Using external low swing clock source with softdevice

[Steve]

Hello,

We're bringing up custom hardware using SiT1532 MEMS 32kHz oscillator.  This part outputs a low swing single ended clock source (300mV +/-55mV p-p).  I'm struggling to find the appropriate way to configure this when using softdevice.

sdk_config.h lists external low swing external source options for the clock driver, but NRF_SDH_CLOCK_LF_SRC has no such option.  When trying to use the 131073 value listed for NRFX_CLOCK_CONFIG_LF_SRC, the compiler throws an error, apparently softdevice can only accept an 8 bit value, even though the LFCLKSRC register documents setting bit 17 for external sources instead of Xtals.  

If I use the clock driver, I'm able to successfully start the clock and measure a good waveform.  But when setting NRF_SDH_CLOCK_LF_SRC to Xtal, my call to nrf_sdh_enable_request() fails with NRF_ERROR_INVALID_PARAM, and it looks like it's propagating up from sd_softdevice_enable().

What's the right way to use an external source with softdevice?  We're on version 16 of the SDK with S140, nRF52840.

Thanks,

Steve

[Håkon Alseth]

Hi,

 

sdk_config.h lists external low swing external source options for the clock driver, but NRF_SDH_CLOCK_LF_SRC has no such option.  When trying to use the 131073 value listed for NRFX_CLOCK_CONFIG_LF_SRC, the compiler throws an error, apparently softdevice can only accept an 8 bit value, even though the LFCLKSRC register documents setting bit 17 for external sources instead of Xtals.  

Yes, this is an unfortunate option that is lacking when initializing the softdevice. The workaround is to set NRF_SDH_CLOCK_LF_SRC to xtal, and manually start the xtal prior to initializing the softdevice.

What's the right way to use an external source with softdevice?  We're on version 16 of the SDK with S140, nRF52840.

You can follow the configuration as posted by my colleague here: https://devzone.nordicsemi.com/f/nordic-q-a/60702/enable-bypass-of-lfclk-crystal-oscillator-with-external-clock-source

You should configure the low swing according to this table: https://infocenter.nordicsemi.com/topic/ps_nrf52840/clock.html?cp=4_0_0_4_3_1#concept_xkj_np1_2r

Please let me know if you run into any issues.

 

Kind regards,

Håkon

[Steve]

This worked, thank you.  We did however, have some issues because it wasn't clear in sdk_config that we needed to set the following when setting source as Xtal

NRF_SDH_CLOCK_LF_RC_TEMP_CTIV 0

NRF_SDH_CLOCK_LF_RC_CTIV 0

Otherwise softdevice returns an error.