That will only work if you have good zero crossing points which with an ADC
you often don't. You can improve matters by filtering the data first if you
know the expected operating frequency range and by adding hysterisis to the
edge detection.

Measuring the number of counts in a time period is only an accurate method
if your time period starts and stops on an input edge. If you want to
calculate the frequency by making a measurement over a fixed period, say 1
sec, then you can improve on the accuracy by measuring the number of counts
and the time period between the last edge and the end of your time period.

If
tn = the time period between the last edge and the end of time period n
Cn = the number of counts in time period n
T = length of time period

Then
Fn = Cn / (T + tn-1 - tn)

I use the above in an 8 bit micro to measure frequencies to 0.01Hz
resolution with an 1 sec update rate.

(There are lots of additional complications if you want to measure
frequencies that are lower than 1/T)



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