App Timer assert - APP_TIMER_SAFE_WINDOW

Question

I have started getting an assert in my code while testing new functionality.

I am using SDK 17.1.0 and the nRF52840 DK

The assert is line 321 in app_timer2.c

        /* If assert fails it suggests that safe window should be increased. */
        ASSERT(app_timer_cnt_diff_compute(drv_rtc_counter_get(p_instance),
                                          drv_rtc_compare_get(p_instance, 0)) < APP_TIMER_SAFE_WINDOW);

Looking at the the description of APP_TIMER_SAFE_WINDOW_MS in sdk_config.h

// <o> APP_TIMER_SAFE_WINDOW_MS - Maximum possible latency (in milliseconds) of handling app_timer event.
// <i> Maximum possible timeout that can be set is reduced by safe window.
// <i> Example: RTC frequency 16384 Hz, maximum possible timeout 1024 seconds - APP_TIMER_SAFE_WINDOW_MS.
// <i> Since RTC is not stopped when processor is halted in debugging session, this value
// <i> must cover it if debugging is needed. It is possible to halt processor for APP_TIMER_SAFE_WINDOW_MS
// <i> without corrupting app_timer behavior.

#ifndef APP_TIMER_SAFE_WINDOW_MS
#define APP_TIMER_SAFE_WINDOW_MS 300000
#endif

Am I correct in thinking this means that this will assert if, for (1024-300) seconds, or ~12 minutes, the app_timer event is not handled?

If so, how do I go about debugging which timer isn't handled?

I initially saw this after leaving a board running for around 36hrs. I think I saw it again, didn't get the error log unforunately, after over 24 hours. I have tried to set up a test to replicate is and saw it withing 4-5 hours. It is still in sat in this state with the debugger attached but I don't know where to begin with working out the cause.

The debugger output is:

[03624616] <error> app: ERROR 3735928559 [Unknown error code] at nRF5_SDK_17.1.0_ddde560/components/libraries/timer/app_timer2.c:321
PC at: 0x000362B3
[03624623] <error> app: End of error report

I want to understand why this is occuring, why it isn't after a fixed period of time and whether increasing APP_TIMER_SAFE_WINDOW_MS is a fix or a bandaid.

Rost · Answer

:) Finally I found something reasonable and repeated the issue with debugging, the CC is incorrectly set as described here by Krzysztof . 
 The fix would be as shown in post above in SDK17.1 in file app_timer2.c:321 
 ASSERT(app_timer_cnt_diff_compute(drv_rtc_counter_get(p_instance), 
 mp_active_timer->end_val & RTC_COUNTER_COUNTER_Msk) < APP_TIMER_SAFE_WINDOW);

App Timer assert - APP_TIMER_SAFE_WINDOW

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