Regarding Nrf52832 Clock Accuracy setting

[Ganabappa]

Hello,

We are using MDBT42Q-512KV2 module which is based on nrf52832. There is Central  and peripheral ( Foot Pedal Switch ) using same module.

We have used crystal from ABRACON ABS07W-32.768khz-K-2-T and 12pF tuning capacitors as per reference design. The NRF_SDH_CLOCK_LF_ACCURACY setting in sdk_config.h was set to 2 i.e. 20ppm.

During development there was no issue it was working fine as expected. When we run a batch of 100 samples and tested, the peripheral device found to be continuously connect and disconnect.

And the disconnect reason code was 8 i.e. BLE_HCI_CONNECTION_TIMEOUT.

After some research we found that the crystals used by our EMS vendor were of different make due to shortage. Further digging we changed NRF_SDH_CLOCK_LF_ACCURACY setting in steps up to 500ppm then it was working fine.

Now the question is how to set the correct NRF_SDH_CLOCK_LF_ACCURACY ? and if it is higher what is the impact to the device?

Thanks and Regards,

Ganesh

[Vidar Berg]

Hello Ganesh,

The Softdevice will use the clock tolerance specified by the application to determine how much clock drift it must compensate for to ensure a stable connection. 

Now the question is how to set the correct NRF_SDH_CLOCK_LF_ACCURACY ?

You will need to refer to the datasheet to find out what the actual accuracy of your new crystal is. Remember to take the crystal's drift over time (aging) and temperature into account. 

and if it is higher what is the impact to the device?

On the peripheral side, the Softdevice will have to increase the RX listening windows to compensate for the lower clock accuracy, and this will have a slight impact on the power consumption. You can use our online power profiler to estimate how much impact it will have on your application.

 Online Power Profiler for Bluetooth LE  

Best regards,

Vidar