nRF52 clock in and out

[Greg Nelson]

I have two related questions about nRF52 (nrf52832) which I have not been able to pinpoint from the documentation.

1. Can the 32MHz crystal connection be replaced by a 32MHz input if there is an existing clock in the system that meets the precision, tolerance, etc? On many processors, one of the two crystal pins may be used as an input while the other is left open if there is a driven external source, but I see no information about this for nRF52.

2. What is the most power-efficient way to get a high frequency clock output from the nRF52? Other ARM processors seem to offer direct "HFCLK" outputs to pins, but I can't find any related features in the nRF52 documentation. The best I've been able to guess is that the SPIM or PWM blocks could be used to provide a clock (8MHz or 16MHz respectively?) but each of those draws additional power and we don't (in this case) need the remaining functionality of the peripherals.

Thank you.

[MartinBL]

The answer to question # 1 is no, it is not supported. It was possible on the nRF51, but the clock module is redesigned and external 32MHz clocks is not a feature of nRF52. You can try at your own discretion, but the new design is so sensitive to jitter and noise that Nordic will not make any guarantees. A crystal is needed to ensure performance.

[mydadisaplumber]

Hi Martin,

I came across this question. I'm confused because in the docs for the Clock of the nRF52 here:

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fclock.html

It seems to suggest you can use a 32 MHz clock with the nRF52 as a source for the HFXO. I've read the clock section of the docs a few time and still don't understand.

If there's no external 32 MHz clock, what's the HFXO source? Internal?

Why is it that all functionality can be done with no HF external clock, but a lot of other forum posts seem to suggest that you can't use Soft Device or BLE if you use an internal RC filter for the LFXO. Is that true? Is the 32.768 kHz external clock required for full nRF52 functionality?

Thanks.