I want to keep running RTC even when nrf51 is in sleep mode,is there any way ?

[Pranay]

I am running RTC in nrf51, after coming out of sleep mode RTC is getting reset.I want this RTC to keep running so that we can use it as wake up signal.

[David]

The RTC peripheral will continue to run as normal if you use System On sleep.

The following code works for me as a 30 second wakeup timer.

Initialize Timer:

NRF_RTC0->TASKS_STOP = 1;							// Stop if running for whatever reason
nrf_delay_us(75);									// We have to wait at least 1 LFCLK tick for stop to take effect.

NRF_RTC0->PRESCALER = 32;							// ~1ms
NRF_RTC0->CC[0] = 29789;							// 30.0 seconds
NRF_RTC0->INTENSET = RTC_INTENSET_COMPARE0_Msk;
NRF_RTC0->TASKS_CLEAR = 1;
NRF_RTC0->TASKS_START = 1;							// Start timer immediately.

NVIC_EnableIRQ(RTC0_IRQn);							// Enable IRQ

Sleep (if SD Disabled):

// Wait for interrupt (System ON sleep)
__WFI();   

Sleep (if SD Enabled)

sd_app_evt_wait();

And you need the interrupt handler for RTC0

void RTC0_IRQHandler(void)
{
	if ((NRF_RTC0->EVENTS_COMPARE[0] != 0) &&
        ((NRF_RTC0->INTENSET & RTC_INTENSET_COMPARE0_Msk) != 0))
    {
		// Do stuff here

		
		// Clear CC flag
		NRF_RTC0->EVENTS_COMPARE[0] = 0;
    }
}

[Pranay]

how can we configure system on mode?

[David]

You don't have to configure anything. Just use one of the two sleep functions from my post.