how to implement accurate milisecond function

[VIKS]

I am new to nrf51822. I am trying to convert my Arduino code into NRF51822

I need something like this

currentMillis = micros(); //microseconds since the Arduino board began running if(currentMillis-past_time>=1000) { do something }

What is the best way to do this in NRF51822? Should I use RTC or go with simple timer? Can anyone share their function if they have already implemented?

Thanks

[Clem Taylor]

I modified app_timer.c to include a overflow count and then I added an access function to (hopefully) atomically access the current overflow count and the RTC1 counter value. From there you can compute seconds and microseconds or whatever you need. I do all my long term timekeeping stuff in seconds & ticks and my short term timekeeping in RTC1 ticks.

One quirk is you always need to keep at least one timer active or the app_timer code will disable RTC1 to save power. I happen to have a timer that is always running, so this wasn't an issue for me.

My access code is:

void app_timer_ticks(uint32_t *p_overflow, uint32_t *p_ticks)
{
    volatile uint32_t overflow0, overflow1;
    uint32_t ticks;
    
    /* spin trying to sample clean counters */
    do
    {
        overflow0 = m_overflow_count;
        ticks = NRF_RTC1->COUNTER;
        overflow1 = m_overflow_count;
    } while (overflow0 != overflow1); // unlikely 
    
    *p_overflow = overflow1;
    *p_ticks    = ticks;
}

I'd imagine this still has a small race window where COUNTER is sampled after it rolls over but before m_overflow_count is updated by the interrupt. Reading the COUNTER twice to make sure the second read is larger then the next might close the window, but I haven't tried this.

I increment m_overflow_count on EVENTS_OVRFLW in RTC1_IRQHandler() and turn on the overflow event in rtc1_start().

This might be a useful feature to add to a future version of the SDK.