I have successfully used a PCA to measure the frequency of an opto coupled AC line voltage present detector. It worked quite well although I was not after any great accuracy because I was only interested in "reasonable assurance that AC present".

I have also use a PCA to measure the tachometer pulse frequency coming from a cooling fan. The huge range of the fan speed (I was also generating a PWM waveform to control the fan speed in accordance with a temperature reading) made it be necessary for me to use the PCA in a way where overflow interrupts were used to increment a software counter to extend the period measurement.

Michael Karas