplifier. For this example, we will use a 16-bit mono wave file of a pure
sine wave of 200Hz (Figure 11.12). The same sine wave as utilized in Section 11.3 can be
used here.
The audio data in a 16-bit mono .wav file is arranged similarly to the other data seen in the
header. The data starts at byte 44 and each 16-bit data value is read as two bytes with the
least significant byte first. Each 16-bit sample can be outputted directly on pin 18 at the rate
defined by SampleRate. It is very important to take good care of data input and output
timing when working with audio, as any timing inaccuracies in playback can be heard as
clicks or stutters. This can be a challenge because interrupts and timing overheads
sometimes make it difficult for the audio to be streamed directly from the SD card at
a constant rate. Therefore, a buffered system is used to enable accurate timing control.
An Introduction to Digital Signal Processing 237
Figure 11.12:
A 200 Hz sine wave .wav file played in Windows Media Player
A good method to ensure accurate timing control when working with audio data is to use
acircular buffer, as showninFigure11.13. Thebufferallows datato bereadinfromthedata
fileandreadoutfromitto aDAC. If thebuffercanholdanumberof audio datasamples, then,
aslongastheDAC outputtimerisaccurate, itdoesnotmattermuchif thetimefordatabeing
read and processed from the SD card is somewhat variable. When the circular buffer write
pointerreachesthelastarray element, itwrapsaroundsothatthenextdataisreadintothefirst
memory elementof thebuffer. Thereisaseparatebufferreadpointerforoutputtingdatatothe
DAC, and this lags behind the write buffer.
It can be seen that two important conditions must be met for the circular buffer method to
work; first, thedatawrittento thebuffermust bewrittenat anequal orfasterratethandatais
read fromthe buffer (so that thewrite pointer stays ahead of the read pointer); and secondly,
Figure 11.13:
Circular buffer example
238 Chapter 11
thebuffermustnevercompletely fill up, orelsethereadpointerwill beovertakenby thewrite
pointer and data will be lost. It is therefore important to ensure that the buffer size is
sufficiently large, or to implement control code to safeguard against data wrapping.
Program Example 11.5 reads a 16-bit mono audio file (called in this example testa.wav,
whichcanbedownloadedfromthebook website) andoutputs at afixedsampleratewhichis
acquiredfromthewavefileheader. As discussedabove, thecircularbufferis usedto ironout
the timing inconsistencies found with reading from the wave data file.
/* Program Example 11.5: 16-bit mono wave player
*/
#include "mbed.h"
#include "SDFileSystem.h"
#define BUFFERSIZE 4096
SDFileSystem sd(p5, p6, p7, p8, "sd");
AnalogOut DACout(p18);
Ticker SampleTicker;
int SampleRate;
float SamplePeriod;
int CircularBuffer[BUFFERSIZE];
int ReadPointer=0;
int WritePointer=0;
bool EndOfFileFlag=0;
void DACFunction(void);
// number of data in circular buffer
// sample period in microseconds
// circular buffer array
// function prototype
int main() {
FILE *fp = fopen("/sd/testa.wav", "rb");
// open wave file
fseek(fp, 24, SEEK_SET);
// move to byte 24
fread(&SampleRate, 4, 1, fp);
// get sample frequency
SamplePeriod=(float)1/SampleRate;
// calculate sample period as float
SampleTicker.attach(&DACFunction,SamplePeriod); // start output tick
while (!feof(fp)) {
// loop until end of file is encountered
fread(&CircularBuffer[WritePointer], 2, 1, fp);
WritePointer=WritePointer+1;
// increment Write Pointer
if (WritePointer>=BUFFERSIZE) {
// if end of circular buffer
WritePointer=0;
// go back to start of buffer
}
}
EndOfFileFlag=1;
fclose(fp);
}
// DAC function called at rate SamplePeriod
void DACFunction(void) {
if ((EndOfFileFlag==0)j(ReadPointer>0)) {
// output while data available
DACout.write_u16(CircularBuffer[ReadPointer]); // output to DAC
ReadPointer=ReadPointer+1;
// increment pointer
if (ReadPointer>=BUFFERSIZE) {
ReadPointer=0;
// reset pointer if necessary
}
}
}
Program Example 11.5 Wave file player with utilizing a circular buffer
An Introduction to Digital Signal Processing 239
(a) raw data output (before correction)
(b) after two’s-complement correction
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Figure 11.14:
Wave file sine wave
When Program Example 11.5 is implemented with a pure mono sine wave audio file,
initially the results will not be correct. Figure 11.14a shows the actual oscilloscope
trace of a sine wave read using this program example. Clearly, this is not an accurate
reproduction of the sine wave data. The error occurs because the wave data is coded in
16-bit two’s-complement form, which means that positive data occupies data values
0 to 0x7FFF and values 0x8000 to 0xFFFF represent the negative data. Two’s
complement arithmetic is introduced in Appendix A. A simple adjustment of the data
is therefore required in order to output the correct waveform as shown in
Figure 11.14b.
n
Exercise 11.7
Modify Program Example 11.5 to include two’s complement correction and ensure
that, when using an audio file of a mono sine wave, the output data observed on an
oscilloscope is that of an accurate sine wave. Appendix A should help you to work out
how to do this.
n
11.6 Summary on DSP
As shown in this chapter, DSP techniques require knowledge of a number of mathematical
and data handling aspects. In particular, attention to detail of timing and data validity is
required to ensure that no dataoverflow errors or timing inconsistencies occur. We have also
looked at a new type of data file which holds signal data to be output at a specific and
240 Chapter 11
controlledrate. Theability to apply simpleDSPtechniques, whetheronadedicatedDSPchip
or on a general-purpose processor like the mbed, hugely expands our capabilities as
embedded system designers.
11.7 Mini-Project: Stereo Wave Player
This project can be done in basic form, or with several extensions.
11.7.1 Basic Stereo Wave Player
A stereo wave player program can be developed by adding additional elements to Program
Example 11.5. You will need to update the programto first analyzewhether thewave data is
stereo or not, and then implement a conditional feature for the case where stereo data is
stored.
Stereo wave datais stored inconsecutivesamples forleft andright stereo playback as shown
inFigure11.15. If usingjustthestandardmbedanalogoutputyouwill needtoaveragetheleft
and right channel data to mono before outputting to the DAC. Full stereo ADC and DAC
chips, such as the Texas Instruments TLV320AIC23b, can be purchased and interfaced with
the mbed through its serial ports.
11.7.2 Stereo Wave Player with PC Interface
Addauserinterfacethatwill display all thewavefilenamesstoredonanSD cardto ahostPC
terminal. The user can then select a file to be played by selecting a number that represents
the chosen data file.
Right channel data
16-bit (LSB-MSB)
Left channel data
16-bit (LSB-MSB)
Right channel data
16-bit (LSB-MSB)
Left channel data
16-bit (LSB-MSB)
Figure 11.15:
Interleaved left and right channel stereo data
An Introduction to Digital Signal Processing 241
11.7.3 Portable Stereo Wave Player with Mobile Phone Display Interface
Experiment with using a liquid crystal display (LCD) mobile phone display and digital
push-buttons to develop your own portable audio player.
Chapter Review
• DSP systems and algorithms are used for managing and manipulating streams of data
and therefore require high precision and timing accuracy.
• A digital filtering algorithm can be used to remove unwanted frequencies from a
data stream. Similar mathematical algorithms can be used for signal analysis, audio/video
manipulation and data compression for communications.
• A DSP systemcommunicates withtheexternal worldthroughanalog-to-digital converters
and digital-to-analog converters, so the analog elements of the system also require
careful design.
• DSP systems usually rely on regularly timed data samples, so the mbed Timer and
Ticker interfaces are useful for programming regular and real-time processing.
• Wave audio files hold high-resolution audio data which can be read from an SD card
and output through the mbed DAC.
• Data management and effective buffering are required to ensure that timing and
data overflow issues are avoided.
Quiz
1.
2.
3.
4.
5.
6.
7.
8.
9.
What does the term MAC refer to and why is this important for DSP systems?
What differentiates a DSP microprocessor from a microcontroller?
What is the difference between an FIR and an IIR digital filter?
What are digital filter coefficients and how can they be acquired for a specific digital
filter design?
Explaintheroleof analogbiasingandanti-aliasingwhenperformingananalog-to-digital
conversion.
What is a reconstruction filter and where would this be found in a DSP system?
What is a circular buffer and why might it be used in DSP systems?
What are the potential effects of poor timing control in an audio DSP system?
A wave audio file has a 16-bit mono data value given by two consecutive bytes. What
will be the correct corresponding voltage output, if this is output through the mbed’s
DAC, for the following data?
(a) 0x35 0x04
(b) 0xFF 0x5F
(c) 0x00 0xE4
242 Chapter 11
10. Drawablock diagramdesignof aDSP process formixingtwo 16-bit datastreams. If the
data output is also to be 16 bit, what consequences will this process have on the output
data resolution and accuracy?
References
11.1.
11.2.
11.3.
11.4.
11.5.
Marven, C. and Ewers, G. (1996). A Simple Approach to Digital Signal Processing. Wiley Blackwell.
Proakis, J. G. and Manolakis, D. K. (1992). Digital Signal Processing: Principles, Algorithms and
Applications. Prentice Hall.
The Mathworks (2010). FDATool e open filter design and analysis tool. http://www.mathworks.com/
help/toolbox/signal/fdatool.html
Fisher, T. (2010). Interactive Digital Filter Design. Online Calculator. http://www-users.cs.york.ac.uk/
~fisher/mkfilter/
Sergeev, I. (2010). Audio Echo Effect. http://dev.frozeneskimo.com/embedded_projects/audio_echo_
effect
CHAPTER 12
Advanced Serial Communications
Chapter Outline
12.1 Introducing Advanced Serial Communication Protocols 243
12.2 Bluetooth Serial Communication 244
12.2.1 Introducing Bluetooth 244
12.2.2 Interfacing the RN-41 and RN-42 Bluetooth Modules 244
12.2.3 Sending mbed Data over Bluetooth 245
12.2.4 Receiving Bluetooth Data from a Host Terminal Application 247
12.2.5 Communicating between Two mbeds on Bluetooth 248
12.3 Introducing USB 251
12.3.1 Using the mbed to Emulate a USB Mouse 252
12.3.2 Sending USB MIDI Data from an mbed 253
12.4 Introducing Ethernet 255
12.4.1 Ethernet Overview 255
12.4.2 Implementing Simple mbed Ethernet Communications 257
12.4.3 Ethernet Communication between mbeds 259
12.5 Local Network and Internet Communications with the mbed 261
12.5.1 Using the mbed as an HTTP Client 262
12.5.2 Using the mbed as an HTTP File Server 264
12.5.3 Using Remote Procedure Calls to Modify mbed Outputs 266
12.5.4 Controlling the mbed using a Remote JavaScript Interface 267
Chapter Review 270
Quiz 271
References 271
12.1 Introducing Advanced Serial Communication Protocols
We have already discussed a number of communication methods in earlier chapters, and in
particularserial communicationswhichallowfastmessagesto besentandreceivedwithonly
a small numberof physical connections, such as universal asynchronous receiver/transmitter
(UART), serial peripheral interface (SPI) and inter-integrated circuit (I2C). Each technique
requiresaprotocol whichdefinestheway inwhichcommunicationsareinitiatedandhowthe
data messages are structured, effectively a code for untangling all the digital 0s and 1s, in
order to understand a message as meaningful information.
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00012-X
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
243
244 Chapter 12
There are a vast number of serial communications protocols and each is developed to
a specific standard and usually with a very specific purpose or application in mind. For this
reason, eachserial communicationmethodhasitsownadvantagesanddisadvantagesforcertain
applications. Universal serial bus (USB) communications, ontheonehand, work very well for
connectinghigh-level peripheral devicesto personal computers(PCs) andhaveasimple‘plugand-play’ method for installation. Ethernet communications, on the other hand, are very high
speed and have been developed to allow computer-to-computer communications, whichled to
the high-speed Internet networks that we nowadays often take for granted.
This chapter discusses a number of advanced serial communications technologies with
respect to thembed, to act as adoorway towards thedevelopment of moreadvanced projects
relying on digital connectivity and data sharing.
12.2 Bluetooth Serial Communication
12.2.1 Introducing Bluetooth
Bluetoothis anovel formof digital radio communication, operatinginthe2.402e2.480GHz
radio band. Bluetooth provides wireless data links between such devices as mobile phones,
computers, wireless audio headsets and systems requiring the use of remote sensors.
Bluetooth’s main characteristics can be summarized as follows:
• The communications range is up to 100m for Class 1 Bluetooth devices and up to 20m
for Class 2 Bluetooth devices.
• Bluetooth consumes relatively low power; Class 1 and 2 devices use around 100mW and
2.5mW, respectively.
• Data rates up to 3Mbps can be achieved.
• Up to eight devices can be simultaneously linked.
• Spread-spectrum frequency hopping is applied, with the transmitter changing frequency
in a pseudo-random manner 1600 times per second.
The Bluetooth standards, controlled by the Bluetooth Special Interest Group (Reference
12.1), dictate that when Bluetooth devices detect one another, they determine automatically
whether they need to interact with each other. Each device has a Media Access Control
(MAC) address which communicating devices can recognize and initialize interaction if
required. Bluetooth systems in contact with each other forma piconet. Once communication
isestablished, membersof thepiconetsynchronizetheirfrequency hopping, sothey remainin
contact. A singleroomcancontainseveral piconetswithanumberof devicescommunicating.
12.2.2 Interfacing the RN-41 and RN-42 Bluetooth Modules
TheRovingNetworksRN-41(showninFigure12.1) andRN-42Bluetoothmodulesareserial
devicesthatallowreplacementof serial wireswithasimpleBluetoothinterface. Forexample,
Advanced Serial Communications 245
Figure 12.1:
Roving Networks RN-41 Bluetooth module. (Image reproduced with permission of Sparkfun Electronics)
most laptop computers have Bluetooth capability installed, so it is possible to replace the
USB cable to an mbed with a Bluetooth device and allow wireless host terminal
communication. The RN-41 and RN-42 modules have identical control and functionality;
however, theRN-41 is aClass 1 Bluetoothdevice, whereas theRN-42is Class 2. TheRN-41
operates at baud rates between 1200bps and 921kbps and includes auto-detect and
auto-connection features. In this section the RN-41 module is used; however, all software
examples should work equally for an RN-42 device.
In order to communicate with a host PC from an RN-41 or RN-42 device, the host PC must
first have Bluetooth enabled and the device added as a known Bluetooth module. When
initializing the connection to the Bluetoothmodule, it will be necessary to specify a passkey,
which is set by default to ‘1234’ as specified in the Advanced User Manual for Roving
Networks Bluetooth Devices (Reference 12.2).
12.2.3 Sending mbed Data over Bluetooth
TheRN-41hasanumberof configurablefeatures. However, itisvery simpleto useoutof the
box as astandardserial interfacewithasimpleTX/RX connection. TheRN-41 thereforecan
be connected to one of the mbed’s UART serial ports and configured with a simple mbed
serial protocol. The wiring for an RN-41 connected to mbed serial port on pins 9 and 10 is
shown in Figure 12.2.
It is possible to configure an mbed to send arbitrary data over a serial interface to verify that
data is being received by a host terminal application such as Tera Term. Program Example
12.1setsupserial communicationswiththeRN-41andsendsdataovertheUART. Thedatais
actually acontinuouscountthroughtheASCII valuesrepresentingnumerical characters0e9.
The onboard mbed light-emitting diodes (LEDs) are also configured to represent the count
value. In the example we convert the ASCII byte to the relevant numerical value by bit
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
mbed Microcontroller
246 Chapter 12
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
Figure 12.2:
Bluetooth RN-41 connected to the mbed (Image reproduced with permission of Sparkfun Electronics)
maskingandclearingthehigher4bits. Thisleavesjustasinglevaluebetween0x00and0x09.
Inreality thebitmaskingof x isnotessential becausetheled BusOut objectcanonly takethe
lower four bits of the 8-bit anyway. We performthe bit mask here just for completeness, but
you could remove it and check that functionality remains the same.
/* Program Example 12.1: Bluetooth serial test data program
*/
#include "mbed.h"
Serial rn41(p9,p10);
BusOut led(LED4,LED3,LED2,LED1);
int main() {
rn41.baud(115200);
while (1) {
for (char x=0x30;x<=0x39;x++){
rn41.putc(x);
led = x & 0x0F;
wait(0.5);
}
}
}
// set baud for RN-41
// ASCII numerical characters 0-9
// send test char data on serial
// set LEDs to count in binary
Program Example 12.1 Bluetooth serial spammer
If aBluetoothconnectionissuccessfully set uponthehost PC (asdetailedin Section12.2.2),
thedatasent overBluetoothfromProgramExample12.1shouldbevisibleonahost terminal
application.
Advanced Serial Communications 247
n
Exercise 12.1
Power the mbed with a battery (acceptable voltage range 4.5e9V) by connecting the
batteryoutputs to pin 1 (GND) and pin 2 (VIN). This will allow you to removetheUSB
cable and demonstrate fully remote communication over Bluetooth.
n
12.2.4 Receiving Bluetooth Data froma Host Terminal Application
As well as sending data to a PC, it is possible to send data from the host PC to an mbed
over Bluetooth. Program Example 12.2 will receive any data from a host PC terminal
application; the remote mbed displays the lower 4 bits of the received byte on the four
onboard LEDs. This program can be referred to as a ‘sniffer’ program as the mbed
continuously monitors (or ‘sniffs’) the serial connection and performs an action when data
becomes available.
/* Program Example 12.2: Bluetooth serial sniffer program
*/
#include "mbed.h"
Serial rn41(p9,p10);
BusOut led(LED4,LED3,LED2,LED1);
int main() {
rn41.baud(115200);
// setup baud rate
rn41.printf("Serial sniffer: outputs received data to LEDs\n\r");
while (1) {
if (rn41.readable()) {
// if data available
char x=rn41.getc();
// get data
led=x;
// output LSByte to LEDs
}
}
}
Program Example 12.2 Bluetooth serial sniffer program
Program Example 12.2 should run and show wireless communications from the PC to the
mbed. If the user presses numerical values between 0 and 9 on the PC keyboard, the
corresponding binary representation should be shown on the remote mbed.
n
Exercise 12.2
Add a batteryand a liquid crystal display(LCD) to theBluetooth-enabled mbed. Update
ProgramExample12.2 to allow a user to typedata that on thePC keyboard will appear
on the wireless LCD.
n
248 Chapter 12
12.2.5 Communicating between Two mbeds on Bluetooth
By implementing Bluetooth it is possible to have two or more mbeds communicating
wirelessly with each other. The configuration for mbed-to-mbed communication is a little
more involved, however, as it is necessary to perform bespoke RN-41 setup procedures in
softwareduringinitialization. Thisthereforerequiresamuchmorethoroughunderstandingof
the previously mentioned user manual (Reference 12.2). In our example we will use two
mbeds and send data from one to the other. We will therefore refer to the mbed sending the
data as the master and the receiving mbed as the slave. The MAC address of the receiving
systemisrequiredto successfully communicatedata. TheMAC addressof theRN-41module
shown in Figure 12.1 can clearly be seen on the module and the passkey is given a default
value specified in the RN-41 user manual (in this case ‘1234’).
ThemastermbedcommunicateswiththemasterRN-41overUART. ThemasterRN-41needs
to be configured to initialize a Bluetooth connection with the slaveRN-41. The serial UART
procedure for doingthis is summarized as follows and discussed in moredetail intheRN-41
user manual.
• Enter command mode by sending ‘$$$’ to RN-41 module.
• Connect to a remote MAC address by sending ‘C,< address> ’ where < address> is the
MAC address of the remote slave module to be connected to.
• Exit connection mode by sending ‘—< cr> ’, where < cr> is the ASCII carriage return
value, i.e. 0x0D.
Note that there are other useful commands which will generate status responses from the
RN-41, such as:
• ‘D’ e displays basic settings: Address, Name, UART Settings, Security, Pin code,
Bonding, Remote Address
• ‘GB’ e returns the Bluetooth Address of the device
• ‘GK’ e returns the current connection status: 1¼connected, 0¼not connected
• ‘SP,<text>’ e setsthesecurity passkey forpairing(note, thedefault ‘1234’ shouldalready
be set).
ProgramExample12.3showsaninitializationroutineforconnectingamasterRN-41module
to a remote slave RN-41 module. The slave RN-41 module in this example has a MAC
address of 00066607ACC1.
/*Program Example 12.3: function to initialize paired Bluetooth connection with RN-41
modules
*/
void initialize_connection() {
rn41.putc(’$’); // Enter command mode
rn41.putc(’$’); //
Advanced Serial Communications 249
rn41.putc(’$’);
wait(0.5);
//
rn41.putc(’C’);
rn41.putc(’,’);
rn41.putc(’0’);
rn41.putc(’0’);
rn41.putc(’0’);
rn41.putc(’6’);
rn41.putc(’6’);
rn41.putc(’6’);
rn41.putc(’0’);
rn41.putc(’7’);
rn41.putc(’A’);
rn41.putc(’C’);
rn41.putc(’C’);
rn41.putc(’1’);
wait(0.5);
//
// Send MAC address
//
//
//
//
//
//
//
//
//
//
//
//
rn41.putc(’-’);
rn41.putc(’-’);
rn41.putc(’-’);
rn41.putc(0x0D);
wait(0.5);
// Exit command mode
//
//
//
}
Program Example12.3 Function to initializepaired Bluetooth connection with RN-41
modules
Once a connection is initialized, it is possible to communicate data wirelessly from the
master to the slave using simple putc( ) commands. Program Example 12.4 shows the main
codefor sendingacontinuous count variableto theslaveover Bluetooth. An overrideswitch
is also included to allow a user to override the count value and show remote control
capabilities for the slave Bluetooth device.
/* Program Example 12.4: Paired Bluetooth master program
*/
#include "mbed.h"
Serial rn41(p9,p10);
BusOut led(LED4,LED3,LED2,LED1);
DigitalIn Din(p26);
// digital switch input on pin 14
char x;
void initialize_connection(void);
int main() {
rn41.baud(115200);
initialize_connection();
while (1) {
if (Din==1) {
// if digital input switched high
x=0x0F;
// override with 0x0F
} else {
250 Chapter 12
x++;
if (x>0x0F) {
x=0;
}
}
rn41.putc(x);
led = x;
wait(0.5);
// else increment and
// output count value
// send char data on serial
// set LEDs to count in binary
}
}
// add function initialize_connection code here.
Program Example 12.4 Paired Bluetooth master program
TheslaveBluetoothsystemcanusethesamesniffercodeasgiveninProgramExample12.2.
Thetwo mbedsystems (similarto thoseshownin Figure12.3) shouldnowconnect andshare
the same count data wirelessly.
n
Exercise 12.3
Add an LCD to each mbed device. Add also a uniquesensor such as an accelerometer or
ultrasonic range finder to each mbed. Implement a two-way communication program
which allows the data from both sensors to be displayed on both displays
simultaneously.
n
(a) comms master with switch input
(b) comms slave without switch
Figure 12.3:
Two battery-powered mbeds communicating via RN-41 Bluetooth modules
Advanced Serial Communications 251
Bluetooth is an exciting technology that allows short-range wireless communication. This
has many valuable applications where wires are intrusive, expensive or difficult to install.
Recent enhancements to Bluetooth have enabled streaming of high-quality audio data and
increased range, so the opportunities and applications for Bluetooth are continuously
growing.
12.3 Introducing USB
The USB protocol was originally introduced to provide a more flexible interconnection
system, whereby itemscouldbeaddedorremovedwithouttheneedforreconfigurationof the
wholesystem. USB is nowubiquitousandvery familiar, widely usedforitsoriginal purpose,
but also to connect all manner of devices, for example digital cameras, MP3 players,
webcams and memory sticks, to the PC.
A USB network hasonehost, andcanhaveoneormany functions, whichinthiscaserefersto
USB-compatible devices that can interact with the host. It is also possible to include hubs,
which can have a number of functions connected to them, and which in turn link back to the
host. Threedatarates arerecognized: highspeedat 480Mbps, full speedat 12Mbps andlow
speedat 1.5Mbps. Thelast of theseis forvery limitedcapability devices, whereonly asmall
amount of data will be transferred. The specification for USB Version 2.0 is defined in
Reference 12.3.
USB uses afour-wire interconnection. Two wires, labeled Dþ and D , carry the differential
signal, andtwo areforpowerandearth. Withincertainlimits, USB functionscandrawpower
from the bus, taking up to 100mA at a nominal 5V. This is supplied by the host. A higher
power demand can also be requested; alternatively, the functions can be self-powered. When
a device is first attached to the bus the host resets it, assigns it an address and interrogates it
(a process known as enumeration). It thus identifies it and gathers basic operating
information, for example device type, power consumption and data rate. All subsequent
datatransfersareinitiatedonly by thehost. Itfirstsendsadatapacketwhichspecifiesthetype
and direction of data transfer, and the address of the target device. The addressed device
responds as appropriate. There is usually then a handshake packet, to indicate success (or
otherwise) of the transfer.
We have already used the mbed USB interface on a number of occasions: first to connect to
the mbed and to download program binary files, but also to communicate serial data to and
from a host PC, in order to interface with a terminal application. The mbed has two USB
ports: one with a standard USB connector, which we have used already to connect to a PC,
and a second on pins 31 and 32 (labeled Dþ and D on the mbed), as seen in Figure 2.1.
Several USB library features are provided through the mbed website; these are particularly
useful for emulating the mbed as a keyboard or mouse, or for working with musical
instrument digital interface (MIDI) systems and advanced audio projects.
252 Chapter 12
12.3.1 Using the mbed to Emulate a USB Mouse
Several mbed USB interfaces are defined within the mbed USBDevice library. In order to
utilize the USB interfaces, the USBDevice library should be imported to a programfromthe
following location:
http://mbed.org/users/samux/libraries/USBDevice/m24owv
Full documentation on all the USB interfaces can be found on the mbed website. One such
interfaceis theUSBMousefeature. WithUSBMouseit is possibleto makethembedbehave
like a standard USB mouse, sending position and button-press commands to the host.
Program Example 12.5 implements a USBMouse interface and continuously sends relative
position information to move the mouse pointer around four coordinates, which make up
a square. The coordinates are defined by the two arrays dx and dy; the mouse is moved to
these positions within a for loop. The mouse, when initialized to its default parameters, uses
arelativecoordinatesystem, soif dy ¼40anddx ¼40, thenthemousepointerisinstructedto
move 40 pixels right and 40 pixels down. Negative coordinates for x and y planes move the
pointer left and up, respectively.
/* Program Example 12.5: Emulating a USB mouse
*/
#include "mbed.h"
// include mbed library
#include "USBMouse.h"
// include USB Mouse library
USBMouse mouse;
// define USBMouse interface
int dx[]={40,0,-40,0};
int dy[]={0,40,0,-40};
int main() {
while (1) {
for (int i=0; i<4; i++) {
mouse.move(dx[i],dy[i]);
wait(0.2);
}
}
}
// relative x position coordinates
// relative y position coordinates
// scroll through position coordinates
// move mouse to coordinate
Program Example 12.5 Emulating a USB mouse
n
Exercise 12.4
Experiment with the other features of the USBMouse library. In particular, implement
the following small programs:
1. Using the scroll library feature, automatically make pages scroll up and down. You
will need to open a document or webpage to test the scroll feature.
2. Add a push-button to your mbed and implement a mouseclick feature.
3. Add an accelerometer to your mbed and use the read acceleration values to update
the mouse pointer position.
n
Advanced Serial Communications 253
12.3.2 Sending USB MIDI Data froman mbed
MIDI is a serial message protocol that allows digital musical instruments to communicate
with digital signal processing systems which can turn those signals into sound. It was first
developed in 1982 by the MIDI Manufacturers’ Association. MIDI data contains a number
of parameters, as described in detail in Reference 12.4. It is still a valuable method for
enabling communications between electronic music systems, particularly given that in 1999
a new MIDI standard was developed to allow messaging through the more modern USB
protocol.
AnexampleinstrumentusingaMIDI interfaceisaMIDI piano-stylekeyboard. Thekeyboard
itself does not makeany sound. Instead, theMIDI signalscommunicatewithasoundmodule
or computer with MIDI software installed, so that the signals can be turned into music. In
a very simple MIDI system the most valuable information is:
• whether a musical note is to be switched on or off
• the pitch of the note.
The method for setting up and interfacing the mbed with MIDI sequencing software varies
subtly dependingonthesoftwareused. Inmost cases, however, theaudio sequencersoftware
(suchas AbletonLive, AppleLogic orSteinbergCubase) automatically recognizes thembed
MIDI interface and allows it to control a software instrument or synthesizer.
A MIDI interface can be created by first importing the USBDevice library (as mentioned
in Section 12.3.1 above), importing theUSBMIDI.h headerfile and initializing theinterface
(in this case named ‘midi’) as follows:
USBMIDI midi;
// initialize MIDI interface
A MIDI message to sound a note is activated by the following command:
midi.write(MIDIMessage::NoteOn(note));
// play musical note
This command uses the Cþ þ scope resolution operator (::), which relates to a number of
advanced Cþ þ programming features. It is not our intention to explore advanced Cþ þ
programmingconcepts, so forthepurposeof this exampleit is sufficient simply to accept the
described syntax for sending a MIDI message from the mbed.
The value note represents notes on a piano keyboard; this is a 7-bit value so there are 128
possiblenotesthatcanbedescribedby MIDI. Thevaluezerorepresentsavery lowCnoteand,
as thereare12notes inthechromatic musical scale, octaves of C occurat multiplesof 12. So
theMIDI notevalue60representsmiddleC (also referredto asC4), whichhasafundamental
frequency of 261.63Hz. An excerpt of the full MIDI note table is shown in Table 12.1.
ProgramExample12.6setsupanmbedMIDI interfacetocontinuously stepthroughthenotes
shown in Table 12.1. The Program Example also implements an analog input, which can be
connected to a potentiometer, so that the speed of the steps can be manipulated.
