32kHz resonator with 12.5 pF - what will happen ?

colsman · ‎2012-10-17

Posted on October 17, 2012 at 11:38

Due to an error in manufacturing we have a lot of modules with a 32khz resonator with 12.5 pF for low speed external clock and two 27pF aditional capacitor to ground.

Prozessor is the STM32F103RCT6. The real time clock unit is running half of time on a coin cell battery.

In the ST documentation it states ''never use a resonator with a load capacitance of 12.5pF''. Well .. seems like we have a problem. Also the clock seem to run at first tests.

Question: What happens due to this large load capacitance ? Does power consumption of the clock circuitry increase or does the clock runs wrong over time ?

Are there quick solutions ? - maybe taking of the additional 27pF capacitors ? Changing the resonator will be very hard to realize.

Thanks for your advise - Matt

zzdz2 · ‎2012-10-17

Posted on October 17, 2012 at 12:46

I would use 15pF if it works, clock frequency may be somewhat different but it should be safer than 27pF for the chip.

Tesla DeLorean · ‎2012-10-17

Posted on October 17, 2012 at 15:41

I think the key problem is that it won't start reliably.

You should be able to benchmark the clock.

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colsman · ‎2012-10-18

Posted on October 18, 2012 at 10:11

Thanks !

What would be a good methode to increase reliability ?

I can't exchange the 12.5pF resonator, but I could change the capacitors. ''Knik'' suggests to use 15pF rather than the 27pF - would that influence the reliability ?

zzdz2 · ‎2012-10-18

Posted on October 18, 2012 at 10:21

It depends on the oscillator amp transconductance.

LSE amp has very low transcond. so 15pF may actually turn out more stable than 27pF.