... It's possible to make very large displays providing you have the pins and the processing power.
Yes, you can make
very big displays using Charlieplexing, but no amount of processing power will overcome it's biggest limitation, 'persistence of vision'.
Because you can only illuminate one LED at a time, in order to display more than one you have to pulse each LED for a short time. For example, if there are 10 LEDs on show, then each is energised for 10% of the time, and for 20 LEDs it's 5%.
When you're down to a 5% duty cycle then flickering becomes noticable, and also the effective brightness is much reduced.
Charlieplexing works best if only a smallish number of LEDs are lit, and it doesn't make any difference how big the full display is, the processing power needed stays the same. ( Another problem is if you need a mixture of different coloured LEDs, because different coloured LEDs need different drive currents. )
However, one great advantage of Charlieplexing on a PICAXE, as well as not using many pins, is that driver chips are not needed. Because only one LED is energised at a time the total current remains the same no matter how many LEDs appear to be lit. This means the 20mA capability of the PICAXE pins is enough to drive the LEDs directly, with just a resistor on each pin to limit current.
Charlieplexing is great. I'd like to build another huge Charlieplexed device, but I can't think of what to make.
( I'm open to suggestions, but NOT another clock !.
Cheers,
Buzby