254 Chapter 12
Table 12.1: MIDI note values and associated musical notes and frequencies
MIDI note
48
49
50
51
52
53
54
55
56
57
58
59
Musical note
C3
C#3
D3
D#3
E3
F3
F#3
G3
G#3
A3
A#3
B3
Frequency (Hz) 130.1 138.6 146.8 155.6 164.8 174.6 185.0 196.0 207.7 220.0 233.1 246.9
MIDI note
60
61
62
63
64
65
66
67
68
69
70
71
Musical note
C4
C#4
D4
D#4
E4
F4
F#4
G4
G#4
A4
A#4
B4
Frequency (Hz) 261.6 277.2 293.7 311.1 329.6 349.2 370.0 392.0 415.3 440.0 466.2 493.9
/* Program Example 12.6: MIDI messaging with variable scroll speed
*/
#include "mbed.h"
#include "USBMIDI.h”
USBMIDI midi;
// initialize MIDI interface
AnalogIn Ain(p19);
// create analog input
int main() {
while (1) {
for(int i=48; i<72; i++) {
midi.write(MIDIMessage::NoteOn(i));
wait(Ain);
midi.write(MIDIMessage::NoteOff(i));
wait(2*Ain);
}
}
}
//
//
//
//
//
step through notes
note on
pause
note off
pause
Program Example 12.6 MIDI messaging with variable scroll speed
The output of Example 12.6 is shown in the MIDI control window of Apple Logic software,
in Figure 12.4.
n
Exercise 12.5
Replace the analog input in Program Example 12.6 with a value measured by an
ultrasonic range finder. It is therefore possible to change the speed of the note
transitions by moving closer to or further away from the sensor.
Also experiment with modifyingthenoteparameters, for examplethedistancemeasured
by an ultrasonic range finder could be used to change the pitch (i.e. the note number)
being played.
n
Other mbed USB library features, which are all supported by the USBDevice library, are
shown in Table 12.2.
Advanced Serial Communications 255
Figure 12.4:
MIDI notes generated by the mbed and recorded into Apple Logic software
Table 12.2: mbed USBDevice library
Mbed USB library
Description
USBMouse
Allows the mbed to emulate a USB mouse
USBKeyboard
Allows the mbed to emulate a USB keyboard
USBMouseKeyboard
A USB mouse and keyboard feature set combined in a single library
USBHID
Allowscustomdata to besent and received froma human interfacedevice(HID)
allowing custom USB features to bedeveloped without theneed for host drivers
to be installed
USBSerial
Emulates an additional standard serial port on the mbed, through the USB
connections
USBMIDI
Allows send and receive of MIDI messages in communication with a host PC
using MIDI sequencer software
USBAudio
Allowsthembed to berecognized asan audio interfaceallowingstreamingaudio
to be read, output or analyzed and processed
USBMSD
Emulates a mass storage device over USB, allowing interaction with a USB
storage device
12.4 Introducing Ethernet
12.4.1 Ethernet Overview
Ethernet is a serial protocol which is designed to facilitate network communications. Any
device successfully connected to the Ethernet can potentially communicate with any other
device connected to the network.
256 Chapter 12
Networks are often described as being one of two types:
• local area network (LAN) e usually for devices connected together in close proximity,
perhaps in the same building and often without Internet access
• wide area network (WAN) e describes a network of devices over a greater geographical
area, usually connected by the Internet.
Ethernet communications are defined by the IEEE 802.3 standard (see Reference 12.5) and
support data rates up to 100 Gigabits per second. Ethernet uses differential send (Tx) and
receive (Rx) signals, resulting in four wires labeled RXþ , RX , TXþ and TX .
Ethernet messages are communicated as serial data packets referred to as frames. Using
framesallowsasinglemessageto holdanumberof datavaluesincludingavaluedefiningthe
length of the data packet as well as the data itself. The Ethernet frame therefore defines its
own size, which means that only the necessary amount of data is communicated, with no
wasted or empty data bytes. Ethernet communications need to pass a large quantity of data
at high rates, so data efficiency is a very important aspect. The use of frames allows an
efficient method. Each frame also includes a unique source and destination MAC address.
The frame is wrapped within a set of preamble and start of frame (SOF) bytes and a frame
check sequence (FCS), which enables devices on the network to understand the function of
eachcommunicateddataelement. Thestandard802.3Ethernetframeisconstructedasshown
in Table 12.3.
The minimum Ethernet frame is 72 bytes; however, the preamble, SOF and FCS are often
discardedonceaframehasbeensuccessfully received. So a72-bytemessagecanbereported
as having just 60 bytes by some Ethernet communications readers.
The Ethernet data takes the form of the Manchester encoding method, which relies on the
directionof theedgetransitionwithinthetimingwindow, asshowninFigure12.5. If theedge
transitionwithinthetimingframeishighto low, thecodedbit isa0; if thetransitionislowto
high then the bit is a 1. The Manchester protocol is very simple to implement in integrated
circuit hardwareand, asthereisalwaysaswitchfrom0to 1or1to 0forevery datavalue, the
clock signal is effectively embedded within the data. As shown in Figure 12.5, even when
a stream of zeros (or ones for that matter) is being transmitted, the digital signal still shows
transitions between high and low states.
Table 12.3: Ethernet frame structure
Preamble
7 e bytes of
10101010
Start of frame Destination Source MAC
delimiter
MAC address
address Length
1 byte of
10101011
6 bytes
6 bytes
2 bytes
Data
46 e
1500
bytes
Frame check Interframe
sequence
gap
4 bytes
Advanced Serial Communications 257
Figure 12.5:
Manchester encoding for Ethernet data
12.4.2 Implementing Simple mbed Ethernet Communications
The mbed Ethernet API is shown in Table 12.4. We can set up an mbed Ethernet systemto
send data and evaluate this on a logic oscilloscope to verify the Ethernet frame structure.
Here, we are not initially concerned with the specific MAC addresses of sending or
receiving devices, we simply want to send some known data and see it appearing on the
logic scope.
Table 12.4: The mbed Ethernet API
Function
Usage
Ethernet
Create an Ethernet interface
write
Writes into an outgoing Ethernet packet
send
Send an outgoing Ethernet packet
receive
Receives an arrived Ethernet packet
read
Read from a received Ethernet packet
address
link
Gives the Ethernet address of the mbed
Returnsthevalue1 if an Ethernet link ispresent and 0 if no link ispresent
set_link
Sets the speed and duplexparameters of an Ethernet link
258 Chapter 12
ProgramExample12.7sendstwo databytesevery 200msfromanmbed’sEthernetport. The
two byte values are arbitrarily chosen as 0xB9 and 0x46.
/* Program Example 12.7: Ethernet write
*/
#include "mbed.h"
#include "Ethernet.h"
Ethernet eth;
// The Ethernet object
char data[]={0xB9,0x46};
// Define the data values
int main() {
while (1) {
eth.write(data,0x02); // Write the package
eth.send();
// Send the package
wait(0.2);
// wait 200 ms
}
}
Program Example 12.7 Ethernet write
Figures12.6and12.7showdetailedanalysisof thedatapacketonafastlogicanalyzer(i.e. an
oscilloscopethat canmeasureat speeds of around10Gbps). Thenoisecaneasily beseenbut
theoscilloscopeaccurately convertstheEthernet signal to anidealizeddigital representation.
In Figure 12.6, the 7-byte preamble and SOF data are identified.
The preamble and SOF delimiter are followed by destination and source MAC addresses
(6 bytes each) and the 2 bytes denoting the data length, which is a minimumof 46 bytes (as
described in Table 12.3). Note that even though in this example we have only sent two data
bytes, theminimumdatasizeof 46bytes is sent. Theremaining44databytes aremadeupof
empty (0x00) data, being followed by the four frame check sequence bytes. (This is not
a particularly efficient use of Ethernet, which is usually required to send large data packets.)
Looking at the end of the data packet, shown in Figure 12.7, we can also see the final zero
padded data and the FCS data.
Figure 12.6:
Ethernet packet showing preamble and SOF data
Advanced Serial Communications 259
Figure 12.7:
Padded Ethernet data and frame check sequence
n
Exercise 12.6
Using a fast logic oscilloscope, identify the data making up the 0xB9 0x46 data values
transmitted in Program Example 12.7. Experiment with different data values and array
sizes, ensuring that each time the correct binary data can be observed on the
oscilloscope.
n
12.4.3 Ethernet Communication between mbeds
An mbed Ethernet port can be used to read data. We can use the simple Ethernet write
program of Program Example 12.7. We will also need a second mbed system to receive
incomingdataanddisplay itto thehostterminal screen, inordertoverify thatthecorrectdata
is being read.
The following program will allow an mbed to read Ethernet data traffic and display the
captured data on the screen.
/* Program Example 12.8: Ethernet read
*/
#include "mbed.h"
Ethernet eth;
// Ethernet object
Serial pc(USBTX, USBRX); // tx, rx for host terminal coms
char buf[0xFF];
// create a large buffer to store data
int main() {
pc.printf("Ethernet data read and display\n\r");
while (1) {
260 Chapter 12
int size = eth.receive();
if (size > 0) {
eth.read(buf, size);
pc.printf("size = %d data = ",size);
for (int i=0;i<size;i++) {
pc.printf("%02X ",buf[i]);
}
pc.printf("\n\r");
}
//
//
//
//
//
//
get size of incoming data packet
if packet received
read packet to data buffer
print to screen
loop for each data byte
print data to screen
}
}
Program Example 12.8 Ethernet read
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
Figure 12.8:
mbed-to-mbed Ethernet wiring diagram
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
mbed Microcontroller
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
mbed Microcontroller
Note that Program Example 12.8 first defines a large data buffer to store incoming data.
During the infinite loop the programuses the eth.receive( ) function to evaluate the size of
any Ethernet data traffic. If the data packet has a size greater than zero then it must be an
Ethernet packet, so the display loop is entered. The size of the data package along with
the read data is then displayed to the host terminal. To communicate successfully
between the two mbeds, a crossed signal connection is required, as shown in Figure 12.8
and Table 12.5.
Advanced Serial Communications 261
Table 12.5: mbed-to-mbed Ethernet wiring table
Data broadcasting mbed
Data receiving mbed
RD
TD
RDþ
TDþ
TD
RD
TDþ
RDþ
Figure 12.9:
Ethernet data successfully communicated between two mbeds and displayed on a host PC
TheEthernet-writembedshouldrunProgramExample12.7, andthedata-receivingmbedruns
12.8. This is connected to Tera Term running on the host computer. When successfully
completed, thehost PC terminal applicationshoulddisplay Ethernet datasimilarto that shown
inFigure12.9. Notethat eventhoughonly two databytes weretransmittedthesizeof thedata
packagereceivedis 60bytes, whichis theminimumEthernet datapackagesize(72) minusthe
preamble, SOF and FCS bytes, which are of no real value once the message is received.
n
Exercise 12.7
Experiment with different datavaluesand arraysizes, ensuringthat each timethecorrect
binary data can be read by the host PC application.
n
12.5 Local Network and Internet Communications with the mbed
It is possible to use the mbed as a network-enabled system, i.e. for both LAN and WAN
communications. The remaining sections of this chapter show some examples of the mbed
being used to access online files, to host files itself, and to communicate data and control
actions through local networks and the Internet. We must, however, advise you of the
challengesassociatedwithcoveringsuchavasttopic inthisbook. Itisimportantto showcase
the power and ability of the mbed, but in many cases here this relies on a very wide set of
262 Chapter 12
knowledge and skills related to Internet and network theory, as well as software engineering
in other languages. So from here onwards in this chapter we will give some interesting
examples but without the capability to evaluate them or their underpinning technologies in
depth. Further support information is provided on the book website to assist with compiling
and completing the Internet communications examples.
The Internet has transformed worldwide communications over the past 20 years, but rather
thanbeingjustasimplenetworkforsharingonlineinformationandcommunicatingviaemail,
the Internet nowadays facilitates advanced interactive applications. The mbed compiler, for
example, is accessed entirely through the Internet, which means that no additional software
needstobeinstalledforadevelopertoworkwiththembed. This, furthermore, facilitatescloud
computingbecausethembedprogramsarestoredonadataserverwhichisheldandmanaged
by ARM. This means that it is no longer the user’s responsibility to back up data, and
developerscanworkonprojectsfromany locationintheworldwithouttheneedtocarry local
copies of files and programs. This also helps with version control as there is only ever one
versionof aspecific project, so thereis less risk of someoneworkingonanoutdatedversion.
Developers can download and archive their programs locally too.
With extended bandwidth and rapid data rates provided by the Internet, it is possible to keep
communications open continuously and to streamdata while still allowing multi-tasking of
otheronlineactivities. It ispossiblenowadaysto link small embeddedsystemsto theInternet
andhencetonetworkdevicesthatareunderembeddedcontrol. Oncethelinkisthere, itcanbe
used for many things: to monitor status or exert control, or even to cause programor data
downloads. Examplesincludethewashingmachinethatcanalerttheengineertoanimpending
fault, thevendingmachinethatcantell theHeadOfficeit isempty, themanufacturerwho can
downloadanewversionof firmwaretoaninstalledburglaralarm, orthehomeownerwhocan
switch on the oven fromthe office or check that the garage door is closed.
12.5.1 Using the mbed as an HTTP Client
Several mbedlibraries exist to allow network communications. It is possibleto usethembed
asanHTTP(hypertexttransferprotocol) clientinordertoaccessdatafromtheInternet. Todo
this we rely on the network interface libraries as shown in Table 12.6.
A standardEthernet socket (RJ45) isrequiredto connect thembedEthernetport to anetwork
huborrouter. TheSparkfunEthernetbreakoutboard(Reference12.6) isusedinthisexample
and can be connected to the mbed as shown in Figure 12.10.
ProgramExample12.9enablesthembedtoconnecttoaremote(online)textfileandaccessatext
stringfromwithinthat file. Thefileis storedat http://www.embeddedacademic.com/mbed/
mbedclienttest.txt. Youcanverify that it exists by accessingit throughastandardInternet
browser.
Advanced Serial Communications 263
Table 12.6: Network interface libraries for HTTP client operation
Mbed library
Library import path
EthernetIF
http://mbed.org/users/donatien/programs/EthernetNetIf/5z422
HTTPClient
http://mbed.org/users/donatien/programs/HTTPClient/5yo73
Figure 12.10:
RJ45 Ethernet connection for mbed
/* Program Example 12.9: mbed HTTP client test
*/
#include "mbed.h"
#include "EthernetNetIf.h"
#include "HTTPClient.h"
EthernetNetIf eth(
IpAddr(192,168,0,101),
//IP Address
IpAddr(255,255,255,0), //Network Mask
IpAddr(192,168,0,1),
//Gateway
IpAddr(192,168,0,1)
//DNS
);
HTTPClient http;
HTTPText txt;
Serial pc (USBTX,USBRX);
int main() {
pc.printf("\r\nSetting up network connection. \n\r");
eth.setup();
pc.printf("\r\nSetup OK. Querying data. \r\n");
// attempt to access file ‘mbedclienttest.txt’ through the Internet.
HTTPResult r=http.get("http://www.embeddedacademic.com/mbed/mbedclienttest.txt", &txt);
pc.printf("Result :\n\r\"%s\"\n\r", txt.gets());
}
Program Example 12.9 mbed HTTP client test
264 Chapter 12
Figure 12.11:
Host terminal results for client test program
Table 12.7: Network interface libraries for HTTP server operation
Mbed library
Library import path
EthernetIF
http://mbed.org/users/donatien/programs/EthernetNetIf/5z422
HTTPServer
http://mbed.org/users/donatien/programs/HTTPServer/5yhmt
Note that the unique Internet protocol (IP) address for the mbed is set in the EthernetIF
declaration; in this example it is chosen as 192.168.0.101. Predominantly, LANs use an IP
with a default address of 192.168.0.0 and the router on that network will usually take the
default address or the next availablevalue above the default address (i.e. 192.168.0.1). Each
system on the network needs its own unique IP address based on this default, so the chosen
value of 192.168.0.101 is unlikely to clash with any other network systems (unless there are
more than 100 other systems).
When the mbed is connected to an active Internet connection the messages shown in
Figure 12.11 should be displayed on a host PC terminal application.
12.5.2 Using the mbed as an HTTP File Server
As well as accessing files on an external server, we can use the mbed to host files to be
accessedfromaremotePC. Forthis weneedto import andutilizethembedlibrariesdetailed
in Table 12.7.
Program Example 12.10 sets up the mbed as a file server.
/* Program Example: 12.10 mbed file server setup
*/
#include "mbed.h"
#include "EthernetNetIf.h"
#include "HTTPServer.h"
LocalFileSystem fs("webfs");
EthernetNetIf eth(
IpAddr(192,168,0,101),
//IP Address
Advanced Serial Communications 265
IpAddr(255,255,255,0),
//Network Mask
IpAddr(192,168,0,1),
//Gateway
IpAddr(192,168,0,1)
//DNS
);
HTTPServer svr;
int main() {
eth.setup();
FSHandler::mount("/webfs", "/"); //Mount webfs path on root path
svr.addHandler<FSHandler>("/");
//Default handler
svr.bind(80);
while(1) {
Net::poll();
// poll for Internet data exchange requests
}
}
Program Example 12.10 mbed file server setup
Notice the important lines, which initialize the Cþ þ server handler routines:
FSHandler::mount("/webfs", "/");
svr.addHandler<FSHandler>("/");
svr.bind(80);
//Mount webfs path on root path
//Default handler
Also notice the Net::poll(); line, which continuously polls the network in order to respond
whenever an external request to access the mbed server arrives.
We need to create a text file to save onto the mbed, so that we can access it fromthe remote
Internet browser. Create a .htm file called, for example, HOME.HTM and enter some
exampletext, so that youwill knowwhentheInternet browserhascorrectly accessedthefile.
An example is shown in Figure 12.12. Create this file in a standard text editor and save it
to the mbed via the standard USB cable connection.
Notethatthembedserverlibrariesonly supportMS-DOS8.3typefilenames. Youwill beable
to access the mbed server from a remote browser by navigating to the specific IP address
and webpage, in this example by entering the following address:
http://192.168.0.101/HOME.HTM
Figure 12.12:
HOME.HTM file to be stored on the mbed server and remotely accessed
266 Chapter 12
We can now access the HOME.HTM file (stored on the mbed) from any other computer
connected to the LAN. To do this we just need to open a web browser application (such as
Internet Explorer) on a connected PC, and type in the navigation address shown above.
Successful navigationtothisnetworkaddressshouldbringupthe.htmtextascontainedinthe
HOME.HTM file.
12.5.3 Using Remote Procedure Calls to Modify mbed Outputs
Remoteprocedurecalls(RPCs) definetheprocesswhereanactiononadeviceorcomputeris
controlled from a remote location. Essentially, this means that, for example, RPCs can be
made fromanetwork PC to switchthe mbed’s LEDs onandoff. Indeed, many of thembed’s
outputs can be controlled and accessed by RPC calls from a remote network PC (see
Reference 12.7).
The mbed HTTPServer library supports this operation. The key aspects for RPC control on
the mbed are:
• Set up an mbed file server as described in Sections 12.5.1 and 12.5.2 above.
• Define mbed interfaces in the extended format with the ‘name’ defined within, e.g.
DigitalOut led1(LED1, "led1");
• MaketherequiredmbedinterfacesavailableoverRPC by addingtheRPC basecommand,
e.g.
Base::add_rpc_class<DigitalOut>();
• Define the RPC Handler, e.g.
svr.addHandler<RPCHandler>("/rpc");
• Manipulate mbed interfaces remotely by using the following browser address format:
http://<mbed-ip-address>/rpc/<Object name>/<Method name> <Value>
If wetakeProgramExample12.10andapply therequiredRPC features describedabove, we
arrive at Program Example 12.11. This enables remote control of a DigitalOut object
assigned to the onboard LED1:
/* Program Example 12.11 Remote
*/
#include "mbed.h"
#include "EthernetNetIf.h"
#include "HTTPServer.h"
LocalFileSystem fs("webfs");
EthernetNetIf eth(
IpAddr(192,168,0,101),
IpAddr(255,255,255,0),
IpAddr(192,168,0,1),
IpAddr(192,168,0,1)
);
Procedure Calls example
//IP Address
//Network Mask
//Gateway
//DNS
Advanced Serial Communications 267
HTTPServer svr;
DigitalOut led1(LED1, "led1");
int main() {
Base::add_rpc_class<DigitalOut>();
eth.setup();
FSHandler::mount("/webfs", "/");
svr.addHandler<FSHandler>("/");
svr.addHandler<RPCHandler>("/rpc");
svr.bind(80);
while(1) {
Net::poll();
}
}
// define mbed object
// RPC base command
// Ethernet setup
// Mount /webfs path on root
//Default handler
// Define RPC handler
Program Example 12.11 Remote procedure calls example
Thevalue of the mbed led1 object can now be changed via a web browser on a network PC,
i.e. remotely. This is done by entering into a web browser the RPCs as shown in Table 12.8.
n
Exercise 12.8
Implement a programto enablemanipulation of a pulsewidth modulation (PWM) duty
cycle by an RPC command. You will need to define a PWM object with the extended
format, for example:
PwmOut pulse(p21, "pulse");
You will also need to include the PWM RPC base command as follows:
Base::add_rpc_class<PwmOut>();
Set the PWM period at the start of the program and check that the duty cycle, as
observed on an oscilloscope, can be manipulated remotely.
Connect thePWM output to a servo motor and show that theservo motor position can
be controlled by RPC commands.
n
12.5.4 Controlling the mbed using a Remote JavaScript Interface
Manipulating mbed objects by entering address commands in the Internet browser is not an
ideal methodforcommunicatingwiththembedonanetwork. Itisslowto typethecommand,
Table 12.8: Commands for RPC control of mbed LED1
Action
Remote procedure call
Remotely switch led1 ON
http://192.186.0.101/rpc/led1/write 1
Remotely switch led1 OFF
http://192.186.0.101/rpc/led1/write 0
268 Chapter 12
Table 12.9: RPC library and header file details
Mbed library
Library import path
Header files to include
RPCInterface
http://mbed.org/users/MichaelW/
libraries/RPCInterface/likpmz
“RPCVariable.h”, “SerialRPCInterface.h”
easy to enter thewrong address and not user intuitive. A better solution is to use a graphical
user interface (GUI) with buttons and displays to allow manipulation and control of mbed
objects. Itisalsouseful tobeableto display thestatusof mbedobjectsonawebpage, to allow
remote monitoring.
The mbed JavaScript interface (described in more detail at Reference 12.8) allows
communicationbetweenthembedandaJava-enabledbrowser, usingtheRPC protocol. This
type of program requires the library shown in Table 12.9 to be imported and the described
header files also to be included in the project.
ProgramExample12.12setsuptwo RPC variables, oneto holdLED statusdataandtheother
to receive remote button-press data.
/* Program Example 12.12 Using RPC variables for remote mbed control
*/
#include "mbed.h"
#include "EthernetNetIf.h"
#include "HTTPServer.h"
#include "RPCVariable.h"
#include "SerialRPCInterface.h"
LocalFileSystem fs("webfs");
EthernetNetIf eth(
IpAddr(192,168,0,101),//IP Address
IpAddr(255,255,255,0),//Network Mask
IpAddr(192,168,0,1), //Gateway
IpAddr(192,168,0,1)
//DNS
);
HTTPServer svr;
DigitalOut Led1(LED1);
// define mbed object
DigitalIn Button1(p21);
// button
int RemoteLEDStatus=0;
RPCVariable<int> RPC_RemoteLEDStatus(&RemoteLEDStatus,"RemoteLEDStatus");
int RemoteLED1Button=0;
RPCVariable<int> RPC_RemoteLED1Button(&RemoteLED1Button,"RemoteLED1Button");
int main() {
Base::add_rpc_class<DigitalOut>();
eth.setup();
FSHandler::mount("/webfs", "/");
svr.addHandler<FSHandler>("/");
svr.addHandler<RPCHandler>("/rpc");
//
//
//
//
//
RPC base command
Ethernet setup
Mount /webfs path root path
Default handler
Define RPC handler
Advanced Serial Communications 269
svr.bind(80);
printf("Listening. \n");
while (true) {
Net::poll();
if ((Button1==1)j(RemoteLED1Button==1)) {
Led1=1;
RemoteLEDStatus=1;
} else {
Led1=0;
RemoteLEDStatus=0;
}
}
}
Program Example 12.12 Using RPC variables for remote mbed control
The RPC variables are created by the following lines of code:
int RemoteLEDStatus=0;
RPCVariable<int> RPC_RemoteLEDStatus(&RemoteLEDStatus,"RemoteLEDStatus");
int RemoteLED1Button=0;
RPCVariable<int> RPC_RemoteLED1Button(&RemoteLED1Button,"RemoteLED1Button");
It canbeseeninProgramExample12.12 that if thepush-buttonconnectedto mbedpin21is
pressed, LED1 is lit. The RPC variable RemoteLED1Button can also activate LED1,
however, if it becomes set. Furthermore, the RPC variable RemoteLEDStatus is set to
maintain the same value as LED1.
A Javaapplication(aJ avaApplet) canbebuiltandlinkedtoa.htmfile, whichisservedby the
mbed. The.htmfilegeneratesthewebpageshowninFigure12.13whenaccessedonthembed
by a remote browser. Creating the .htmfile and Java application code is outside the scope of
this book; however, these specific examples can be downloaded from the book website to
allow you to implement the example. In this example, the small LED graphic will always be
in the same state as LED1 on the mbed.
Figure 12.13:
Remote interface Java Applet: LED1 ¼off
270 Chapter 12
Figure 12.14:
Remote interface Java Applet: LED1¼on
If the user performs a mouse click within the button area on the webpage, the mbed’s LED1
will also toggle. This is because within the Java Applet it is possible to connect the user
mouse click action to the RPC variable named RemoteLED1Button. Clicking the button
area on the webpage sends RemoteLED1Button high and hence causes the if statement in
Program Example 12.12 to be entered, lighting the LED on the mbed.
Whenever LED1 on the mbed is lit, the LED indicator displayed on the Java webpage
becomes filled in, as shown in Figure 12.14. This is because the Java Applet is capable of
usingaconditional drawingcommandto fill theLED indicatorwheneverRemoteLEDStatus
is set high.
Here, we are now controlling the mbed LED1 output by a remote web application, and the
JavaApplicationalso maintains acorrect status indication of thembed’s LED. This example
is quitecomplex andprogrammingskillswithC/Cþ þ , HTML andJava, beyondthescopeof
this book, are required to build on this example. This does, however, serve as an example of
the power of the mbed as a device which can be uniquely accessed and controlled through
networks and the Internet.
Chapter Review
• A number of serial communications protocols exist, each with its advantages and disadvantagesandeachaimedat specific applications. TheseincludeUART, SPI, I2C, USB and
Ethernet.
• Bluetooth communications allow standard serial messages to be communicated in a
wireless manner.
• The USB protocol is designed specifically for allowing plug-and-play communications
between a computer and peripheral devices such as a keyboard or mouse.
• There are several mbed USB libraries allowing the mbed to operate as a mouse or a
keyboard, or as an audio or MIDI interface, for example.
Advanced Serial Communications 271
• Ethernet is a high-speed serial protocol which facilitates networked systems and
communications between computers, either within a local network or through the
World Wide Web.
• The mbed can communicate through Ethernet to access data from files stored on a data
server computer.
• Thembedcanbeconfiguredto act as anEthernet fileserveritself, allowingdatastoredon
the mbed to be accessed through a network.
• Thembed RPC interfaceand libraries allow variables and outputs to be manipulatedfrom
an external remote interface, written in Java, for example.
Quiz
1.
2.
3.
4.
5.
6.
What is the communication range of a Class 1 Bluetooth device?
What does the term MAC address refer to?
What does MIDI stand for, and how is this relevant to USB communications?
What is the MIDI note value for C2 (fundamental frequency of 65.3Hz)?
Describe the Manchester digital communication format for Ethernet signals.
Sketch the following Ethernet data streams, as they would appear on an analog
oscilloscope, labeling all points of interest:
(a) 0000
(b) 0101
(c) 1110
7. What are the minimum and maximum Ethernet data packet sizes, in bytes?
8. Draw a table listing the advantages and disadvantages of using USB or Ethernet for
communicating between two mbeds.
9. What does the term RPC refer to?
References
12.1. The official Bluetooth website. http://www.bluetooth.com
12.2. Roving Networks Advanced User Manual. Version 4.77, 21 November 2009. rn-bluetooth-um
12.3. Universal Serial Bus Specification, Revision 2.0. April 2000. Compaq, Hewlett-Packard, Intel, Lucent,
Microsoft, NEC, Philips.
12.4. Summary of MIDI Messages. http://www.midi.org/techspecs/midimessages.php
12.5. Ethernet Working Group. IEEE 802.3. http://www.ieee802.org/3/
12.6. Breakout Board for RJ45. http://www.sparkfun.com/products/716
12.7. Interfacing Using RPC. http://mbed.org/cookbook/Interfacing-Using-RPC
12.8. Interfacing with JavaScript. http://mbed.org/cookbook/Interfacing-with-JavaScript
CHAPTER 13
An Introduction to Control Systems
Chapter Outline
13.1 Control Systems 273
13.1.1 Closed and Open Loop Control Systems 274
13.1.2 Closed Loop Cruise Control Example 275
13.1.3 Proportional Control 276
13.1.4 Proportional Integral Derivative Control 278
13.2 Closed Loop Digital Compass Example 280
13.2.1 Using the HMC6352 Digital Compass 281
13.2.2 Implementing a 360 Degree Rotation Servo 283
13.2.3 Implementing a Closed Loop Control Algorithm 285
13.3 Communicating Control Data over the Controller Area Network 288
13.3.1 The Controller Area Network 288
13.3.2 CAN on the mbed 289
Chapter Review 294
Quiz 294
References 295
13.1 Control Systems
We havealready implemented a number of electromechanical control systems, which can be
broadly defined as electronic and/or software systems that ar e designed to control
a designated physical variable. For example, in Chapter 4 we discussed a systemthat moved
a servo to a desired position by applying the correct pulse width modulation (PWM)
waveform. Electromechanical control systems are very common, with applications ranging
from home and office equipment, to industrial machinery, automotivevehicles, robotics and
aerospace. Think of how much control there must be in a humble inkjet printer: drawing in
and guiding the paper, shooting the print cartridges backwards and forwards with
extraordinary precision, and squirting just the right amount of ink, of the right color, in just
the right place. The accuracy of a control system is a very important aspect, as errors and
inaccuracy can have significant relevance to performance, wear and safety.
