APPTIMER works even after RTC rollover, why?

[haxxa]

This is a question of curiosity rather than a specific concern:

In my sdk_config.h I have my prescaller set to 0:

// <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

Based on this, the RTC1 used for AppTimer should rollover every 512 seconds:

http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Frtc.html

Yet I have an AppTimer set for 30 minutes which works? 

#define GAS_WRITE_MS (1000 * 60 * 30) ///< calibration delay for the gas sensor [ms].
app_timer_start(gas_calib_timer_id, APP_TIMER_TICKS(GAS_WRITE_MS), NULL);

Can anyone explain why this works? Could I have AppTimers set for weeks or Months? I thought the RTC1 prescaler would limit the longest rollover time permitted for each AppTimer.

[Einar Thorsrud]

Hi,

That is not supported by the app timer implementation as far as I know. Are you sure this works for you? If so, how have you verified it, which SDK version do you use, and have you modified the apptimer implementation?

(It would of course be possible to implement, for instance by counting how many times the RTC has wrapped around.)

[haxxa]

Yes I am sure it runs for 30 minutes, (I timed it directly with my phone).

I am using the latest SDK (15.2) with the NRF52 DK. In fact, I have seen the same code used with the Nordic Thingy: [ https://github.com/NordicSemiconductor/Nordic-Thingy52-FW/blob/master/source/modules/m_environment.c#L66 ] 

and it has the prescaler configured at 0 as well:
[https://github.com/NordicSemiconductor/Nordic-Thingy52-FW/blob/master/project/pca20020_s132/config/sdk_config.h#L4588].

I have made no modifications to the AppTimer Code. 

I must be overlooking something?

[Einar Thorsrud]

Hi,

I have looked a bit more at this, and it turns out that the app timer implementation is inconsistent in this case. A timer with a timeout >= 2^24 ticks will work provided that another timer with a shorter timeout is running. If not, it will not work. I have reported this internally, so that it can be improved or at the very least documented in a feature SDK release.

[haxxa]

Interesting, are there any limits to the timer exceeding 2^24 ticks? Could it run for a week or longer?, [given I had a second repeating Timer (within the 2^24 ticks rollover limit)]?

[Einar Thorsrud]

Hi,

There does not seem to be any limitations other than that what follows from the fact that the tick count used in the apptimer module is 32 bit. However, using such long timeouts is a bit dodgy as it makes one app timer instance dependent on the other, based on some intricacies in the module itself. Also, the behavior could change in a updated SDK.