Is it possible to dynamically detect presence of low frequency crystal for Soft Device?

Question

Is it possible to detect the presence of a Low Frequency Crystal and configure softdevice to use if present, otherwise use the internal RC oscillator? 
 A proposal would be to probe presence of the crystal prior to sd_enable() and change the LFCLKSRC definition to use the crystal or RC depending on the result. Would this pattern work?

koniho · Accepted Answer

It looks like this is related (though for nrf51) https://devzone.nordicsemi.com/f/nordic-q-a/56844/in-our-product-we-would-like-to-select-lfclk-from-the-following-choices-instead-of-external-xtal-however-we-have-some-manufacturing-issues-with-the-xtal-chosen-and-so-we-muct-use-the-lf_src-to-be-the-rc/231321#231321

Code posted there is:

NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
NRF_CLOCK->TASKS_LFCLKSTART = 1;

// Wait for the low frequency clock to start
nrf_delay_ms(1000);

if(NRF_CLOCK->EVENTS_LFCLKSTARTED == 1)
{
// hurray there is an external LFCLK present.
// you can set external LFCLK when enable softdevice
NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
}
else
{
// no external lfclk present, you can use internal LFCK when enable softdevice
}

Vidar Berg · Answer

Hello,

Yes, the approach above should work. Also, if it makes sense for your application, you may consider making this a one-time check by storing the detected clock source to flash for subsequent startups. This will allow you to reduce the startup time and power consumption a bit.

Something like this (code is untested)

#include "nrf_delay.h"

/* Make sure selected UICR CUSTOMER[n] register is not used elsewhere
 * in Application or Bootloader code.
 */
#define UICR_CLOCK_SETTING_ADDRESS NRF_UICR->CUSTOMER[0] 
#define UICR_CLOCK_SRC_XTAL        0x1
#define UICR_CLOCK_SRC_RC          0x2

static bool lfclk_is_xtal_available(void)
{
    bool has_xtal = false;

    /* Check if we need to update the UICR register. True on first boot */
    if (UICR_CLOCK_SETTING_ADDRESS == 0xFFFFFFFF)
    {

        NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
        NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
        NRF_CLOCK->TASKS_LFCLKSTART = 1;

        nrf_delay_ms(1000);

        if (NRF_CLOCK->EVENTS_LFCLKSTARTED == 1)
        {
            // hurray there is an external LFCLK present.
            // you can set external LFCLK when enable softdevice
            NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
            has_xtal = true;
        }

        /* Use NVMC to update the UICR register with out LF clk source setting */
        NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy)
        {
        }
        UICR_CLOCK_SETTING_ADDRESS = (has_xtal == true) ? UICR_CLOCK_SRC_XTAL : UICR_CLOCK_SRC_RC;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy)
        {
        }
        NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy)
        {
        }
    }
    else
    {
        has_xtal = (UICR_CLOCK_SETTING_ADDRESS == UICR_CLOCK_SRC_XTAL) ? true : false;
    }

    return has_xtal;
}

Is it possible to dynamically detect presence of low frequency crystal for Soft Device?

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