The whole topic of control systems is a very detailed and broad field, which draws on
advanced mathematical theory as well as electronics, digital communications, applied
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00013-1
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
273
274 Chapter 13
mechanics, and sensor and actuator design. It is not possible to give a thorough insight into
control systems in this book, but the mbed is a valuable device which can be used to
implement some simple control algorithms and communication methods. This chapter
thereforeacts as astartingpoint forthe readerwho may wish to go onto study orimplement
control systems at a later date.
13.1.1 Closed and Open Loop Control Systems
Closed loop control systems employ internal error analysis to achieve accurate control of
actuators, given continuous sensor input readings. For example, this means that the system
measures the position of an actuator, compares it with a signal representing the position it is
meantto beat(calledthesetpoint), andfromthisinformationtriesto movetheactuatorto the
correct position. The fundamental components of a closed loop system are therefore an
actuator, whichcausessomeformof action; asensor, whichmeasurestheeffectof theaction;
and a controller, to compute the difference between setpoint and actual position. Although
wesay position, any physical variablecanbecontrolled. Forexample, aheaterelementmight
be used to heat an oven to a specified temperature and a temperature sensor used to measure
the actual heat of the oven. If the closed loop controller tells the oven to be at 200 C then
the heater will provide heat until the sensor signals the controller that 200 C has been
achieved. If the sensor measurement shows that the actuator has achieved its desired action
(i.e. 200 C is achieved), then the error between the desired setpoint and the actual reading is
zero and no further action is required. If, however, the sensor reads that the desired action
has not been achieved (e.g. the temperature is only 190 C), then the control system knows
that more action is required to meet the desired setpoint.
Anopenloopcontrol systemis onethat employsno analysis sensorandrelies onacalibrated
actuator setting to achieve its desired action. For example, a 5V electric motor might rotate
at different speeds over the voltage input range, say 0V gives 0 revolutions per second (rps)
and 5V gives 100rps. If we want to rotate the motor at 50rps we might simply assume
thattheresponsecharacteristicof themotorislinear(orhaveacalibratedlook-uptableforits
response profile) and provide 2.5V. However, we will not actually know if the motor is
rotating at the desired speed; for different operating temperatures, with different friction
loading and with component drift over time, the speed achieved given 2.5V input cannot be
accurately predicted. Here, an improved closed loop system with a speed sensor could be
used to modify the input voltage and ensure that an accurate 50rps is achieved.
Advantages of closed loop systems are improved accuracy of a controlled variable and the
ability to automatically makecontinuousadjustment. Infact, many actuators arequiteerratic
and non-linear in terms of performance, so in many cases closed loop control is a necessity
rather than a desire. Closed loop systems built around fast microcontrollers can also allow
advanced control of systems that were previously thought uncontrollable. For example, the
An Introduction to Control Systems 275
Figure 13.1:
The Segway personal transporter (www.segway.com). (Imagereproduced with permission of SegwayInc.)
Segway personal transportation systems (Figure 13.1, Reference 13.1) use sensitive
gyroscopes to ensure that the standing platform always remains horizontal. If weight shifts
forward (and the gyroscope shows an imbalance) then the motor in the wheels moves
forwards marginally to compensate and stops moving when a horizontal position has been
achieved again. The microcontroller, sensors and actuators inside read and compute so
rapidly that this process can be performed faster than a human’s motion can displace their
weight, so the controller can always ensure that the platform stays stable.
13.1.2 Closed Loop Cruise Control Example
Anotherexample of closed loop control is cruisecontrol inmany automotivevehicles. Here,
thedriverchoosesadesiredspeedandthevehicleautomatically maintainsthatspeeduntil the
driver disables the mode, either by switching it off or by applying the brake.
A key point to noteis that theactuatorperformsadifferent operationfromthespecific sensor
reading that is being used for comparison with the setpoint. For example, the setpoint might
be a vehicle speed of 80kph, but the actuator is actually the electronic throttle valve, which
determinestheamountof airandhencefuel thatisdrawnintotheengineoneachpistoncycle.
So a complex chain of events occurs, from automatically moving the throttle, to sucking air
and fuel into the engine, engine combustion, torque being applied to the wheels, the vehicle
moving, and the vehicle motion finally being measured and converted to a horizontal speed
value in kph. Many factors affect the accuracy of this process, for example changes in air
pressure, fuel type, engine wear, tire size and tire pressure, as well as road friction and
276 Chapter 13
environmental factors suchas dragandgravity. Anopenloopsystemcouldneverbepossible
for cruise control.
A closedloopcontrol systemcanbequitesimple. All weneedto do, givenamicroprocessorbased system, is to writeacontrol algorithmwhichtakes asetpoint (desiredspeed chosenby
the driver) and calculates the error between the setpoint and the actual speed. The algorithm
then uses this error to calculate the required action. For example, if the error is negative
(actual speedis less thanthedesiredsetpoint) thentheactionmust bepositiveto increasethe
amount of fuel to the engine, increase the torque to thewheels and increase the car speed. If
the error is positive, then the action must be negative, to reduce the car speed to the setpoint.
Theuseof negativefeedback, wherethemeasuredpositionissubtractedfromthesetpoint (in
order to calculate the error), is therefore found in most closed loop control algorithms.
The mechanical system, between the actuator and the sensor, involves a chain of events with
an inherent response time. For a cruise control system, the response time is the time it takes
for anincreaseinfuel injectedto the engineto causeanincrease inspeedof thevehicle e in
some cases this might be of the order of seconds, or at least hundreds of milliseconds. The
slow response time can cause problems for an automatic control system, because once the
desired setpoint is achieved, it is likely to overshoot. With cruise control, once the desired
speedhasbeenachieveditwill takeasmall amountof timefortheengineto reduceitstorque
output (as fuel will already have been injected into the engine), so the controller could
overshoot andcauseanerrorintheoppositedirection. Thecontrollerwill automatically then
attempt to reversetheactionandreducetheerrorback towardszero again, butmay overshoot
in the opposite direction too. In a poorly designed cruise control system the controller may
never even achieve a satisfactory and steady 80kph as desired by the driver. Indeed, the
system may oscillate continuously between, say, 78 and 82kph, which would be an
unpleasant driving experience.
It follows that the performance of the control system depends on the effectiveness of the
control algorithmemployed. Designandtuningof thecontrol algorithmisadelicateprocess,
requiring knowledge of the system characteristics and usually experimental testing to
understand response times. When final calibration is performed effectively, however, very
stable and responsive control systems can be developed.
13.1.3 Proportional Control
The simplest form of closed loop control uses a proportional algorithm to form a linear
relationship between the measured error and the actuator control. Figure 13.2 describes the
electromechanical process of a closed loop cruise control system. The driver tells the
microcontroller the speed at which they would like the vehicle to cruise. At the same time,
the actual vehicle speed is calculated from a wheel speed sensor reading. The speed
An Introduction to Control Systems 277
Figure 13.2:
Closed loop cruise control example. PWM: pulse width modulation;
ADC: analog-to-digital converter
setpoint is compared to the actual vehicle speed to calculate the error, which is then
multiplied by a calibrated value K. The calculated value is then used to set a PWM output
of the microcontroller, and there may be a conversion look-up table here to ensure that
a suitable PWM output is set. The PWM output controls a solenoid, which can move the
engine’s throttle position. The position of the throttle valve determines how much air and
fuel are drawn into the engine, and hence the amount of torque or power generated. The
engine torque, via the vehicle gearbox, determines how much power is delivered to the
wheels, which subsequently dictates the speed of the vehicle. If a setpoint is chosen then,
with a continuously sampling microcontroller, the control loop will automatically selfadjust so that a speed error of zero is achieved. In reality, a zero error is not quite achieved,
but the control loop will get as close as possible. If suddenly the setpoint is changed, for
example from 80kph to 90kph, there is as an immediate step change in the setpoint. It will
take some time for the vehicle to speed up to the new setpoint, and the time taken for
the step response is dependent on a number of factors. It will depend predominantly on the
value of K, the physical response time of the mechanical system and the speed of the
microprocessor software loop.
278 Chapter 13
Figure 13.3:
Proportional control system
Figure13.2also showstheelementsof theelectronic control system(i.e. themicrocontroller)
andthemechanical system, whichissometimesreferredto asaplant. Inmany casestheplant
itself is modeled mathematically and implemented in another microcontroller, so that
engineers can test their control algorithm in simulation before implementing it on a real
vehicle.
A moregeneral representationof theclosed loopcontrol systemis shownin Figure13.3. We
can see that when the error is positive (i.e. the measured value is above the setpoint), the
control value is reduced (owing to negative feedback) and vice versa for negative errors.
Furthermore, whentheerrorislarge, thecontrol valueischangedby alargeamount; whenthe
error is small, the control value is adjusted by only a small amount. The value of the gain
determines how quickly the system responds to errors.
Thefactthatthecontrol valueisalteredproportionally isagoodthingwhentheerrorislarge;
however, this causes problems when the error is small. As the error gets smaller, the control
value adjustment get smaller too, which results in the desired setpoint never actually being
achieved. With proportional only control there is always therefore a steady-state error. The
steady-state error can be reduced by making the gain larger, but unfortunately this could
easily result in overshoot and a longer settling period, and even continuous instability if the
gainistoo high. Figure13.4showsanexamplestepresponseof aproportional control system
with high and low gain values.
To overcome this error with controlled overshoot and stability it is necessary to add integral
and derivative terms to the control algorithm.
13.1.4 Proportional Integral Derivative Control
Proportional integral derivative(PID) control is similarto proportional control, but with the
addition of algorithm components relating to the integral and derivative values of the error
An Introduction to Control Systems 279
(a)
(b)
1.8
1.8
1.6
response
1.4
1.4
1.2
1.2
amplitude
amplitude
1.6
1
0.8
input
0.6
1
0.8
0.6
0.4
0.4
0.2
0.2
0
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8
2
0
response
input
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
time (s)
time (s)
Figure 13.4:
Step response with (a) high proportional gain; (b) low proportional gain
data. This adds an element of history to the algorithm, rather than it being responsive to the
current error value alone. A PID control system is shown in Figure 13.5.
Figure13.6 shows that theerroris integrated andmultipliedby anintegral gainvalueas well
as differentiated and multiplied by a differential gain. These are then summed together
with the proportional gain term to give a control value which takes into account not only
Figure 13.5:
PID control system
280 Chapter 13
1.8
1.6
1.4
amplitude
1.2
1
0.8
0.6
response
input
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
time (s)
1.2
1.4
1.6
1.8
2
Figure 13.6:
PID controller tuned to give step response with low overshoot and low steady-state error
current position, but also the rate of change of the position and the longer term history of
change. Nowthetuningof thesystembecomes even morecomplex, but theresults can bean
accurate control system with low steady error and low overshoot. The gain terms can be
calculated by experimentation and by employing software tools. Complex mathematical
methods can help to determine the best PID gain values (see Reference 13.2, for example),
althoughsomefinemanual tuningis oftenrequiredalso. Figure13.6shows thestepresponse
of aPID control systemwhichhas its gainvalues tuned to giveafast responsetimewith low
overshoot and low steady-state error.
13.2 Closed Loop Digital Compass Example
A closedloopsystemrequiresasensorandanactuator,whereby thesensor’spositionorstateis
altered by the actuator’s action. A simple practical example is that of a digital compass
integrated circuit chip, positioned onaservo. As the servo rotates, thecompass readings will
alteraccordingly.Wecandesignaclosedloopsystemtoautomaticallyadjusttheservoposition
to a desired compass reading and use a simple algorithmto ensure accurate positioning and
smooth movement. Implementing a full PID closed loop controller is rather complex and
outside the scope of this book. However, it is possible to implement a simple proportional
control algorithmfor the purpose of introducing closed loop control on the mbed.
Inthis example, wewill use thembedwithanHMC6352 digital compass modulewitha360
degreerotationservo. Youwill also need a6V battery supply fortheservo. It is important to
An Introduction to Control Systems 281
Figure 13.7:
Closed loop compass system
think carefully about connections and wiring to ensure that the wires do not become tangled
as the servo rotates. The compass sensor also needs to be kept as far away as possible from
any unwanted magnetic fields. An example test setup is shown in Figure 13.7.
13.2.1 Using the HMC6352 Digital Compass
Honeywell’sHMC6352compassisanotherhighly integratedsensor, similarinconceptto the
ADXL345 accelerometer and TMP102 temperature sensor introduced in Chapter 7. Fully
integrated onto a single integrated circuit (IC), it contains sensors, signal conditioning,
microprocessorandinter-integratedcircuit(I2C) serial interface. Thetwo sensors, mountedat
right angles to sense the components of the surrounding magnetic field, are based on the
magnetoresistive principle. This relies on the fact that the apparent resistance of a conductor
canbeseento changeinthepresenceof amagnetic field. Weuseit onatiny breakout board,
as shown in Figure 13.8. Full information is given in Reference 13.3.
Figure 13.8:
HMC6352 mounted on a breakout board. (Image reproduced with permission of Sparkfun Electronics)
282 Chapter 13
Table 13.1: Pin connections for HMC6352 digital compass
HMC6352 pin
mbed pin
2
SCL (I C)
27
SDA (I2C)
28
VCC (3.3 V)
40
GND (0 V)
1
The HMC6352 compass can be connected to the mbed via an I2C interface, as described by
the pin connections in Table 13.1. Before building a complete system, we will first write
a program to ensure that the compass initializes and reads data correctly.
As detailed in the HMC6352 datasheet (Reference 13.3), the base address for the compass is
0x42. The datasheet also describes that in order to initialize the IC we write an ASCII ‘G’
(0x47) commandto informthedevicethatwewishtowriteto aparticularRAMaddress. This
is followed by the operation mode register address (0x74), followed by the operation mode
control byte. Wewill initialize the compass to update in continuous mode at a rate of 20Hz,
and we also want to enable the Set/Reset function. This gives a data bytevalue of 0x72. The
compass setup can therefore be executed by an I2C write message containing the three
command bytes as a single array, i.e:
cmd[0] = 0x47;
cmd[1] = 0x74;
cmd[2] = 0x72;
// ‘ G’ write to RAM address
// Operation mode register address
// Op mode = 20H, no S/R, continuous
A test program to set up and read the HMC6352 is shown in Program Example 13.1.
/* Program Example 13.1: HMC6352 Compass Setup and Read
*/
#include "mbed.h"
// mbed objects
I2C compass(p28, p27);
// sda, scl
Serial pc(USBTX, USBRX);
// tx, rx
// variables
const int addr = 0x42;
// define the I2C write Address
char cmd[3];
// command array for read and write
float pos;
// measured position
// main code
int main() {
cmd[0] = 0x47;
// ’ G’ write to RAM address
cmd[1] = 0x74;
// Operation mode register address
cmd[2] = 0x72;
// Op mode = 20H, no S/R, continuous
compass.write(addr,cmd, 3); // Send operation
while (1) {
compass.read(addr, cmd, 2); // read the two-byte echo result
An Introduction to Control Systems 283
pos = 0.1 * ((cmd[0] << 8) + cmd[1]);
if (pos>180){
pos=pos-360;
}
pc.printf("deg = %.1f\n", pos);
wait(0.3);
//convert to degrees
}
}
Program Example 13.1 HMC6352 compass setup and read
To read data from the compass we use an infinite loop and output data values to a host PC
terminal application. Inside the infinite loop we simply perform a 2-byte read which
returns the 16-bit data value (in tenths of a degree). To form a single number from these 2
bytes we shift the first byte left by 8 (as it is the more significant byte) and add the second
byte; thisformsa16-bitnumber. Inthesamelineof codethisisthenmultipliedby 0.1, togive
a result in degrees. It is actually more useful to have degrees in the range 180 to þ 180
(rather than 0 to 360), so values greater than 180 have the 360 subtracted.
With the compass IC connected to mbed pins 27 and 28, Program13.1 should initialize and
run, allowingyouto verify that correct compassreadings areshownonthePC terminal. Zero
degrees indicates north and 180 degrees (or 180 degrees) indicates south, and hence 90
degrees represents due east and 90 degrees represents due west. Be careful to ensure that
large magnets or power sources do not interfere with the compass’s sensitivity. If you are in
a steel-framed building or a well-equipped laboratory, you may find that the earth’s field is
totally overwhelmed by what is around you!
13.2.2 Implementing a 360 Degree Rotation Servo
Wecanimplementanopenlooppositioncontrollerwitha360degreeservoandapotentiometer
to control. Servoswith360degreemotioncanbesourced(as detailedinAppendix D), or
astandard180degreemotionservocanbemodifiedtoallowcompleterotation, asdescribedby
Reference13.4. The360degreeservo requiresthesamembedconnectionsasthestandard180
degreeservo(asshowninFigure4.10). However, the360degreeversionisamoreerraticdevice
andmorechallengingtocontrol. WhenconnectingaPWMsignal tothedevicealowduty cycle
signal will causetheservotocontinuously turnanti-clockwise, whereasahigherduty cyclewill
causetheservotorotatecontinuouslyclockwise.Therewill beasinglePWMdutyvaluewhichis
a‘zero’ point at whichtheservo holds stationary.
We will now implement a scheduled program to allow different rate functions. In
particular, we will use a fast rate function to control the servo positioning and to read the
sensor data (100Hz), but a slower rate function to send data to the host terminal
application (5Hz). We will therefore use two Ticker objects and two task functions to
control and implement the program timing. In this program we will use a potentiometer to
284 Chapter 13
allow open loop control of the servo; we will also read and display compass data to the
screen to quantify the direction in which the compass is pointing. We therefore need to
define an analog input (to read the potentiometer) and a PWM output (to actuate the
servo). We will also need to implement the same HMC6352 initialization and read
commands as used in Program Example 13.1.
Start a new project and enter the code shown in Program Example 13.2.
/* Program Example: 13.2 Open Loop Compass
*/
#include "mbed.h"
//***** mbed objects *****
I2C compass(p28, p27);
// sda, scl
PwmOut PWM(p25);
AnalogIn Ain(p20);
Serial pc(USBTX, USBRX);
// tx, rx
Ticker s100hz_tick;
// 100 Hz (10ms) ticker
Ticker s5hz_tick;
// 5 Hz (200ms) ticker
//***** variables *****
const int addr = 0x42;
// define the I2C write Address
char cmd[3];
float pos;
// measured position
float ctrlval;
// PWM control value
//***** function prototypes *****
void s100hz_task(void);
// 100 Hz task
void s5hz_task(void);
// 5 Hz task
//***** main code *****
int main() {
// initialize and setup data
PWM.period(0.02);
cmd[0] = 0x47;
// ’ G’ write to RAM address
cmd[1] = 0x74;
// Operation mode register address
cmd[2] = 0x72;
// Op mode = 20H, S/R, continuous
compass.write(addr,cmd, 3);// Send operation
s100hz_tick.attach(&s100hz_task,0.01);
// attach 100 Hz task
s5hz_tick.attach(&s5hz_task,0.2);
// attach 5 Hz task
while(1){
// loop forever
}
}
//***** function 100hz_task *****
void s100hz_task(void) {
compass.read(addr, cmd, 2);
// read the two-byte compass data
pos = 0.1 * ((cmd[0] << 8) + cmd[1]);
//convert to degrees
if (pos>180)
pos=pos-360;
convert to 180deg
ctrlval=Ain;
// set control value (also try ctrlval=Ain/4)
PWM=ctrlval;
// output control value to PWM
}
An Introduction to Control Systems 285
//***** function 5hz_task *****
void s5hz_task(void) {
pc.printf("deg = %.1f PWM = %.4f\n", pos, ctrlval);
}
Program Example 13.2 Open loop compass
If a potentiometer input is now connected to pin 20 and the PWM output on pin 25 is
connected to a powered servo, your compiled code should allow the potentiometer to move
the servo clockwise and anti-clockwise. Position data will also be sent to a host PC running
Tera Term.
You will notice that the motion of the servo is quite sensitive and erratic; it is actually
very difficult to achieve an exact stationary position. Indeed, the analog input gives values
between 0.0 and 1.0, but we know fromour experience of servos in Chapter 4 that the servo
only usesthePWMduty cyclerangeof about5to 20%. Itisthereforepossibleto decreasethe
open loop sensitivity by implementing either a scaling factor or some hard limits to the
possible PWM control values.
n
Exercise 13.1
1. Modify Program Example 13.2 so that the control value ctrlval is only a fraction of
Ain by dividing by a fixed number. Try different values and see whether the control
and stability can be improved.
2. Looking at theTera Termoutput, find thePWM value(ctrlval) that causes theservo
to hold stationary. This is a constant for theservo you areusing. Makea noteof this
‘zero’ value, as we will need it later on, at which time we will call it PWM_zero.
n
13.2.3 Implementing a Closed Loop Control Algorithm
We can use a simple proportional control algorithm to implement a closed loop system. To
demonstrate this we will set our desired compass position to be 0 degrees, i.e. the compass
will always point north (much like a real compass). The servo will move the position of the
compass automatically to ensure that it does this. First, we will need to define a few
variables for calculating error, the desired setpoint (¼0) and associated PWM control value,
as follows:
float
float
float
float
float
setpos=0; // desired setpoint position = zero (North)
error;
// calculated error
ctrlval; // PWM control value
kp=0.0002;
// proportional gain (to be tuned)
PWM_zero=0.075;
// zero value (as deduced in Exercise 13.1)
ProgramExample13.3implementstheclosedloopalgorithm, whichretainsmuchof thecode
from Program Example 13.2.
286 Chapter 13
/* Program Example 13.3: Closed loop compass program
*/
#include "mbed.h"
// mbed objects
I2C compass(p28, p27);
PwmOut PWM(p25);
AnalogIn Ain(p20);
Serial pc(USBTX, USBRX);
Ticker s100hz_tick;
Ticker s5hz_tick;
// variables
const int addr = 0x42;
char cmd[3];
float pos;
float setpos=0;
float error;
float ctrlval;
float kp=0.0002;
float PWM_zero=0.075;
// sda, scl
// tx, rx
// 100 Hz (10ms) ticker
// 5 Hz (200ms) ticker
// define the I2C write Address
// measured position
// setpoint position = zero (North)
// calculated error
// PWM control value
// proportional gain
// zero value
// function prototypes
void s100hz_task(void);
void s5hz_task(void);
// 100 Hz task
// 5 Hz task
// main code
int main() {
// initialize and setup data
PWM.period(0.02);
cmd[0] = 0x47;
// ’ G’ write to RAM address
cmd[1] = 0x74;
// Operation mode register address
cmd[2] = 0x72;
// Op mode = 20H, S/R, continuous
compass.write(addr,cmd, 3); // Send operation
// assign timers
s100hz_tick.attach(&s100hz_task,0.01); //attach 100 Hz task to 10ms tick
s5hz_tick.attach(&s5hz_task,0.2);
//attach 5Hz task to 200ms tick
while(1){
// loop forever
}
}
// function 100hz_task
void s100hz_task(void) {
compass.read(addr, cmd, 2); // read the two-byte echo result
//convert data to degrees
pos = 0.1 * ((cmd[0] << 8) + cmd[1]);
if (pos>180)
pos=pos-360;
error = setpos - pos;
// get error
ctrlval = (kp * error);
// calculate ctrlval (proportional)
ctrlval = ctrlval + PWM_zero; // add control value to zero position
PWM = ctrlval;
// output to PWM
}
An Introduction to Control Systems 287
// function 5hz_task
void s5hz_task(void) {
pc.printf("deg = %.1f error=%.1f ctrlval=%.4f\n",pos,error,ctrlval);
}
Program Example 13.3 Closed loop compass program
The 100Hz task includes the closed loop control algorithm. Immediately after the pos
variable has been calculated, the error between the actual and desired setpoint positions,
i.e. the proportional control value ctrlval, is calculated as follows:
error = setpos - pos;
ctrlval = (kp * error);
// get error
// calculate ctrlval (proportional)
The control value is a calculated position that automatically adjusts to make up for the
difference between the desired and actual in a smooth and controlled manner. However, we
first need to add the zero position to the control value in the following line:
ctrlval = ctrlval + PWM_zero;
// add control value to zero position
Thezero position, PWM_zero, is asimplecalibratablevaluewhichwededucedinthesecond
partof Exercise13.1. ItdefinesthePWMduty cyclethatholdstheservoinastationary position.
Itnowactsasanoffsetto thecontrol value, so anegativeerrorcausestheservo to rotateto one
sideof thezero position, andapositivevaluecausesitto rotateto theotherside. Whenthereis
zero errorinthecontrol algorithmtheservo stays still. Inthis examplethezero offset valueis
calibratedas PWM_zero[ 0.075, althoughyourownservo may requireadifferent value.
n
Exercise 13.2
1. Modify the value of Kp and see how this affects the system stability. In general, if
Kp is too large then overshoot and instability are observed, whereas if Kp is too
small, then the servo response is slow and inaccurate.
2. Investigate the required accuracy of the PWM_zero value. Are three decimal places
sufficient or does refining this value to four or five decimal places enhance the
controller’s performance?
n
n
Exercise 13.3
1. LCD output e Add a liquid crystal display(LCD) to your systemto displayposition,
error and PWM data. You can also use the 6 V servo battery pack as a portable
supply voltage for the mbed. This will allow you to disconnect the mbed from the
host PC and have a fully mobile digital compass.
2. Position control e Develop your system to allow a user to input a desired position
(in degrees) in a terminal application, and the servo will immediately move to
that position.
n
288 Chapter 13
13.3 Communicating Control Data over the Controller Area Network
13.3.1 The Controller Area Network
The Controller Area Network (CAN) is another type of serial communications protocol that
was developed within the automotive industry to allow a number of electronic units on
a single vehicle to share essential control data. A vehicle nowadays uses many
microcontrollers for autonomous control systems. Each microcontroller systemis referred to
as an electronic control unit (ECU) and these include the engine management ECU, an
anti-braking system (ABS) ECU, a dashboard ECU, active suspension and the radio/CD
player, forexample. Eachof theseECUsmanagesitsowncontrol strategies, butthey all need
to access informationrelevant to their own operation, for example drawn fromengine speed,
throttle position, brake pedal position and engine temperature. However, an automotive
vehicle generates a high level of electromagnetic interference and a wide temperature and
humidity range; this is a hostile environment for any signal and indeed for any electronic
device. Moreover, very highreliability isessential asvehiclesaresafety-critical systems. The
serial standards developed for the benign environment of home or office, such as UART, SPI
and I2C, are completely inappropriate for this hostile environment, and a new standard was
therefore needed. Initially, CAN was developed by the German company Bosch. They
publishedVersion2.0of thestandardin1991, andin1993itwasadoptedby theInternational
Organization for Standardization (ISO), as ISO 11898. At the time of writing, CAN
specificationVersion2.0B canbedownloadedfromtheBoschwebsite(Reference 13.5). The
CAN standard has a high level of data security, it is inevitably complex and just the briefest
overview is given here. The main features are listed below.
• Communication is asynchronous, half duplex, with (for a given system) a fixed bit rate.
The maximum for this is 1Mbit/s.
• The configuration is ‘peer to peer’, i.e. all nodes are viewed as equals. There is, however,
a mechanism for prioritization. Master and slave designation is not used.
• Logicvaluesonthebusaredefinedas‘dominant’ or‘recessive’, wheredominantoverrides
recessive. Physical interconnect is not otherwise defined.
• The bus access is flexible. With all nodes being peers, any can start a message. An
ingenious arbitrationprocess is appliedinthecaseof simultaneous access, whichdoes not
lead to loss of time or data. The arbitration process recognizes prioritization.
• There is an unlimited number of nodes.
• Bus nodes do not have addresses, but apply ‘message filtering’ to determine whether
data on the bus is relevant to them.
• Data is transferred in frames, which have a complex format. This starts with identifier
bits, during which arbitration can take place. Eight data bytes are allowed per frame.
• There is an exceptionally high level of data security, with exhaustive error checking.
A node that recognizes that it is faulty can disconnect itself from the bus.
An Introduction to Control Systems 289
Figure 13.9:
An example vehicle CAN bus network
Figure13.9 shows anexamplevehicleCANbus network. Thetypes of control datathat need
sharing along the bus can vary. For example, the dashboard ECU will need to access data
includingenginespeed, enginetemperature, vehiclespeedanddiagnostic information, which
will be predominantly provided by sensors attached to the engine management and gearbox
ECUs, aswell asdooropen/closeddatafromthedoorECUs. Furthermore, if thisvehicleisto
implementanautomatic doorlock strategy onceathresholdvehiclespeedhasbeenachieved,
then the door lock ECUs will need to access vehicle speed data from the CAN bus.
So it can be seen that a very reliable and thorough communications protocol for sharing
control data around an automotive vehicle is required, and CAN provides this. CAN is also
usedinindustrial andfactory applicationswherenoiseissuesandreliability requirements are
similar to those of the automotive industry.
