App timer prescaler

[Chris]

I know this should be simple, but the prescaler is confusing me...

If I initialize my timers with a prescaler of 0, APP_TIMER_INIT(0, ...)

Then the max timer I can have is 512 seconds?

What if I do: app_timer_start(timer_id, 58982400, NULL)? Will that give me a 30 minute timer or not?

Thanks.

[FormerMember]

Yes. I hope you call app_timer_create to create timer_id.

[Chris]

By yes, do you mean I will get a 30 minute timer? Or that 512 seconds is the max and anything above that could be unpredictable?

Yes, I forgot to include that I call app_timer_create. I mean, the program works, but I have not sat down and actually timed anything longer than 512 seconds.

[FormerMember]

Sorry, my bad. 512 seconds is max with 0 prescalar. You need a prescalar of 3 so that LFCLK is divided by 4, which will give enough to count for 30 min. I've to sleep soon I guess. ;)

[RK]

no it'll give you a 264 second timer. Everything internally is '&' with the maximum timer count which is 0x00FFFFFF so you'll end up with 58982400 mod 16777216 timer ticks.

[Susheel Nuguru]

Just an addition to RK answer APP_Timer is 24 bit. The number you mentioned 58982400(0X3840000) does not fit, trying to write it will write (0X840000) which is 8650752 timer ticks. With prescaler 0 each tick is 30.516us. So like RK mentioned you will end of with 264 second timer.

If you change the prescaler say to 7, then there are 4096 ticks per seconds and 7372800(0X7080000) ticks for 30 minutes. This should fit into the timer well.