RE: 32.768Khz crystal frequency offset detection

james — Tue, 14 Feb 2023 01:24:29 GMT

Hi haakonsh

Thanks so much.It helps me a lot.

RE: 32.768Khz crystal frequency offset detection

haakonsh — Mon, 13 Feb 2023 13:08:07 GMT

You can f.ex. measure the crystal's signal directly by using an oscilloscope to get start-up time, frequency offset, jitter, etc.

One pitfall here is that a typical probe adds 9-12pF load to the signal. This can be overcome by either using an inductive probe, active probe (1-2pF load), or temporarily change the value of the crystal's loading capacitor so that the total capacitive load matches the original load cap value.

If you only need to measure offset you can can set up a GPIOTE channel to toggle a pin on RTC compare/overflow event, and then measure the pin with an analog or digital scope. Here you'll want to run the RTC without prescaling and toggle the pin at large time periods, in order to average out the timing "error" of the PPI peripheral. PPI is bound to the 16MHz clock and need one clock cycle from a received EVENT to trigger a given TASK (min 61.25ns, max 122.5ns).

An RTC overflow event will happen every 1s. PPM can then be calculated: ( 1s - (1s / (1s + error))) * 10^6.
The maximum measurement error introduced by the PPI system is -0.1225PPM at 1s, which should be more good enough for clock accuracy verification.