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RTC Time Drifts Even Though LSCO looks good

kile
Associate II

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). 

11 REPLIES 11

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?

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