Nuts & Volts - March 2007

Getting Started With PICAXE Microcontrollers

functions, we first need to construct a simple PICAXE-08M circuit (either in an empty space on the Terminal breadboard or on a second small breadboard). Figure 5 presents the schematic for this circuit, which is simply the PICAXE-08M programming circuit. Because we are transmitting data back to the PC on the programming connector’s Serial Out line (pin 7), no additional connections to the 08M are necessary.

If you have a second serial port available on your PC, you can use two serial connections at the same time without having to move your three-pin programming adapter back and forth between the 08M circuit and the 18X circuit. Simply construct a second three-pin adapter, connect it between the 08M circuit of Figure 5 and your PC, and you are ready to download Sertxd.bas to the PICAXE-08M. If you only have one available serial port, you will have to “cable swap” whenever you switch between downloading the 08M software and the 18X software, or vice versa.

When you have constructed the circuit of Figure 5 and have Sertxd.bas open in the Programming Editor, make sure you have the Options set for the PICAXE-08M and the correct serial port. Also, in the Editor’s View menu, select Options and then select the Editor tab. In the Serial Terminal section, check the “Automatically open Serial Terminal after download” option, so you will be able to view the serial output from the PICAXE-08M. Download Sertxd.bas to the 08M; you should see the words “One” through “Ten” repetitively displayed in the Terminal Window. If you get an error message, recheck the Option settings and try again.

Once Sertxd.bas is working properly on the 08M, we’re ready to modify it to output the data from output 4 (pin 3) on the 08M to the one remaining input on the 18X (input 2 on pin 1). The only changes we need to make in the Sertxd program are to replace the first Sertxd command with “Serout 4, N2400 (outchar)” (which transmits our serial data on output 4 at 2400 baud) and to remove the second Sertxd command [Sertxd (cr,lf)]. You may want to save the new file as Serout.bas, so you

have both of them to experiment with.

In case you were wondering, “cr” stands for “carriage return” and “lf” stands for “line feed;” both definitions are built into PICAXE Basic, but I don’t think they are documented — I stumbled across them accidentally! If the Serout syntax is confusing, read through the documentation in Part 2 of the PICAXE manual; it should help clarify things.

To connect the 08M and the 18X, just disconnect the power from both circuits and add a jumper wire from output 4 (pin 3) on the 08M to input 2 (pin 1) on the 18X. If your two circuits are on separate breadboards, it is also necessary to connect the grounds of both breadboards with a second jumper wire.

On the 18X, the SerinLCD.bas program will receive the 08M’s serial transmission and display it on the LCD. In effect, a PICAXE-18X with SerinLCD.bas loaded into its program memory converts any HD44780 LCD into a serial LCD for use in your projects. In addition, you can easily modify the program to include additional features you find useful. You could also construct a complete circuit on a stripboard the same size as the LCD and attach it to the LCD board to make your own serial LCD module.

Of course, the characters displayed on the LCD by SerinLCD.bas are specific to the keyboard being used, so they may not be appropriate for your keyboard. To correct this, locate the branch statement in SerinLCD.bas and

replace the data characters with ones that match your keyboard.

Now that we have the two circuits connected, it’s time to download the software and test the system. First, set up the options in the Programming Editor for the 18X circuit; make sure you have set the correct serial port on the PC. Download SerinLCD.bas to the 18X, change the Option settings (and your programming cable if you are using only one), and download Serout.bas to the 08M. After a brief pause, the words “One” through “Ten” should be sequentially displayed on the LCD, along with an annoying beep for each word. If not, you will need to troubleshoot the system, which is getting a little complex by now!

Implementing the Complete I/O Terminal

At this point, we are ready to begin working on our stand-alone data terminal. First, we need to power-down both circuits again and add one more jumper wire connecting output 0 (pin 6) on the 18X to input 2 (pin 5) on the 08M. This connection will provide for serial output from the Terminal to the Target. It’s a good idea to include a 1K resistor in series in this connection; pin 5 on the 08M is a bidirectional I/O pin, so it’s possible for it to be accidentally set to be an output pin. The direct connection of two output pins (with no current-limiting resistor) can result in a high-level out-