Calculating ppm offset between the 32khz and the 16mhz clocks

RE: Calculating ppm offset between the 32khz and the 16mhz clocks

Einar Thorsrud — Tue, 31 Jul 2018 14:02:57 GMT

The way I read your code, you should only be affected by the compare-compare jitter as the timer is started and stopped by compare events via PPI. You could perhaps let the test run for a longer time to get a more exact average.

If the accuracy you have measured is correct, then there could be an issue with your LF oscillator. Are you using a crystal or the internal RC? If you use the internal RC, then you should calibrate it regularly. If you are using a crystal, then you need to make sure that you have proper load capacitors.

RE: Calculating ppm offset between the 32khz and the 16mhz clocks

Omri Friedman — Mon, 30 Jul 2018 08:22:53 GMT

I have implemented the ppm test code mentioned above, but i get offset of ~4000ppm between the 32768hz and the 16mhz clocks.

I read this http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Frtc.html and wondered if that has to do with my results.

I saw in the link 15 to 46 μs delay on task START and +/- 15 μs on "START to COUNTER increment" and +/- 62.5 ns on "COMPARE to COMPARE".

I start the RTC and use CC[0] and CC[1] to trigger capture of TIMER2 (via PPI). then i use the delta to calculate ppm offset.

Which of the above jitters/delays should i expect? just the +/- 62.5 ns?

RE: Calculating ppm offset between the 32khz and the 16mhz clocks

Omri Friedman — Tue, 24 Jul 2018 06:00:58 GMT

Thanks a lot.

I have written a quick code without testing it yet.

Is it in line with your suggestion?

void ppm_test(void)
{
  // user RTC2 to count 32khz tick via its compare registers 
  NRF_RTC2->PRESCALER = 0;
  NRF_RTC2->EVTENSET = RTC_EVTENCLR_COMPARE0_Enabled | RTC_EVTENCLR_COMPARE1_Enabled;
  NRF_RTC2->CC[0] = 1;  // get 1st tick 
  NRF_RTC2->CC[1] = 21; // get 21th tick 
  
  // get interrupt on RTC2 CC[1] 
  NRF_RTC2->INTENSET = RTC_INTENSET_COMPARE1_Enabled; 
  NVIC_ClearPendingIRQ(RTC2_IRQn);
  NVIC_EnableIRQ(RTC2_IRQn);
  
  // use TIMER2 capture counting 16mhz ticks  
  NRF_TIMER2->MODE = (TIMER_MODE_MODE_Timer << TIMER_MODE_MODE_Pos); 
  NRF_TIMER2->BITMODE = TIMER_BITMODE_BITMODE_32Bit;
  NRF_TIMER2->PRESCALER = 0; // 16mhz 
  NRF_TIMER2->TASKS_CLEAR = 1;

  // use ppi to hook RTC compare and to timer capture 
  NRF_PPI->CH[0].EEP = (uint32_t) &NRF_RTC2->EVENTS_COMPARE[0]; 
  NRF_PPI->CH[0].TEP = (uint32_t) &NRF_TIMER2->TASKS_CAPTURE[0]; 
  NRF_PPI->CH[1].EEP = (uint32_t) &NRF_RTC2->EVENTS_COMPARE[1]; 
  NRF_PPI->CH[1].TEP = (uint32_t) &NRF_TIMER2->TASKS_CAPTURE[1]; 
  NRF_PPI->CHENSET = (PPI_CHENSET_CH0_Enabled << PPI_CHENSET_CH0_Pos) | 
                     (PPI_CHENSET_CH1_Enabled << PPI_CHENSET_CH1_Pos); 

  // enable it all 
  NRF_TIMER2->TASKS_START = 1; 
  NRF_RTC2->TASKS_START = 1; 
}

// interrupt on compare 1 
void TIMER2_IRQHandler(void)
{
  NVIC_ClearPendingIRQ(TIMER2_IRQn);
  NVIC_DisableIRQ(TIMER2_IRQn);

  int delta = NRF_TIMER2->CC[1] - NRF_TIMER2->CC[0];
  
  // in there are 100000 16mhz ticks in 20 32khz tick (as if it is a 1.6khz)
  ppm = (delta - 100000) * ( 1000000 / 100000 );

  // disable it all 
  NRF_TIMER2->TASKS_STOPT = 1; 
  NRF_RTC2->TASKS_STOP = 1; 
  NRF_PPI->CHENCLR = (PPI_CHENCLR_CH0_Clear << PPI_CHENCLR_CH0_Pos) | 
                      PPI_CHENCLR_CH1_Clear << PPI_CHENCLR_CH1_Pos);
}

RE: Calculating ppm offset between the 32khz and the 16mhz clocks

Turbo J — Mon, 23 Jul 2018 17:49:07 GMT

You can try connecting the COMPARE0/COMPARE1 events in an RTC (I'd use RTC2) to the CAPTURE[0]/CAPTURE[1] task in a timer via two PPI channels. Most examples leave TIMER2 instance free to use. The time will be measured in the CC[1]-CC[0] difference of the running timer.