At 06:49 AM 5/27/2005 -0700, you wrote:


>  A 20MHz clocked PIC would update the PWM counters every 50 cycles
>* 0.2uS/cycle = 25 uS. The PWM period is 256 * 25uS ~ 6.4mS which
>translates into a PWM frequency of 1/6.4mS = 156Hz. Triggering the LED
>refresh at this frequency should be fast enough to remove perceivable
>flicker.

Scott, 156Hz is nowhere near fast enough in most cases. You have to multiply
the minimum "flicker" rate times the number of digits being sequentially muxed
and apply a healthy "quality appearance" multiplier if you don't want the 
display
to "break up" visually when it or the user's head moves a bit. 1000Hz is more
like it (or perhaps 500Hz in this case, since he's got only 3 digits).

This could be still done by the PWM routine, by triggering off of every n'th
update, and making sure that the display routine takes less than the
cycles or so available until you have to return so the next interrupt can
take place on schedule. Probably would have less issues than trying to use
nested interrupts or whatever. There would be a bit more overhead in each
interrupt to decide whether to scan the display this int or not. Of course
there would be more room to play with a 40MHz (or 120MHz) PIC.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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