> > datasheet for the 16F87, and it says the internal oscillator is
> > +- 1% accurate, I think, at 25 degrees C.  I think something
> > must be very wrong for it go get that far out.
> 
> Do the calculation - half a second in 2 minutes is 0.4%, well in
> spec. You'll need to use a crystal to keep accurate time

Or use OSCTUNE or adjust your timing block to compensate
for the 0.4%. Depends on what periods you want to time. Even
a crystal will not give give accurate long-term stability unless it's
calibrated and temperature-compensated. 10ppm is ~ 1s / day

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