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.

[MartinBL]

I have tested with PCA10056 running the app_ble_template example and S140 v5.0.0-2.alpha and I'm still not able to reproduce this. With a logic analyzer I can still see that the interrupt triggers at 300ms intervals. With the PPK though, I do see some variations in the order of 10-20ms. However, I talked to the PPK HW guy, and he told me that this is to be expected because the time axis in the PPK software is not that accurate. Especially if you are running the software on Windows 10.