Introduction to PICAXE Programming and Projects in Software

Make Denso QR Bar Code in Software Introduction to PICAXE Programming and Projects

I HAVE NEVER BEEN A FAN of microcontroller project books that take three or four chapters to get to the first project, so we re going to tackle the handson part as soon as possible However, there are a few things about the PICAXE programming system that we need do to cover before we jump into our first project Essentially, the required information can be divided into four areas:

surprising amount of computing power in its little eight-pin package, which is shown in Figure 1-1 The 08M2 can operate with a power supply anywhere between +18V and +50V, so the simplest way to power it is to use a two- or threeAA cell battery pack, which is what we ll do in our first project

Choosing a PICAXE processor Interfacing a project with your Mac or PC Using RevEd s free Programming Editor or AXEpad software Programming in PICAXE BASIC

For our first project, we want to get started as quickly as possible, so we re going to use the PICAXE-08M2 It s the smallest (and therefore the simplest) processor in the PICAXE lineup, but don t let that fool you the 08M2 packs a

The power connections for the 08M2 are made to pins 1 (+V) and 8 (0V or Ground) The Serial In and Serial Out pins are used for downloading programs from your Mac or PC (We ll get to that shortly) Once a program is downloaded to the 08M2, the Serial Out pin can also function as an output for your program, and the Serial In pin can function as an input, which gives the 08M2 a total of six I/O pins: output C0, inputs C3 and C5, and I/O pins C1, C2, and C4 The different numbering of the I/O pins and external pins can be

Figure 1-1

a little confusing at first, but it s necessary because of the underlying structure of the Microchip PIC microprocessor on which the 08M2 is based As you can see in Figure 1-1, many of the 08M2 s I/O pins have multiple functions We ll get into the details as the need arises; right now, I just want to mention that the function listed closest to each pin is always that pin s default function In other words, when a chip is first powered up, each I/O pin is configured to implement the function that is adjacent to it, as shown in Figure 1-1 Any pin that can be either an input or an output always starts up as an input This is a safety precaution that prevents the activation of any peripheral devices until your program is properly configured If you have any previous experience with microcontrollers, you will notice the absence of dedicated crystal or resonator pins in Figure 1-1 Part of the simplicity of the 08M2 is its internal 4MHz resonator (which can also be switched to 8, 16, or 32MHz by your program) The internal resonator is not as accurate as the external one found on many of the larger PICAXE processors, but it s accurate enough for the vast majority of 08M2 projects If greater accuracy is needed, there s a BASIC command (calibfreq) that allows you to fine-tune the 08M2 s operating frequency Of course, you would need a frequency counter or oscilloscope to make the necessary adjustments

long before the serial port is totally obsolete Fortunately, the PICAXE programming interface is essentially the same for serial or USB connections Actually, there are two versions of the programming interface basic and enhanced both of which are presented in Figure 1-2 The basic version only includes the 10k and 22k resistors; the enhanced version adds two optional parts (the BAT85 diode and the 180 resistor) to improve the accuracy of the serial download circuit For USB connections, the 10k and 22k resistors are all that s required, but the enhanced circuit will also function correctly in this situation as well If you still use a serial connection to your computer, my recommendation would be to try the basic interface first If programs download reliably to your PICAXE processors, you re all set If a download occasionally fails, try including the 180 resistor and the BAT85 diode If you do include the diode, be sure to install it backwards, that is, with its anode (rather than its cathode) connected to Ground, as shown in Figure 1-2 If your computer only has USB ports, you will need a USB-to-serial adapter to program any PICAXE chip Before you run out and buy one, however, you need to know that most currently available adapters simply don t work with PICAXE processors Fortunately, RevEd produces the AXE027 USB programming cable, which is available at wwwsparkfuncom (SKU: PGM-08312) and elsewhere The AXE027 USB programming cable terminates in a 35-mm mini-stereo plug that provides the necessary serial in, serial out, and Ground connections for programming all PICAXE processors Unfortunately, a mini-stereo plug is obviously not what you would call breadboard-friendly Since most of the projects in this book will be implemented on breadboards, we re going to need a way to adapt the mini-stereo plug for breadboard use We will do exactly that in our Hello World! project later in this chapter