> I'm still not sure why the result varies approximately a quarter to
> a half hertz though

I'm trying to think of possible causes

Frequeny wobble in the incoming wave

Not always exactly 50:50 duty cycle. For example a short edge-
edge period at the start and end of the count would make the
frequency appear higher, vice versa for a longer edge-edge period

Irregular rise / fall time

Small s/w error accumulating

My best guess would be that the PIC is too slow -

4000.00Hz => 1250.00 cycles @ 5MHz
3999.75Hz =>  1250.08 cycles @ 5MHz

It seems to me that the edge could very easily come at any point in
the cycle and some time would be lost

Put another way, in the example above, if you were trying to measure
3999.75Hz and 4000Hz, the discrimination is between 250.000us and
250.016us, using a measurement tool with a coarse 0.200us resolution

wbr, Joe
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