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13th 2003f October 2003
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  PicAxe MicroController - Eric Patching

Date:October 10, 2003
Time:7:00 pm.
Place:WEA, 223 Angas Street, ADELAIDE
Presenter:Eric Patching

Eric Patching who is a long serving SAMG committee member enjoys hands-on, leading edge technology, so it is no surprise to find that he is using PICAXE chips. He provided a very interesting presentation and demonstration of some of the chips in this series at our October meeting.

Eric describing the instruction set of the PICAXE

The PICAXE concept is very appealing. It uses low cost Microchip PIC chips such as the PIC12F675/629 preloaded with a Basic interpreter in their flash RAM. Your program then loads into the remaining Flash RAM over a simple 3 wire interface from your PC where a suite of very clever software gives you the choice of programming in PICAXE Basic, programming using flowcharts, or debugging. PICAXE chips are really cheaper Basic Stamp clones.

There are 8, 18, and 28 pin PICAXE chips in the series so far with increasing features and I/O capabilities as the pin count goes up. The 8 pin the PICAXE-08 uses the PIC 12F629, the 18 pin PICAXE-18A uses the PIC 16F819, and the PICAXE-28A uses the PIC 16F872. (The PICAXE-28 is particularly interesting as it can provide eight continuous and simultaneous servo outputs making it very useful in robot and model engineering.)

A 3 wire interface connects to the serial port on your PC for programming . The signal wire from the PICAXE to the PC connects directly, whereas the signal from the PC to the PICAXE uses a couple of resistors to reduce the voltage level. The third wire is ground.

Because most of the chips in the series use the interal RC oscillator (set to 4 MHz for the PICAXE-08) there is not much more than the power supply (2.5 to 6 volts at ~2mA), the 3 programming wires (only if you do in-circuit programming) and your I/O lines that are required for a working circuit. The 8 pin PICAXE has 64 bytes RAM, 128 bytes EEPROM and 5 I/Os including a limited ADC.

Eric warned us about something he learned the hard way which is that the serial-in pin of the PICAXE should never be allowed to float when the chip is in use - you need to ensure that there is always either a path to ground via the resistors in the 3 wire programming circuit or by a solid ground.

The PICAXE Basic high level language uses a compact yet powerful instruction set to enable you to quickly write a wide range of applications. The commands include analog, digital, and serial I/O, flow control, table lookup, EEPROM R/W, power control, and debug. The amount of code is limited by the amount of Flash RAM in the particular chip -- the smallest 8 pin chip has room for about 40 lines of Basic while the biggest 28 pin chip can accommodate up to 600 lines.

Some of the more interesting commands are SOUND that can produce "music" and READADC that can read an input voltage (while this is somewhat limited in the 8 pin chips, the bigger chips have full 8 bit ADC resolution).

Eric used a short program that he had written with the flowchart software and downloaded into an 8 pin PICAXE to demonstrate an application that could be used in, say, a shop to monitor when people enter the shop by playing a distinctive tune and playing another tune when they press an "attention" button.

Development board implementing sound generator and serial communication

Eric also has been experimenting the temperature sensor (DS18B20) and with two PICAXE chips communicating with each other over a serial link. There is the limitation that he can not use full duplex and has to use simlex due to a limitation of the PICAXEs.

These chips obviously fill a niche where quick, cheap, easy solutions are required and, as Eric is willing to admit, are fascinating to use as well.

There is more information and the free downloadable suite of programming software at Revolution Education Ltd. (originators of the PICAXE), and http://www.picaxe.co.uk The chips are now available in Australia from several sources such as Altronics, Oatley, and possibly Aztronics (Sturt St Adelaide) for prices starting at about $3-50 for the 8 pin chips.

. . . . Rick Matthews