Timer event capture clarification

[Nick_RA]

I want to use a timer to generate an event at a rate of 16kHz. So just as a test I'm setting up TIMER4 and a CC event to toggle a gpio pin. As I understand the clock source should be 16MHz so I'm loading the CC register with 1000 to generate an interrupt @16kHz.

I'm expecting the gpio to toggle at a frequency if 16kHz (giving a 8 kHz squarewave) but I'm getting an actual output of 6.74 kHz on my PCA10040 board. Am I misunderstanding the operation of the timer module, or doing something wrong?

This is my timer initialization

void timers_init()
{
  NRF_TIMER4->MODE =      TIMER_MODE_MODE_Timer;
  NRF_TIMER4->PRESCALER = 0;                                    
  NRF_TIMER4->TASKS_CLEAR = 1;                                  
  NRF_TIMER4->BITMODE =   TIMER_BITMODE_BITMODE_32Bit;		
  NRF_TIMER4->CC[0] =     1000;
  NRF_TIMER4->INTENSET =  (TIMER_INTENSET_COMPARE0_Enabled << TIMER_INTENSET_COMPARE0_Pos);
  NVIC_EnableIRQ(TIMER4_IRQn);
  NRF_TIMER4->TASKS_START = 1;               // Start TIMER
}

and IRQ

void TIMER4_IRQHandler(void)
{
  if ((NRF_TIMER4->EVENTS_COMPARE[0] != 0) && ((NRF_TIMER4->INTENSET & TIMER_INTENSET_COMPARE0_Msk) != 0)) 
  {
    NRF_TIMER4->EVENTS_COMPARE[0] = 0; //Clear compare register 0 event
    NRF_TIMER4->TASKS_CLEAR = 1;
    NRF_TIMER4->TASKS_START = 1;
    nrf_gpio_pin_toggle(PIN_OUT);
  }
}

[Dmitry]

Hi,

your interrupt handler takes some time, that's why frequency is lower. Instead of restarting timer in interrupt handler, add a shortcut for clearing it by COMPARE[0] event (timer is not stopped on CLEAR task):

  NRF_TIMER4->SHORTS = TIMER_SHORTS_COMPARE0_CLEAR_Msk;

Also you can toggle a pin with GPIOTE module, without waking up CPU at all.

[haakonsh]

Dmitry said:

Also you can toggle a pin with GPIOTE module, without waking up CPU at all.

 The PPI — Programmable peripheral interconnect can connect any event from any peripheral to any task of any peripheral. In your case, you'll want to connect the CC[0] event to a GPIOTE channel's OUT task. 

Read the PPI and GPIOTE chapters in the Product specification, and study the examples/peripheral/ppi example. 

[Nick_RA]

This works, thanks. 

(@haakonsh thanks, but the GPIO is only for a test, to check the sample rate is as expected)