13.3.2 CAN on the mbed
The mbed has two CAN controller interfaces on pins 9e10 and 30e29; note that only the
lattertwopinsappearasaCANinterfaceinFigure2.1. Thepinsrefertoreceive(RD orRXD)
andtransmit (TD orTXD) signals, respectively. TheCANinterfacecanbeusedto writedata
words out of a CAN port and will return the data received from another CAN device. The
290 Chapter 13
Table 13.2: Selected CAN API functions
Function
CAN CANMessage
Usage
Creates a CAN interface connected to specific pins
Creates an empty CAN message (for reading) or a CAN message with
specific content (for writing):
id
the message ID
data
len
space for 8-byte payload
length of data in bytes
format
type
frequency write read
defines if the message has standard or extended format
defines the type of a message
Set the frequency of the CAN interface
Write a CANMessage to the bus. Returns 0 if write failed, 1 if write was
successful
Read a CANMessage from the bus. Returns 0 if no message arrived,
1 if message arrived
CAN clock frequency can also be configured. A selection of the CAN application
programming interface (API) functions is shown in Table 13.2.
When using the mbed CAN API, we first have to define a CAN object and then define
a message (CANMessage) to be written to or read from the CAN bus. The mbed CAN
controllers alone, however, cannot beusedto communicatedirectly withaCANnetwork. To
do this weneedto usethembedCANinterfaceto control aspecific CAN transceiver IC such
as the Microchip MCP2551. This is shown in Figure 13.10 and described in Reference 13.6.
The CAN bus implementation used is differential, and connects to the CANH and CANL
pins. An external resistor connected at Rs controls the slew rate of the data signal, with
aslowerrateminimizingelectromagneticinterference, althoughforsimpletestingthiscanbe
connected directly to ground. The mbed connects from its CAN controllers to the
transceiver’s RXD and TXD pins.
Figure 13.10:
Microchip MCP2551 CAN transceiver IC
An Introduction to Control Systems 291
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
MCP2551
TXD
Rs
VSS CANH
VDD CANL
RXD VREF
MCP2551
The CAN bus
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
mbed Microcontroller
mbed B: Read
TXD
Rs
VSS CANH
VDD CANL
RXD VREF
Gnd
Vin
Vb
nR
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
mbed Microcontroller
mbed A: Write
Vout
Vu
IFIF+
RDRD+
TDTD+
DD+
30
29
28
27
26
25
24
23
22
21
Figure 13.11:
Hardware build for mbed-to-mbed CAN communication
To demonstrate CAN with the mbed we will communicate some data between two mbeds
over a CANnetwork. One mbed will send data to the CANbus while the other will receive
data from the CAN bus. The hardware connections required to create a simple CAN
network test are shown in Figure 13.11 and Table 13.3. Notice that the ‘CAN bus’ is
defined as the wires CANH and CANL, which sometimes require a termination resistance
of 100e200U between them over long wires. In this simple, noise-free example the short
connections mean that a termination resistor is not necessary. Note also that we use the
mbed CAN controller on pins 30 and 29 for the send system and the CAN controller uses
pins 9 and 10 e these are arbitrarily chosen, predominantly to make the hardware
connection diagram neat!
Having built the CAN test system described in Figure 13.11 and Table 13.3, Program
Example 13.4 can be implemented to regularly send data values to the CAN bus.
/* Program Example 13.4: CAN data write e sends an incrementing count value to the CAN
bus every second
*/
#include "mbed.h"
Serial pc(USBTX, USBRX);
// tx, rx for Tera Term output
DigitalOut led1(LED1);
// status LED
CAN can1(p30, p29);
// CAN interface
char counter = 0;
int main() {
printf("send. ");
while (1) {
// send value to CAN bus and monitor return value to check if CAN
// message was sent successfully. If so display, increment and toggle
292 Chapter 13
if (can1.write(CANMessage(1, &counter, 1))) {
pc.printf("Message sent: %d\n", counter);
// display
counter++;
// increment
led1 = !led1;
// toggle status LED
}else{
can1.reset();
}
wait(1);
}
}
Program Example 13.4 CAN data write
Here, onevery importantlineof codeistheif statement, whichperformsanumberof actions:
if (can1.write(CANMessage(1, &counter, 1))) {
First, this line creates a CANMessage with id[ 1, holding the data from the address of
variable counter and of length 1 byte. The message is then written to the CAN bus and the
Table 13.3: Pin connections for mbed-to-mbed CAN communications
MCP2551 pin
Description
Pin connection
Write system
1 TXD
Transmit Data Input
mbed p29 CAN TD
2 VSS
Ground
mbed p1 GND
3 VDD
Supply Voltage
mbed p39 Vu (5V)
4 RXD
Receive Data Output
mbed p30 CAN RD
5 Rs
Reference Output Voltage
mbed p1 GND
6 CANL
CAN Low-Level Voltage I/O
CAN bus low
7 CANH
CAN High-Level Voltage I/O
CAN bus high
8 VREF
Slope-Control Input
e
Read system
1 TXD
Transmit Data Input
mbed p10 CAN TD
2 VSS
Ground
mbed p1 GND
3 VDD
Supply Voltage
mbed p39 Vu (5V)
4 RXD
Receive Data Output
mbed p9 CAN RD
5 Rs
Reference Output Voltage
mbed p1 GND
6 CANL
CAN Low-Level Voltage I/O
CAN bus low
7 CANH
CAN High-Level Voltage I/O
CAN bus high
8 VREF
Slope-Control Input
e
An Introduction to Control Systems 293
return value of the write operation is monitored. Only if the write operation returns
a successful response (by returning a value 1) does the counter increment and light-emitting
diode (LED) toggle.
Program Example 13.5 creates an empty CANMessage. It continuously monitors the CAN
bus and displays any read CAN messages to Tera Term on a host PC.
/* Program Example 13.5: CAN data read e reads CAN messages from the CAN bus
*/
#include "mbed.h"
Serial pc(USBTX, USBRX);
// tx, rx for Tera Term output
DigitalOut led2(LED2);
// status LED
CAN can1(p9, p10);
// CAN interface
int main() {
CANMessage msg;
// create empty CAN message
printf("read. \n");
while(1) {
if(can1.read(msg)) {
// if message is available, read into msg
printf("Message received: %d\n", msg.data[0]); // display message data
led2 = !led2;
// toggle status LED
}
}
}
Program Example 13.5 CAN data read
You can now implement the hardware connections shown and Program Examples 13.4 and
13.5 to show two mbeds communicating over CAN.
n
Exercise 13.4
1. Enhance the CAN write program to send a second data value to the CAN bus. Give
the second data value a different ID value and make it two bytes in size and at
a different message rate (say every 5 seconds). Evaluate how the CAN reader
program interprets the CAN messages it receives.
2. Implement a procedure in the CAN reader program so that it can interpret the two
different CAN messages based on the message ID, and uniquely display a text
message in Tera Term based on the ID of the message received.
n
For a practical implementation of a CAN messaging system, such as for an automotive
system, a CAN specification will define the messages that can be communicated between
ECUs. Eachmessage, forexamplevehiclespeed, enginespeed, enginetemperatureanddoor
open/closed status, will have a unique ID as well as a definition of the size of the data and
any conversion equation required in order to turn the raw data into a sensible value. Each
ECU will bepre-programmedwiththeCANspecificationso that whenamessageis received
it can immediately know what the message is and how to interpret the data.
294 Chapter 13
Several CAN interface devices have been developed and are available commercially for
example to read diagnostic information from vehicles. It is also possible to use an mbed to
read and write data messages to and from a vehicle CAN bus with the correct interface, and
hence even control a vehicle this way. Of course, to do this you will need to know the CAN
specification for the vehicle that is being interfaced with. A number of interesting examples
and an mbed CAN interface board can be found in Reference 13.7.
Chapter Review
• Closed loop control systems use negative feedback of sensor data to ensure that a specific
actuator position is accurate.
• When responding to a step change in desired setpoint, a closed loop control system
may respond too slowly or overshoot, or have a steady-state error. Tuning the control
algorithm can ensure a best-possible step response.
• The time taken for the step response is dependent on the control algorithm design, the
physical response time of the mechanical system and the speed of the microprocessor
software loop.
• Proportional, integral and derivative control can be tuned to give smooth control of
many electromechanical systems.
• Control data can be shared on a network of electronic control units (ECUs) over the
Controller Area Network (CAN) protocol.
• An automotive vehicle may have a number of ECUs for controlling, for example,
enginemanagement systems, brakesystems, thedashboard, airbagcontrol anddoorlocks.
These can all be connected by CAN.
• CAN is an extremely reliable communications protocol with high tolerance to noise
and interference.
• To implement CAN on the mbed, additional CAN transceiver ICs must be used to
create the CAN bus.
Quiz
1.
2.
3.
4.
5.
What are the main differences between an open loop and a closed loop control system?
What do the terms setpoint, plant and negative feedback refer to in closed loop control?
Where and why might closed loop control be required in the design of a helicopter?
What does ‘the step response’ of a control system refer to?
What benefits does CAN communication have over other serial protocols such as I2C
and USB?
6. Give five example ECUs that may be found on a modern car.
An Introduction to Control Systems 295
References
13.1. The Segway website. http://www.segway.com
13.2. Copeland, B. R. (2008). The Design of PID Controllers using ZieglereNichols Tuning. http://www.eng.
uwi.tt/depts/elec/staff/copeland/ee27B/Ziegler_Nichols.pdf
13.3. 2-Axis Compass with Algorithms. HMC6352. Document #900307. Rev. D. January 2006. http://www.
magneticsensors.com/
13.4 Servo modification for 360 degree rotation. http://www.embeddedtronics.com/servo.html
13.5. The CAN section of the Bosch website. www.can.bosch.com
13.6. Microchip Technology (2003). MCP2551 High-Speed CAN Transceiver Data Sheet. Document
DS21730E.
13.7. mbed CAN-Bus Demo Board. http://mbed.org/forum/news-announce
CHAPTER 14
Letting Go of the mbed Libraries
Chapter Outline
14.1 Introduction 297
14.2 Control Register Concepts 298
14.3 Digital Input/ Output 299
14.3.1 mbed Digital Input/Output Control Registers 299
14.3.2 A Digital Output Application 301
14.3.3 Adding a Second Digital Output 302
14.3.4 Digital Inputs 303
14.4 Getting Deeper into the Control Registers 305
14.4.1 Pin Select and Pin Mode Registers 305
14.4.2 Power Control and Clock Select Registers 308
14.5 Using the DAC 310
14.5.1 mbed DAC Control Registers 310
14.5.2 A DAC Application 311
14.6 Using the ADC 312
14.6.1 mbed ADC Control Registers 313
14.6.2 An ADC Application 313
14.6.3 Changing ADC Conversion Speed 317
14.7 A Conclusion on using the Control Registers 318
Chapter Review 319
Quiz 319
Reference 319
14.1 Introduction
The mbed library contains many useful functions, which allow us to write simple and
effectivecode. Thisseems agoodthing, but it is also sometimeslimiting. What if wewant to
use a peripheral in a way not allowed by any of the functions? Therefore it is useful to
understand how peripherals can be configured by direct access to the microcontroller’s
registers. In turn, this leads to a deeper insight into some aspects of how a microcontroller
works. As a by-product, and because we will be working at the bit and byte level, this study
develops further skills in C programming.
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00014-3
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
297
298 Chapter 14
It is worthissuingavery clearhealthwarningat this early stage: this chapteris insomeways
more complex than any of the others that have gone before. It introduces some of the
complexity of theLPC1768microcontroller, whichliesattheheartof thembed, acomplexity
which the mbed designers rightly wish to keep from you. Your own curiosity, ambition or
your professional needs may, however, lead you to want to work at this deeper level.
Working with the complexity of this chapter may result in two opposing feelings. One is
a sense of gratitude to the writers of the mbed libraries, that they have saved you the
complexity of controlling the peripherals directly. At the opposite extreme, getting to grips
with the chapter should also be a very liberating experience, like throwing away the water
wingsafteryouhavelearnt to swim. At themomentyouprobably think that youcannot write
any program unless you have the library functions at the ready. When you are through with
this chapter youwill realize that you are no longerdependent on the library; youuse it when
you want, and write your own routines when you want e the choice becomes yours!
This chapter may be read in sequencewith all other chapters. Alternatively, it can be read in
different sections as extensions of earlier chapters. We will refer to Reference 2.3, the
LPC1768 datasheet, and even more Reference 2.4, its user manual. Because we are now
workingatmicrocontrollerlevel, ratherthanmbedlevel, wewill havetotakemorecareabout
how the microcontroller pins connect with the mbed pins. Therefore you will also need to
have the mbed schematics, Reference 2.2, ready.
14.2 Control Register Concepts
It is useful at this stageto understandalittle more about how the microcontroller central
processingunit(CPU)interactswithitsperipherals.Eachof thesehasoneormoresystemcontrol
registers which act as thedoorway betweenthe CPU and the peripheral. To the CPU, these
registerslookjustlikememory locations.They canusually bewrittentoandreadfrom,andeach
has its ownaddress withinthe memory map. The cleverpart is that eachof the bits inthe
registeriswiredacrosstotheperipheral. They mightcarry control information, sentby theCPU
to the peripheral, orthey might return status information fromtheperipheral. They might also
provideanessential pathfor data. Thegeneral idea of this is illustrated in Figure 14.1. The
microcontrollerperipherals also usually generate interrupts, for example to flag whenan
analog-to-digital conversion is complete oranew word has been received by aserial port.
Early in any program, the programmer must write to the control registers, to set up the
peripherals to the configuration needed. This is called initialization, and turns the control
registers froma general-purpose and non-functioning piece of hardware into something that
is useful for the project in hand. In the mbed, this task is undertaken in the mbed utilities; in
thischapter, wemoveto doingthisworkourselves. Inall of this, animportantquestionarises:
what happens in that short period after power has been applied, but before the peripherals
Letting Go of the mbed Libraries 299
Register’s Unique Address
Control
Information
Status
Information
Peripheral
Peripheral
The CPU
Data Bus
Data
Interchange
Interrupt(s)
Figure 14.1:
The principle of a control register
havebeenset upby theprogram? At thistimeanembeddedsystemispowered, but not under
complete control. Fortunately, all makers of microcontrollers design in a reset condition
for each control register. This generally puts a peripheral into a predictable, and often
inactive, state. We can look this up, and write the initialization code accordingly. On some
occasions, the reset state may be the state that we need.
A central theme of this chapter is the exploration and use of some of the LPC1768 control
registers. Although we draw information from the LPC1768 data, the chapter is
self-contained, withall necessary dataincluded. However, weshowwhichtableinthemanual
the data is taken from, so it is easy to cross-refer. We hope this gives you the confidence
and ability to move on to explore and apply the other registers.
14.3 Digital Input/Output
14.3.1 mbed Digital Input/Output Control Registers
Thedigital input/output (I/O) is auseful placeto start ourstudy of control registers, as this is
simpler than any other peripherals on the LPC1768. This has its digital I/O arranged
nominally as five 32-bit ports; yes, that implies a stunning 160 bits. Only parts of these are
300 Chapter 14
used, however, for example Port 0 has 28 bits implemented, Port 2 has 14, and Ports 3 and 4
have only two each. In the end, among its 100 pins, the LPC1768 has around 70 general
purposeI/Opinsavailable. However, thembedhasonly 40accessiblepins, soonly asubsetof
themicrocontrollerpinsactually appearsonthembedinterconnect, many of whichareshared
with other features.
As we have seen, it is possible to set each port pin as an input or as an output. Each port
has a 32-bit register that controls the direction of each of its pins. These are called the
FIODIR registers. To specify which port the register relates to, the port number is
embedded within the register name, as shown in Table 14.1. For example, FIO0DIR is the
direction register for Port 0. Each bit in this register then controls the corresponding
bit in the I/O port, for example bit 0 in the direction register controls bit 0 in the port.
If the bit in the direction register is set to 1, then that port pin is configured as an output;
if the bit is set to 0, the pin is configured as an input.
It is sometimes moreconvenient not to work withthefull 32-bit directionregister, especially
whenwemight just bethinkingof oneortwo bitswithintheregister. Forthisreasonit is also
possible to access any of the bytes within the larger register, as single-byte registers. These
registers have a number code at their end. For example, FIO2DIR0 is byte 0 of the Port 2
direction register, also seen in Table 14.1. Fromthe table you can see that the address of the
whole word is shared by the address of the lowest byte.
Table 14.1: Example digital I/ O control registers
Register name
Register function
Register address
FIOnDIR
Sets the data direction of each pin in Port n, where n takes value
0 to 4. A port pin is set to output when its bit is set to 1, and
as input when it is set to 0. Accessible as word. Reset value¼0,
i.e. all bits are set to input on reset
e
FIO0DIR
FIO2DIR
Example of above for Port 0
Example of above for Port 2
0x2009C000
0x2009C040
FIOnDIRp
Sets the data direction of each pin in byte pof Port n, where
ptakes value0 to 3. A port pin is set to output when its bit is set
to 1. Accessible as byte
e
FIO0DIR0
FIO0DIR1
FIO2DIR0
Example of above, Port 0 byte 0
Example of above, Port 0 byte 1
Example of above, Port 2 byte 0
0x2009C000
0x2009C001
0x2009C040
FIO0PIN0
Sets the data value of each bit in least significant byte of Port
0 or 2. Accessible as byte. Reset value¼0
0x2009C014
FIO2PIN0
0x2009C054
Letting Go of the mbed Libraries 301
A second set of registers, called FIOPIN, holds the data value of the microcontroller’s pins,
whether they have been set as input or output. Again, two are seen in Table 14.1, with the
same naming pattern as the FIODIR registers. If a port bit has been set as an output, then
writing to its corresponding bit in its FIOPIN register will control the logic value placed on
that pin. If thepinhasbeensetasinput, thenreadingfromthatbitwill tell youthelogic value
assertedat thepin. TheFIODIR andFIOPINregistersaretheonly two registersetsweneed
to worry about for our first simple I/O programs.
14.3.2 A Digital Output Application
A digital output can be configured and then tested simply by flashing a light-emitting diode
(LED). We did this near the beginning of the book, in Program Example 2.1. We will look
at the complete method for making this happen, working directly with the microcontroller
registers. This first program, Program Example 14.1, will use Port 2 pin 0 as the
digital output. The mbed schematic (Reference 2.2) shows that this pin is routed to the
mbed pin 26.
Follow through Program Example 14.1 with care. Notice that for once we are not
writing #include "mbed.h"! At the program’s start the names of two registers from
Table 14.1 are defined to equal the contents of their addresses. This is done by
definingtheaddressesaspointers(seeSectionB.8.2), usingthe* operator. Theformatof thisis
not entirely simple, and for the present purposes we can use it as shown. Further information
can, however, be found in Reference 14.1. Having defined these pointers, we can now refer to
the register names in the code rather than worrying about the addresses. The C keyword
volatile is also applied. This is used to define a data type whose value may change outside
program control. This is a common occurrence in embedded systems, and is particularly
applicable to memory-mapped registers, such as we are using here, which can be changed by
the external hardware.
C code
feature
Within the main( ) function the data direction register of Port 2, byte 0 is set to
output, by setting all bits to Logic 1. A while loop is then established, just as in
ProgramExample 2.1. Within this loop, pin 0 of Port 2 is set high and low in turn.
Check the method of doing this, if you are not familiar with it, as this shows one way of
manipulatingasinglebit withinalargerword. To set thebit high, it is ORedwithLogic 1. All
other bits are ORed with Logic 0, so will not change. To set it low, it is ANDed with binary
11111110. Thishastheeffectof returningtheleastsignificantbit(LSB) to 0, whileleavingall
other bits unchanged. The 1111 1110 value is derived by taking the logic inversion of 0x01,
using the C ~operator.
C code
feature
The delay function should be self-explanatory, and is explored more in Exercise 14.1. Its
function prototype appears early in the program.
302 Chapter 14
/*Program Example 14.1: Sets up a digital output pin using control registers, and
flashes an led.
*/
// function prototypes
void delay(void);
//Define addresses of digital i/o control registers, as pointers to volatile data
#define FIO2DIR0
(*(volatile unsigned char *)(0x2009C040))
#define FIO2PIN0
(*(volatile unsigned char *)(0x2009C054))
int main() {
FIO2DIR0=0xFF; // set port 2, lowest byte to output
while(1) {
FIO2PIN0 j= 0x01; // OR bit 0 with 1 to set pin high
delay();
FIO2PIN0 &= ~0x01; // AND bit 0 with 0 to set pin low
delay();
}
}
//delay function
void delay(void){
int j;
//loop variable j
for (j=0;j<1000000;j++) {
j++;
j--;
//waste time
}
}
Program Example 14.1 Manipulating control registers to flash an LED
Connect anLED between an mbed pin 26 and 0 V, and compile, download and run the code.
Press reset and the LED should flash.
n
Exercise 14.1
With an oscilloscope, carefully measure the duration of the delay function in Program
instructions. Adjust the
Example14.1. Estimatetheexecution timefor thejþ þ and j
delay function experimentally so that it is precisely 100 ms. Then create a new 1 s delay
function, which works by calling the 100 ms one 10 times. Now write a library delay
routine, which gives a delay of n ms, where n is the parameter sent.
n
14.3.3 Adding a Second Digital Output
Following the principles outlined above, we can add further digital outputs. Here, we add
a second LED and make a flashing pattern. Port 2 pin 1 is used, which connects to mbed pin
25. AddasecondLED to thispin, preferably of adifferentcolor. Wehavealready definedthe
Letting Go of the mbed Libraries 303
direction control and pin I/O registers for Port 2, so we do not need to add any new registers.
We do, however, add a variable to allow us to generate an interesting flashing pattern.
Copy Program Example 14.1 to a new program, and add into it the following variable
declaration, prior to the main program function:
char i;
Also add the for loop of Program Example 14.2 at the end of the while loop. Notice that
now we are toggling bit 1 of the FIO2PIN0 register. We do this as before by ORing with
0x02, and then ANDing with the inverse of 0x02. This is where our second LED is
connected.
for (i=1;i<=3;i++){
FIO0PIN0 j= 0x02;
delay();
FIO0PIN0 &= ~0x02;
delay();
}
// set port 2 pin 1 high (mbed pin 25)
// set port 2 pin 1 low
Program Example 14.2 (code fragment) Controlling a second LED output
Runtheprogram. ItshouldflashthefirstLED once, followedby threeflashesof thesecond. If
you have LEDs of different colors this will give you a pattern something like:
green e red e red e red e green e red e red e red e green e .
n
Exercise 14.2
In the mbed circuit diagrams (Reference 2.2) identify the LPC1768 pins that drive the
onboard LEDs. RewriteProgramExample14.2 so that theonboard LEDs areactivated,
instead of using external ones.
n
14.3.4 Digital Inputs
We can create digital inputs simply by setting a port bit to input, using the correct bit in an
FIODIR register. Program Example 14.3 now develops the previous example, by including
a digital input from a switch. The state of the switch changes the loop pattern, determining
which LED flashes three times and which flashes just once in a cycle. This then gives
a control system which has outputs that are dependent on particular input characteristics. It
uses the outputs already used, and adds bit 0 of Port 0 as a digital input. Looking at the
schematic, we can see that this is pin 9 of the mbed.
It should not be difficult to follow Program Example 14.3 through. As before, we see the
necessary register addresses defined. Directly inside the main function Port 0 byte 0 is set
as a digital input, noting that a Logic 0 sets the corresponding pin to input. We have set all
304 Chapter 14
of byte 0 to input by sending 0x00; we could of course set each pin within the byte
individually if needed. Moreover, this setting is not fixed; we can, if needed, change a pin
from input to output as a program executes. After all this, a reading of Table 14.1
reminds us that the reset value of all ports is as input; therefore, this little bit of code is not
actually necessary e try removing it when you run the program. However, it is good
practice to reassert values that are said to be in place due to the reset; it gives you the
confidence that the value is in place, and it is a definite statement in the code of a setting
that you want.
The while loop then starts, and at the beginning of this we see the if statement testing the
digital input value. The if condition uses a bit mask to discard the value of all the other pins
on Port 0 byte 0 and simply returns a high or low result dependent on pin 8 alone. The
variablesa andb will holdvalueswhichwill changedependingontheswitchposition. A little
later, we see the a and b values used to define the port values for the green and red LEDs.
If b has beenset to 0x01beforethefor loop, thenthered LED will flashthreetimes; if it has
been set to 0x02, then the green LED will flash three times.
/* Program Example 14.3: Uses digital input and output using control registers, and
flashes an LED. LEDS connect to mbed pins 25 and 26. Switch input to pin 9.
*/
// function prototypes
void delay(void);
//Define Digital I/O registers
#define FIO0DIR0 (*( volatile unsigned char *)(0x2009C000))
#define FIO0PIN0 (*( volatile unsigned char *)(0x2009C014))
#define FIO2DIR0 (*(volatile unsigned char *)(0x2009C040))
#define FIO2PIN0 (*(volatile unsigned char *)(0x2009C054))
//some variables
char a;
char b;
char i;
int main() {
FIO0DIR0=0x00;
// set all bits of port 0 byte 0 to input
FIO2DIR0=0xFF;
// set port 2 byte 0 to output
while(1) {
if (FIO0PIN0&0x01==1){ // bit test port 0 pin 0 (mbed pin 9)
a=0x01;
// this reverses the order of LED flashing
b=0x02;
// based on the switch position
}
else {
a=0x02;
b=0x01;
}
FIO2PIN0 j= a;
delay();
FIO2PIN0 &= ~a;
delay();
Letting Go of the mbed Libraries 305
for (i=1;i<=3;i++){
FIO2PIN0 j= b;
delay();
FIO2PIN0 &= ~b;
delay();
}
}
}
void delay(void){
//program continues
//end while loop
//delay function.
Program Example 14.3 Combined digital input and output
To run this program, set up the circuit of Figure 3.6, except that the green and red LEDs
shouldconnecttombedpins25and26, respectively, andtheswitchinputisonpin9. Compile
andrun. Youshouldseethat thepositionof theswitchtogglestheflashingLEDs betweenthe
patterns
green e red e red e red e green e red e red e red e green e .
and
green e green e green e red e green e green e green e red e green e .
n
Exercise 14.3
Rewrite Program Example 14.3 so that it runs on the exact circuit of Figure 3.6.
n
14.4 Getting Deeper into the Control Registers
Tocontinuewiththischapterweneedtogetfurtherintotheuseof control registers.Thissection
therefore looks at some of the registers that control features across the microcontroller e
informally called ‘global’ registers e that will be used in later sections. These relate to the
allocation of pins, setting clock frequency and controlling power. This is by no means
a complete survey, and wewill not even see or make use of many of the features of this
microcontroller.
14.4.1 Pin Select and Pin Mode Registers
One of the reasons that modern microcontrollers are so versatile is that most pins are
multifunctional. They can be allocated to different peripherals and used in different ways.
Withthembedlibrary, this flexibility is (quitereasonably) moreorless hiddenfromtheuser;
306 Chapter 14
Table 14.2: PINSEL1 register
PINSEL1
Pin
name
Function
when 00
Function
when 01
Function
when 10
Function
when 11
Reset
value
1:0
P0.16
GPIO Port 0.16
RXD1
SSEL0
SSEL
00
3:2
P0.17
GPIO Port 0.17
CTS1
MISO0
MISO
00
5:4
P0.18
GPIO Port 0.18
DCD1
MOSI0
MOSI
00
7:6
1
P0.19
GPIO Port 0.19
DSR1
Reserved
SDA1
00
9:8
P0.201
GPIO Port 0.20
DTR1
Reserved
SCL1
00
11:10
P0.211
GPIO Port 0.21
RI1
Reserved
RD1
00
13:12
P0.22
GPIO Port 0.22
RTS1
Reserved
TD1
00
15:14
1
P0.23
GPIO Port 0.23
AD0.0
I2SRX_CLK
CAP3.0
00
17:16
P0.241
GPIO Port 0.24
AD0.1
I2SRX_WS
CAP3.1
00
19:18
P0.25
GPIO Port 0.25
AD0.2
I2SRX_SDA
TXD3
00
21:20
P0.26
GPIO Port 0.26
AD0.3
AOUT
RXD3
00
23:22
P0.271,2
GPIO Port 0.27
SDA0
USB_SDA
Reserved
00
25:24
1,2
P0.28
GPIO Port 0.28
SCL0
USB_SCL
Reserved
00
27:26
P0.29
GPIO Port 0.29
USB_Dþ
Reserved
Reserved
00
29:28
P0.30
GPIO Port 0.30
USB_D_
Reserved
Reserved
00
31:30
e
Reserved
Reserved
Reserved
Reserved
00
1
Not available on 80-pin package.
Pins P0.27 and P0.28 are open drain for I2C bus compliance.
RedrawnfromNXP SemiconductorsUM10360 LPC17xxUser Manual, Table79: Pinfunctionselect register 1 (PINSEL1 e address0x4002
C004) bit description, with permission of NXP.
2
the libraries tidily make the allocations for you, without you even knowing. If they did not,
youwouldbefacedwithabewilderingchoiceof possibilitiesevery timeyoutriedto develop
an application. As our expertise grows, however, it is good to know that some of these
possibilities are available.
Two important sets of registers used in the LPC1768 are called PINSEL and PINMODE.
ThePINSEL registercanallocateeachpinto oneof fourpossibilities. Anexampleof part of
one register that we will be using soon, PINSEL1, is shown in Table 14.2. This controls the
upper half of Port 0. The first column shows the bit number within the register; each line
details two bits. The second column shows the microcontroller pin that is being controlled.
The two bits under consideration can have four possible combinations; each of these
connects the pin in a different way. These are shown in the next four columns. Do not worry
if some of the abbreviations shown have little meaning to you; we will pick out the ones we
need, when we need them.
Letting Go of the mbed Libraries 307
Table 14.3: PINMODE0 register
PINMODE0
Symbol
1:0
P0.00MODE
3:2
Value
Description
Reset value
Port 0 pin 0 on-chip pull-up/down
resistor control
00
00
P0.0 pin has a pull-up resistor enabled
01
P0.0 pin has repeater mode enabled
10
P0.0 pin has neither pull-up nor
pull-down
11
P0.0 pin has a pull-down resistor
enabled
P0.01MODE
Port 0 pin 1 control, see P0.00MODE
00
continued to P0.15MODE
RedrawnfromNXP SemiconductorsUM10360LPC17xxUser Manual, Table87: PinModeselect register 0 (PINMODE0 e address0x4002
C040) bit description, with permission of NXP.
