RTC Time Drifts Even Though LSCO looks good

[kile]

Hi Everyone,

I have a project utilizing a STM32G030F6P6 with a 32.768kHz crystal and the LSE to source the RTC clock. It is exhibiting 2% clock error (which is huge!) over a 10 minute period despite using a 20ppm crystal. I am measuring this error by interrogating the RTC and communicating it to the rest of my system via i2c.

Now, I have read all of the crystal oscillator and load capacitor horror stories, but I have validated the actual crystal frequency by configuring the LSCO to output the LSE clock and it measures a stable 32.77 kHz (limited by the precision of my oscilloscope). In any case, much better than 2% error.

What gives? Can anyone confirm the expected duty cycle of the LSCO signal (I am measuring 40% and cannot find documentation as to what I should expect). 

[kile]

It's solved now anyway, but I'm not sure how that would help. If the LSCO is correct, then there should be nothing further to do with the crystal, no?

[STOne-32]

Correct ! I concur what you said . Prescaler is digital before RTC peripheral. Nothing to do in impacting LSCO and LSE