events (interrupts) from timer

Question

Dear sirs! 
 I need that timer (for example timer 3 or 4) will generate interrupt each 5 ms very precisely with precision of crystal (20 ppm). In 10 hours I cannot allow deviation more than 1 second. 
 I cannot reload timer registers in interrupt routine because it will cause unpredictable delays. I need that timer will reset to 0 automatically when arrive compare register value. Or may be there is another way to do it 
 
 What is the best way to do it with lower power consumption? Do you have any example? 
 Unfortunately, I cannot find how to do it.

user14926 · Accepted Answer

Hi, 
 The TIMER peripheral have SHORTS from each COMPAREx event to the CLEAR task, which means that the TIMER will be cleared when the event is generated, if the corresponding short is enabled. This happens in HW, which means there is no delay from handling it in interrupt handler. 
 You can enable the short using the timer driver API nrf_drv_timer_extended_compare ()/ nrfx_timer_extended_compare () , where you pass NRF_TIMER_SHORT_COMPAREx_CLEAR_MASK to the 4th argument ( timer_short_mask) . 
 Note that the TIMER peripheral is not particularly low power, as it requires the 16 MHz HFCLK to be running. The current consumption will typically be in the 3-500 uA range. 
 To achieve low power, you need to use the RTC peripheral, which runs off the 32.768 kHz LFCLK. This will give you much lower resolution (~30.5 us), which may not be enough for your requirements. You can set the PRESCALER to 163, which will give you a period of 164/32768=0.00500488281s, or ~4.88us off your desired period for each event. For a 10 hour period, this will accumulate to ~35s (10h*60min*60sec*200periods/s*4.88us=35.14s). This is without accounting for the accuracy of the crystal/clock source. 
 Best regards, Jørgen

events (interrupts) from timer

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