Let us take as an example the line showing the effect of bits 21:20, i.e. line 11 of the table;
these control bit 26 of Port 0. Column 3 shows that if the bits are 00, the pin is allocated to
Port 0 bit 26, i.e. the pin is connected as general-purpose I/O. Importantly, the final column
showsthatthisisalso thevaluewhenthechipisreset. Inotherwords, aslongasweonly want
to use digital I/O, we do not need to worry about this register at all, as the reset value is the
valuewewant. If the bits are set to 01, the pin is allocated to input 3 of the analog-to-digital
converter (ADC). If set to 10, the pin is used for analog output, i.e. the digital-to-analog
converter (DAC) output. If set to 11, the pin is allocated as Receiver input for universal
asynchronous receiver/transmitter (UART) 3.
Turning to the PINMODE registers, partial details of one of these are shown in Table 14.3.
This is PINMODE0, whichcontrols theinput characteristics of thelowerhalf of Port 0. The
patternisthesameforevery pin, sothereisno needforrepetition. Itiseasy toseethatpull-up
and pull-down resistors (as seen in Figure 3.5) are available, and this is explored in Program
Example 14.4. The repeater modeis a neat little facility which enables pull-up resistor when
the input is a Logic 1, and pull-down if it is low. If the external circuit changes so that the
input is no longer driven, then the input will hold its most recent value.
n
Exercise 14.4
Change the hardware for Program Example 14.3 so that you use an SPST (single-pole,
single-throw) switch, for example a push-button, instead of the toggle (single-pole,
double-throw, SPDT) switch. Connect it first between pin 9 and ground, and run the
program with no change. This should run as before, because you are depending on
308 Chapter 14
the pull-up resistor being in place owing to the reset value of the PINMODE register.
In diagrammatic terms, you have moved from the switch circuit of Figure 3.5a to that
of Figure 3.5b. Now change the setting of PINMODE0 so that the pull-down resistor
is enabled, and connect the switch between pin 9 and 3.3 V, i.e. applying Figure 3.5c.
The program should again work, but with the changed input mode selection.
n
14.4.2 Power Control and Clock Select Registers
Power control and clock frequency are very closely linked. Every clock transition causes the
circuit of the microcontroller to take a tiny pulse of current; the more transitions, the more
the current taken. Hence, a processor or peripheral running at a high clock speed will
cause high power consumption; one running at a low clock frequency will consume less
power. Onewithits clock switchedoff, evenif it is powered up, will (if apurely digital circuit
using CMOS technology) take negligible power. To conserve power, it is possible to turn
off the clock source to many of the LPC1768 peripherals. This power management is
controlled by the PCONP register, seen in part in Table 14.4. Where a bit is set to 1, the
Table 14.4: Power Control register PCONP (part of)
Bit
Symbol
Description
Reset value
0
e
Reserved
NA
1
PCTIM0
Timer/Counter 0 power/clock control bit
1
2
PCTIM1
Timer/Counter 1 power/clock control bit
1
3
PCUART0
UART0 power/clock control bit
1
4
PCUART1
UART1 power/clock control bit
1
5
e
Reserved
NA
6
PCPWM1
PWM1 power/clock control bit
1
2
7
PCI2C0
I C0 interface power/clock control bit
1
8
PCSPI
SPI interface power/clock control bit
1
9
PCRTC
RTC power/clock control bit
1
10
PCSSP1
SSP 1 interface power/clock control bit
1
11
e
Reserved
NA
12
PCADC
A/D converter (ADC) power/clock control bit
Note: Clear the PDN bit in the AD0CR before
clearing this bit, and set this bit beforesetting PDN
0
continued to bit 28
RedrawnfromNXP SemiconductorsUM10360 LPC17xxUser Manual, Table46: Power Control for Peripheralsregister (PCONP e address
0x400F C0C4) bit description, with permission of NXP.
Letting Go of the mbed Libraries 309
peripheral is enabled; when set to 0 it is disabled. It is interesting to note that some
peripherals, such as the serial peripheral interface (SPI), are reset in the enabled mode, and
others, such as the ADC, are reset disabled.
Asidefrombeingabletoswitchtheclocktoaperipheral onoroff, thereissomecontrol overthe
peripheral’sclock frequency itself. Thiswill control theperipheral’sspeedof operationaswell
as its power consumption. This clock frequency is controlled by the PCLKSEL registers.
Peripheral clocksarederivedfromtheclockthatdrivestheCPU,whichiscalledCCLK. Forthe
mbed, CCLK normally runsat96MHz. Partial detailsof PCLKSEL0areshowninTable14.5,
which shows that two bits are used per peripheral to control the clock frequency to each.
ThefourpossiblecombinationsareshowninTable14.6. ThisshowsthattheCCLK frequency
itself can be used to drive the peripheral. Alternatively, it can be divided by 2, 4 or 8. CCLK
is derived fromthe main oscillator circuit, and can be manipulated in a number of interesting
ways; however, we do not go into the details of that in this book.
Table 14.5: Peripheral Clock Selection register PCLKSEL0
1
Bit
Symbol
Description
Reset value
1:0
PCLK_WDT
Peripheral clock selection for WDT
00
3:2
PCLK_TIMER0
Peripheral clock selection for TIMER0
00
5:4
PCLK_TIMER1
Peripheral clock selection for TIMER1
00
7:6
PCLK_UART0
Peripheral clock selection for UART0
00
9:8
PCLK_UART1
Peripheral clock selection for UART1
00
11:10
e
Reserved
NA
13:12
PCLK_PWM1
Peripheral clock selection for PWM1
00
15:14
PCLK_I2C0
Peripheral clock selection for I2C0
00
17:16
PCLK_SPI
Peripheral clock selection for SPI
00
19:18
e
Reserved
NA
21:20
PCLK_SSP1
Peripheral clock selection for SSP1
00
23:22
PCLK_DAC
Peripheral clock selection for DAC
00
25:24
PCLK_ADC
Peripheral clock selection for ADC
00
27:26
PCLK_CAN1
Peripheral clock selection for CAN11
00
29:28
PCLK_CAN2
Peripheral clock selection for CAN21
00
31:30
PCLK_ACF
Peripheral clock selection for CAN acceptance
filtering1
00
PCLK_CAN1 and PCLK_CAN2 must have the same PCLK divide value when the CAN function is used.
RedrawnfromNXP SemiconductorsUM10360 LPC17xxUser Manual, Table40: Peripheral Clock Selectionregister (PCLKSEL0 e address
0x400F C1A8) bit description, with permission of NXP.
310 Chapter 14
Table 14.6: Peripheral Clock Selection register bit values
PCLKSEL0 and PCLKSEL1
individual peripheral’s clock
select options
Function
Reset value
00
PCLK_peripheral ¼CCLK/4
00
01
PCLK_peripheral ¼CCLK
10
PCLK_peripheral ¼CCLK/2
11
PCLK_peripheral ¼CCLK/8, except for
CAN1, CAN2 and CAN filtering when
“11” selects¼CCLK/6
RedrawnfromNXP SemiconductorsUM10360 LPC17xxUser Manual, Table42: Peripheral ClockSelectionregister bit values, with
permission of NXP.
14.5 Using the DAC
We now turnto controlling the DAC throughits registers, tryingto replicateand develop the
work doneinChapter4. Remindyourself of thegeneral block diagramof theDAC, asseenin
Figure4.1.ItisworthmentioningherethatthepositivereferencevoltageinputtotheLPC1768,
sharedby bothADCandDAC, iscalledVREFP, andappearsonpin12of themicrocontroller. A
careful look at the mbedcircuit diagrams (Reference 2.2) shows that this is connected to the
power supply 3.3 V, filtered to limit noise input. This is asensibleconnection, and we are
unabletochangeit.Thenegativereferencevoltageinput,calledVREFN,appearsonpin15of the
microcontroller. Onthe mbedit is connected directly to ground.
14.5.1 mbed DAC Control Registers
Aswithall peripherals, theDAC hasaset of registersthat control itsactivity. Intermsof the
‘global’ registersthatwehavejustseen, theDACpowerisalwaysenabled, sothereisnoneed
toconsiderthePCONP register. TheonlypinthattheDACoutputisavailableonisPort0pin
26, sowemustallocatethispinappropriately throughthePINSEL1register, asseeninTable
14.2. TheDAC output is labeledAOUT here. It is no surpriseto seeinthembedschematics
that this pin is connected to mbed pin 18, the only mbed analog output.
The only register specific to the DAC that we will use is the DACR register. This is
comparatively simpleto grasp, andisshowninTable14.7. Wecanseethatthedigital inputto
the DAC must be placed in here, in bits 6e15. Most of the rest of the bits are unused, apart
from the bias bit, explained in the table.
Applying Equation 4.1 to this 10-bit DAC, its output is given by:
V0 ¼ðVREFP
DÞ=1024 ¼ð3:3
DÞ=1024
where D is the value of the 10-bit number placed in bits 15e6 of the DACR register.
14.1
Letting Go of the mbed Libraries 311
Table 14.7: The DACR register
Bit
Symbol
5:0
Description
Reset value
e
Reserved, user software should not write ones to
reserved bits. Thevalueread froma reserved bit is
not defined
NA
15:6
VALUE
After the selected settling time after this field is
written with a new VALUE, the voltage on the
AOUT pin (with respect to VSSA) is
VALUE ((VREFP VREFN)/1024) þ VREFN
0
16
BIAS1
0
The settling time of the DAC is 1 ms maxand the
maximum current is 700 mA. This allows
a maximum update rate of 1 MHz
0
1
The settling time of the DAC is 2.5 ms max, and
the maximum current is 350 mA. This allows
a maximum update rate of 400 kHz
0
Reserved, user software should not write ones to
reserved bits. Thevalueread froma reserved bit is
not defined
NA
31:17
Value
e
1
The settling times noted in the description of the BIAS bit are valid for a capacitance load on the AOUT pin not exceeding
100 pF. A load impedance value greater than that value will cause settling time longer than the specified time.
RedrawnfromNXP SemiconductorsUM10360LPC17xxUser Manual, Table539: D/A Converter register (DACRe address0x4008C000)
bit description, with permission of NXP.
14.5.2 A DAC Application
Let’s try now a simple program to drive the DAC, accessing it through the microcontroller
control registers. ProgramExample14.4replicatesthesimplesawtoothoutput, whichwefirst
achieved in Program Example 4.2. The program follows the familiar pattern of defining
register addresses, and then setting these appropriately early in the main function. The
PINSEL1 register is set to select DAC output on port bit 0.26. An integer variable, called
dac_value, is then repeatedly incremented and transferred to the DAC input, in register
DACR. It has to be shifted left six times, to place it in the correct bits of the DACR register.
Betweeneachnewvalueof DAC input adelay is introduced. Theeffect of this is exploredin
Exercise 14.5.
/* Program Example 14.4: Sawtooth waveform on DAC output. View on oscilloscope. Port
0.26 is used for DAC output, i.e. mbed Pin 18
*/
// function prototype
void delay(void);
// variable declarations
int dac_value;
//the value to be output
//define addresses of control registers, as pointers to volatile data
#define DACR (*(volatile unsigned long *)(0x4008C000))
312 Chapter 14
#define PINSEL1 (*(volatile unsigned long *)(0x4002C004))
int main(){
PINSEL1=0x00200000; //set bits 21-20 to 10 to enable analog out on P0.26
while(1){
for (dac_value=0;dac_value<1023;dac_value=dac_value+1){
DACR=(dac_value<<6);
delay();
}
}
}
void delay(void) //delay function.
//program continues
Program Example 14.4 Sawtooth output on the DAC
Compile the program and run it on an mbed; no external connections are needed. View the
output from the mbed pin 18 on an oscilloscope.
n
Exercise 14.5
Measure the period of the sawtooth waveform. How does it relate to the delay
value you measured in Exercise 14.1? Try varying the period by varying the delay value,
or removing it altogether. Can you estimate how long a single digital-to-analog
conversion takes?
n
14.6 Using the ADC
We now turn to controlling the ADC throughits registers, trying to replicate and develop the
work doneinChapter5. Itisworthglancingback atFigure5.1, asthisrepresentsmany of the
featuresthatwewill needto control. Thiswill includeselecting(oratleastknowingthevalue
of) the voltage reference, clock speed and input channel, starting a conversion, detecting
a completion and reading the output data. The ADC has a set of registers that control all this
activity. The LPC1768 has eight inputs to its ADC, which appear e in order frominput 0 to
input 7 e on pins 9e6, 21, 20, 99 and 98 of the microcontroller. A study of the mbed circuit
shows that the lower six of these are used, connected to pins 15e20 inclusive of the mbed.
14.6.1 mbed ADC Control Registers
The LPC1768 has a number of registers that control its ADC, particularly in its more
sophisticated operation. However, wewill only apply two of these, the ADC control register
(ADCR) andtheGlobal DataRegister(ADGDR). ThesearedetailedinTables14.8and14.9.
As we have seen, the ADC can also be powered down; indeed, on microcontroller reset it is
Letting Go of the mbed Libraries 313
Table 14.8: The AD0CR register
Bit
Symbol
7:0
Value
Description
Reset value
SEL
Selects which of the AD0.7:0 pins is (are) to be sampled
and converted. For AD0, bit 0 selectsPin AD0.0, and bit 7
selects pin AD0.7. In software-controlled mode, onlyone
of these bits should be 1. In hardware scan mode, any
value containing 1 to 8 ones is allowed. All zeroes is
equivalent to 0x01
0x01
15:8
CLKDIV
TheAPB clock(PCLK_ADC0) isdivided by(thisvalueplus
one) to produce the clock for the A/D converter, which
should be less than or equal to 13 MHz. Typically,
software should program the smallest value in this field
that yieldsaclockof 13 MHzor slightlyless, but in certain
cases (such as a high-impedanceanalog source) a slower
clock may be desirable
0
16
BURST
The AD converter does repeated conversions at up to
200 kHz, scanning(if necessary) through thepinsselected
by bits set to ones in the SEL field. The first conversion
after thestart corresponds to theleast significant 1 in the
SEL field, then higher numbered 1-bits (pins) if
applicable. Repeated conversions can be terminated by
clearing this bit, but the conversion that’s in progress
when this bit is cleared will be completed
0
1
Remark: START bits must be 000 when BURST ¼1 or
conversions will not start
0
20:17
e
21
PDN
Conversionsaresoftwarecontrolled and require65 clocks
Reserved, user softwareshould not writeones to reserved
bits. The value read from a reserved bit is not defined
NA
1
The A/D converter is operational
0
0
The A/D converter is in power-down mode
23:22
e
Reserved, user softwareshould not writeones to reserved
bits. The value read from a reserved bit is not defined
NA
26:24
START
When the BURST bit is 0, these bits control whether and
when an A/D conversion is started:
0
000
No start (this value should be used when clearing
PDN to 0)
001
Start conversion now
Further moreadvanced optionsfor START control areavailable, which also then applyEDGE
RedrawnfromNXP SemiconductorsUM10360 LPC17xxUser Manual, Table531: A/D Control Register (AD0CRe address0x4003 4000)
bit description, with permission of NXP.
314 Chapter 14
switchedoff. Therefore, to enableit wewill haveto set bit 12inthePCONP register, seenin
Table 14.4.
14.6.2 An ADC Application
Program Example 14.5 provides a good opportunity to see many of the control registers in
action. Channel 1 of the ADC is applied, which connects to pin 16 of the mbed.
The configuration of the ADC must be done with care, so let’s read the program
with diligence, starting fromthe comment ‘initialize the ADC’. The ADC channel
wewant ismultiplexedwithbit 24of Port 0, so first wemust allocatethispinto the
ADC. This is done through bits 17 and 16 of the PINSEL1 register (Table 14.2). We then
enable the ADC, through the relevant bit in the PCONP register (Table 14.4). There follows
quite a complex process of configuring the ADC, through the AD0CR register. We could
set this register by transferring a single word. Instead, we shift in the relevant bits in turn.
Check each with Table 14.8.
C code
feature
The data conversion then starts in the while loop which follows. The comments contained in
the program listing should give you a good picture of each stage.
/* Program Example 14.5: A bargraph meter for ADC input, using control registers to set
up ADC and digital I/O
*/
// variable declarations
char ADC_channel=1; // ADC channel 1
int ADCdata;
//this will hold the result of the conversion
int DigOutData=0;
//a buffer for the output display pattern
// function prototype
void delay(void);
//define addresses of control registers, as pointers to volatile data
//(i.e. the memory contents)
#define PINSEL1
(*(volatile unsigned long *)(0x4002C004))
#define PCONP
(*(volatile unsigned long *)(0x400FC0C4))
#define AD0CR
(*(volatile unsigned long *)(0x40034000))
#define AD0GDR
(*(volatile unsigned long *)(0x40034004))
#define FIO2DIR0
(*(volatile unsigned char *)( 0x2009C040))
#define FIO2PIN0
(*(volatile unsigned char *)( 0x2009C054))
int main() {
FIO2DIR0=0xFF;// set lower byte of Port 2 to output, this drives bargraph
//initialize the ADC
PINSEL1=0x00010000; //set bits 17-16 to 01 to enable AD0.1 (mbed pin 16)
PCONP j= (1 << 12);
// enable ADC clock
AD0CR =
(1 << ADC_channel)
// select channel 1
j (4 << 8)
// Divide incoming clock by (4+1), giving 4.8MHz
j (0 << 16)
// BURST = 0, conversions under software control
Letting Go of the mbed Libraries 315
j (1 << 21)
j (1 << 24);
// PDN = 1, enables power
// START = 1, start A/D conversion now
while(1) {
AD0CR = AD0CR j 0x01000000;
// infinite loop
//start conversion by setting bit 24 to 1,
//by ORing
// wait for it to finish by polling the ADC DONE bit
while ((AD0GDR & 0x80000000) == 0) { //test DONE bit, wait till it’ s 1
}
ADCdata = AD0GDR;
// get the data from AD0GDR
AD0CR &= 0xF8FFFFFF;
//stop ADC by setting START bits to zero
// Shift data 4 bits to right justify, and 2 more to give 10-bit ADC
// value - this gives convenient range of just over one thousand.
ADCdata=(ADCdata>>6)&0x03FF; //and mask
DigOutData=0x00;
//clear the output buffer
//display the data
if (ADCdata>200)
DigOutData=(DigOutDataj0x01); //set the lsb by ORing with 1
if (ADCdata>400)
DigOutData=(DigOutDataj0x02); //set the next lsb by ORing with 1
if (ADCdata>600)
DigOutData=(DigOutDataj0x04);
if (ADCdata>800)
DigOutData=(DigOutDataj0x08);
if (ADCdata>1000)
DigOutData=(DigOutDataj0x10);
FIO2PIN0 = DigOutData;
delay();
// pause
}
// set port 2 to Digoutdata
}
void delay(void){
//program continues
//delay function
Program Example 14.5 Applying the ADC as a bargraph
Connect an mbed with a potentiometer between 0 and 3.3 V with the wiper connected to
mbed pin 16. Connect five LEDs between pin and ground, from pin 22 to pin 26 inclusive.
Compile and download your code to the mbed, and press reset. Moving the potentiometer
should alter the number of LEDs lit, from none at all to all five.
n
Exercise 14.6
Add a digital input switch as in the previous example to reverse the operation of the
analoginput. With thedigital switch in oneposition theLEDswill light fromright to left,
but with the switch in the alternate position the DigOutData variable can be inverted
to light LEDs in the opposite direction, from left to right.
n
316 Chapter 14
Table 14.9: The AD0GDR register
Bit
Symbol
Description
Reset value
3:0
e
Reserved, user softwareshould not writeones to reserved
bits. The value read from a reserved bit is not defined
NA
15:4
RESULT
When DONE is 1, this field contains a binary fraction
representingthevoltageon theAD0[n] pin selected bythe
SEL field, asit fallswithin therangeof VREFP to VREFN. Zero
in thefield indicates that thevoltageon theinput pin was
less than, equal to, or closeto that on VREFN, while0x3FF
indicatesthat thevoltageon theinput wascloseto, equal
to, or greater than that on VREFP
NA
23:16
e
Reserved, user softwareshould not writeones to reserved
bits. The value read from a reserved bit is not defined
NA
26:24
CHN
These bits contain the channel from which the RESULT
bits were converted (e.g. 000 identifies channel 0, 001
channel 1 . )
NA
29:27
e
Reserved, user softwareshould not writeones to reserved
bits. The value read from a reserved bit is not defined
NA
30
OVERRUN
This bit is 1 in burst mode if the results of one or more
conversions was (were) lost and overwritten before the
conversion that produced the result in the RESULT bits.
This bit is cleared by reading this register
0
31
DONE
Thisbit isset to 1 when an A/D conversion completes. It is
cleared when this register is read and when the ADCR is
written. If theADCR iswritten whilea conversion isstill in
progress, this bit is set and a new conversion is started
0
RedrawnfromNXP SemiconductorsUM10360 LPC17xxUser Manual, Table532: A/D Global DataRegister (AD0GDRe address0x4003
4004) bit description, with permission of NXP.
n
Exercise 14.7
Extend the bargraph so that it has eight or 10 LEDs.
n
14.6.3 Changing ADC Conversion Speed
One of the limitations of the mbed ADC library is the comparatively slow speed of
conversion. Thiswas exploredinExercise5.6. Let’s try nowto vary this conversionspeedby
adjusting the ADC clock speed.
Letting Go of the mbed Libraries 317
Table 14.8 tells us that the ADC clock frequency should have a maximumvalue of 13 MHz,
and that it takes 65 cycles of the ADC clock to complete a conversion. A quick calculation
shows that the minimum conversion time possible is therefore 5 ms. It takes a very careful
reading of the LPC1768 user manual (Reference 2.4) to gain a full picture of how the ADC
clock frequency is controlled. The ADC clock is derived from the main microcontroller
clock; there are several stages of division that the user can control in order to set up
afrequency ascloseto13 MHz aspossible. ThefirstisthroughregisterPCLKSEL0, detailed
in Tables 14.5 and 14.6. Bits 25 and 24 of PCLKSEL0 control the ADC clock division. We
haveseenthat formost peripherals, includingtheADC, theclock canbedividedby 1, 2, 4or
8. On power-up the selection defaults to divide by 4. The clock may be further divided
through bits 15e8 of the AD0CR register, seen in Table 14.8.
Program Example 14.6 replicates Program Example 5.5, with some interesting results. It is
also a useful example, as it combines ADC, DAC and digital I/O, therefore illustrating how
these can be used together. It is made up of elements from programs earlier in this chapter,
sometimes with adjustments; it should be possible to follow it through without too much
difficulty. As sections of the programrepeat fromearlier examples, only the newer parts are
reproduced here. The full program listing can be downloaded from the book website.
/* Program Example 14.6: Explore ADC conversion times, programming control registers
directly. ADC value is transferred to DAC, while an output pin is strobed to indicate
conversion duration. Observe on oscilloscope
*/
. .
. .
int main() {
FIO2DIR0=0xFF;
// set lower bits port 2 to output
PINSEL1=0x00210000; //set bits 21-20 to 10 for analog output (mbed p18)
//and bits 17-16 to 01 to enable ADC channel 1 (AD0.1, mbed pin 16)
//initialize the ADC.
. .
. .
while(1){
// infinite loop
// start A/D conversion by modifying bits in the AD0CR register
AD0CR &= (AD0CR & 0xFFFFFF00);
FIO2PIN0 |= 0x01;
// OR bit 0 with 1 to set pin high
AD0CR |= (1 << ADC_channel) | (1 << 24);
// wait for it to finish by polling the ADC DONE bit
while((AD0GDR & 0x80000000) == 0) {
}
FIO2PIN0 &= ~0x01;
// AND bit 0 with 0 to set pin low
ADCdata = AD0GDR;
// get the data from AD0GDR
AD0CR &= 0xF8FFFFFF;
//stop ADC by setting START bits to zero
// shift data 4 bits to right justify, and 2 more to give 10-bit ADC value
ADCdata=(ADCdata>>6)&0x03FF; //and mask
318 Chapter 14
DACR=(ADCdata<<6);
//delay();
}
}
. .
//could be merged with previous line,
// but separated for clarity
//insert delay if wished
Program Example 14.6 Applying ADC, DAC and digital output, to measure
conversion duration
You can use the same mbed configuration for this as you did for Program Example 14.5,
although only the LED on pin 26 is necessary. Compile and run the program. First put an
oscilloscope probe on pin 18, the DAC output. The voltage on this pin should change as the
potentiometeris used. This confirms that theprogramis running. Nowmovetheprobeto pin
26; youwill see thepin pulsinghighfor the durationof theADC conversion. If youmeasure
this, you should find it is 14 ms, or just under.
To calculate the ADC clock frequency, and hence conversion time, remember that the mbed
CCLK frequency is 96 MHz. We have not touched the PCLKSEL0 register, so the clock
settingfortheADCwill be96 MHz dividedby theresetvalueof 4, or24 MHz. Thisisfurther
divided by 5 in the ADC0CR setting seen in the programexample, leading to an ADC clock
frequency of 4.8 MHz, or a period of 0.21 ms. Sixty-five cycles of 0.21 ms leads to the
measurement duration mentioned in the previous paragraph.
n
Exercise 14.8
1. Adjust the setting of the CLKDIV bits in AD0CR in Program Example 14.7 to give
the fastest permissible conversion time. Run the program, and check that your
measured value agrees with the predicted value.
2. Can you now account for the value of ADC conversion time you measured in
Exercise 5.6?
n
14.7 A Conclusion on using the Control Registers
Thischapterhasexploredtheuseof theLPC1768control registers,inconnectionwithuseof the
digital I/O, ADC andDAC peripherals. This discussionhas demonstratedhowtheseregisters
allowtheperipheralstobecontrolleddirectly, withoutusingthembedlibraries.Thishasallowed
greaterflexibilityof useof theperipherals,atthecostof gettingintothetinydetail of theregisters
andprogrammingat thelevel of thebitsthatmakethemup. Ordinarily, weprobably wouldnot
want to programlikethis; it is timeconsuming, inconvenient anderrorprone. However, if we
needaconfigurationorsettingnot offeredby thembedlibraries, this approachcanbeaway
forward. Whilewehaveonly workedinthisway inconnectionwiththreeof theperipherals, itis
Letting Go of the mbed Libraries 319
possibletodoitwithany of them. Itisworthmentioningthatthesethreeperipheralsaresomeof
thesimplerones; someof theothers requireevenmoreattentionto detail.
Chapter Review
• This chapter introduced a different way of controlling the mbed peripherals. It demands
a much deeper understanding of the mbed microcontroller, but allows for much greater
flexibility.
• Some registers relate to just one peripheral and others relate to microcontroller
performance as a whole.
• We have begun to implement features that are not currently available in the mbed
library, for example in the change of the ADC conversion speed.
• The chapter only introduces a small range of the control registers that are used by
the LPC1768. However, it should have given you the confidence to look up and begin to
apply any that you need.
Quiz
1. The initialization section of a certain program reads:
FIO0DIR0=0xF0;
PINMODE0=0x0F;
PINSEL1=0x00204000;
Explain the settings that have been made.
2. An LPC1768 is connected with a 3.0 V reference voltage. Its DAC output reads 0.375 V.
What is its input digital value?
3. On an LPC1768, the ADC clock is set at 4 MHz. How long does one conversion take?
4. A userwants to sampleanincomingsignal withanmbedADC at 44 kHz orgreater. What
is the minimum ADC clock value?
5. Describe how the ADC clock frequency calculated in Question 4 can be set up.
Reference
14.1. ARM Technical Support Knowledge Articles. Placing C variables at specific addresses to access memorymapped peripherals. http://infocenter.arm.com/help/index.jsp?topic¼/com.arm.doc.faqs/ka3750.html.
Accessed January 2012.
CHAPTER 15
Extension Projects
Chapter Outline
15.1 Where do We Go from Here? 321
15.2 Pololu Robot for mbed 321
15.3 Advanced Audio Projects 323
15.4 The Internet of Things 323
15.5 Introducing the mbed LPC11U24 325
15.6 From mbed to Manufacture 326
15.7 Closing Thoughts 329
References 329
15.1 Where do We Go fromHere?
A vast number of peripheral devices and applications can be used with mbed projects. In the
scope of this book alone it is not possible to explore every technology or peripheral device,
but the design and programming techniques covered will allow you to investigate wider
concepts to develop advanced projects using the mbed.
This chapter discusses a number of extension topics which suggest other areas for
investigating and enjoying, as well as helping to bridge the gap between laboratory
prototyping and the world of mass manufacturing. It is not possible to cover each of
these aspects in detail, but an overview is provided to allow you to be creative in your
approach to mbed projects and to give a springboard to more advanced and innovative
systems.
15.2 Pololu Robot for mbed
Pololu manufactures a number of robotics and electronics kits for developing embedded
systems. Inparticular, thePololum3pi robot, whichfeaturesanmbedcontrol boardandahost
of sensorsandactuators, isshowninFigure15.1. Therobothastwowheelsandisableto turn
on its central point by rotating the wheels in opposite directions. For full details, see
Reference 15.1.
Fast and Effective Embedded Systems Design. DOI: 10.1016/B978-0-08-097768-3.00015-5
Copyright Ó 2012 Elsevier Ltd. All rights reserved.
321
322 Chapter 15
(a)
(b)
Figure 15.1:
(a) Pololu m3pi robot with (b) mbed control board (Reference 15.1). (Image reproduced with
permission of Pololu Corporation)
The mp3i has an 8x2 character liquid crystal display (LCD) screen built in, as well as
a number of buzzers and switches for input and output control. There are also several
reflective sensors on the underside to allow development of line-tracking and maze-solving
systems. Beyondthis, thereis auniversal serial bus (USB) port andanXBeesocket allowing
wireless connectivity. Indeed, various projects have been prototyped by ARM to show the
capabilities of the Pololu m3pi, in particular a system that allows control of the robot via
a Nintendo Wii controller (see Reference 15.2).
