Repeated app_timer vs RTC with maximum prescaler

RE: Repeated app_timer vs RTC with maximum prescaler

ovrebekk — Thu, 29 Aug 2019 13:11:19 GMT

[quote user="myNrfDoesntWork"]If I use a repeated app_timer of one second, shouldn't the RTC itself never overflow?[/quote]

It will overflow at one point, but the overflow is handled automatically by the app_timer library so that you don't have to think about it. The app_timer callbacks will continue to run as normal even after the overflow occurs.

I agree with your math. If you use a uint32 for counting seconds, and start running it now, it will not overflow until 136 years have past.

If you are thinking about reserving a full 20 bytes for the second counter then the sun should burn out long before your counter overflows ;)

Best regards
Torbjørn

RE: Repeated app_timer vs RTC with maximum prescaler

myNrfDoesntWork — Thu, 29 Aug 2019 07:00:50 GMT

Hi Torbjørn,

If I use a repeated app_timer of one second, shouldn't the RTC itself never overflow?

Using repeated app_timer I would be incrementing a counter variable every second, and an integer variable can overflow. But if the maximum unsigned integer value in C is 4294967295 then a counter variable of 4294967295 seconds would still be 136 years. If I am trying to transmit the value over BLE, then the maximum integer value is 2^(20 bytes*8 bits) - 1, which is still a lot of years before overflow.

Is my logic correct, or am I missing any important details? Thanks!

RE: Repeated app_timer vs RTC with maximum prescaler

ovrebekk — Thu, 29 Aug 2019 06:27:39 GMT

To me it seem the main difference between these two approaches is how often it overflows. At one point it will overflow regardless.

The main impact on average current consumption will be your wakeup frequency, and the choice of prescaler should not make a big difference.

I would suggest setting a wakeup frequency suitable for the sensor sampling and other things you need to do, and update a variable according to the wakeup frequency.

Best regards
Torbjørn

RE: Repeated app_timer vs RTC with maximum prescaler

myNrfDoesntWork — Wed, 28 Aug 2019 16:40:04 GMT

Hi Torbjørn,

Thanks, I will check it out! But are there any significant advantages or disadvantages with using a repeated app_timer with a duration of one/two seconds over using an app_timer with maximum prescaler (4095) & maximum duration of 582.542 hours?

RE: Repeated app_timer vs RTC with maximum prescaler

ovrebekk — Wed, 28 Aug 2019 07:04:40 GMT

I made a small calendar example for the nRF52 series a while back. There are no project files for the nRF51 unfortunately, but I don't think it should be a lot of work to make it run on the nRF51 instead.

It allows you to use the app_timer just as you describe, and fire of an interrupt every second. Then it will update an internal variable, and use the time.h library to decode this into a time and date.

The example can be found here:
https://github.com/NordicPlayground/nrf5-calendar-example

Best regards
Torbjørn