I have been following this thread with great interest, however I do have
one question to throw out there to all of you experts:

If someone wanted to use this frequency counter as a general purpose lab
instrument (hobbiest quality of course...), what would be the best way to
condition the input signal for measuring an analog signal of unknown
amplitude and waveform?

My first thoughts lead me to a rail-to-rail output op-amp running open loop
with the signal applied to the non-inverting input and also clamped to
VDD & VSS to prevent "fold-over" from distorting the waveform as well
as protecting the op-amp.  The output ofthe op-amp, of course tied
to RA4.

Hopefully this would allow weak as well as strong signals to be counted.

Thanks...

Dan