Sharing LF CLOCK with an SPI sensor

[Mehmet]

I have an SPI sensor that benefits from having a precise clock source, and has a muxed pin that can have a clock source.

The peripheral states that this pin can handle frequencies between 31 kHz to 50kHz, but the examples are all 32.768 RTC clocks. 

So, I was wondering whether I can share the LF clock with the sensor by not compromising from jitter. 

My questions from other posts and the Reference:

• I have seen the register LFCLKSRC, and that there are registers EXTERNAL and BYPASS. These are for external active oscillators, right? Their descriptions are a bit vague to me. Somehow this should disable the feedback circuit that rings the XTAL, right? What is actually bypassed? May I use a TCXO and hook it to both nRF and the sensor?
• If I avoided to share the clock, can I use PWM peripheral for the same effect? What is the accuracy that I can reach there?
• And would any one of these options, is there a big compromise on power efficiency?

Thanks!

MA 

[Kenneth]

Hello,

You may find these two tables useful:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/clock.html#concept_xkj_np1_2r and
https://infocenter.nordicsemi.com/topic/ps_nrf52840/clock.html#unique_567737179 

Note that the BLE softdevice may not have the api to configure BYPASS and EXTERNAL register, so you need to follow this description if you want to use external clock source:
https://devzone.nordicsemi.com/f/nordic-q-a/60702/enable-bypass-of-lfclk-crystal-oscillator-with-external-clock-source/247385#247385 

But to answer your questions:

Unfortunately I think sharing a crystal driven by the nRF52840 will difficult since the amplitude will be around ~200mV, in addition the capacitance etc must be taken account for to ensure frequency accuracy. It's not possible to output a 32kHz from the nRF52840 in any low power way (it must be generated by the HFCLK in such case), so I think that is not an option either.

My suggestion if you want to share, then look into using a MEMS to supply 32kHz clock intput to XL1, just make sure to set BYPASS and EXTERNAL as in the devzone case linked above.

Best regards,
Kenneth

[Mehmet]

Thank you, Kenneth. I will take a look at the links now.

You are referring to a MEMS oscillator, right? (My sensor happens to be a MEMS accelerometer, that is why.