> So why not calibrate in software?  Measure the real frequency
> at production time, burn some sort of calibration factor into
> EEPROM or program memory if need be, then have the firware
> adjust accordingly

That's one thing I've been considering. I have another product
in which a central xtal-controlled 18F452 puts out a 1s timing
period to calibrate and synchronise satellite 16F88s and 12F675s
that all run on IntRC

For this product I have a couple of spare pins that are terminated
with a pin (unused until now) that I could inject a long accurate
period into, perhaps a day. It would be a minor pain to set up but
you'd get a better product out at the end. s/w inside the F88 can
work out how to reproduce that day. For example, TMR1 re-load
value is presently $4000 ( = -$C000) to give a 0.01s IRQ period.
For the TOD function this might now become some combination of
 X * $4000 and Y * 3FFB, or whatever it turns out to be per second


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