Nuts and Volts - October 2010

Our second program ( keypad2.bas) incorporates all the changes we just discussed. Open it in the Programming Editor and change the theoretical mid-point values in each of the case conditions to the values you calculated from the results of running keypad1.bas. Also note that the sertxd statement has changed; instead of sending the individual digits of the ADC value, we’re now transmitting the appropriate ASCII value for each character to the terminal window. Download the program to your breadboard circuit and test all the keypresses. You should see the correct character appear in the terminal window in response to each keypress. If not, a little trouble-shooting is in order.

WHAT’S AN EVIL GENIUS TO DO?

the convenient layout for the four arrow keys and the fact that the “Enter” key is in the lower right-hand corner where it should be.

Next, the modified keypad was used to develop a programmable, multi-function peripheral device (MPD) that can be easily connected to any breadboard circuit (see Figure 9). Because the MPD is fully programmable, it can be used to implement a variety of helpful utility and troubleshooting applications for use with any PICAXE project. The following are just a few of the many possibilities: • Stand-alone I/O terminal with sound • Digital frequency generator • Frequency measurement of project signals • Timing of project routines and waveforms That’s as far as we’re going to take our ADC keypad project this month. I’m sure it’s no surprise that the project we tackled in the Evil Genius book was to design a serialized version that simplified the interface with a master processor by freeing the master from the chore of repetitively monitoring an ADC input. Next, we performed a little “surgery” on our keypad. We disassembled it and rearranged its innards, so that the connector was moved to the top of the keypad and its keys were shuffled into a more convenient pattern. The results of our little operation are shown in Figure 8.

COMING ATTRACTIONS

In addition to the publication of PICAXE® Microcontroller Projects for the Evil Genius, there’s another major event that’s about to occur in the PICAXE world. Revolution Education has announced its newest processor — the PICAXE-18M2 — and it’s scheduled to be available by the time you’re reading this column. The 18M2 is a revolutionary product with many impressive hardware and software enhancements. I don’t have space this month to get into the details, but take a look at Figure 10. I

■ FIGURE 10. Comparison of 18M2 vs. 18M selected features.

18M2 18M General Purpose Vars (bytes) 28 14 Storage Variables (bytes) 256 48 EEPROM Storage (bytes) 256 256-prog

Max Operating Speed (MHz) 32 8 Min Operating Voltage (V) 1.8 3.0

PICAXE PRIMER

think you’ll be impressed.

The 18M2 also includes several software enhancements; I’ll just mention two of them to whet your appetite. There’s a new built-in time variable that keeps track of elapsed time in the background while your program is carrying out other tasks. This means that a project will be able to include a real-time clock feature without necessitating additional hardware or complicated software interrupt routines. Also, the 18M2 will be able to run up to four different program tasks in parallel at the same time by rapidly switching among the various program “threads.” This new capability isn’t multitasking in the strict sense, because program instructions are still being executed one at a time. However, the automatic switching among program threads will occur so rapidly that it will create the illusion of multitasking.

I have saved the most impressive specification of the new 18M2 processor until last; it’s slated to retail for about the same price as the older 18M processor. To be able to purchase that amount of processing power for less than $4 is truly amazing. I’m eager to get my hands on one, and as soon as I do (which will almost certainly be in time for the next Primer column), I’ll get to work on reporting what I learn about this impressive new chip. See you then ... NV