Nuts and Volts - February 2013

PICAXE PRIMER

■ FIGURE 10. PNP and NPN switching circuits (Version 2).

■ FIGURE 9. PNP and NPN switching circuits (Version 1).

• Make It Fast — In our earlier experiment with the two discrete LEDs, we discovered that we needed to switch the LEDs at a rate greater than 50 Hz in order to avoid seeing any flickering, so our main multiplexing loop must be able to execute at least 50 times per second. Another way to say the same thing is that the loop can't take more than 20 ms per iteration.

to discussing the details of the program.

As we've done in the past, the number of each of the following comments refers to the corresponding number in brackets at the right-hand edge of the program listing.

• Keep It Balanced — The portion of the code that lights the ten's digit should execute in the same amount of time as the portion of the code that lights the one's digit. If the timings aren't equal, one displayed digit will appear brighter than the other.

( LEDmultiCount99.bas) is available for download at the article link. We still need to discuss how the program implements the three points listed above, but I'm sure you will want to run the program first. When you download LEDmultiCount99.bas to your breadboard setup, the display should repetitively count up from 00 to 99 at a rate of one count per second. If not, you will need to check the wiring on your breadboard.

When you're sure your system is working correctly, we can move on

1) I think we've discussed the time variable in a previous installment, but in case I'm mistaken, let me explain. Time is a built-in word variable that's available in all • Avoid Ghosting — We need to make sure that we never change the data that's being output on port B while something is being displayed on the LEDs. If we were to do that, some of the display segments that should be off would appear to be faintly lit — a phenomenon known as ghosting.