Decrease the timer resolution of SDK 14

[Atul Bansal]

How can we decrease the resolution of the timer to 500 ms in SDK 14? The minimum resolution I could get to was 1 ms by changing the APP_TIMER_CONFIG_RTC_FREQUENCY macro to 31. 

Thanks

Atul

[superpak]

The RTC peripheral has a 12 bit prescaler. So the max value for prescaler will be (2^12) - 1 = 4095. This will give you a counter resolution of 125 ms with an overflow of 582.542 hours.

If you want the l̶o̶w̶e̶s̶t̶ highest counter resolution, you can set the prescaler (APP_TIMER_CONFIG_RTC_FREQUENCY) to 0. This will give you a counter resolution of 30.517 us and an overflow of 512 seconds.

[Atul Bansal]

The counter resolution of 30.517 us is the highest resolution right, not the lowest one. 

Is there any way in which I can get a resolution of 500 ms? And even if I make do with the 125 ms resolution, where should I change the PRESCALER register? I didn't see any such command in the documentation of SDK 14. I could change the APP_TIMER_CONFIG_RTC_FREQUENCY macro, but in the description, there are some values provided to us, and I suppose, only those can be used as the possible values of the macro. I am attaching the code from the sdk_config file. 

// <o> APP_TIMER_CONFIG_RTC_FREQUENCY  - Configure RTC prescaler.
 
// <0=> 32768 Hz 
// <1=> 16384 Hz 
// <3=> 8192 Hz 
// <7=> 4096 Hz 
// <15=> 2048 Hz 
// <31=> 1024 Hz 

#ifndef APP_TIMER_CONFIG_RTC_FREQUENCY
#define APP_TIMER_CONFIG_RTC_FREQUENCY 31        
#endif

[superpak]

The app_timer library sets the prescaler for you. Look at the rtc1_init(..) routine in app_timer.c

The APP_TIMER_CONFIG_RTC_FREQUENCY is the prescaler value for app_timer library.

static void rtc1_init(uint32_t prescaler)
{
    NRF_RTC1->PRESCALER = prescaler;
    NVIC_SetPriority(RTC1_IRQn, RTC1_IRQ_PRI);
}

This is called by app_timer_init(...)

ret_code_t app_timer_init(void)
{
.....

rtc1_init(APP_TIMER_CONFIG_RTC_FREQUENCY);

....
}

[Atul Bansal]

Cool. Got it! But 500 ms resolution is impossible then? 125 ms is the minimum we could get?

[superpak]

Yes, I believe so. What is the reasoning for having a 500 ms resolution? Just curious!

[Atul Bansal]

Want to send the timestamp values in the advertisement packets and I don't want to have large timer values as the packet sizes are very small. About half a second resolution would have been pretty good. But, I guess I will try to work with 125 ms.