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MC 401 LECTURE 2
Embedded Systems Hardware
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Lecture learning outcomes
• Identify and describe the main subsystems of a typical microcontroller
• Describe the primary function of each subsystem in the µC
• Describe the differences among common µC architectures
• Sketch the functional block diagram of the µC
• Describe the differences between the RISC and CISC instruction set
• Describe the main bus structures available in µCs (address, bus and
control)
• Describe the various types of memory
• Describe the relationship between width of data and address buses
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Microcontroller details
• This is an entire computer system contained within a single integrated
circuit. There are various types of microcontrollers (µCs): atmega, PICs,
Arduinos etc). The goal is to pick the most economical with the desired
parameters and features required for an application.
• Range of µCs is from limited 4-bit processors to high speed, full featured 32
bit processors.
• A µC contains several subsystems to ensure correct operation. The main
subsystems include:
a. A central processing unit (CPU)
b. A bus system
c. A memory system
d. A bus system
e. A port system
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µCs overview cont.
E
Figure 2.1: Microcontroller basic structure
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Basic Architecture of a µCs
• Figure 2.1 showed the basic architecture of a µC. A µC is equipped with ports which
enable the designer to link the outside world to the µC. Generally, ports are
bidirectional and have alternate functions such as ADC, serial communications and
flexible timing system capability.
• Just like a computer a µC is also equipped with various memory components which
shall be looked at in greater detail.
• To allow interruptions of internal events by external events, the µC has external
interrupt pins to integrate interrupt systems. Interrupt systems enable µCs to
respond to high priority events and later return to normal processing operation.
• The µC can be programmed using a development kit/board which is linked to a
personal computer via In System Programming (ISP) features.
• Another critical item of the µC structure is the timing subsystem. An operating time
base of a µC can be provided via an external crystal oscillator or resonator.
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µC CPU cont.
• The central processing unit (CPU) which is part of the µC subsystem is a
complex sequential circuit whose function is to execute programs which are
stored within its Flash EEPROM(Electrically Erasable Programmable Read
Only Memory).
• Worthwhile mentioning here that a program is simple a series of
instructions to perform a specific task.
• What you will learn in this course is to program. You as the µC designer will
use a program development tool or integrated development environment
(IDE) hosted on a computer to develop programs.
• Once the program is developed it is written/downloaded/burnt into the µC
and allows the µC to be a stand-alone processing system.
• Every time a µC is reset, the CPU fetches a program from memory and
decodes the instruction’s content and then executes.
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µC CPU Architectures
Figure 2.2: Various CPU architecture configurations
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µC CPU cont.
• The CPU is the main control centre of the µC. Apart from executing
instructions the CPU calls upon available subsystems to perform their tasks as
required by instructions in the program.
• Figure 2.2 showed the classifications of CPU architectures available for µCs.
Each architecture has its own advantages and disadvantages.
The following classes exist: (refer to fig. 2.2)
I. Accumulator-based architecture: In an accumulator-based architecture,
instructions begin and end in specially designated registers called
accumulators (A and B). An operation is performed in which one operand is
found in an accumulator and the other is fetched from memory. The result
is then saved in the accumulator. It is slower than the other configurations
because values have to be constantly fetched from an already slower
running memory module compared to the main processor. This forces the
processor to also slow down so as to accommodate an operand memory
fetch. It has the ability to do fairly complex instructions and can be
modified to work with values stored in a register and one in memory.
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µC CPU cont.
• Register-based architecture: Here, both operands are stored in the register
that are typically collocated with the CPU. The result is also stored in a
register. Since the CPU and the registers operate at the same speed, the
processor does not to slow down to read or write operands. Register
contents are read from and written to memory using a background
operation.
• Stack-based architecture: In a stack-based architecture, both operands and
the operation to be performed are stored on the stack. The result is then
placed back on the stack. The stack may be based in dedicated registers or
maybe a special portion of RAM.
• Pipeline architecture: This architecture consists of hardware subsystems
arranged in stages to fetch an instruction from memory, decode the
instruction, fetch instruction operands from memory or registers, execute
the instruction, then write back the results to memory. Each stage
simultaneously executes a portion of the instruction set such that the
overall result is a completed execution of an instruction every clock cycle.
So in a five stage pipeline, five instructions are simultaneously processed
through the pipeline. Instructions are simple instructions easily processed
by a single stage.
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RISC versus CISC instruction set
Just like hardware has an architecture, instruction sets also have an
architecture. The are two based architectures:
• Reduced Instruction Set Computer (RISC) – It has a complement or
simple building block instructions. They are used in systems with less
complex CPU architectures.
• Complex Instruction Set Computer (CISC): It has a complement of a
fuller feature and handles more complex instructions.
It is difficult to predict whether a given program will be more efficiently
coded with a RISC- or CISC-based instruction set. It largely depends on
how well the specific algorithm matches the feature set of a given
processor.
Knowledge of the hardware and software architecture of a given
microcontroller is very important when coding in assembly unlike when
using a high level language like C.
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REGISTER SET
• Most microcontrollers have a number of registers designated as a register
set. The register set is the interface between the use and the different
subsystems in the µC.
