Timer1 NRF_TIMER1->CC[1] not having an effect

[Mahesh Venkitachalam]

I am using the nRF51-DK and have Timer1 setup as follows:

void start_timer(void)
{		
  NRF_TIMER1->MODE = TIMER_MODE_MODE_Timer;  
  NRF_TIMER1->TASKS_CLEAR = 1;       
  NRF_TIMER1->PRESCALER = 4; 
  NRF_TIMER1->BITMODE = TIMER_BITMODE_BITMODE_16Bit;
  NRF_TIMER1->CC[1] = 1000;
		
  // Enable interrupt on Timer 2, both for CC[0] and CC[1] compare match events
  NRF_TIMER1->INTENSET = (TIMER_INTENSET_COMPARE1_Enabled << TIMER_INTENSET_COMPARE1_Pos);

  NVIC_EnableIRQ(TIMER1_IRQn);
		
  NRF_TIMER1->TASKS_START = 1;               // Start TIMER1
}
		

void TIMER1_IRQHandler(void)
{
	if (NRF_TIMER1->EVENTS_COMPARE[1] && 
          NRF_TIMER1->INTENSET & TIMER_INTENSET_COMPARE1_Msk) {

  
    //Clear compare register event	
    NRF_TIMER1->EVENTS_COMPARE[1] = 0;

    if(nrf_gpio_pin_read(21)) {
      nrf_gpio_pin_clear(21);
    }
    else {
      nrf_gpio_pin_set(21);
    }
           
  }
}

I have an oscilloscope connected to pin 21, and am finding that no matter what I set to NRF_TIMER1->CC[1], the pulse width remains the same.

I am also finding that for NRF_TIMER1->PRESCALER set to 0, I am getting a pulse of period ~8 ms. That seems incorrect. Again, NRF_TIMER1->CC[1] has no effect.

Overall I think I am not setting up the timer correctly. I'd appreciate any help with this.

I am using this code with SoftDevice S110.

(Please note that I don't want to use the nrf_drv_timer methods as I find them to be confusing, and would rather use the registers so I can understand the feature consistent with the data sheet.)

Thanks

[Susheel Nuguru]

you have the enable the SHORT that clears the the TIMER value after CC[0] event is triggered. Else no matter what you set in CC register, the timer will go circular over 16bit CC register

[Mahesh Venkitachalam]

Thanks, Aryan. I guess calling NRF_TIMER1->TASKS_CLEAR = 1; manually also would work? So what is NRF_TIMER1->EVENTS_COMPARE[1] = 0; doing? It's clearing the compare event, but it doesn't effect the timer counting?

[Susheel Nuguru]

With prescalar 0, 1 timer tick is 62 nano second long. If you put 1000 in CC, it should generate event in 62 micro second. Since you are toggling pin at every event, your pin period will be 125 micro second.

[Mahesh Venkitachalam]

Great! I am seeing 0.138 ms. I appreciate your patience in explaining this to me. One more question - I had to call start_timer() after starting BLE advertizing, or it doesn't advertize. Is that expected?

[Susheel Nuguru]

I do not understand that dependency. Make sure that you are not using timer1 for something else.

[Mahesh Venkitachalam]

OK, thanks.