low frequency 32Khz XTAL

[johananh]

Out of 10 prototype boards, we have 3 not working well due to 32 KHz xtal issue.

7 units are OK, but the other 3 units works only when configured with internal RC osc.

We use a XTAL which is used in other project and has 6 pF load capacitance.

Could this be the problem (I know commandeered is 9 pF..)

We tried a few low capacitors in parallel without much help.

Please comment. What is the best approach for a reliable system?

Regards

Johanan

[johananh]

We use XTAL with 6 pf CL. 

I tried C11 C12 10pF without results.

Thanks

Johanan

[Kenneth]

If you set LF accuracy to NRF_CLOCK_LF_ACCURACY_500_PPM when enable softdevice does it work then?

I think either the frequency is wrong or possible some hardware issues (e.g. soldering issue?).

[Nguyen Hoan Hoang]

try the caps in the range of 8pF instead.  It is approx calculated as 2 * Cl - 4pF which is 8pF in your case.

[KariMa3]

Is it allowed to use nonsymmetric capacitors to get more accurate 32k ? With 2x10pF we run some 35ppm too fast and 2x12pF the clock is 10ppm slow. We could use non standard 11pF capacitors or use 10pF+12pF to get very accurate clock. Is there any recommendation on this ?

[Kaja]

Please make a new case on DevZone, to get an engineer assigned to your question. 

Best regards,
Kaja