s110 and max value of app timer

[Adam]

Hi,

What is the largest possible value of "MAX_VAL" for application timer given below:

"#define APP_TIMER_PRESCALER 0"

"#define M_DELAY APP_TIMER_TICKS(MAX_VAL, APP_TIMER_PRESCALER)"

Is it 32 bit value?

[Clem Taylor]

The RTC timer is only 24 bits and app_timer does all its timing modulo 2^24. So the largest value for your M_DELAY is (2^24)-1. However due to the way the modulo counting is done the max value is a bit smaller then this. The value I use is:

#define RTC_TICKS_BITS                24 // size of RTC[01] counter in bits 
#define TIMER_MAX_TIMEOUT \
    ((1 << RTC_TICKS_BITS) - APP_TIMER_MIN_TIMEOUT_TICKS * 10) // fudge

How much time this represents is a function of the prescaler chosen. APP_TIMER_TICKS converts from miliseconds to ticks.

The max value in miliseconds is something like:

  (2**24) * (APP_TIMER_PRESCALER+1) * 1000 / 32768 - fudge

This works out to <512 seconds for prescaler of 0.

Personally I think app_timer could really use a thorough rewrite. It wouldn't take much for it to support timeout intervals longer then the roll-over period of the timer and in the process the code could be greatly simplified. Only one timer is available for the user and I need it to provide both a monotonic real time counter (which requires a small prescaler), and timeouts/time scheduled events (which requires a larger prescaler for longer intervals) .

[Adam]

Many thanks for explanation. So the easiest way to get timeout > 512 seconds is to increase APP_TIMER_PRESCALER, but on the other hand if e.g. APP_TIMER_PRESCALER=4095 I can only write "MAX_VAL" taht is multiple value of 125 ms counter period? As you mentioned the problem appears when I have a few timers that need to generate different delays (e.g 1ms and a few hours). Is there any good solution for this problem? (or should I use small prescaler and start timer many times in timeout_handler in a conditional loop to get the bigger delay).

[Clem Taylor]

Short of rewriting app_timer, I'm not sure what the best option is. I ended up using a prescaler of 0 because I needed to use RTC1 for a 'not-quite high enough precision' monotonic counter and I gave up on doing long timers directly with app_timer.

At one point I started to rewrite app_timer with something that better matched my needs but decided I didn't want to put the time (sic) to properly test it.

[Susheel Nuguru]

Hi Clem,

What is this fudge factor you are talking about?

[Clem Taylor]

At the time it was unclear to me if the app_timer code would accept a timeout up near the limit of 2**24 ticks, so it seemed like a good idea to add a little bit of headroom.

But then I found that I was having timers misfire when the timeout was > 2**(24-1) ticks. See: devzone.nordicsemi.com/.../

Now I never use a timer > 2**23 ticks.