nRF52840 SDK16 - What is the practical purpose of prescaler in app_timer?

[Nikosant03]

Hi everyone,

I know that I can configure the resolution of app_timer by adjusting the prescaler form sdk_config.h

// <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 0
#endif

However, what is the practical purpose of prescaler? Could give me an example where is usefull to adjust the prescaler and change the timer's resolution?

Thanks

Nick

[Daniel Chisholm]

The prescaler give the option of reducing resolution in return for increasing the amount of time between overflows/wraparound.

If you are trying to timestamp something which happens quite slowly, a lower frequency clock (e.g. 1024 Hz) is just as fit for the purpose as the fastest setting (32768Hz) - for example, if you are recording the opening and closing of a door.

When your ticks happen 1024 times per second, a 16-bit timer value will overflow and wrap around every 64s, whereas a 32,768Hz clock will overflow/wrap every 2s.

[Nikosant03]

Thank you for your answer Daniel,

So in general is better to keep overflow time as high as possible? Is there any advantage of it?

[Einar Thorsrud]

I would say the answer is yes, ass long as you don't need a better time resolution. But in most cases this will not mean much in practice when using the app timer. The only reasons I can think of for wanting a low RTC frequency with the app timer is if you have very long timeouts making it necessary (at least with the old app timer implementation in app_timer.c).

[Nikosant03]

Thank you for your inputs guys!!