LFCLK (32kHz) on a GPIO pin for precision measurement?

[Sébastien Despont]

Dear Nordic Developer Zone,

Is there a way to set the LFCLK (32kHz) on a GPIO for precision measurement?

Best regards

[Audun]

Hi Sebastien,

yes, you can connect the LFCLK to a GPIO using the RTC and GPIOTE:

For example: RTC tick event can be configured to trigger a GPIOTE task which toggles a pin. See code example below which starts up the internal 32 kHz RC and configures RTC Tick event to toggle gpio 0:

// Start 32 kHz clock
NRF_CLOCK->TASKS_LFCLKSTART = 1;

// Configure GPIOTE to toggle gpio 0
NRF_GPIOTE->CONFIG[0] = (GPIOTE_CONFIG_MODE_Task       << GPIOTE_CONFIG_MODE_Pos)     | 
                        (GPIOTE_CONFIG_OUTINIT_Low     << GPIOTE_CONFIG_OUTINIT_Pos)  |
                        (GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos) | 
                        (0                             << GPIOTE_CONFIG_PSEL_Pos);

// Configure RTC1 to generate tick event
NRF_RTC1->PRESCALER = 0;
NRF_RTC1->EVTENSET  = RTC_EVTENSET_TICK_Msk;

// Connect RTC1 tick event to GPIOTE toggle task
NRF_PPI->CH[0].EEP = (uint32_t) &NRF_RTC1->EVENTS_TICK;
NRF_PPI->CH[0].TEP = (uint32_t) &NRF_GPIOTE->TASKS_OUT[0];
NRF_PPI->CHEN      = PPI_CHENCLR_CH0_Msk;
                     
// Start RTC1
NRF_RTC1->TASKS_START = 1;

while(1)
{
}

The pulse width of the signal measured on gpio 0 corresponds to LFCLK frequency (1 / 32768 seconds).

Note that the internal RC needs to be calibrated (using calibration task NRF_CLOCK->TASKS_CAL) to achieve 250 ppm accuracy. External 16 MHz crystal should be running during calibration for best results.

[Sébastien Despont]

Thanks for your answer.

[Susheel Nuguru]

i think you get only half of the LFCLK here on GPIO. 1 TICK is one toggle on GPIO which converts to two RTC ticks converting to one cycle on GPIO.

[Yashur]

I'm measuring only 16.67KHz using the above code! :(

[Stephlj33]

Hello - is this method also applicable to the nRF52840?