Extension Projects 323
15.3 Advanced Audio Projects
So far, the audio projects we have discussed have used very simple implementations of the
analog-to-digital converter (ADC), the digital-to-analog converter (DAC) or the pulse width
modulation ports. In addition, some very simple analog circuitry has been used when
implementingsimpledigital audiosamplingandanalogreconstructionsystems. However, the
LPC1768 ADC is only 12-bit and the DAC is only 10-bit resolution, whereas high-quality
audio generally requires 16-bit or even 24-bit resolution. Furthermore, having only
asingleDAC meansthatthembedcannotdirectly outputstereo (two-channel) audio. If highquality stereo audio projects are to be developed it is possible to use a specialist ADC/DAC
integrated circuit, such as the Texas Instruments TLV320AIC23b (Reference 15.3), which
boasts stereo 24-bit resolution at sampling frequencies up to 96kHz.
RS Components has announced the release of an mbed demo board (Reference 15.4) which
includes a TLV320AIC23b ADC/DAC, and ARM has developed a number of libraries and
support pages to interface the device (Reference 15.5). The mbed communicates with the
TLV320 through an inter-integrated circuit sound (I2S) port. This is a serial interface
specifically designedforconnectingdigital audio devicestogether. Wehavenotuseditinthis
book, as it is not part of the ‘official’ mbed interconnection of Figure 2.1. However, it does
appear in the LPC1768 block diagram of Figure 2.3, and there are means of routing it
externally.
15.4 The Internet of Things
TheInternetof Thingsisaphrasethathasevolvedto referto asystemof everyday objectsand
sensors being connected to the Internet, allowing remote access to information that can be
used to control and enhance everyday activities, while interacting with the mobile Internet
devices that people are now carrying (for example smartphones). Initially, the Internet of
Things referred to global logistical systems that allow advanced stock control and real-time
tracking of delivery items. For example, when an itemis purchased froman online shop, the
shop will have a detailed inventory of all the items held in stock. In general, when a shop
receives items to its warehouse, the items are scanned in by an electronic system which
evaluatessomebarcodedataandautomatically maintainsanaccuratestock register. Whenan
itemisdispatchedto acustomer, theitemisscannedoutof thewarehouseandissubsequently
scannedat various checkpoints along thedelivery route, allowingthecustomerto loginto an
onlineportal andtrack thedelivery of theirpurchase. Ondelivery, thecustomerwill writean
electronic signature into a portable device which will update the online delivery system to
identify theitemasdelivered, andperhapsgenerateaninvoiceto besentto thecustomer. The
whole process happens automatically, allowing the item to have an online status and the
customeraccess to thestatus information. Thetechnology that allows this to happenis inthe
324 Chapter 15
barcodes, the barcode scanners, handheld scanning and signature systems, as well as the
Internet server hardware and software that manage the tracking information.
Technical advances in scanning and tracking processes are expected to continue,
particularly with the development of cheaper and more advanced barcode scanners and
the introduction of Quick Response (QR) codes which allow more data than a standard
barcode to be encoded into an image. Figure 15.2 shows a QR code which, when scanned,
will open the ARM mbed website. One advantage of using QR codes is that they effectively
allow a one-way communication protocol with no wires and no electronic requirement in
the slavesystem, which is very cost effective. They are also very tolerant to noise and image
degradation. Using a QR code printed on an item, a QR scanner can find out a number of
details about that item, essentially in a similar way to requesting a status messagevia any of
the serial communication protocols previously discussed. People also have access to very
accurate code scanners these days, as most smartphones are enabled with high-resolution
cameras which, when equipped with barcode scanning software, such as that fromRedLaser
(Reference 15.6), allow many people to scan items with little additional hardware cost. The
QR code cannot be changed, of course, so there are limitations, but dynamic QR codes may
be an innovation for the future!
More recently, the Internet of Things concept has expanded to include everyday objects
attachedto theInternet; forexample, allowingthedatafromatemperaturesensorononeside
of the world to be used in a bespoke mobile phone application being accessed on the other
side of theworld; or allowing people to control their household lighting and heating systems
remotely over the Internet or via a smartphone, bringing potential fuel efficiency and home
security improvements. In some cases the ideal application for this globally connected
networkof dataandinformationhasyettoberealized, butitiswidely thoughtthatthistypeof
Figure 15.2:
QR code linking to www.mbed.org
Extension Projects 325
data access and control will become commonplace, especially if systems are made open
access (i.e. at no cost) to software developers and network users.
ARM has recently developed a new concept for the mbed platform called ‘WebSockets’
(Reference15.7). WebSocketsallowmbeddevelopersto prototypesystemsthat adhereto the
Internet of Things concept by giving webserver access to wireless- and Ethernet-equipped
mbed systems. An mbed system can push sensor data to an ARM-managed WebSocket (a
remote server) and the real-time data can be accessed globally by any Internet-equipped
device. It is also possible to push data to the mbed through a WebSocket, hence allowing
remote Internet control of the mbed system’s features.
15.5 Introducing the mbed LPC11U24
In early 2012 ARM released a second mbed platform, the mbed LPC11U24, based on the
ARM Cortex-M0 microprocessor architecture. This device is aimed predominantly at
prototyping low-power, portable applications and specifically those requiring USB
functionality. The NXP LPC11U24 microcontroller is a 48MHz device with 8kB of RAM
and 32kB of flashmemory. It also supports USB, two serial peripheral interface(SPI) buses,
inter-integrated circuit (I2C) and universal asynchronous receiver/transmitter (UART) serial,
and has six analog inputs and up to 30 digital input/output pins. The full pinout diagramfor
the mbed LPC11U24 is shown in Figure 15.3.
ThembedLPC11U24isdesignedto havethesameformfactorandpinlayoutastheoriginal
mbed LPC1768, but with reduced functionality. So, in general, any programs compiled to
run on the lower spec mbed LPC11U24 should execute with identical functionality on the
mbed LPC1768. The mbed LPC11U24 is also specifically designed to have lower power
consumption, so enabling prototyping of portable systems. Libraries for enabling sleep
modeandinterrupt-drivenwakefunctionshavealso beenimplementedby ARM, andrecent
tests showed the device drawing 16mA while awake and less than 2mA while asleep.
However, it must be remembered that the device itself is for prototyping and any resultant
production design based on the LPC11U24 could readily do away with a number of mbed
hardware features and potentially reduce power consumption to lower levels still.
A further application of the mbed LPC11U24 is as a key enabler for the Internet of Things
and WebSockets concepts discussed earlier. The Internet of Things relies heavily on
portable sensing devices and mass commercial take-up, so mobile communications,
portability, low cost and low power prototyping are essential for these technologies to
evolve. In addition to the mbed LPC11U24, ARM has developed a number of WebSockets
libraries to allow mobile connectivity to the Internet, using the Roving Networks RN-131C
WiFly module, which communicates with the mbed over standard UART serial (see
Reference 15.8).
326 Chapter 15
Figure 15.3:
mbed LPC11U24 pinout diagram. (Image reproduced with permission of ARM Holdings)
15.6 Frommbed to Manufacture
The main thrust of this book has been focused towards rapid prototyping of embedded
systems, using the ARM mbed. Rapid prototyping is an essential part of the design process,
as it is the point where engineers need to seewhether a design concept will work and how it
should be implemented in hardware and software. The speed of this process is important,
because companies need to know where to invest their research and development resources
and which products to develop into mass-market systems. The mbed is an excellent device
for accelerating this process as its ease of use and high-level libraries allow continuous
testing during the prototyping and proof-of-concept cycle. The easier it is to test a system,
the easier it is to develop accurate solutions in a short space of time.
There comes a time, however, when, if a prototype proves successful, a developer will wish
to consider how the systemcould be engineered for mass manufacture and consumer use.
At this stage a number of issues should be considered, in particular size, cost, power
consumption, manufacturing methods, component availability and reliability, and quality
control. So, the move froma working prototype to a commercially ready product is not an
insignificant one, and thesoonertheseconsiderations can beincorporatedinto thedesignthe
Extension Projects 327
lowertheriskof drasticfeaturechangesbeingrequiredatalaterstage. Figure15.4summarizes
aproductdesigncycle, frominitial ideaandconceptdevelopment, throughprototypingtofinal
commercialization. The role of the mbed in proof of concept can bevery clearly seen.
Althoughthembedis designed as arapid prototypingdevice, it is intended to allow asimple
development path from prototyping to commercialization. In large companies, research and
development engineers will prototype and evaluate design concepts and a different set of
engineers may be required to take successful prototypes to commercialization. However, in
small industries it is not uncommon for a single engineer to see a product through the entire
development cycle, so engineers must also have a working knowledge of the manufacturing
constraints when working on prototyping and product development.
Brainstorming
Concept
Development
IDEAS
PROOF
OF
CONCEPT
Rapid
Prototyping
Market research
Prototype
Testing
Design for
Manufacture
Schematic and printed
circuit board design
Component sourcing
Negotiating costs and
partnerships
COMMERCIALIZATION
Field
Testing
Product
Launch
Figure 15.4:
A simplified product development cycle
328 Chapter 15
If only a few commercial units are to be built, then it is quite feasible to use the mbed as it
stands as the core controller of the system. However, where thousands of units are to be
developed and sold, cost and size become very important factors. It is therefore desirable to
implement the mbed hardware components on a custom printed circuit board (PCB) design,
inorderto minimizesizeandcost. Indeed, anumberof hardwarefeaturesonthembedcanbe
removedforcertainhardwareimplementations, if developingabespokePCB, hencereducing
cost and size further still.
Guidanceontheprocess involvedintakinganmbedprototypeto themanufacturestagecan
be found in Reference 15.9, while Reference 15.10 gives a good example. Essentially, the
USB bootloader hardware features on the mbed (those which allow drag-and-drop
downloading of the .bin file) are only required for prototyping. Furthermore, if the
application does not use Ethernet, then the Ethernet features can be left off the PCB design
too. In a production system, the program binary file needs downloading to the LPC1768
only once, although this is an important part of the manufacturing process. It would be
inefficient to populate the bespoke PCB with all the USB capabilities of the mbed
simply to program the device once, but thankfully it is possible to use an actual mbed as
a gateway device to the LPC1768 on the PCB, allowing the chip to easily be programmed.
Martin Smith’s description in Reference 15.10 of the development of a bespoke PCB
containing all mbed features, except for Ethernet, JTAG and USB, is a useful guide for
anyonelookingto developtheirownmbedPCBs. TheresultingPCB isshowninFigure15.5.
The process of programming the LPC1768 on this PCB through a second mbed is also
described.
Figure 15.5:
Bespoke PCB based on the mbed, showing the LPC1768 in place. (Image reproduced with permission
of Martin Smith)
Extension Projects 329
15.7 Closing Thoughts
The mbed is shown to be a flexible and powerful device for the rapid prototyping of
a number of electronic systems from electromechanical control to advanced Internet
communications. The possibilities for projects involving the mbed are perhaps endless, and
these latter chapters are intended to widen the developer’s creative perspective and give
some examples of how the technology described can be applied to bigger and more
advanced projects.
The mbed itself is an innovative design, using a novel online compiler and a simple
drag-and-drop USB interface. The aim is to make working with embedded systems simple,
as often the learning curve is so steep that non-engineers rarely overcome the initial hurdles.
The mbed is an excellent device to use for prototyping, education (both at early learning
and higher education levels) and engagement between artistic and technological fields,
where developers may not have aconventional engineering background. The idea is to make
embedded systems design fun, accessible, yet productive, which the mbed does with ease.
References
15.1.
15.2.
15.3.
15.4.
15.5.
15.6.
15.7.
15.8.
15.9.
15.10.
Pololu m3pi robot with mbed socket. http://www.pololu.com/catalog/product/2151
mbed Robot Racing Wii. http://mbed.org/cookbook/mbed-Robot-Racing-Wii
TLV320AIC23B low-power stereo CODEC with HP amplifier. http://www.ti.com/product/tlv320aic23b
mbed now does audio. http://www.designspark.com/content/mbed-now-does-audio
TLV320AIC23B. http://mbed.org/cookbook/TLV320AIC23B
RedLaser is a scanning application for iPhone. http://redlaser.com/
The Internet of Things. http://mbed.org/cookbook/IOT
Roving Networks RN-131 Wi-Fi module http://www.rovingnetworks.com/products/RN_131
Prototype to hardware. http://mbed.org/users/chris/notebook/prototype-to-hardware/
Turning an mbed into a custom PCB. http://mbed.org/users/ms523/notebook/turning-an-mbed-circuitinto-a-custom-pcb/
APPEN DIX A
Some Number Systems
A.1 Binary, Decimal and Hexadecimal
Thenumbersystemwearemost familiarwith, decimal, makesuseof 10different symbolsto
represent numbers, i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. Each of these symbols represents
anumber, andwemakelargernumbersby usinggroupsof symbols. Inthiscasethedigitmost
to the right represents units, the next represents tens, the next hundreds, and so on. For
example, the number 249, shown in Figure A.1, is evaluated by adding the values in each
position:
2 hundredsþ 4 tensþ 9 units ¼249
or
2
102 þ 4
101 þ 9
100 ¼249
The base or radix of the decimal system, just described, is 10. We almost certainly count in
the decimal system owing to the accident of having 10 fingers and thumbs on our hands.
Thereis nothingintrinsically correct orsuperiorabout it. It is quitepossibleto count inother
bases, and the world of digital computing almost forces us to do this.
The binary counting system has a base or radix of 2. It therefore uses just two symbols,
normally 0 and 1. These are called binary digits, or bits. Numbers are made up of groups of
digits. The value each digit represents again depends upon its position in the number.
Therefore, the 4-bit number 1101, shown in Figure A.2, is interpreted as:
1
8þ 1
4þ 0
2þ 1
1 ¼8þ 4þ 1 ¼13
Similarly, thevalue0110binary ¼6decimal. Notethatwereferto theunitsdigitas‘bit0’, or
the least significant bit (LSB). The twos digit is called ‘bit 1’ and so on, up to the most
significant bit (MSB).
Figure A.1:
The decimal number 249
331
332 Appendix A
Figure A.2:
The binary number 1101
We are obviously interested in binary representation of numbers, because that is how digital
machines performmathematical operations. But sometimes therearejust too many 0s and1s
to keep track of. We oftengroup bits in 4s, as we oftenlook at 4-bit numbers, 8-bit numbers,
12, 16, 20, 24, 28 and 32-bit numbers.
A single byte is made up of 8 bits. With one byte we can count up to 255 in decimal; for
example:
0000 0000 binary ¼0 decimal
1111 1111 binary ¼255 decimal
1001 1100 binary ¼128 +16 +8 +4 ¼156
The range of 0 to 255, offered by a single byte, is not very great, however. To perform
mathematical calculations to ahigh accuracy we need to work with 16, 24 or32-bit systems.
With 16 bits we can count or resolve up to 65535; for example:
1111 1111 1111 1111 binary ¼65535 decimal
0111 0111 0111 0110 binary ¼30582 decimal
Working with large binary numbers is something that is just not easy for a human; we just
cannot absorball those1s and0s. A very convenient alternativeis to usehexadecimal, which
works to base 16. This means that we can now count up to 15 (decimal) before there is an
overflow, and each new overflow column is an increased power of 16. Consider the
hexadecimal number 371 as shown in Figure A.3. Here, we see that 371 in hexadecimal is
(3 256) +(7 16) +1¼881 in decimal.
Now we need to represent 16 numbers with just a single digit. We use the decimal digits
for numbers 0 to 9, but to represent numbers 10 to 15 we use letters: ‘A’ or ‘a’ is adopted
to represent decimal 10. Similarly, B or b¼11, C¼12, D ¼13, E¼14 and F¼15. As
convention, we put ‘0x’ before a hexadecimal number; this means that all the bits before the
Figure A.3:
Hexadecimal number system
Some Number Systems 333
Table A.1: 4-bit values in binary, hexadecimal and decimal
4-bit binary number
Hexadecimal equivalent
Decimal equivalent
0
0
0
0
0x0
0
0
0
0
1
0x1
1
0
0
1
0
0x2
2
0
0
1
1
0x3
3
0
1
0
0
0x4
4
0
1
0
1
0x5
5
0
1
1
0
0x6
6
0
1
1
1
0x7
7
1
0
0
0
0x8
8
1
0
0
1
0x9
9
1
0
1
0
0xA
10
1
0
1
1
0xB
11
1
1
0
0
0xC
12
1
1
0
1
0xD
13
1
1
1
0
0xE
14
1
1
1
1
0xF
15
number are zero or clear. For example, 0xE5¼14 16+5¼229. Table A.1 shows the
equivalent 4-bit values in binary, hexadecimal and decimal.
The great advantage of hexadecimal numbers is that we can represent 4-bit numbers with
a single digit. An 8-bit number is represented with 2 hexadecimal digits, and 16-bit numbers
with 4 digits. You can then see that by individually looking at groups of four bits, we can
easily generate the hexadecimal equivalent, as with the following examples:
255 decimal ¼1111 1111 ¼0xFF
156 decimal ¼1001 1100 ¼0x9C
65535 decimal ¼1111 1111 1111 1111 ¼0xFFFF
30582 decimal ¼0111 0111 0111 0110 ¼0x7776
While we correctly use the two values of 0 and 1 in all binary numbers above, it is worth
noting that different terminology is sometimes used when we apply electronic circuits to
represent thesenumbers. Thisisdoneparticularly by thosewho arethinkingmoreintermsof
the circuit than of the numbers. This terminology is shown in Table A.2.
334 Appendix A
Table A.2: Some terminology for logic levels
Logic 0
Logic 1
0
1
Off
On
Low
High
Clear
Set
Open
Closed
A.2 Representation of Negative Numbers: Two’s Complement
Simple binary numbers allow only the representation of unsigned numbers, which under
normal circumstances are considered to be positive. Yet we must have away of representing
negative numbers as well. A simple way of doing this is by offsetting the available range of
numbers. We do this by coding the largest anticipated negative number as zero and counting
up from there. In the 8-bit range, with symmetrical offset, 128 can be represented as
00000000, 1000000 then represents zero, and 11111111 represents +127. This method of
coding is called offset binary, and is illustrated in Table A.3. It is used on occasions (e.g. in
Table A.3: Two’s complement and offset binary
Two’s complement
Decimal
Offset binary
0111 1111
+127
1111 1111
0111 1110
+126
1111 1110
:
:
0000 0001
+1
1000 0001
0000 0000
0
1000 0000
1111 1111
1
0111 1111
1111 1110
2
0111 1110
1000 0010
126
0000 0010
1000 0001
127
0000 0001
1000 0000
128
0000 0000
:
:
Some Number Systems 335
analog-to-digital converter outputs), but its usefulness is limited, as it is not easy to do
arithmetic with.
Let us consider an alternative approach. Suppose we took an 8-bit binary down counter, and
clocked it from any value down to, and then below, zero. We would get this sequence of
numbers:
Binary
Decimal
0000 0101
5
0000 0100
4
0000 0011
3
0000 0010
2
0000 0001
1
0000 0000
0
1111 1111
1?
1111 1110
2?
1111 1101
3?
1111 1100
4?
1111 1011
5?
This gives a possible means of representing negative numbers: effectively, we subtract the
magnitude of the negative number from zero, within the limits of the 8-bit number, or
whatever other size is in use. This representation is called two’s complement. It can be
shown that using two’s complement leads to correct results when simple binary addition
and subtraction is applied. Two’s complement notation can be applied to binary words of
any size.
Thetwo’scomplementof ann-bitnumberisfoundby subtractingitfrom2n; thatiswherethe
terminology comes from. Rather than doing this error-prone subtraction, an easier way of
reaching the same result is to complement (i.e. change 1 to 0, and 0 to 1) all the bits of the
positive number, and then add 1. Hence, to find 5 we follow the procedure:
original number
0000 0101 (+5)
complement all
/
1111 1010
add one
/
1111 1011 ( 5 in two’s complement)
Toconvertback, simply subtract1andcomplementagain. Notethatthemostsignificantbitof
atwo’scomplementnumberactsasa‘signbit’, 1fornegative, 0forpositive. The8-bitbinary
range, shown for both two’s complement and offset binary, appears in Table A.3.
336 Appendix A
A.2.1 Range of Two’s Complement
Ingeneral, therangeof ann-bittwo’scomplementnumberisfrom 2(n 1) to+{2(n
Table A.4 summarizes the ranges available for some commonly used values of n.
1)
1}.
A.3 Floating Point Number Representation
The numbers described so far all appear to be integers; we have not been able to represent
any sort of fractional number, and their range is limited by the size of the binary word
representing them. Suppose we need to represent really large or small numbers? Another
way of expressing a number, which greatly widens the range, is called floating point
representation.
Ingeneral, anumbercanberepresentedasa re, whereaisthemantissa, r istheradix, ande
is the exponent. This is sometimes called scientific notation. For example, the decimal
number 12.3 can be represented as:
1.23
.123
12.3
123
1230
101 or
102 or
100 or
10 1 or
10 2
Floating point notation adapts and applies scientific notation to the computer world. The
name is derived fromtheway the binary point can be allowed to float, by adjusting thevalue
of theexponent, to makebest useof thebits availableinthemantissa. Standardformats exist
for representing numbers by their sign, mantissa and exponent, and a host of hardware and
software techniques exists to process numbers represented in this way. Their disadvantage
lies in their greater complexity, and hence usually slower processing speed and higher cost.
For flexible use of numbers in the computing world they are, however, essential.
The most widely recognized and used format is the IEEE Standard for Floating-Point
Arithmetic (known as IEEE 754). In single precision form this makes use of 32-bit
Table A.4: Number ranges for differing word sizes
Number of bits
Unsigned binary
Two’s complement
8
0 to 255
128 to +127
12
0 to 4095
2048 to +2047
16
0 to 65535
32768 to +32 767
24
0 to 16777 215
8388 608 to +8388 607
32
0 to 4 294967 295
2147 483648 to +2147483 647
Some Number Systems 337
Figure A.4:
IEEE 754 32-bit floating point format
representation for a number, with 23 bits for the mantissa, 8 bits for the exponent and a sign
bit, as seeninFigureA.4. Thebinary point is assumedto bejust to theleft of theMSB of the
mantissa. A further bit, always 1 for a non-zero number, is added to the mantissa, making it
effectively a24-bitnumber. Zeroisrepresentedby 4zerobytes. Thenumber127issubtracted
fromthe exponent, leading to an effective range of exponents from 126 to +127. Exponent
255 (leading to 128, when 127 is subtracted) is reserved to represent infinity. The value of
a number represented in this format is then:
ð 1Þsign
2ðexponent
127Þ
1:mantissa
This allows number representation in the range:
1:175494
10
38
to
3:402823
10þ 38
APPEN DIX B
Some C Essentials
B.1 A Word about C
This appendix aims to summarize all features of the C language used in this book, though
not of the language as a whole. It is intended to be just adequate for the purpose of
supporting the book, and should not be viewed as complete. With care and experimentation
it can be used as an adequate introduction to the main features of the language. If you
are a C novice, however, it is well worth having another reference source available,
particularly as you consider the more advanced features. References B.1 and B.2 are
both good.
As youprogress throughthebook, youwill findyourself jumpingaround withinthematerial
of this appendix. Do not feel you need to read it sequentially. Instead, read the different
sections as they are referenced from the book.
B.2 Elements of a C Program
B.2.1 Keywords
C has a number of keywords whose use is defined. A programmer cannot use a keyword for
any otherpurpose, forexampleasadataname. KeywordsaresummarizedinTablesB.1e B.3.
B.2.2 ProgramFeatures and Layout
Simply speaking, a C program is made up of:
Declarations
All variables in C must be declared before they can be applied, giving as a minimum
the variable name and its data type. A declaration is terminated with a semicolon. In
simple programs, declarations appear as one of the first things in the program. They can
also occur within the program, with significance attached to the location of the
declaration.
For example:
float exchange_rate;
int new_value;
339
340 Appendix B
Table B.1: C keywords associated with data type and structure definition
Word
Summary meaning
Word
Summary meaning
char
A single character, usually 8-bit
signed
A qualifier applied to char or int (default for
char and int is signed)
const
Data that will not be modified
sizeof
Returns the size in bytes of a specified item,
which may be variable, expression or array
double
A ‘double precision’ floatingpoint number
struct
Allows definition of a data structure
enum
Defines variables that can only
take certain integer values
typedef
Creates new name for existing data type
float
A ‘single precision’ floatingpoint number
union
A memory block shared by two or more
variables, of any data type
int
An integer value
unsigned
A qualifier applied to char or int (default for
char and int is signed)
long
An extended integer value; if
used alone, integer is implied
void
No value or type
short
A short integer value; if used
alone, integer is implied
volatile
A variable which can be changed by factors
other than the program code
Table B.2: C keywords associated with program flow
Word
Summary meaning
Word
Summary meaning
break
Causes exit from a loop
for
Defines a repeated loop e loop is
executed as long as condition
associated with for remains true
case
Identifies options for selection within
a switch expression
goto
Program execution moves to labeled
statement
continue
Allows a programto skip to theend of
a for, while or do statement
if
Starts conditional statement; if
condition is true, associated statement
or code block is executed
default
Identifies default option in a switch
expression, if no matches found
return
Returns program execution to calling
routine, causingalso return of anydata
value specified by function
do
Used with while to create loop, in
which statement or code block
following do is repeated as long as
while condition is true
switch
Used with case to allow selection of
anumber of alternatives; switch hasan
associated expression which is tested
against a number of case options
else
Used with if, and precedes alternative
statement or codeblockto beexecuted
if if condition is not true
while
Defines a repeated loop e loop is
executed as long as condition
associated with while remains true
Some C Essentials 341
Table B.3: C keywords associated with data storage class
Word
Summary meaning
Word
Summary meaning
auto
Variable exists only within block
within which it isdefined. Thisis
the default class
register
Variable to be stored in a CPU register;
thus, address operator (&) has no effect
extern
Declares data defined elsewhere
static
Declares variable which exists throughout
program execution; the location of its
declaration affects in what part of the
program it can be referenced
declare a variable called exchange_rate as a floating point number, and another
variable called new_value as an integer. The data types are keywords seen in
Tables B.1e B.3.
Statements
Statements are where the action of the program takes place. They perform mathematical
or logical operations, and establish program flow. Every statement that is not a block
(seebelow) endswithasemicolon. Statementsareexecutedinthesequencethey appearinthe
program, except where program branches take place.
For example, this line is a statement:
counter = counter + 1;
Space and Layout
There is no strict layout format to which C programs must adhere. The way the program is
laidoutandtheuseof spacearebothusedto enhanceclarity. Blank linesandindentsinlines,
for example, are ignored by the compiler, but used by the programmer to optimize the
program layout.
Asanexample, theprogramthat thembedcompileralwaysstartsupwith, shownasProgram
Example 2.1, could bewritten as shown here. It would not be easy to read, however. It is the
semi-colonsattheendof eachstatement, andthebrackets, whichinreality definemuchof the
program structure.
#include "mbed.h"
DigitalOut myled(LED1); int main() {while(1) {myled = 1; wait(0.2); myled = 0; wait(0.2);}}
Comments
Two ways of commentingareused. Oneis to placethecomment betweenthemarkers /* and
*/. This is useful for a block of text information running over several lines. Alternatively,
when two forward slash symbols (//) are used, the compiler ignores any text that follows on
that line only, which can then be used for comment.
342 Appendix B
For example:
/*A program which flashes mbed LED1 on and off,
Demonstrating use of digital output and wait functions. */
#include "mbed.h" //include the med header file as part of this program
Code Blocks
Declarations andstatements canbegroupedtogetherinto blocks. A block is containedwithin
braces, i.e. { and }. Blocks can and are written within other blocks, each within its own pair
of braces. Keepingtrackof thesepairsof bracesisanimportantpastimeinCprogramming, as
in a complex piece of software there can be numerous ones nested within each other.
B.2.3 Compiler Directives
Compiler directives are messages to the compiler, and do not directly lead to programcode.
Compiler directives all start with a hash, #. Two examples follow.
#include
The#includedirectivedirectly insertsanotherfileinto thefilethatinvokesthedirective. This
provides a feature for combining a number of files as if they were one large file. Angled
brackets(< > ) areusedto enclosefiles heldinadirectory different fromthecurrent working
directory, hence often for library files not written by the current author. Quotation marks are
used to contain a file located within the current working directory, hence often user defined.
For example:
#include "mbed.h"
#define
The #define directive allows use of names for specific constants. For example, to use the
numberp ¼3.141592intheprogram, wecouldcreatea#defineforthename‘PI’ andassign
that number to it, as shown:
#define PI
3.141592
Thename‘PI’ is thenusedinthecodewheneverthenumberis needed. Whencompiling, the
compiler replaces the name in the #define with the value that has been specified.
B.3 Variables and Data
B.3.1 Declaring, Naming and Initializing
Variables must be named, and their data type defined, before they can be used in a program.
Keywords from Table B.1 are used for this. For example,
Some C Essentials 343
int MyVariable;
defines ‘MyVariable’ as a data type int (integer).
It is possible to initialize the variable at the same time as declaration; for example,
int MyVariable = 25;
initializes MyVariable, and sets it to an initial value of 25.
It is possible to give variables meaningful names, while still avoiding excessive length, for
example ‘Height’, ‘InputFile’, ‘Area’. Variable names must start with a letter or an
underscore; no other punctuation marks are allowed. Variable names are case sensitive.
B.3.2 Data Types
Whenadatadeclarationismade, thecompilerreservesforitasectionof memory, whosesize
depends onthe typeinvoked. Examples of thelink betweendatatype, numberrange and
memory size are shown in Table B.4. It is interestingto comparethesewithinformationon
numbertypesgiveninAppendix A. Notethattheactual memory sizeappliedtodatatypescan
vary between compilers. A full listing forthe mbed compilercan befound inReference B.3.