• Each register is a collection of flipflops (FF) which can either be set to logic
one or zero. In software engineering, the flipflop can be seen as a switch
that can be turned on by sending a logic level 1 and turned off by sending a
logic level zero.
• A subsystem contains a set of FFs making a set of registers associated with
a given subsystem. To configure a subsystem, a designer determines that
appropriate bit within the register.
• The function of each register and register bit are documented in the µCs
library. To program the bits in a register, you have to set the name of the
register to a desired bit pattern.
• Each µC has a number of header files which provide a link between the
name of the specific register used within a program and its location within
the microcontroller.
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BUS Structure
• A bus is a collection of parallel conductors that have a similar function.
Most µCs are built with an address bus, a data bus and a control bus.
Address Bus
This bus provides a connection between the CPU and the memory subsystem
aboard the µC. The number of conductors in the address bus sets the upper
limit of memory locations that may be linearly addressed by the µC. The first
address in the memory subsystem will be all zeroes while the final address
will be all logic ones. The number of individually addressable memory
addresses may be determined by evaluating:
2address lines= addressable locations.
For example, a microcontroller equipped with a 16-bit address bus is capable
of addressing 65 536 (64 kB) separate locations. The first address in this
memory space is (0000)16 while the last address in this space will be
(FFFF)16.
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Address bus cont.
To expand the span of addressable memory locations, some
microcontrollers employ a paged memory addressing scheme. In paged
addressing, a memory system is subdivided into memory pages.
Memory page length is usually some smaller block of memory such as a
4 kB page. Many 4 kB pages of memory can be assigned to the same
4,096 addresses in the linearly addressable memory space. To select a
specific memory page for access additional addressing or select bits are
required.
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Data Bus
• This is used to route or transfer parallel data about different
subsystems with the µC. Common data bus widths are 4, 8, 16 or 32
bit. The width of the data path generally determines the size of a data
argument that the microcontroller can process. The largest unsigned
integer that may be stored in a microcontroller with an eight-bit data
path is 255. It should be emphasized that a 32-bit microcontroller is
not a better microcontroller than its four-bit counterpart. Recall that
the primary objective of the embedded system designer is to choose
the most economical microcontroller that accomplishes the
requirements of a specific design.
• For example, a four-bit microcontroller may be an ideal choice to host
the control algorithm for an irrigation scheme. However, a 32-bit
processor may be required to host the control features required for a
cell phone.
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Control Bus
• Microcontrollers are equipped with paths to send and receive a
collection of control signals designated as the control bus.
• These signal lines carry control signals to different subsystems
throughout the microcontroller.
• Most of the control signals are internal to the microcontroller
integrated circuit (chip).
• However, they are often conveniently provided at a microcontroller
port so that external components may be added to the processor.
• Control signals are issued by the CPU in response to program
instructions to insure the instruction is properly executed.
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Memory
• As previously mentioned the number of uniquely addressable
memory locations in a microcontroller is determined by the width of
the address bus. This span of addressable memory usually contains
several different types of memory including Static Random Access
Memory
(SRAM),
byte-addressable
Electrically
Erasable
Programmable Read Only Memory (EEPROM), and bulk
programmable Flash EEPROM. It should be emphasized that these
memory components are contained within the confines of the
microcontroller chip.
• To keep a track of the memory locations in use and the type of
memory present within the system, a visual tool called a memory
map is employed. The memory map provides the size in bytes of each
memory component and its start and stop address within the
memory system.
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Components of µC memory
• RAM : This is volatile memory. . That is, if the microcontroller loses power,
the contents of RAM memory are lost. It can be written to and read from
during program execution. It is typically used during system development to
store a program. Once development is complete, the completed program is
stored in non-volatile memory such as Flash EEPROM. When the program is
running, RAM is used to store global variables, to provided dynamic memory
allocation of variables and to provide location for a stack.
• Byte-addressable EEPROM: This type of memory is used to permanently
store and recall variables during program execution. It is especially useful for
logging system malfunctions and fault data during program execution. It is
also useful for storing data that must be retained during a power failure but
might need to be changed periodically. Examples where this type of memory
is used are found in applications to store system parameters, electronic lock
combinations, and automatic garage door electronic unlock sequences.
• Flash EEPROM: Bulk programmable Flash EEPROM is used to store programs.
It can be erased and programmed as a whole. Some microcontroller systems
provide a large complement of both RAM and Flash EEPROM. Therefore, a
system program can be developed in RAM and then transferred to Flash
EEPROM when complete.
Figure 2.3: Memory map example and the
types of memory in use.
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More on memory
• Other microcontrollers provide only a large Flash EEPROM and a
smaller RAM component. With this memory configuration, system
development takes place in Flash EEPROM. Flash EEPROM is typically
programmed using In System Programming (ISP) techniques. That is, a
host PC is connected via a cable to a microcontroller while it is
resident within its application circuit. The host PC downloads the
program to the microcontroller.
• A few skills students are expected to have is the ability to use dynamic
linked lists to store information in dynamic memory (we will do an
example when we tackle embedded C)
• Bootstrapping is loading a set of instructions when the
microcontroller starts up. These can be preliminary variables that can
be loaded necessary for the main program to run on start up.
End of Lecture 2!
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