My favorite way to handle this 'non-commensurate' intervals problem is to
steal a concept from the Bresenham line drawing algorithm.

Using the current case: 4.00 MHz crystal

1Mhz instruction rate

256 cycles and prescaler of 64

each overflow of the timer represents

256*64 == 16384 microseconds

You start with a counter set to 1,000,000 (1 second in microseconds)

On each timer interrupt you subtract 16384 from the counter.

If the counter goes negative you update the time by one second and then add
1,000,000 back in to the counter.

This technique will work for any interval. It can be made perfect for
intervals that have a rational relationship to the instruction cycle time,
and can be abitrary close to perfect even for irrational ratios (anybody got
any SQRT(2) Mhz crystals handy?)

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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