Multiplexing was Re: Information on the PPS-4/1 4 bit microprocessor

On Oct 31, 2011, at 6:17 PM, Tony Duell wrote: Most of the time, these days, that's all it's used for; I've seen some app notes where, if you're really clever, you can drive 4 LEDs per pin (plus 1 extra) without any additional external electronics. Very nice for either pin-constrained micros or large sets of LEDs. For example, most cheap clock radios have one 4-pin DIP running the whole show (and it's pretty much been the same chip since the '80s). It uses the 60/50 Hz zero crossing on the AC line to keep time, which gives it very precise time (Laurent Hammond, inventor of the AC synchronized motor which ran both home clocks and the Hammond organ, took advantage of this and now the power companies must make sure that the power cycles average 60/50 Hz over the day to a few ppm, if anecdotes inform correctly) and also gives it a handy time base for multiplexing half the LED segments. Normally, as mentioned earlier, a 60 Hz refresh frequency might cause a headache, but I imagine there are holdup caps on the LEDs so they're not quite so obnoxious. - Dave