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

[Matt Barr]

Here's another solution using Nordic's app_timer API to RTC1.

uint32_t watchdog_timer;
uint32_t app_timer_cur;
uint32_t app_timer_diff;

/* Set watchdog timer */
app_timer_cnt_get(&watchdog_timer);

/* Other processing occurs... */

/* Check for a watchdog timeout */
app_timer_cnt_get(&app_timer_cur);
app_timer_cnt_diff_compute(app_timer_cur, watchdog_timer, &app_timer_diff);
if (app_timer_diff > APP_TIMER_TICKS(WATCHDOG_TMOUT,APP_TIMER_PRESCALER))
{
    /* Watchdog timeout */
}

WATCHDOG_TIMOUT is in milliseconds, here it is a constant but it could be a variable.

The cnt_diff_compute function handles timer range and rollover. This of course has the same constraint that RTC1 is running when you are tracking elapsed time.

[Clem Taylor]

My problem was the 24 bit counter with a 32khz clock rolls over every 512s. I needed to keep track of time for the power on life of the device. You divide the 32khz down but it depends on how much resolution you need.