B.3.3 Working with Data
In C we can work with numbers in binary, fixed or floating point decimal, or
hexadecimal format, depending on what is most convenient, and what number type and
Table B.4: Example C data types, as implemented by the mbed compiler
Data type
Description
Length (bytes)
Range
char
Character
1
0 to 255
signed char
Character
1
unsigned char
Character
1
short
Integer
2
unsigned short
Integer
2
int
Integer
4
2147483648 to +2147483647
long
Integer
4
2147483648 to +2147483647
unsigned long
Integer
4
float
Floating point
1.17549435
3.40282347
double
Floating point, double
precision
2.22507385850720138
1.79769313486231571
128 to +127
0 to 255
32768 to +32767
0 to 65 535
0 to 4294967295
10 38 to
10+38
10 308 to
10+308
344 Appendix B
range is required. For time-critical applications it is important to remember that floating
point calculations can take much longer than fixed point. In general, it is easiest for us
to work in decimal, but if a variable represents a register bit field or a port address, then
it is usually more appropriate to manipulate the data in hexadecimal. When writing
numbers in a program, the default radix for integers is decimal, with no leading 0 (zero).
Octal numbers are identified with a leading 0. Hexadecimal numbers are prefixed
with 0x.
For example, if a variable MyVariable is of type char we can perform the following
examples to assign a number to that variable:
MyVariable = 15; //a decimal example
MyVariable = 0x0E; //a hexadecimal example
The value for both is the same.
B.4 Functions
A function is a section of code that can be called from another part of the program. So,
if a particular piece of code is to be used or duplicated many times, we can write it
once as a function, and then call that function whenever the specific operation is
required. Using functions saves coding time and improves readability by making the
code neater.
Data can be passed to functions, and returned from them. Such data elements, called
arguments, must beof atypethat is declaredinadvance. Only onereturnvariableis allowed,
whose type must also be declared. The data passed to the variable is a copy of the original.
Therefore, the function does not itself modify the value of the variable named. The impact
of the function should thus be predictable and controlled.
return type
function
name
argument(s) type(s)
and names(s)
Close of function
Figure B.1:
Function example
opening of function
Some C Essentials 345
A functionisdefinedinaprogramby ablockof codehavingparticularcharacteristics. Itsfirst
line forms the function header, with the format:
Output_type function_name (variable_type_1 variable_name_1, variable_type_2
variable_name_2,. )
An example is shown in Figure B.1. The return type is given first. In this example, the
keyword float is used. After the function name, in brackets, one or more data types may be
listed, which identify the arguments which must be passed to the function. In this case two
arguments are sent, one of type char and one of type float. Following the function header,
apairof braces encloses thecodewhichmakes upthefunctionitself. This couldbeanything
fromasinglelineto many pages. Thefinal statement of thefunctionmay beareturn, which
will specify thevaluereturnedtothecallingprogram. Thisisnotessential if no returnvalueis
required.
B.4.1 The main Function
Thecorecodeof any C programis containedwithinits‘main’ function. Otherfunctionsmay
be written outside main( ), and called from within it. Program execution starts at the
beginning of main( ). It must follow the structure just described. However, as main( )
containsthecentral program, onedoesnotexpect to sendanythingto it orto receiveanything
from it. Therefore, usual patterns for main( ) are:
void main (void){
void main (){
int main (){
The keyword void indicates that no data is specified. The mbed main( ) function applies the
third option, as in C++int is the return type for main( ) normally used.
B.4.2 Function Prototypes
Just like variables, functions must be declared at the start of a program, before the main
function. The declaration statements for functions are called prototypes. Each function in
the code must have an associated prototype for it to run. The format is the same as for the
function header.
For example, the following function prototype applies to the function header seen above:
float conversion(char currency, float number_of_pounds)
This describes a function that takes inputs of a character value for the selected currency and
a floating point (decimal) value for the number of pounds to be converted. The function
returns the decimal monetary value in the specified currency.
346 Appendix B
B.4.3 Function Definitions
The actual function code is called the function definition. For example:
float conversion(char currency, float number_of_pounds) {
float exchange_rate;
switch(currency) {
case 'U': exchange_rate = 1.50;
// US dollars
break;
case 'E': exchange_rate = 1.12 ); // Euros
break;
case 'Y': exchange_rate = 135.4); // Japan yen
break;
default: exchange_rate = 1);
}
exchange_value=number_of_pounds*exchange_rate;
return(exchange_value);
}
This function can be called any number of times from within the main C program, or from
another function, for example in this statement:
ten_pounds_in_yen=conversion(‘ Y’ ,10.00);
B.4.4 Using the static Storage Class with Functions
The static data type is useful for defining variables within functions, where the data inside
the function must be remembered between function calls. For example, if a function within
a real-time system is used to calculate a digital filter output, the function should always
remember its previous data values. In this case, data values inside the function should be
defined as static; for example:
float movingaveragefilter(float data_in) {
static float data_array[10];
// define static float data array
for (int i=8;i>=0;i--) {
data_array[i+1]=data_array[i]; // shift each data value along
}
// (the oldest data value is discarded)
data_array[0]=data_in;
// place new data at index 0
float sum=0;
for (int i=0;i<=9;i++) {
sum=sum+data_array[i];
// calculate sum of data array
}
return sum/10;
// return average value of array
}
B.5 Operators
C has a wide set of operators, shown in Table B.5. The symbols used are familiar, but their
application is not always the same as in conventional algebra. For example, a single ‘equals’
symbol, ¼, is used to assign a value to a variable. A double equals sign, ¼¼, is used to
represent the conventional ‘equal to’.
Some C Essentials 347
Table B.5: C operators
Precedence
and order
Operation
Symbol
Precedence
and order
Operation
Symbol
Parentheses and array access operators
1, L to R
Function calls
()
1, L to R
Point at member
X-> Y
1, L to R
Subscript
[]
1, L to R
Select member
X.Y
Arithmetic operators
4, L to R
Add
X+Y
3, L to R
Multiply
X*Y
4, L to R
Subtract
X Y
3, L to R
Divide
X/Y
2, R to L
Unary plus
+X
3, L to R
Modulus
%
2, R to L
Unary minus
6, L to R
Less than or equal to
X< ¼Y
X
Relational operators
6, L to R
Greater than
X> Y
6, L to R
Greater than or equal to X> ¼Y
7, L to R
Equal to
X¼¼Y
6, L to R
Less than
7, L to R
Not equal to
X!¼Y
2, R to L
NOT (1 if X¼0)
!X
X< Y
Logical operators
11, L to R
AND (1 if both X and Y X&&Y
are not 0)
12, L to R
OR (1 if either Xor Yare XjjY
not 0)
Bitwise operators
8, L to R
Bitwise AND
X&Y
2, L to R
Ones complement
(bitwise NOT)
zX
10, L to R
Bitwise OR
XjY
5, L to R
Right shift. X is shifted
right Y times
X[ Y
9, L to R
Bitwise XOR
XˆY
5, L to R
Left shift. Xisshifted left X[ Y
Y times
Assignment operators
14, R to L
Assignment
X¼Y
14, R to L
Bitwise AND assign
X&¼Y
14, R to L
Add assign
X+¼Y
14, R to L
Bitwise inclusive OR
assign
Xj¼Y
14, R to L
Subtract assign
X ¼Y
14, R to L
Bitwise exclusive OR
assign
Xˆ¼Y
14, R to L
Multiply assign
X * ¼Y
14, R to L
Right shift assign
X [ ¼Y
Continued
348 Appendix B
Table B.5: C operatorsd cont’d
Precedence
and order
Operation
14, R to L
Divide assign
14, R to L
Remainder assign
Increment and decrement operators
2, R to L
Preincrement
2, R to L
Predecrement
Conditional operators
13, R to L
Evaluate either X (if
Zs 0) or Y (if Z¼0)
“Data interpretation” operators
2, R to L
The object or function
pointed to by X
2, R to L
Cast e the value of X,
with (scalar) type
specified
Precedence
and order
Operation
Symbol
X/¼Y
X%¼Y
14, R to L
Left shift assign
X
++X
X
2, R to L
2, R to L
Postincrement
Postdecrement
X++
X
Z?X:Y
15, L to R
Evaluate X first,
followed by Y
X,Y
*X
2, R to L
The address of X
&X
(type) X
2, R to L
The size of X, in bytes
Sizeof X
Symbol
¼Y
Operators have a certain order of precedence, shown in Table B.5. The compiler applies this
orderwhenitevaluatesastatement. If morethanoneoperatoratthesamelevel of precedence
occurs in a statement, then those operators are evaluated in turn, either left to right or right
to left, as shown in the table. For example, the line
counter = counter + 1;
contains two operators. Table B.5 shows that the addition operator has precedence level
4, while all assign operators have precedence 14. The addition is therefore evaluated
first, followed by the assign. The outcome is that the variable counter is incremented
by 1.
B.6 Flow Control: Conditional Branching
Flowcontrol coversthedifferentformsof branchingandloopingavailableinC. Asbranching
and looping can lead to programming errors, C provides clear structures to improve
programming reliability.
B.6.1 If and else
If statements always start with use of the if keyword, followed by a logical condition. If the
condition is satisfied, then the code block that follows is executed. If the condition is not
satisfied, thenthecodeisnotexecuted. Theremay ormay not also befollowingelseorelseif
statements.
Some C Essentials 349
Syntax:
if (Condition1){
. C statements here
}else if (Condition2){
. C statements here
}else if (Condition3){
. C statements here
}else{
. C statements here
}
The if and else statements are evaluated in sequence, i.e.:
• else if statements are only evaluated if previous if or else if conditions have failed
• else statements are only executed if all previous conditions have failed.
For example, in the above example, the else if (Condition2) will only be executed if
Condition1 has failed.
Example:
if (data > 10){
data += 5;
}else if(data > 5){
data -= 3;
}else{
nVal = 0;
}
//If we reach this point, data must be > 10
//If we reach this point, data must be <= 10
//If we reach this point, data must be <= 5
B.6.2 Switch Statements, and using break
Theswitch statementallowsaselectionto bemadeof oneoutof several actions, basedonthe
value of a variable or an expression given in the statement. An example of this structure has
already appeared, in the example function in Section B.4.3. The structure uses no fewer than
four C keywords. Selection is made from a list of case statements, each with an associated
label e note that a colon following a text word defines it as a label. If the label equals the
switch expression then the action associated with that case is executed. The default action
(which is optional) occurs if none of the case statements is satisfied. The break keyword,
which terminates each case condition, can be used to exit from any loop. It causes program
execution to continue after the switch code block.
B.7 Flow Control: ProgramLoops
B.7.1 while Loops
A whileloopisasimplemechanismforrepeatingasectionof code, until acertainconditionis
satisfied. The condition is stated in brackets after the word while, with the conditional code
block following. For example:
350 Appendix B
i=1
while (i<10) {
. C statements here
i++
//increment i
}
Here, thevalueof i isdefinedoutsidetheloop; itisthenupdatedwithintheloop. Eventually i
increments to 10, at which point the loop will terminate. The condition associated with the
while statement is evaluated at the start of each loop iteration; the loop then only runs if
the condition is found to be true.
B.7.2 for Loops
The for loop allows a different form of looping, in that the dependent variable is updated
automatically every timetheloopisrepeated. Itdefinesaninitializedvariable, aconditionfor
looping and an update statement. Note that the update takes place at the end of each loop
iteration. If the updated variable is no longer true for the loop condition, the loop stops and
program flow continues. For example:
for(j=0; j<10; j++) {
. C statements here
}
Here, theinitial conditionis j=0 andtheupdatevalueis j++, i.e. j is incremented. This means
that j increments with each loop. When j becomes 10 (i.e. after10 loops), the condition j<10
is no longer satisfied, so the loop does not continue any further.
B.7.3 Infinite Loops
Weoftenrequireaprogramtoloopforever, particularly inasuper-loopprogramstructure. An
infinite loop can be implemented by either of the following loops:
while(1) {
. continuously called C statements here
}
or
for(;;) {
. continuously called C statements here
}
B.7.4 Exiting Loops with break
The break keyword can also be used to exit froma for or while loop, at any timewithin the
loop. For example:
while(i>5) {
. C statements here
if (fred = = 1)
break;
Some C Essentials 351
. C statements here
}
//end of while
//execution continues here loop completion, or on break
B.8 Derived Data Types
Inadditiontothefundamental datatypes, therearefurtherdatatypesthatcanbederivedfrom
them. Example types that we use are described in this section.
B.8.1 Arrays and Strings
An array is a set of data elements, each of which has the same type. Any data type can be
used. Array elements are stored in consecutive memory locations. An array is declared with
its name and the data type of its elements; it is recognized by the use of the square brackets
following the name. The number of elements and their value can also be specified. For
example, the declaration
unsigned char message1[8];
defines an array called message1, containing eight characters. Alternatively, it can be left to
the compiler to deduce the array length, as seen in the two examples here:
char item1[] = "Apple";
int nTemp[] = {5,15,20,25};
In each of these the array is initialized as it is declared.
Elements withinanarray canbeaccessedwithanindex, starting withvalue0. Therefore, for
the first example above, message1[0] selects the first element and message1[7] the last. An
access to message [8] would be outside the boundary of the array, and would give invalid
data. The index can be replaced by any variable that represents the required value.
Importantly, thenameof anarray is set equal to theaddress of theinitial element. Therefore,
when an array name is passed in a function, what is passed is this address.
A string is a special array of type char that is ended by the NULL (\0) character. The
null character allows code to search for the end of a string. The size of the string array
mustthereforebeonebytegreaterthanthestringitself, tocontainthischaracter. Forexample,
a 20-character string could be declared:
char MyString[21];
// 20 characters plus null
B.8.2 Pointers
Insteadof specifyingavariableby name, wecanspecify itsaddress. InCterminology suchan
addressiscalledapointer. A pointercanbeloadedwiththeaddressof avariableby usingthe
unary operator ‘&’, like this:
my_pointer = &fred;
352 Appendix B
Thisloadsthevariablemy_pointer withtheaddressof thevariablefred; my_pointer isthen
said to point to fred.
Doing things the other way round, the value of the variable pointed to by a pointer can
be specified by prefixing the pointer with the ‘*’ operator. For example, *my_pointer
can be read as ‘the value pointed to by my_pointer’. The * operator, used in this way,
is sometimes called the dereferencing or indirection operator. The indirect value of
a pointer, for example *my_pointer, can be used in an expression just like any other
variable.
A pointer is declared by the data type it points to. Thus,
int *my_pointer;
indicates that my_pointer points to a variable of type int.
We can also use pointers with arrays, because an array is really just a number of data values
stored at consecutive memory locations. So, if the following is defined:
int dataarray[]={3,4,6,2,8,9,1,4,6};
int *ptr;
ptr = &dataarray[0];
// define an array of arbitrary values
// define a pointer
// assign pointer to the address of
// the first element of the data array
given the previous declarations, the following statements will therefore be true:
*ptr = = 3;
*(ptr+1) = = 4;
*(ptr+2) = = 6;
// the first element of the array pointed to
// the second element of the array pointed to
// the third element of the array pointed to
Thus, array searching can be done by moving the pointer value to the correct array offset.
Pointersarerequiredforanumberof reasons, butonesimplereasonisbecausetheC standard
doesnotallowusto passarraysof datato andfromfunctions, sowemustusepointersinstead
to get around this.
B.8.3 Structures and Unions
Structures and unions are both sets of related variables, defined through the C keywords
struct andunion. Inaway they arelikearrays, but inbothcasesthey canbeof dataelements
of different types.
Structure elements, called members, are arranged sequentially, with the members occupying
successive locations in memory. A structure is declared by invoking the struct keyword,
followed by an optional name (called the structure tag), followed by a list of the structure
members, each of these itself forming a declaration. For example:
struct resistor {int val; char pow; char tol;};
Some C Essentials 353
declares a structure with tag resistor, which holds the value (val), power rating (pow) and
tolerance (tol) of a resistor. The tag may come before or after the braces holding the list of
structure members.
Structure elements are identified by specifying the name of the variable and the name of the
member, separated by a full stop (period). Therefore, resistor.val identifies the first member
of the example structure above.
Likeastructure, aunioncanholddifferenttypesof data. Unlikethestructure, unionelements
all beginatthesameaddress.Hence,theunioncanrepresentonlyoneof itsmembersatanyone
time, andthesizeof theunionis thesizeof thelargest element. It is upto theprogrammerto
trackwhichtypeiscurrentlystored.Unionsaredeclaredinaformatsimilartothatof structures.
Unions, structures and arrays can occur within each other.
B.9 C Libraries and Standard Functions
B.9.1 Header Files
All but the simplest of C programs are made up of more than one file. In general, many files
are combined together in the process of compiling, for example original source files
combining with standard library files. To aid this process, a key section of any library file is
detached and created as a separate header file.
Header filenames end in.h; thefile typically includes declarations of constants and function
prototypes, and links on to other library files. The function definitions themselves stay in the
associated .c or.cpp files. In order to use the features of the header file and the file(s) it
invokes, itmustbeincludedwithinany programaccessingit, using#include. Weseembed.h
being included in every program in the book. Note also that the .c file where the function
declarations appear must also include the header file.
B.9.2 Libraries, and the C Standard Library
BecauseC isasimplelanguage, muchof itsfunctionality derivesfromstandardfunctionsand
macros that are available in the libraries accompanying any compiler. A C library is a set
of precompiled functions that can be linked in to the application. These may be supplied
withacompiler, availablein-company, ormay bepublicdomain. Notably, thereisaStandard
Library, definedintheC ANSI standard. Thereareanumberof standardheaderfiles, usedfor
different groups of functions within the standard library. For example, the < math.h> header
file is used for a range of mathematical functions (including all trigonometric functions),
while < stdio.h> contains the standard input and output functions, including the printf
function.
354 Appendix B
B.9.3 Using printf
This versatile function provides formatted output, typically for sending display data to a PC
screen. Text, data, formatting and control formatting can be specified. Only summary
informationisprovidedhere; afull statementcanbefoundinReferenceB.1. Examplesbelow
are taken from book chapters. In each case the function appears in the form pc.printf( ),
indicating that printf( ) is being used as a member function of a C++class pc created in the
example program. This does not affect the format applied.
Simple Text Messages
pc.printf("ADC Data Values. \n\r"); \\send an opening text message
Thisprintsthetextstring‘ADCDataValues. ’ toscreen, andusescontrol characters\nand\r
to force a new line and carriage return, respectively.
Data Messages
pc.printf("%1.3f",ADCdata);
Thisprintsthevalueof thefloat variableADCdata. A conversionspecifier, initiatedby the%
character, defines the format. Within this, the ‘f’ specifies floating point, and the .3 causes
output to three decimal places.
pc.printf("%1.3f \n\r",ADCdata); \\send the data to the terminal
As above, but includes \n and \r to force a new line and carriage return.
Combination of Text and Data
pc.printf("random number is %i\n\r", r_delay);
Thisprintsatextmessage, followedby thevalueof int variable(indicatedby the‘i’ specifier)
r_delay.
pc.printf("Time taken was %f seconds\n", t.read()); //print timed value to pc
This prints a text message, followed by the return value of function t.read( ), which is of
type float.
B.10 File Access Operations
B.10.1 Overview
In C we can open files, read and write data and also scan through files to specific
locations, even searching for particular types of data. The commands for input and
output operations are all defined by the C stdio library, already mentioned in connection
with printf( ).
Some C Essentials 355
The stdio library uses the concept of streams to manage the data flow. All streams have
similar properties, even though the actual application of the data flow may be very varied.
Streams are represented in the stdio library as pointers to FILE objects, which uniquely
identify the stream. We can store data in files (as chars) or we can store words and strings
(as character arrays). A summary of useful stdio file access library functions is given in
Table B.6.
B.10.2 Opening and Closing Files
A file can be opened with the following command:
FILE* pFile = fopen("datafile.txt","w");
This assigns a pointer with name pFile to the file at the specific location given in the
fopen statement. In this example the access mode is specified with a ‘w’, and
a number of other file open access modes, and their specific meanings, are shown in
Table B.7. When ‘w’ is the access mode (meaning write access), if the file does not
already exist then the fopen command will automatically create it in the specified
location.
When you have finished using a file for reading or writing it is good practice to close it,
for example with
fclose(pFile);
Table B.6: Useful stdio library functions
Function
Format
Summary action
fclose
int fclose ( FILE * stream );
Closes a file
fgetc
int fgetc ( FILE * stream );
Gets a character from a stream
fgets
char * fgets ( char * str, int num, FILE *
stream );
Gets a string from a stream
fopen
FILE * fopen ( const char * filename, const
char * mode);
Opens the file of type file and name
filename
fprintf
int fprintf ( FILE * stream, const char *
format, . );
Writes formatted data to a file
fputc
int fputc ( int character, FILE * stream );
Writes a character to a stream
fputs
int fputs ( const char * str, FILE * stream );
Writes a string to a stream
fseek
int fseek ( FILE * stream, long int offset,
int origin );
Moves the file pointer to the specified
location
str: an arraycontainingthenull-terminated sequenceof charactersto bewritten; stream: pointer to aFILE object that identifies
the stream where the string is to be written.
356 Appendix B
Table B.7: Access modes for fopen
Access mode
Action
‘r’
Open an existing file for reading
‘w’
Create a new empty file for writing. If a file of the same name already exists it will be
deleted and replaced with a blank file
‘a’
Append to a file. Writeoperationsresult in data beingappended to theend of thefile. If
the file does not exist a new blank file will be created
‘r+’
Open an existing file for both reading and writing
‘w+’
Createa new emptyfilefor both reading and writing. If a fileof thesamenamealready
exists it will be deleted and replaced with a blank file
‘a+’
Open a filefor appendingor reading. Writeoperationsresult in data beingappended to
the end of the file. If the file does not exist a new blank file will be created
B.10.3 Writing and Reading File Data
If the intention is to store numerical data, this can be done in a simple way by storing
individual 8-bit data values. The fputc command allows this, as follows:
char write_var=0x0F;
fputc(write_var, pFile);
Thisstoresthe8-bitvariablewrite_var tothedatafile. Thedatacanalso bereadfromafileto
a variable as follows:
read_var = fgetc(pFile);
Using the stdio.h commands, it is also possible to read and write words and strings with
fgets( ) and fputs( ), and to write formatted data with fprintf( ).
It is also possible to search or move through files looking for particular data elements.
When reading data from a file, the file pointer can be moved with the fseek command.
For example, the following command will reposition the file pointer to the 8th byte in
the text file:
fseek (pFile , 8 , SEEK_SET ); // move file pointer to 8 bytes from the start
Thefirst termof thefseek( ) functionis thestream(thefilepointername), thesecondtermis
thepointeroffset valueandthethirdtermis theoriginforwheretheoffset shouldbeapplied.
There are three possible values for the origin as shown in Table B.8. The originvalues have
predefined names as shown.
Further details on the stdio commands and syntax can also be found in References B.1
and B.2.
Some C Essentials 357
Table B.8: fseek origin values
Origin value
Description
SEEK_SET
Beginning of file
SEEK_CUR
Current position of the file pointer
SEEK_END
End of file
B.11 Towards Professional Practice
Thereadability of aC programis muchenhancedby goodlayout onthepageorscreen. This
helps to produce good and error-free code, and enhances the ability to understand, maintain
and upgrade it. Many companies impose a ‘house style’ on C code written for them; such
guides can be found online.
Thevery simplestyleguideadoptedinthisbookshouldbeexemplifiedinany of theprograms
reproduced. It includes:
•
•
•
•
•
•
Courier New font applied
opening header text block, giving overview of program action
blank lines used to separate major code sections
extensive commenting
opening brace of any code block placed on the line which initiates it
code within any code block indented 2 spaces compared with code immediately outside
the block
• closing brace stands alone, indented to align with line initiating code block.
Versioncontrol is essential practiceinany professional environment. Forclarity, wehavenot
displayed version control information within the programs as reproduced in the book,
althoughthiswasdoneinthedevelopmentprocess. Minimal versioncontrol withinthesource
code (spaced within the header text block) would include the date of origin, name of the
original author, name of the person making the most recent revision, and date.
References
B.1. Prinz, P. and Kirch-Prinz, U. (2003). C Pocket Reference. O’Reilly.
B.2. The C++Resources Network. www.cplusplus.com
B.3. ARM. RealViewÒ CompilationTools. Version4.0. CompilerReferenceGuide. December2010. DUI 0348C
(ID101213).
APPEN DIX C
mbed Technical Data
C.1 Summarizing Technical Details of the mbed
The mbed is built around an LPC1768 microcontroller, made by NXP Semiconductors. The
LPC1768 is, in turn, designed around an ARM Cortex-M3 core, with 512kB flash memory,
64kB RAM, and a wide range of interfaces, including Ethernet, universal serial bus (USB)
device, Controller Area Network (CAN), serial peripheral interface (SPI), inter-integrated
circuit (I2C) and other input/output (I/O). It runs at 96MHz.
Full technical details of the mbed and LPC1768 can be found in References 2.1 to 2.4.
Summary mbed operating conditions are given directly below.
Package:
• 40-pinDIPpackage,with0.1inchspacingbetweenpins,and0.9inchesbetweenthetworows.
• Overall size: 2inches 1inch, 53mm 26mm.
Power:
• Powered through the USB connection, or 4.5e9.0V applied to VIN (see Figure 2.1).
• Power consumption < 200mA (100mA approx. with Ethernet disabled).
• Real-time clock battery backup input VB; 1.8e3.3V at this input keeps the Real
Time Clock running. This requires 27m A, which can be supplied by a coin cell.
• 3.3V regulated output on VOUT available to power peripherals.
• 5.0V from USB available on VU, available only when USB is connected.
• Total supply is current limited to 500mA.
• Digital I/O pins are 3.3V, 40mA each, 400mA maximum total.
Reset:
• nR e Active-low reset pin with identical action to the reset button. Pull-up resistor is on
the board.
C.2 LPC1768 Electrical Characteristics
To interface successfully to any microcontroller, certain electrical conditions must be
satisfied. Many digital componentsaredesignedto becompatiblewitheachother, so inmany
situations we do not need to worry about this. Once it comes to applying non-standard
359
360 Appendix C
devices, however, it is very important to gain an understanding of interfacing requirements.
Input signals haveto liewithincertain thresholds in order to be correctly interpreted as logic
levels. We need also to understand the ability of an output to drive external loads that may
be connected to it.
These operating conditions for the mbed are referred to in Chapter 3 and from time to time
onwards. They are specified precisely, and in very great detail, by the manufacturer of any
electronic component or integrated circuit, in the relevant datasheet. It is part of the skill of
aprofessional designengineertoknowwhereto accessthesedetails, to knowhowto interpret
them and to be able to design to meet the criteria specified. The hobbyist can engage
creatively orintuitively insometrial anderror, andhopefor thebest. The professional needs
to be able to predict analytically the performance of a design.
Inside the mbed is the LPC1768 microcontroller; when interfacing with the mbed we are
actually directly interfacing with an LPC1768. We therefore turn to the LPC1768 datasheet
(Reference 2.3). For a novice reader this is a very complex document. We have therefore
extracted a very limited amount of data which contains some of the main details required.
Some of these lead to the mbed figures quoted above; others indicate operating conditions
within which the mbed figures apply.
C.2.1 Port Pin Interface Characteristics
Table C.1 is drawn mainly from Table 7 of Reference 2.3, and defines port pin operating
characteristics. The format of Reference 2.3 is retained as far as possible, but for simplicity
afewof thefinerdetailsof operationareremoved. Eachparameterthat appearsinthetableis
described here.
• Supply voltage (3.3 V), VDD(3V3) e This is the power supply voltage connected to the
VDD(3V3) pin of the microcontroller. Although nominally 3.3V, the minimum acceptable
supply voltage for this pin is seen to be 2.4V, and the maximum 3.6V. This pin is one
of several supply inputs, and supplies the port pins.
• LOW-level input current, IIL e This is the current flowing into a port pin, with pull-up
resistor disabled, when the input voltage is at 0V. This very low current implies a very
high input impedance.
• HIGH-level input current, IIH e This is the current flowing into a port pin, with
pull-down resistor disabled, when the input voltage is equal to VDD(3V3). This very low
current implies a very high input impedance.
• Input voltage, VI e This indicates the legal range of input voltages to any port
pin. Unsurprisingly, the minimum is 0V. Interestingly, the maximumis 5V, showing that
the pin input can actually exceed the supply voltage. This is a very useful feature, and
allows interfacing to a system supplied from 5V.
mbed Technical Data 361
Table C.1: Selected port pin characteristics
Symbol
Parameter
Conditions
Minimum
Typical
Maximum
Unit
VDD(3V3)
Supply voltage (3.3 V)
External rail
2.4
3.3
3.6
V
IIL
LOW-level input
current
VI ¼ 0 V; on-chip
pull-up resistor
disabled
e
0.5
10
nA
IIH
HIGH-level input
current
VI ¼ VDD(3V3);
on-chip pull-down
resistor disabled
e
0.5
10
nA
VI
Input voltage
pin configured to
provide a digital
function
0
e
5
V
VO
Output voltage
output active
0
e
VDD(3V3)
V
VIH
HIGH-level input
voltage
0.7VDD(3V3)
e
e
V
VIL
LOW-level input
voltage
e
e
0.3VDD(3V3)
V
VOH
HIGH-level output
voltage
IOH ¼
e
V
VOL
LOW-level output
voltage
IOL ¼ 4mA
e
0.4
V
IOH
HIGH-level output
current
VOH ¼ VDD(3V3)
e
e
mA
IOL
LOW-level output
current
VOL ¼ 0.4V
4
e
e
mA
IOHS
HIGH-level shortcircuit output current
VOH ¼ 0V
e
e
IOLS
LOW-level shortcircuit output current
VOL ¼ VDD(3V3)
e
e
50
mA
Ipd
Pull-down current
VI ¼ 5 V
10
50
150
m A
Ipu
Pull-up current
VI ¼ 0 V
85
m A
4mA
VDD(3V3)
e
0.4V
4
15
0.4 e
45
50
mA
• Output voltage, VO e This indicates the range of output voltages that a port pin
can source. The limits are 0V and the supply voltage.
