RTC oscillator STM32F407

[Michael99]

Hello,
I am using a CC4V-T1A-32.768KHZ-12.5PF-20PPM-TA-QC crystal with 22pF caps on a STM32F407ZGT6.problem with some devices the crystal does not oscillate.could the crystal or the capacitors be wrongly designed?
What can we change?
Thanks for your help

[Peter BENSCH]

Welcome @Michael99, to the community!

Problems with the very sensitive LSE have often been discussed here on the community. They occur if you do not follow the recommendations of AN2867 closely enough, where sections 7.1 and 7.2 in particular provide detailed instructions.

What does your layout around the LSE Crystal and the STM32 look like?

Regards
/Peter

[Michael99]

Hello Peter,
Thanks for the quick reply.
Attached is a part of the board with the LSE Oscillator.
We have tested leaving out the two capacitors on the quartz, result: the quartz oscillates.
We did the stability calculations for the AN2867 and the result was ok.
Is it better to use a crystal with a smaller load capacitance, perhaps 6pF?

Greetings
Michael