Timer measurement using PPI on nRF51822

Question

Hi All

I'm trying to measure the Ton and the Toff of a signal present on an input pin of a nRF51822.

The idea is to use PPI, Timer 1 and save timer on every falling/rising edge of the input pin into CC[3]

I wrote a simple code , attached below, referring to : https://devzone.nordicsemi.com/f/nordic-q-a/8707/how-to-use-timer1-capture-function

int main(void)
{    
  Init_HFCLK();			// HF clock launching	
	Init_RTC();				// RTC Timer interrupt setting // DAFARE inserire timer per IR led
	Init_IO();				// I/O pinout setting
	Init_PPI();
			
	while(1)
	{
		__NOP();
	}
}

void Init_PPI(void)
{
    NRF_PPI->CH[1].EEP = (uint32_t)&NRF_GPIOTE->EVENTS_IN[0];
//		NRF_PPI->CH[1].TEP = (uint32_t)&NRF_TIMER1->TASKS_COUNT;
		NRF_PPI->CH[1].TEP = (uint32_t)&NRF_TIMER1->TASKS_CAPTURE[3];

    NRF_PPI->CHEN = (PPI_CHEN_CH0_Enabled << PPI_CHEN_CH0_Pos);
}

// RTC Timer interrupt setting
void Init_RTC(void)
{
  NRF_TIMER1->MODE = TIMER_MODE_MODE_Timer;  // set Timer mode

  NRF_TIMER1->TASKS_CLEAR = 1;               // clear the task first to be usable for later

	NRF_TIMER1->PRESCALER = 4;
	NRF_TIMER1->BITMODE = TIMER_BITMODE_BITMODE_24Bit; //Set counter 24 bit resolution
	
	NRF_TIMER1->CC[1] = 1000;
  NRF_TIMER1->CC[2] = 250; 
	
	NRF_TIMER1->INTENSET = (TIMER_INTENSET_COMPARE2_Enabled << TIMER_INTENSET_COMPARE2_Pos) | (TIMER_INTENSET_COMPARE1_Enabled << TIMER_INTENSET_COMPARE1_Pos);
		
	NVIC_EnableIRQ(TIMER1_IRQn);    
  NRF_TIMER1->TASKS_START = 1;   // Start TIMER
}


void Init_IO(void)
{
	// OUTPUT  	
	nrf_gpio_cfg_output(LED_FB);
	nrf_gpio_pin_clear(LED_FB);	

	// INPUT		
	nrf_gpio_cfg_input(IN_pin,NRF_GPIO_PIN_NOPULL);
	nrf_gpiote_event_config(0, IN_pin, NRF_GPIOTE_POLARITY_TOGGLE); //GPIOTE gives an interrupt on pin change
}

void Init_HFCLK(void)
{
//	NRF_CLOCK->XTALFREQ = 0x0;
	  NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while(NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
}
void TIMER1_IRQHandler (void)
{
	if ((NRF_TIMER1->EVENTS_COMPARE[2] != 0) && ((NRF_TIMER1->INTENSET & TIMER_INTENSET_COMPARE2_Msk) != 0))
  {
		NRF_TIMER1->EVENTS_COMPARE[2] = 0;
		
		NRF_TIMER1->CC[2] += 250;//TMR_BASE;	// retrigger compare interrupt
		
		nrf_gpio_pin_set(LED_FB);
  }
	
	if ((NRF_TIMER1->EVENTS_COMPARE[1] != 0) && ((NRF_TIMER1->INTENSET & TIMER_INTENSET_COMPARE1_Msk) != 0))
  {
		NRF_TIMER1->EVENTS_COMPARE[1] = 0;
		
		NRF_TIMER1->CC[1] += 1000;
		
		nrf_gpio_pin_clear(LED_FB);
				
//		NRF_TIMER1->TASKS_START = 1;
	}	
	
}

This is the first time I'm using the PPI and I'm a bit confused about its working.

It seems Timer1 interrupt works as expected, as far as LED_FB (output pin) toggles, but I supposed that, when the input signal changes its level value, from L to H or H to L, an interrupt on Timer 1 is triggered and capture timer is saved in CC[3]. This, actually, doesn't happen.

How do I have to modify the code to save capture timer in CC[3], or in some other register?

Thanks in advance for your support.

Kind regards

Cristian