Would calculation affect timer

[mich_x]

I am using a power profiler to measure the timing of my program and I found something wrong. So I tested with a simple pure timer.

So first I set the timer during to 300ms

#define half_epoch_during		APP_TIMER_TICKS(300)
err_code = app_timer_create(&half_epoch_timer, APP_TIMER_MODE_SINGLE_SHOT, half_epoch_timer_handler);
err_code=app_timer_start(half_epoch_timer,half_epoch_during,NULL);

And I did nothing in the callback function

static void half_epoch_timer_handler (void * p_context)
{}

The timing is all right. I measure a periodic change in current every 300ms. Then I add a for loop to simulate calculations in the callback function.

static void half_epoch_timer_handler (void * p_context)
{
	int d;
	for (int i =0; i<3000000; i++)
	{	
		d = i;
	}
}

And now, the timing is wrong. I can measure a current change every 240ms. So my question is why would calculation make the timer shorter?

ps: My board is nRF52840 PDK.

[AmbystomaLabs]

Or so you don't have to interface to the device, just have it toggle a gpio and then you can measure the rate with an o-scope. I would assume you will see some jitter in the basic timing since the ble stack has highest interrupt priority but the basic rate should still be 300msec.