NRF52810: external 32.768khz tolerance requirement for BLE

[zhang.sandy]

what is the external 32.768khz crystal tolerance range for BLE? we have a product, it easily disconnect with another one when the distance is 5m, the PPM is -120ppm, fine tune the PPM, solved the problem.  and now we have a lot of product, the all ppm is around 70ppm, is it safe? 

[Einar Thorsrud]

Hi,

The sleep clock accuracy concept in BLE maps to the 32.768 oscillator accuracy on the nRF circuitry. If you had been using the internal LFRC, that would have been 500 ppm. For a crystal it is typically much less. You do need to confiture the accuracy, though. And this should cover the worst case frequency offset you can have. The only down side of specifying a slightly larger power consumption due to window widening (you can experiment with the online power profiler to see the effects it has.

If you are using the nRF Connect SDK, set the CONFIG_CLOCK_CONTROL_NRF_K32SRC_... option that matches your reference clock. You can for instance use CONFIG_CLOCK_CONTROL_NRF_K32SRC_150PPM in this case. If you are using the nRF5 SDK, you want to adjust the NRF_SDH_CLOCK_LF_ACCURACY define in sdk_config.h. If you e this to 2, it covers everything up to 150 ppm.

As a side note I want to mention that if you are seeing a worse clock accuracy than what is specified for the crystal(?), then that could indicate that you have not loaded the crystal correctly. See Crystal and load capacitors in the General PCB design guidelines for nRF52 series.

[zhang.sandy]

for the 32.768khz crystal, we used load capacitor 12.5pf, and the max value of load capacitor in datasheet is 12.5pf, is it OK? 

[Einar Thorsrud]

The load cap needs to match the crystal you are using. Which exact crystal do you use, which tolerance does it have, and what is the CL specified for it in it's datasheet? Once you have that, you should calculate the value of the two load caps on the board like this: 2 * Cl - C_pcb - C_pin, where C_pcb + C_pin is approximately 4 pF.