> Factor number two is that 1% at 200 Hz is 50us

I'd suggest 50us would be very very dim indeed and I really do
doubt the occassional few usec disturbance would be discernible.

After all, it's 4950us until the next 'on' refresh, so the "20%" error
isn't part of the 'on' pulse. The disturbance could well be in the 'off'
time. If I were doing it, and the s/w has noted a delay is in effect,
then you might have something like

pulse - 4960us - pulse - 4940us - pulse

where the s/w compensates for the 10us delay by shortening the
next interval by 10us (if the timing has that resolution). The overall
error is 0.2% in the position of the pulse within the 5000us period.
Assuming only 1 LED is lit that is ; you'd take off processing time
for selecting and lighting each additional LED

Even if the resolution is 50us that's still only a 1% shift in position

However, to notice this displacement, you'd need to be looking at
the display, in which case you'd have to factor in the eye's persistence
of vision - at that dimness and at 200Hz

As this is all being done under interrupt, there shouldn't be any
noticeable delay anyway with a fast processor. 40MHz is a cycle
time of 100ns, 500 instructions per 50us

Easy to test with a single LED of equal brightness

Joe
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.