>Is it normal that the capture module varies ~1 unit
>the capture of an stable sqare signal in all
>the range of frecuencies?
>4000Hz 250 and  251
>2000Hz 500 and  501
>800Hz 1250 and 1251
>What this capture means?
>There is something wrong in my interrupt routine?

Nothing wrong, just that your sampling and the measured signal are not
synchronous.

Any digital measurement system is likely to show +/-1 in the least
significant digit over a number of measurements for a stable input signal
unless there is some special synchronising hardware to make sure the
measurement always starts at a given point on the measured waveform. Even
then at certain points there will be some uncertainty which will cause what
you are seeing.

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