LFCLK External Xtal and Soft Device

[stevebedford]

I am attempting to enable the external LFCLK. I want the clock to swing rail to rail.  I have the following code in place.

    /* Start low frequency crystal oscillator for app_timer(used by bsp)*/
    NRF_CLOCK->LFCLKSRC            = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
    NRF_CLOCK->LFCLKSRC            = (CLOCK_LFCLKSRC_EXTERNAL_Enabled  << CLOCK_LFCLKSRC_EXTERNAL_Pos);
    NRF_CLOCK->LFCLKSRC            = (CLOCK_LFCLKSRC_BYPASS_Enabled << CLOCK_LFCLKSRC_BYPASS_Pos);
    NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_LFCLKSTART    = 1;

    while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0)
    {
        // Do nothing.
    }
    NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;

I am using the s140 v 7.2 soft device and the nRF5_SDK_17.0.2_d674dde version of the sdk.

Is there some additional configuration step I'm missing?

[Sigurd Hellesvik]

Hi Steve

Documentation on the clock configuration is described in CLOCK — Clock control.

Does the code you describe run as expected?

Regards,
Sigurd Hellesvik

[stevebedford]

Sigurd, I am seeing a 32kHz 200mV peak to peak signal on the crystal.  200mV is smaller than we expected, but I do see that the electrical characteristics for the LFXO lists a peak to peak amplitude between 200mV and 1V. 

Is there a way increase that amplitude to the 1V end of the range?  Our application is monitoring the 32kHz clock signal, buffered by an op-amp, and were expecting that signal to be in the 1V peak to peak range.

Also, I've read in other discussion threads that if the 32kHz signal is not configured correctly that the soft device will not advertise.  I am seeing the device advertising when scanning; does that suggest that the clock actually is enabled correctly?