> So if my end goal was to have the main ISR execute every
> second, then an additional counter loaded with 64 should
> do the trick

You won't be able to make the "main ISR" execute every second
with that fast a clock using even the maximum prescaler. Did you
mean "main do-something" routine ?

The longest timer period is 256 count *256 prescale. A 262,144Hz
crystal would give you 1 IRQ/sec ( 262144 / 4 / 256 /256 ) without
using an IRQ counter. You can go slower than that, eg 100kHz and
32.768kHz crystals are available, if you use a lower prescaler or
reloading for "decimal" crystals

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