> The 555 is for giving 60Hz without any complicated divisors. I
> had thought of doing it in software, but I chose to go the other way.

To get an "exact but average" 60Hz with a 20MHz isn't too difficult but
there are easier ways

60Hz = 83333(recurring)us period = (83333+83333+83334)/3, which
could be done using a cycle of 3 timing loops

However, if you change to a hex value crystal something close to your
20MHz (18.432MHz or 19.6608MHz) it becomes quite simple

(18432000/4) / 256 = 18000 TMR0 IRQs / sec = 150 per 1/2 cycle

Similarly 19.6608 -> 160 IRQs / 1/2 cycle

Allow TMR0 to free-run and have an interrupt to bump a RAM counter

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