Can the nRF52 output the 32.768kHz clock on a GPIO?

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Fri, 02 Sep 2016 10:17:47 GMT

i am not aware of any method that you can do it with the MCU.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Thu, 01 Sep 2016 13:56:54 GMT

I see; didn't realize the delayed clock was "recalibrated" every clock cycle, which in essence gives you a delayed RTC clock. I suppose this should work; thanks! There's no way to do the XOR internally to the MCU though, right?

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Thu, 01 Sep 2016 06:18:28 GMT

They should not drift, like you can see in the code, the rising and falling edge of the generated clock through RTC are resets for the generating the delayed clock edges (In a way the first one is calibrating the second delayed generated clock every cycle). So it should be perfectly calibrated every cycle. I verified it over 50 seconds log and i can confirm that, you can also try this and look in the signal analyzer.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Wed, 31 Aug 2016 14:46:25 GMT

XOR is a passive gate; it does not affect timing (other than prop delay).

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Wed, 31 Aug 2016 14:44:22 GMT

One is toggling a GPIO @ 16.384kHz; the other @ 16.394kHz, right? If you take a long capture, won't you see the pulses deviate from each other?

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Wed, 31 Aug 2016 06:33:44 GMT

are you sure they drift over time? these are outputs from two XTALs (HFCLK and LFCLK) you think XORing them will drift?

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Wed, 31 Aug 2016 01:15:11 GMT

I don' think that this will work; the two different clock frequencies (32.768kHz and 32.787kHz) drift over time.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Tue, 30 Aug 2016 10:31:16 GMT

Try this code

#define LFCLK_OUTPUT             (11U)
#define DELAYED_LFCLK_OUTPUT     (12U)


void start_timer(void)
{       
    
        /* Start 16 MHz crystal oscillator */
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART    = 1;

    /* Wait for the external oscillator to start up */
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
    {
        // Do nothing.
    }
    
  NRF_TIMER1->MODE = TIMER_MODE_MODE_Timer;  
  NRF_TIMER1->TASKS_CLEAR = 1;       
  NRF_TIMER1->PRESCALER = 0; 
  NRF_TIMER1->BITMODE = TIMER_BITMODE_BITMODE_16Bit;
  
    // Timer expires 1
  NRF_TIMER1->CC[0] = 244;  // expires  (1/4)*(1/16384) microseconds
 
}

void lfclk_config()
{
  NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
  NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
  NRF_CLOCK->TASKS_LFCLKSTART = 1;
  while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {}
      
  NRF_RTC1->PRESCALER = 0; //1kHz frequency
  NRF_RTC1->EVTEN     = (RTC_EVTENSET_TICK_Msk ); /* event on COMPARE0 and TICK */

}



int main(void) {

  lfclk_config();
    
  start_timer();

  // configure GPIOTE
  NRF_GPIOTE->CONFIG[0] = ( (GPIOTE_CONFIG_MODE_Task      << GPIOTE_CONFIG_MODE_Pos)
                          | (LFCLK_OUTPUT                  << GPIOTE_CONFIG_PSEL_Pos)
                          | (GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos) )
                          | (0 << GPIOTE_CONFIG_OUTINIT_Pos);

  // configure timer CC event to toggle  DELAYED_LFCLK_OUTPUT
  NRF_GPIOTE->CONFIG[1] = ( (GPIOTE_CONFIG_MODE_Task      << GPIOTE_CONFIG_MODE_Pos)
                          | (DELAYED_LFCLK_OUTPUT         << GPIOTE_CONFIG_PSEL_Pos)
                          | (GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos) )
                          | (1 << GPIOTE_CONFIG_OUTINIT_Pos);
    
  // configure PPI  channel 2 convert timer CC event to GPIOTE task out
  NRF_PPI->CH[2].EEP    = (uint32_t)(&NRF_TIMER1->EVENTS_COMPARE[0]);
  NRF_PPI->CH[2].TEP    = (uint32_t)(&NRF_GPIOTE->TASKS_OUT[1]);
  NRF_PPI->CHENSET    |= PPI_CHENCLR_CH2_Msk;  // enable   
  
  // configure PPI  
  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->CHENSET      = PPI_CHENCLR_CH0_Msk;


  // configure PPI  channel 1 to clear timer counter
  NRF_PPI->CH[1].EEP    = (uint32_t)(&NRF_RTC1->EVENTS_TICK);
  NRF_PPI->CH[1].TEP    = (uint32_t)(&NRF_TIMER1->TASKS_CLEAR);
  NRF_PPI->CHENSET    |= PPI_CHENCLR_CH1_Msk;  // enable   
        
  NRF_RTC1->TASKS_START = 1;
  NRF_TIMER1->TASKS_START = 1;               // Start TIMER1
  
    while (true)
    {
        __SEV();
        __WFE();
        __WFE();
    }
}

output is like below

XOR both outputs and you should have LFCLK with required accuracy. Hopefully the propagation delays within the chip for rerouting events are negligible.

I understand the TIMER1 (HFCLK) is used to create the second signal, but XORing it with first signal which is generated from RTC (LFCLK). So the signal that is generated by XORing these two should have better accuracy.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Mon, 29 Aug 2016 20:11:27 GMT

Let me try to do this tomorrow morning. Maybe we can do it.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Mon, 29 Aug 2016 14:14:31 GMT

I am not following your latest comment...in order to get a 32.768kHz clock I was thinking of XORing the 16.384kHz clock with a delayed version of itself. The link you posted shows how to get a 16.384kHz clock based on the LFCLK, right?

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Fri, 26 Aug 2016 16:12:43 GMT

That could potentially work if the LFCLK-GPIO can be XORed with another delayed LFCLK-GPIO. Can this be done internal to the nRF52?

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Fri, 26 Aug 2016 06:56:31 GMT

this might be of little help here.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Tosa — Thu, 25 Aug 2016 23:01:51 GMT

I don't see how to get 32.768 kHz from this solution. The timer runs off 16MHz (or 1MHz for slow f_timer). The closest I can get is 32.787kHz (if using 16MHz clock), which is over 500ppm off.

RE: Can the nRF52 output the 32.768kHz clock on a GPIO?

Susheel Nuguru — Thu, 25 Aug 2016 19:55:41 GMT

here it gives you an idea of how to generate 8Mhz clock from HFCLK using timer and gpiote. On nrf52 it should be the same logic. Ofcourse to get the lower frequency out you need higher prescaler and different value in TIMER CC