You modify what I posted the other day

20MHz/4 = 5MHz (200ns instruction cycle)

=> 5,000,000 / 256 = 19,531.25 IRQs / sec. Leave as is if you
don't mind losing (0.25 * 256 * 200ns) = 12.8us per second. IOW
the 'second' will actually be 999.9872ms

If you use a 19.6608MHz crystal and set TMR0 pre-scaler to 256

(19,660,800/4) / 65536 = exactly 75 IRQs / second

I've got something like that running right in front of me, generating
150 IRQ/sec for a project. Reload the timer with $8000 (bsf bit15),
divide by 3, => 50Hz

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