Timer value not accurate using app_timer.c in nRF51822?

[Mo A]

Hi All

I am facing one issue using app_timer.c, When I put the PRESCALER value as 327 and started timer as app_timer_start(timer_ID, 100, NULL);, It was giving only 920ms instead of 1s? why is this so?

Here is my code

#include <stdint.h>
#include "nrf.h"
#include "app_timer.h"
#include "app_error.h"
#include "nrf_gpio.h"

//	Timer settings
#define APP_TIMER_PRESCALER             327		
#define APP_TIMER_MAX_TIMERS            1     	
#define APP_TIMER_OP_QUEUE_SIZE         1     

static app_timer_id_t         timer_ID;


void app_error_handler(uint32_t error_code, uint32_t line_num, const uint8_t * p_file_name)
{
}


void timeOut(void * p_context)
{
	nrf_gpio_pin_toggle(4);
	
}


int main(void)
{  
  	 nrf_gpio_cfg_output(4);


	//	TIMER init	
	APP_TIMER_INIT(APP_TIMER_PRESCALER, APP_TIMER_MAX_TIMERS, APP_TIMER_OP_QUEUE_SIZE, false);
	NRF_CLOCK->TASKS_LFCLKSTART = 1;
	
	while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0);

	app_timer_create(&timer_ID, APP_TIMER_MODE_REPEATED, timeOut);

	app_timer_start(timer_ID, 100, NULL);
	
  while(true)
  {
		
  }

}

Please help me on this, Mo

[FormerMember]

The basis of using a 32.768kHz frequency for RTC is that it is a power of 2. So, having a PRESCALAR of 255 and waiting for 128 ticks would result in accurate 1s interval.

[FormerMember]

If you are using a development board of sorts, it will most likely have crystals for both LF and HF clock. Then adding a line specifying the clock source for LFCLK will make it use the crystal oscillator. The code snippet would be (edited, source must be specified before starting):

NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos); NRF_CLOCK->TASKS_LFCLKSTART = 1; while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0);