EEE305 Microcontroller Systems
Lecture 9B: GPIO on the Arduino
Teaching resources are at www.eej.ulst.ac.uk
My office 5B18, telephone 028 90 366364
My email IJ.McCrum@ulster.ac.uk
What is an Arduino?
• A small circuit board containing an ATMEL AVR
microprocessor. AVR chips are similar to PICs
• Typically with a specific physical size and form
factor, with the I/o pins arranged in a specific way
to support many add on boards (called shields)
• Typically with a board loader using RS232 or USB
so a programmer is not needed
• Typically capable of running Arduino Software,
from a nice simple IDE, in a sanitised language
called sketch (based on C and a tiny bit of C++)
• Everything is opensource and free (boards are
cheap). Designed for designers and artists…
Arduino boards
Now several variants, often with Italian names reflecting their Origin. See Arduino.cc for info.
There are also bigger MEGA boards, and
boards with ARM chips, Galileo PC
compatible boards and even a PIC version
that carries a PIC32 – a very powerful chip.
The top two above are the more normal
types – Buy from Farnell.co.uk or Maplin
Hardware of basic Arduino
The pins are labelled as shown, irrespective of
what actual pins on the physical chip are – that
depends on the package, surface mount, Dual in
Line (DIL) and whether a 28 pin or 40 pin device.
The special software environment uses the
connection pins – labelled here beside the black
connectors so the software “abstracts” you away
from the actual chip.
Arduino is meant to be used by non-specialists,
artists, hobbists, and children. Nonetheless it is
very convenient – even for heavyweight
embedded systems.
Various power options…you program them by
holding down the reset button and releasing as
you you download code from the PC – a
bootloader
The original Connection system was based on a ATMEGA168 from
ATMEL but we now typically use different more modern chips.
The real pin names are on
the chip datasheet but
users never read that – the
arduino way of doing
things is give the users a
simple way of achieving
rapid results. – even of
they may not understand
how things works. This
works fine usually, and
there is a massive online
community to help if you
are doing something tricky
Standardised
hardware was
easier to
teach with.
Also the
All (99%) Arduinos already have an LED on pin 13
So a user can get the board to do something at once Software tools
were stripped
Two buttons on the TOOLBAR, one to
down to
compile and one to download. Also a
something
simple serial monitor on the PC
allowed use of printf – using the
very simple
same hardware as the bootloader.
Also even the language was made simple – based on C and C++ but all the
tricky stuff was abstracted out into a powerful library system. This meant that
beginners could get a lot done quickly. Libraries came with examples, there
are dozen of examples found under the (simple) menu system.
Also every arduino program has a setup section, run once and a loop section,
this continually reruns once power is applied, you don’t need a do… while(1).
Resembles C but you don’t need
to tell it abou the standard
libraries. Note you MUST have a
setup() and a loop() function but
don’t need a main()
pinMode( 13, OUTPUT); // sets up PIN 13
digitalWrite(13,HIGH); // puts +5 out on pin 13
You tell the IDE which board and
what clock frequency, it works out
the right time delay
Note that the loop function runs
and runs automatically…
These programs are called
“sketches” and have a .ino
extension.
There are many libraries – you just ensure they are
installed in the right place and they become available
You access library functions using
a dot ‘.’ Each library has different
member functions. This resembles
object orientated programming.
You access library functions using
a dot ‘.’ Most people grab an
examlple program, guess what
things mean and just modify
(hack) the code to want they
want.
There are 5 tabs on the IDE menu – the Help has “getting started” and “reference” sections
Actually the Arduino IDE is built on top of a complex set of software tools given
away free by ATMEL. Known as AVRStudio they are analogous to Microchip’s MPLAB
used to program PIC microprocessors.
You can write normal C in the Arduino environment and it will work, you may need
to #include <avr/io.h> and other libraries to make it function correctly.
Equally valid is to use the AVRStudio direcly, it is based on the industry standard
Eclipse IDE and is easily customisable. It containes a compiler, linker and a make
utility which helps automate the building and programming of a hex file.
Or you can use a good programming editor such as “The Programmers Notepad”.
This can be customised to call the compiler and downloader directly.
If you do not use a downloader you can get an AVR hardware programmer for
under £20. These use a 3x2 connector which you can see on the photograph of the
Arduinos in slide 3 and 4.
Interrupts in sketches
You might need to read the AVR/ATMEL datasheet
carefully to use the timers, but broadly speaking
they are similar to the PIC.
An Interrupt function in the sketch language
initialises the peripheral, uses an “attachinterrupt”
function and other similar functions.
Note that timer0 is used by the internal delay
functions so avoid using it …
http://www.instructables.com/id/Arduino-Timer-Interrupts/step1/Prescalers-and-the-CompareMatch-Register/
There are three timers.
Timer0 and timer2 are 8 bit timers, meaning they can store a maximum counter
value of 255. Timer1 is a 16 bit timer, meaning it can store a maximum counter
value of 65535. Once a counter reaches its maximum, it will tick back to zero (this is
called overflow). This means at 16MHz, even if we set the compare match register
to the max counter value, interrupts will occur every 256/16,000,000 seconds
(~16us) for the 8 bit counters, and every 65,536/16,000,000 (~4 ms) seconds for the
16 bit counter.
The prescaler can equal 1, 8, 64, 256, and 1024.
interrupt frequency (Hz) = (Arduino clock speed 16,000,000Hz) /
(prescaler * (compare match register + 1))
Rather than continue with digging up the data, reading the datasheets carefully and
using “googled code” we will avoid reinventing the wheel – there is a library we can
use. This is the arduino way to get things done but you learn less… look at
http://playground.arduino.cc/Main/MsTimer2#.U0HjPfldWSo
MsTimer2 is a small and very easy to use library to interface Timer2 with humans. It's
called MsTimer2 because it "hardcodes" a resolution of 1 millisecond on timer2.
(After downloading the MsTimer2 library from arduino.cc)
Unzip it into the libraries
folder and restart the IDE
This function is treated as an ISR, in our
miniproject it can output a multiplex pattern
to the digit select lines of the 7 segment
displays and poll the columns of the keypad
We will want a
5millisecond interval, not
a 500 millisecond
interval…