Olin, using the example below, can you expand/clarify please

> Your timer 1 overflow period is 16.4mS.  All you need is some code
> that is guaranteed to run a little more often than that

Say its period is 0xFF00 (16.320ms), and the frequency being measured
is ~59.4Hz, ie a period of 16.832ms, or 0x10700 * 250ns. So that's (1
rollover + 0x0700) of TMR1 between rising edges

> This code grabs the timer 0 [1 ? typo ?] high byte, finds the delta from
> the last time it did that, then adds that delta into a software
accumulator
>
> The procedure above makes the software accumulator up to date with
> the latest high byte of timer 1

Assuming the previous sample left 0x00 in the s/w acc, which was loaded
into TMR1H (as per paragraph below)

This sample of TMR1H is 0xFF

delta = 0x01 -> into s/w acc (*** did you mean absolute delta ?)

Now, next edge comes along when TMR1H is 0x07 (rolled through 00).
So total increments since last edge is 0x010700

> In the capture interrupt, the total period is the period indicated by the
> timer 1 low byte difference, plus the timer 1 high byte difference from
> its last updated value, plus the software accumulator

TMR1H difference = 0x07 (last update) + 0x01 (s/w acc)

> You then clear the software accumulator, update the timer 1 old low
> byte, and update the timer 1 old high byte used by the software
> accumulator update routine

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