32 kHz square wave in Lieu of Crystal : compatibility matrix ?

[Vincent Belaïche]

Dear Jared ,

Following your suggestion to create a new ticket : 

We are also considering to make a design where the nRF52840 would use for the LFCLK and external 32,768kHz square wave source from an external RTC.
We would like to know from which nRF5SDK and SoftDevice versions this combination is supported, and whether legacy BLE SDK function can be used, or migrating to the newer nrfx_… API is mandated (just in case some of the legacy functions are not fully wrapped).

One more point of interest to us is how to handle the case of an external RTC that does not provide the 32kHz clock at reset. We are considering to configure the SDK to use the internal RC oscillator by default, and then switch to the HFCLK, stop LFCLK, enable external 32kHz, and then reconfigure LFCLK to be based on external 32kHz. Do you have any opinion on this approach ?

[Sigurd]

Hi,

It's not an officially supported configuration for the SoftDevice, but it's possible to workaround. See e.g. this post: https://devzone.nordicsemi.com/f/nordic-q-a/61284/using-external-low-swing-clock-source-with-softdevice/249415#249415

[Vincent Belaïche]

Dear Sigurd , 

Thank you so much for providing this work-around. I have one more question : is that possible to use the LFCLK calibration against HFCLK when the 32k source is external and when we use it with the SoftDevice by means of your work-around.

Could you please also provide from which nRF5SDK and SoftDevice versions your workaround is applicable, as we are not using the latest version for either of them.

[Sigurd]

is that possible to use the LFCLK calibration against HFCLK when the 32k source is external and when we use it with the SoftDevice by means of your work-around.

No.

Could you please also provide from which nRF5SDK and SoftDevice versions your workaround is applicable, as we are not using the latest version for either of them.

I have not tested it, but to my knowledge it should be applicable for all versions.

[Vincent Belaïche]

Thank you for the feedback.