accuracy of application timer with schedular on nRF52

Question

I use the application timer with a schedular on the nRF52 with SoftDevice S132 1.0.0.3 alpha and SDK v0.9.2. 
 The application timer is running in repeated mode and should have a timeout of 1 ms. 
 Defines: 
 #define APP_TIMER_PRESCALER 0 
#define APP_TIMER_MAX_TIMERS (3 + BSP_APP_TIMERS_NUMBER) 
#define APP_TIMER_OP_QUEUE_SIZE 4 
 
 Initialize of the timer: 
 // Initialize timer module, making it use the scheduler
APP_TIMER_APPSH_INIT(APP_TIMER_PRESCALER, APP_TIMER_MAX_TIMERS, APP_TIMER_OP_QUEUE_SIZE, true);

err_code = app_timer_create(&m_data_timer_id, APP_TIMER_MODE_REPEATED, timer_data_timeout_handler);
APP_ERROR_CHECK(err_code);
 
 The timer is started with this code: 
 err_code = app_timer_start(m_data_timer_id, APP_TIMER_TICKS(1, APP_TIMER_PRESCALER), NULL);
APP_ERROR_CHECK(err_code);
 
 Every time the timout handler is called a GPIO PIN is toggeled.. 
 // Timeout handler for the repeated timer
static void timer_data_timeout_handler(void * p_context)
{
	nrf_drv_gpiote_out_set(PIN_OUT);
	nrf_drv_gpiote_out_clear(PIN_OUT);
	ms_counter++;
	
}
 
 When I measure the time between the peaks with a osciloscope the time between each peak is between 1 ms and 6 ms and not as expacted always 1 ms. 
 Have you any idea which causes that timeout handler is not called excatly every millisecond?

Susheel Nuguru · Accepted Answer

I am not sure of the exact accuracy of the app_timer library but I can for sure say that it does not add delay of 5ms. The interrupt handler for the app_timer is called from lower priority than the softdevice activity, so if there is some high priority activity going on in the softdevice, then your RTC handler will be delayed until that is finished. 
 If you want to quickly verify this, then run your app_timer code with softdevice disabled and you will see that the accuracy is acceptable.