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

[Kenneth]

Hello,

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

If yes, then likely the problem here is the series capacitors on the crystal, you may want to increase them yes. For instance if you are using a 32.768kHz crystal with CL=9pF, then the series capacitors (C11 and C12) should be 12pF:
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52810.ps/ref_circuitry.html?cp=2_2_0_6_2 

What is the CL for the 32.768kHz crystal you are using? 

Best regards,
Kenneth

[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 ?