To measure the period and/or frequency of a fast waveform you have two basic 
solutions:

1: Use the CCP with the 'every 16th edge' option. Compute the difference of 
the counts every N interrupts. You now are determining the period of 16*N 
cycles of the original waveform.

2: Connect your input to a Timer/Counter. Enable the counter for a known 
time interval based on the crystal frequency of the PIC. The value in the 
counter will be proportional to the frequency of the input.

Of course to go from period to frequency or the reversion you just need a 
division.

Bob Ammerman
RAm Systems


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