A
picaxe is a PIC microcrontroller with some embedded firmware that
allows things like:
- Programming without a hardware programmer
- Support for I2C, Serial and Keyboard
interfaces
- BASIC, Flowcharts or assembler programming.
I chose it because it was reasonably powerful, cheap and easy to use.
It was developed in the UK to support education in electronics and has
moved into the hobbyist and even commercial markets. They are also
widely available across the world. Locate them by using google
or using the distributer list on the UK site. You will
want to check out the UK site anyway which has loads of documentation:
www.picaxe.co.uk
The latest versions of the picaxe are the 'x' series - the 18x, 28x and
40x. Together with a greater program memory, these chips also offer
some important complex operations such as the I2C interface that we
will be using. I believe that I could use a 28x for this project
however I have decided to pay the extra $AU5.00 to get the 40x. In
mapping the functions I would need I decided that the 28x would do
however there would be no room for expansion.
I am building this project on vero or strip board. I would have
preferred to use wirewrap however this seems to have gone out of
fashion and the supplies are hard to get. On completion of this step
you will have a working picaxe that can be programmed and reset.
Power
In the final build I will use a PC power supply because I need +5v for all the digital
circuitry and +12v at up to 4.5a for the motors. If you want to do the
same there are many sites on how to alter a power supply for purpose. I
would suggest though that you use a ballast resister (say 10ohm 5watts)
in the 5v circuit rather than the car bulb suggested by many of them.
Alternatively read on the UK picaxe site for power circuits they have
both battery and transformer based versions.
For development I have created a rectified power supply based on a 9v transformer. Details can be found here.
Components:
| Picaxe 40x |
| momentary on switch |
| 1x 4k7 resister |
| 1x 10K resister |
| 1x 22K resister |
| 1x 180 resister |
| 1x BAT85 or equivalent shottky diode |
| 1x 4Mhz oscillators |
| socket for a serial cable - stereo jack,
db-9 female etc |
| vero board, hookup wire, solder, sockets |
Build the circuit according to the
schematic.
Then test it. I found this part straight forward.
1)
Reset Switch
The picaxe requires a high logic level on the reset pin to run. The 4k7
pullup resister ensures this level except when the switch connects the
pin to ground. A low logic level will cause the picaxe to stop and when
it again gets a high logic level it resets and starts.
2)
Serial Circuit Most
of the documentation on the UK picaxe site uses a simpler serial
circuit. This one is also on their site on page 22 of the first part of
the user manual. It is described as the enhanced serial interface.
To test the circuit you will need to download the software from the
UK picaxe site.
You will need the editor and any patches they have posted. Note for the
picaxe 40x specify the 28x. I have some simple test programs on my
software page.
The next step, once this is working, is to add a real time clock.
