Electronic – How to measure the frequency of a watch crystal using the oscilloscope


I am trying to measure the frequency of a 32.768 kHz watch crystal which is connected to my AVR MCU. This is so that I can fine-tune the crystal's capacitor values to get a correct frequency. However, I don't seem to be getting any output on my oscilloscope (a Rigol DS1052E with the factory probes).

I have tried measuring between both crystal legs and also between one leg and ground and don't get any output. I have set the fuses correctly for the external crystal, and the MCU is working.

Also, even if I could measure the frequency, could I be sure that the measured frequency is what it would run at, or would the oscilloscope probes end up giving me a distorted reading somehow? If so, how can I tune the caps to get a stable 32.768 kHz frequency?

Best Answer

If the AVR or any MCU has a way to output the clock to an external pin, even if it divides it down, I would measure that pin. Measuring at a clock pin will interfere with the clock frequency due to capacitance of the scope probe. This way it’s buffered.