"Dave Plowman (News)" wrote:

I have a device where the crystal and associated network is external to
the CPU. Which isn't working. Looking across the two connections with a
scope there's no approx 30 kHz as should be - but what looks like very
dirty mains hum. Any way of easily testing the crystal other than by
substitution?


You need a special probe to keep from loading down the circuit, and
if it is a typical 32.768 kHz watch crystal the waveform isn't clean
even when it is working properly. It is a tuning fork type of crystal,
not the typical cut used at higher frequencies. A high input impedance
FET amplifier with a fraction of a pF coupling capacitor is typically
used to prevent loading, but you can often see a waveform at the drive
side of the crystal. They use an unbuffered inverter and a resistor to
bias it into the linear range, but the AC voltage in the circuit pushes
it out of the linear range.


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