> 20% x 10e-6

Are you sure it is 20 *percent* ? Because it is usually 20 x 10e-6 which
is 20 ppm which is +/- 0.65536 Hz at 32768Hz and takes a counter with at
least 100 second gate time to make sense of (and those are not common -
try to get one that has a period meter perhaps). Imho build a specialised
counter using a PIC and do NOT probe the osc pins, provide a separate
clock output on a pin instead (even if it is slower and takes longer to
measure).

Peter

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