• HIGH-level input voltage, VIH e This parameter defines the range of voltages for an
input to be recognized as a Logic 1. Any input voltage exceeding (VDD(3V3) 0.4) V is
interpreted as Logic 1. In this context there is no need for a typical or maximum value,
although the maximum is contained within the definition for VI. VIH is illustrated in
Figure 3.1.
362 Appendix C
• LOW-level input voltage, VIL e This parameter defines the range of voltages for an
input to be recognized as a Logic 0. Any input voltage less than 0.4V is interpreted as
Logic 0. In this context there is no need for a typical or minimum value, although the
minimum is contained within the definition for VI. VIL is illustrated in Figure 3.1.
• HIGH-level output voltage, VOH e This indicates the output voltage, for a Logic 1
output. Thevalue is defined for a load current of 0.4mA, where the convention is applied
thatacurrentflowingoutof alogicgateterminal isnegative. Considerthatacircuitsuchas
Figure 3.3b applies. For this output current the minimum output voltage is (VDD(3V3)
0.4) V. With no output current the output voltage will be equal to VDD(3V3). The values
quotedimply anapproximateoutput resistanceof 100U undertheseoperatingconditions.
• LOW-level output voltage, VOL e Thisindicatestheoutput voltage, foraLogic 0output.
The value is defined for a load current of 0.4mA, where the convention is applied that
a current flowing out of a logic gate terminal is positive. Consider that a circuit such as
Figure 3.3c applies. For this output current the maximumoutput voltage is 0.4V. With no
output current the output voltage will be equal to 0V. The values quoted imply an
approximate output resistance of 100U under these operating conditions.
• HIGH-level output current, IOH e This gives the same information as the HIGH-level
output voltage, VOH.
• LOW-level output current, IOL e This gives the same information as the LOW-level
output voltage, VOL.
• HIGH-level short-circuit output current, IOHS e This gives the maximum
output current if the output is at Logic 1, but is short-circuited to ground.
• LOW-level short-circuit output current, IOLS e This gives the maximum output
current if the output is a Logic 0, but is connected to the supply, VDD(3V3).
• Pull-down current, Ipd e This is the current which flows due to the internal pull-down
resistor, when enabled, when VI ¼ 5V.
• Pull-up current, Ipu e This is the current whichflows due to theinternal pull-upresistor,
when enabled, when VI ¼ 0V.
Table C.2: Selected limiting values
Symbol
Parameter
Conditions
Minimum
Maximum
Unit
VDD(3V3)
Supply voltage (3.3 V)
External rail
2.4
3.6
V
VI
Input voltage
5 V tolerant I/O pins; only valid
when the VDD(3V3) supplyvoltage is
present
+5.5
V
IDD
Supply current
Per supply pin
e
100
mA
ISS
Ground current
Per ground pin
e
100
mA
0.5
mbed Technical Data 363
C.2.2 Limiting Values
The values in the previous section showed the limits to which operating conditions could be
taken, while maintaining normal operation. Limiting values (Table C.2), also called absolute
maximumvalues, definethelimitsthatmustbemaintained, otherwisedevicedamageoccurs.
These must, of course, always be observed. For example, although a single port pin can
supply up to 45mA when short-circuited, one must also take note of the limiting value for
supply and ground currents.
APPEN DIX D
Parts List
To getstartedwiththepractical exercisesinthisbookyouwill of courseneedanmbed. There
is some flexibility in almost any other component or subsystem after that. The items
listed below are the ones we used, sourced from the supplier shown; in most cases it is easy
to substitute others. As is the way with electronics, some of these parts are likely to become
obsolete and you will have to seek an alternative. Although we identify certain suppliers,
there are many alternatives.
Table D.1: Components and devices used in the book
Description
Supplier
Supplier code
NXP mbed prototyping board
LPC1768
www.rs-online.com
703-9238
Prototyping breadboard
www.rapidonline.com
34-0664 OR 34-0550
Jumper wire kit
www.rapidonline.com
34-0495 OR 34-0555
Chapter 2
Chapter 3
LED, 5 V, red
www.rapidonline.com
56-1500
LED, 5 V, green
www.rapidonline.com
56-1505
Switch PCB mount SPDT
www.rapidonline.com
76-0200
Photointerrupter/Slotted Optoswitch
www.rapidonline.com
58-0944 OR 58-0303
Kingbright 7-segment display
www.rapidonline.com
57-0115
Switching transistor, ZVN4206
www.rapidonline.com
47-0162
Motor, 6 V, DC
www.rapidonline.com
37-0161
Battery box, 4 AA
www.rapidonline.com
18-2913 OR 18-2909
Diode, IN4001
www.rapidonline.com
47-3420
Chapter 4
Servo Hitec HS-422
www.active-robots.com
HS-422
Piezo transducer (‘buzzer’)
www.rapidonline.com
35-0200
Chapter 5
10k linear potentiometer
www.rapidonline.com
65-0715
Continued
365
366 Appendix D
Table D.1: Components and devices used in the bookd cont’d
Description
Supplier
Supplier code
NORPS12 light-dependent resistor
www.rapidonline.com
58-0132
10k resistor
www.rapidonline.com
62-2146
LM35 temperature sensor
www.rapidonline.com
82-0240
Chapter 6
(No new items)
Chapter 7
(two mbeds and breadboards required
for some builds)
Triple-axis accelerometer breakout,
ADXL345
www.sparkfun.com
SEN-09836
4k7 resistor
www.rapidonline.com
62-2130
Push-button switch, red
www.rapidonline.com
78-0160
Push-button switch, green
www.rapidonline.com
78-0155
Digital temperature sensor breakout,
TMP102
www.sparkfun.com
SEN-09418
Ultrasonic range finder, SRF08
www.rapidonline.com
78-1086
Chapter 8
PC1602 alphanumeric LCD display
www.rapidonline.com
57-0913
Nokia 6610 display
www.coolcomponents.
co.uk
000147
Chapter 9
ICL7611 op amp
www.rapidonline.com
82-0782
Chapter 10
Breakout board for MicroSD transflash
www.sparkfun.com
BOB-00544
Transcend MicroSD card
www.rapidonline.com
19-9123
Chapter 11
1.5k resistor
www.rapidonline.com
62-2106
47k resistor
www.rapidonline.com
62-2178
4.7nF capacitor
www.rapidonline.com
08-0995
10 mF capacitor
www.rapidonline.com
11-0840
0.1mF capacitor
www.rapidonline.com
08-1020
Parts List 367
Chapter 12
RN-41 Bluetooth module (2 required)
www.sparkfun.com
WRL-10559
Ethernet RJ45 8-pin connector
www.sparkfun.com
PRT-00643
Ethernet RJ45 breakout board
www.sparkfun.com
BOB-00716
Chapter 13
HMC6352 digital compass module
www.sparkfun.com
SEN-07915
Servo e continuous rotation
www.oomlout.co.uk
NA
MCP2551 CAN transceiver (2
required)
www.farnell.com
9758569
Chapter 14
(No new items)
Chapter 15
Pololu m3pi robot with mbed socket
www.pololu.com
2151
WiFly GSX breakout RN-131C
www.sparkfun.com
WRL-10050
NXP mbed prototyping board
LPC11U24
www.rs-online.com
751-0725
Note: Order codes and suppliers change very rapidly. Check that each code is correct for the part you wish to order,
before placing the order.
APPEN DIX E
The Tera TermTerminal Emulator
E.1 Introducing Terminal Applications
A terminal emulator allows a host computer to send or receive data from another computer,
through a variety of links. Terminal emulators simply take the form of a software package
runningonthehost computerwhichcreates animageonthecomputerscreen, throughwhich
settings can be selected. It then provides a context for data transfer, using the host computer
keyboard for data input. Such a terminal is particularly useful for displaying messages from
the mbed to a computer screen. It can become a very useful tool for user interfacing and
software debugging. For example, status or error messages can be embedded into a program
running on the mbed, and transferred to the terminal emulator when certain points in the
program are reached.
Whileanumberof terminal emulatorsareavailable, TeraTermis recommendedby thembed
team. This is afreeandopensourceterminal emulatorwhichallows host PC communication
withthembed. Inorderto useTeraTermwithaWindowsPC, youwill firsthaveto install the
WindowsSerial Driver; thiscanbeinstalledfromtheHandbook sectionof thembedwebsite,
at time of writing http://mbed.org/handbook/Windows-serial-configuration. Note that you
needtoruntheinstallerforevery mbed, asWindowsloadsthedriverbasedonthembedserial
number. The Tera Term support page can be viewed at http://logmett.com/
E.2 Setting up and Testing Tera Term
Open the Tera Term application. You will need to perform the following configuration:
• Select File / New Connection (or just press Alt+N).
• Select the Serial radio buttonand select thembed Serial Port fromthe drop-down menu,
as seen in FigureE.1.
• Click OK.
If mbed Serial Port is not in the drop-down menu, the Windows Serial Driver may not be
installed.
Set up New-line format (to print out new line characters correctly) by following:
• Setup / Terminal .
• Under ‘New-line’, set Receive to ‘LF’.
369
370 Appendix E
Figure E.1:
Setting up the Tera Term connection
To test Tera Term running with an mbed, create a new project of suitable name in the
compiler, andenterthecodeof ProgramExampleE.1. Thiscodesimply greetstheworld, and
then reads your keyboard input and displays it to Tera Term. To show that the character
value is being seen by the mbed, the program adds one to the ASCII code, meaning that
every letterisechoedbacktotheterminal astheletterfollowingitinthealphabet: binsteadof
a, c instead of b, and so on.
// Print to the PC, then pass back characters (slightly modified!)
*/
#include "mbed.h"
Serial pc(USBTX, USBRX);
// define transmitter and receiver
int main() {
pc.printf("Hello World!");
while(1) {
pc.putc(pc.getc() + 1); //adds 1 to the ASCII code, and returns it
}
}
Program Example E.1 Transferring keyboard characters to the screen
A more advanced program using terminal access is given in Program Example E.2. This is
taken from the mbed site, and uses a pulse width modulated signal to increase and decrease
the brightness of a light-emitting diode (LED). The pulse width modulation (PWM) is
controlled from the keyboard and displayed on the Tera Term screen. To test the program,
connect an LED between pin 21 and ground, and enable Tera Term.
/*Connects to the mbed with a Terminal program and uses the 'u' and 'd' keys to make LED1 brighter
or dimmer
*/
#include "mbed.h"
Serial pc(USBTX, USBRX);
// define transmitter and receiver
The Tera TermTerminal Emulator 371
PwmOut led(p21);
float brightness=0.0;
int main() {
pc.printf("Press 'u' to turn LED1 brightness up, 'd' for down\n\r");
while(1) {
char c = pc.getc();
wait(0.001);
if((c == 'u') && (brightness < 0.1)) {
brightness += 0.001;
led = brightness;
}
if((c == 'd') && (brightness > 0.0)) {
brightness -= 0.001;
led = brightness;
}
pc.printf("%c %1.3f \n \r",c,brightness);
}
}
Program Example E.2 Controlling PWM setting from the keyboard
Index
Note: Page numbers with “f” denote figures; “t” tables
A
Accelerometer
data logging, on exceeding
threshold, 214e215
threshold interrupt, 198e199,
see also Interrupts
Acorn computer, 12
Address bus, 8
Advanced RISC Machines Ltd,
11e15
ADXL345 accelerometer, 125,
126f
pin connections to mbed, 121f
program, developing,
125e128
registers, 126t
Aliasing, 81e82, 81f
American Standard Code for
Information Interchange
(ASCII), 104, 155, 156t
Analog input, 77e93
combined with analog output
LED brightness by PWM,
controlling, 84
LED brightness by variable
voltage, controlling, 82e83,
83f
PWM frequency, controlling,
84e85
data conversiontime, estimating,
90e92
data on LCD, displaying,
162e163
interrupts from, 180e181, 180f
with mbed, 82
mini project, 92
processing data from, 86e88
ADC outputs to recognized
units, scaling, 87
averaging to reduce noise,
applying, 87e88
displaying values on computer
screen, 86
Analog output, 57e75
combined with analog input
LED brightness by PWM,
controlling, 84
LED brightness by variable
voltage, controlling, 82e83,
83f
PWM frequency, controlling,
84e85
on mbed, 59e65
API summary for, 59, 59t, 67t
constant output voltages,
creating, 60
DAC resolution, testing, 63e64
sawtooth waveforms, 60e62
sine wave, generating, 64e65
Analog sensors, 88e90
integrated circuit temperature
sensor, 89e90, 90f
light-dependent resistor, 88e89,
89f
Analog-to-digital conversion,
77e82
quantization error, 79e80
range of, 79e80
resolution of, 79e81
sampling frequency, 81e82
Analog-to-digital converter
(ADC), 77e79
control registers, 312e318
applications of, 314e316
conversion speed, changing,
316e318
mbed, 82, 305e308, 312, 313t
LPC1768, 323
373
Apple, 12
Application programming
interface (API), 25e26,
32e33
Arithmetic logic unit (ALU), 7e8
ARM
ARM Holding, 12e13
Cortex core, 12e15, 14f, 178
history of, 12e13
mbed, see mbed
RVDS (RealView Development
Suite) compiler, 25
Arrays, 351
Assembler, 10
Asynchronous serial data
communication,
140e144, 141f
on mbed, 141e144, 142t, see
also Serial data
communication
Audio projects, advanced, 323
Averaging to reduce noise,
applying, 87e88
B
Binary number system, 331, 332f
Bistable circuit, 201e202
Bluetooth serial communication,
244e251
communication between two
mbeds, 248e251
data from host terminal
application, receiving, 247
defined, 244
RN-41 module, 244e245, 245f
RN-42 module, 244e245
sending mbed data over,
245e247, see also Serial
communication
374 Index
Break statement, 349e351
Byte, 332
C
C language, 9e10, 339e357
code blocks, 342
comments, 341e342
compiler directives, 342
#define, 342
#include, 342
data types, 343, 343t
file access operations,
352e353
closing, 355
opening, 355, 356t
reading file data, 356
standard library functions,
355t
writing file data, 356
flow control, conditional
branching, 348e349
break statement, 349
if and else, 348e349
switch statement, 349
flow control, program loops,
349e351
arrays, 351
derived data types, 351e353
exiting loops with break,
350e351
for loops, 350
infinite loops, 350
pointers, 351e352
strings, 351
structures, 352e353
unions, 352e353
while loops, 349e350
functions, 344e346, 344f
definitions, 346
main function, 345
prototypes, 345
static storage class with,
346
#ifndef directive, 108
header files, 353
keywords, 339, 340t
layout, 341
library functions, 205, 205t
mbed development environment
using, 26
mbed objects, global use of,
109
operators, 346e348
preprocessor, 107e108, 107f
preprocessordirectives, 107e108
printf command, 354
program compilation, 106e107,
106f
space, 341
standard libraries, 353
statements, 341
using multiple files in, 105e109
variables
declaration, 339e343
initializing, 342e343
naming, 342e343
working with data, 343e344
C++language, 9e10
#ifndef directive, 108
library functions, 205, 205t
mbed development environment
using, 26
mbed objects, global use of, 109
preprocessor, 107e108, 107f
preprocessor directives,
107e108
program compilation, 106e107,
106f
using multiple files in, 105e109
Central processing unit (CPU), 7
Circular buffer, 236, 237f
CISC, 13
Clear display command, 155,
155f
Clock select registers, 308e309,
309t, 310t
Closed loop control system, 274
cruise, 275e276, 277f
digital compass, 280e287, 281f
HMC6352 digital compass,
281e283, 281f, 282t
implementation algorithm,
285e287
360 degree rotation servo,
283e285
See also Control systems
Cloud computing, 262
Code blocks, 342
Color LCDs, 163, 164t, see also
Liquid crystal displays
Comments, 341e342
Compiler
directives, 342
#define, 342
#include, 342
mbed, 25e26
Complex instruction set computer
(CISC), 13
Constant output voltages,
creating, 60
Context, 172e173
Controller area network (CAN),
288e289, 289f
data read, 293
on mbed, 289e294
mbed-to-mbed communication
hardware build for, 291f
pin connections for, 292t
Microchip transceiver IC,
290f
Control register(s), 298e299,
299f
analog-to-digital converter,
312e318
applications of, 314e316
conversion speed, changing,
316e318
mbed, 305e308, 312, 313t
clock select registers, 308e309,
309t, 310t
digital input, 303e305
digital output
applications of, 301e302
second digital output, adding,
302e303
digital-to-analog converter,
308e309
mbed, 310
mbed digital input/output,
299e301, 300t
pin mode registers, 307e308,
307t
pin select registers, 305e307,
306t
power control registers,
308e309, 308t
Control systems, 273e295
advantages of, 274e275
closed loop, 274
cruise, 275e276, 277f
Index 375
controller area network,
288e294, 289f, 290f, 291f,
292t
electromechanical, 273
open loop, 274
proportional, 276e278, 278f,
279f
proportional integral derivative,
278e280, 279f, 280f
Cortex microprocessor core,
12e15, 14f, 178
Counter
digital counter, 181e182, 182f
as timer, 182e183
Cruise control system, 275e276,
277f, see also Control
systems
D
Data
acquisition system, 78f
bus, 8
conversion, 57e58
derived data types, 351e353
memory, 7e8, 7f, 201, see also
Memory
types, 343, 343t
working with, 343e344
Debounce, 196e198, 196f
Decimal number system,
331e332, 331f
Decoupling capacitor, 224
Delay/echo effect, 230f, 232f
Derived data types, 351e353
Development cycle, of embedded
systems, 10e11, 11f
Digital compass closed loop
control system, 280e287,
281f
HMC6352 digital compass,
281e283, 281f, 282t
implementation algorithm of,
285e287
360 degree rotation servo,
283e285, see also Control
system; Closed loop control
system
Digital counter, 181e182, 182f,
see also Counter
Digital filtering, 220e222
Digital input, 29e55, 36f
control registers, 303e305, see
also Control register(s)
DigitalIn API, 40, 40t
switches to logic inputs,
connecting, 39, 39f
using if to respond to a switch
input, 40e41
Digital output, 29e55, 36f
control registers, see also
Control register(s)
applications of, 301e302
second digital output, adding,
302e303
using external pins, 37e39, 38f
using LEDs, 36e37, 37f
Digital signal processing (DSP),
219e242
defined, 219e220
delay/echo effect, 229e232
digital data, input/output of,
222e224, 226f
high-pass filter, adding,
220e221, 229
low-pass filter, adding,
224e228
push-button activation, adding,
228e229
signal reconstruction, 224
stereo wave player, 240e241
basic, 240
with PC interface, 240
portable, with mobile phone
display interface, 241
wave audio file, 232e239
information header,
233e234
with mbed, reading,
235e236
mono wave data, reading and
outputting, 236e239
Digital spirit level, in LCDs, 166
Digital-to-analog converter
(DAC), 58, 58f, 59f
control registers, 308e309
applications of, 59e65,
311e312
mbed, 59e65, 310
resolution, testing, 63e64
TLV320AIC23b, 323
Disabling interrupts, 179e180,
see also Interrupts
Displaying values on computer
screen, 86
Display mode control register,
154e155, 155f
Duty cycle, 65
Dynamic random access memory
(DRAM), 203, see also
Random access memory
E
Electrically erasable
programmable read only
memory (EEPROM), 204,
see also Read-only
memory (ROM)
Electrically programmable read
only memory (EPROM),
204, see also Read-only
memory (ROM)
Electromechanical control
systems, 273, see also
Control systems
Electronic memory, 202e204, see
also Memory
Embedded system, 4f
defined, 3e5
development processes in, 9e11
development cycle, 10e11,
11f
programming languages,
9e10
example, 5e6, 6f, see also mbed
Ethernet, 255e261
communicationsbetweenmbeds,
257e261
data, Manchester encoding for,
256, 257f
overview of, 255e256
Event-triggered tasks, 171, 171t
responding to, 171e173
polling, 171e172, 172f
See also Task
Extension projects, 321e329
advanced audio projects, 323
Pololu robot, for mbed,
321e322, 322f
External pins, digital output
using, 37e39, 38f
376 Index
F
Field effect transistor (FET), 204
Field programmable gate array
(FPGA), 15
File(s)
access, 207e208, 352e353
closing, 355
opening, 355, 356t
reading file data, 356
standard library functions,
355t
string, 208e209
writing file data, 356
closing, 206
data
formatted data, using,
209e210
reading, 206e207
writing, 206e207
opening, 206
Filter(s)
cut-off frequency, 220e221
finite impulse response,
221e222, 221f
high-pass, 220e221
infinite impulse response, 225,
226f
low-pass, 220e221, 224e228
order of, 220e221
pass-band, 220e221
roll-off rate, 220e221
stop-band, 220e221
taps, 221
Finite impulse response (FIR)
filter, 221e222, 221f, see
also Filter(s)
Flash memory, 204, see also
Memory
Floating point, 59
number representation, 9f,
336e337
Flowcharts, 97, 97f, 98f
Flow control
conditional branching, 348e349
break statement, 349
if and else, 348e349
switch statement, 349
program loops, 349e351
exiting loops with break,
350e351
infinite loops, 350
for loops, 350
while loops, 349e350
For loops, 350
Formatted data, 209e210
Framecheck sequence(FCS), 256
Frames, 256
Full duplex, 118
Functions(s), 96e97, 344e346,
344f
call, 96f
definitions, 346
liquid crystal displays, 157e158
main function, 345
on mbed, working with,
100e105
mode control register, 154, 154f
prototypes, 345
reuse, 102e103
static storage class with
functions, 346
H
Half duplex, 129e130
Hardware, 4e5
#define directive, 342
#ifndef directive, 108
#include directive, 342
HD44780, 149
data transfer, 150f
4-bit interface, timing for, 150f
instruction transfer, 150f
user interface lines, 150f
Header files, 353
Hexadecimal number system,
332e333, 332f
High-level language (HLL), 10
High-pass filter (HPF), 220e221,
229, see also Filter(s)
Hitachi
HD44780, 149, 150f
HMC6352 digital compass
control system, 281e283,
281f
pin connections for, 282t
setup and read, 282e283
Hot electron injection (HEI), 204
HTTP client, mbed as, 262e264
HTTP file server, mbed as,
264e266
I
IBM, 12
If and else, 348e349
Infinite impulse response (FIR)
filter, 225, 226f, see also
Filter(s)
Infinite loops, 350
Instruction set, 8
Integrated circuit (IC), 3
Integrated circuit temperature
sensor, 89e90, 90f
Intelligent instrumentation, 125
Inter-integrated circuit (I2C) bus,
129e134, 129f
data link, setting up, 131e134
data transfer, 130e131, 131f
evaluation of, 140
on mbed, 131, 132t
multi-node, 144
temperature sensor,
communicating with,
135e138
Inter-integrated circuit sound
(I2S) port, 323
Internet of Things, 323e325
Interrupts, 170e173, 175e181
accelerometer threshold,
198e199
from analog inputs, 180e181,
180f
disabling, 179e180
latency, 176e179
on LPC1768 microcontroller,
178
masked, 176
on mbed, 173e175
nested, 176
pending, 176e177
prioritized, 176
timer, repetitive, 183
vector, 172e173
Interrupt service routine (ISR),
172e173
K
Keywords, 339, 340t
L
Latency, 176e177
testing, 178e179
Index 377
Layout, 341
Library wait functions, 31t
Light emitting diode (LED),
147e148
brightness by pulse width
modulation, controlling, 84
brightness by variable voltage,
controlling, 82e83, 83f
digital output using, 36e37, 37f
Light-dependent resistor (LDR),
88e89, 89f
Lighttracking,two-dimensional,92
Linear pulse code modulation,
233
Liquid crystal displays (LCDs),
147e167
adding data to specified location,
158e159, 159f
analog input data on, displaying,
162e163
character displays, 149
color, 163, 164t
definition of, 156e157
display data to, sending,
155e156
functions, utilizing, 157e158
HD44780, 149, 150f
initializing, 154e155
clear display, 155, 155f
display mode, 154e155,
155f
function mode, 154, 154f
mbed TextLCD library, using,
160e161
modular coding to interface,
153e154
PC1602F LCD, 150e159, 151t,
152t
SPI LCD mobile phone display,
controlling, 163e166
Local area network (LAN), 256
Local file system, defining,
205e206
Low-pass filter (LPF), 220e221,
224e228, see also
Filter(s)
LPC11U24 mbed, 325, 326f
LPC1768 microcontroller,
17e19, 18f, 21e22, 21f,
173e176, 298e299, 323
electrical characteristics of,
359e362
limiting values, 362t
port pin interface
characteristics, 354e356,
355t
interrupts on, 178, see also
Microcontroller(s)
M
Machine code, 8
Manchester encoding method,
256, 257f
Masked interrupts, 176, see also
Interrupts
mbed, 17e27, 18f
architecture of, 19e20, 20f
as HTTP client, 262e264
as HTTP file server, 264e266
two mbeds on Bluetooth,
communicating between,
248e251
Media access control (MAC)
address, 244
Memory, 201e204
access with mbed, 205e207
external, 210e212
data, 7e8, 7f, 201
electronic, 202e204
flash, 204
function types of, 201e202
non-volatile, 7e8, 201e202
program, 7, 201e202
dynamic, 203
static, 203
read-only, 201e202
electrically erasable
programmable, 204
electrically programmable,
204
programmable, 204
volatile, 7e8, 201e202
Microchips, 3
Microcontroller(s), 4, 8e9
features of, 9f
LPC1768, 17e19, 18f, 21e22,
21f, 173e176, 298
electrical characteristics of,
359e362, 361t
interrupts on, 178
Microelectromechanical system
(MEMS), 125
Microprocessor(s), 3, 6e9
interrupt response, 172e173,
173f, 177e178, 177f
Microwire, 117e118
Mono wave data, reading and
outputting, 236e239
Motorola, 117e118
Motor with mbed, switching,
50e51
Multi-node I2C bus, 144, see also
Inter-integratedcircuitbus
Multi-tasking, 170
in metronome program,
194e195, see also
Task
N
National Semiconductors,
117e118
Negative feedback, 276
Negative numbers, representation
of, 334e336
Nested interrupts, 176, see also
Interrupts
Nested Vectored Interrupt
Controller (NVIC),
178
Non-volatile memory, 7e8,
201e202, see also
Memory
Nordheim Fowler Tunneling,
204
Nyquist sampling criterion,
81e82, 90
O
Offset binary, 334e335, 334t
Open loop control system, 274,
see also Control systems
Operators, 346e347
Opto-sensors, 42e48
to mbed, connecting, 43e44, 44f
reflective, 42e43, 43f
transmissive, 42e43, 43f
P
PC1602F LCD, 150e159
to mbed, connecting, 152e153
378 Index
PC1602F LCD (Continued)
physical pin layout, 153f
pin descriptions, 151t, 152t
Philips
I2C bus, 129e134
Piconet system, 244
Piezo transducer, outputting to,
72e74
Pin mode registers, 307e308,
307t
Pin select registers, 305e307,
306t
Plant, 278
Pointers, 212e214, 351e352
Polling, 171e172, 172f
Pololurobot, for mbed, 321e322,
322f
Powercontrol registers, 308e309,
308t
Printf command, 354
Prioritized interrupts, 176, see
also Interrupts
Product development cycle, 327f
Program design
benefits of, 95
defining code structure, using
flowcharts, 97, 97f, 98f
modular program example,
109e112
pseudocode, 99
using multiple files in C/C++,
105e109
Program memory, 7, 201e202,
see also Memory
Program structure, benefits of, 95
Programmable read only memory
(PROM), 204, see also
Read-only memory
Programming languages, 9e10,
see C language, see also
C++language
Proportional control system,
276e278, 278f
stepresponse of, 276e278, 279f,
see also Control systems
Proportional integral derivative
(PID) control system,
278e280, 279f, 280f, see
also Control systems
Pseudocode, 99, 99f
Pulse width modulation (PWM),
65e66
averaging circuits, 66f
controlling LED brightness by,
84
frequency, controlling,
84e85
on mbed, 66e74
outputting to piezo transducer,
72e74
servo control, 71e72
small motor, speed control of,
68
in software, generating,
70e71
sources, 66e67
trial outputs, 67e68
waveform, 65f
Push-button activation, adding,
228e229
Q
Quantization error, 79e80
Quick response (QR) codes, 324,
324f
R
Random access memory (RAM),
7e8, 201e202
dynamic, 203
static, 203, see also Memory
Rapid prototyping, 326e327
Read-only memory (ROM), 7e8,
201e202
electrically erasable
programmable, 204
electrically programmable,
204
programmable, 204, see also
Memory
Real Time Clock (RTC), 195
RedLaser, 324
Reduced instruction set computer
(RISC), 13e14
Remote JavaScript interface,
controlling mbed using,
267e270
Remote procedure calls (RPCs)
to modify mbed outputs,
266e267
Repetitive interrupt timer, 183,
see also Timer(s)
RISC, 13e14
RN-41 module, 244e246, 245f,
246f, 248
RN-42 module, 244e245
S
Sampling frequency, 81e82
Sawtooth waveforms, 60e62
Segway personal transporter, 275f
Self-contained metronome, 198
Serial communication, 115e145,
243e271
asynchronous, 140e144, 141f
on mbed, 141e144, 142t
Bluetooth, 244e251
data from host terminal
application, receiving, 247
defined, 244
RN-41 module, 244e245,
245f
RN-42 module, 244e245
sending mbed data over,
245e247
two mbeds, communicating
between, 248e251
Ethernet, 255e261
communications between
mbeds, 257e261
data, Manchesterencodingfor,
256, 257f
overview of, 255e256
protocols, advanced, 243e244
synchronous, 115e117
with host computer, 144
universal serial bus, 251e254
MIDI data from mbed,
253e254
mouse, mbed usage for
emulating, 252
Serial peripheral interface (SPI),
117e124, 210e211
ADXL345 accelerometer, 125,
126f
pin connections to mbed, 121f
program, developing,
125e128
registers, 126t
data link, creating, 121e124