How does the nRF52840 indicate when HFXO is stable?

[kermit the fog]

I'm trying to measure the startup time of the 32 MHz XTAL at the pins XC1 and XC2.  Often a microcontroller will indicate in a register that the XTAL and internal PLL are stable either through an event, or interrupt, or status register.

How does the nRF52840 indicate that the XTAL is stable?

Another way to ask this is, how do I know what to set HFXODEBOUNCE to?

[Kenneth]

Hi,

The simplest may be something like shown below, you may also check that HFCLKSTAT register if external crystal is running:
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/clock.html?cp=2_1_0_18_2_3#register.HFCLKSTAT 

Kenneth

    // Stop external HFCLK if it's already running
    NRF_CLOCK->TASKS_HFCLKSTOP = 1;        
    nrf_delay_us(1000); // Just some random delay to ensure it's stopped    
    
    // Start HFCLK 
    // Note: The BLE softdevice require that the startup time is <1.5ms   
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART = 1;    
    while(NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
        ;    
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    
    // External HFCLK is now running