Suggestions for low power time keeping on nRF51?

RE: Suggestions for low power time keeping on nRF51?

Nicolas Brunner — Wed, 09 Aug 2017 08:18:48 GMT

Doing the following can result in error when the RTC overflow:

total_time = RAM_counter << 24 | RTC_counter;

If you are doing the read in a lower interrupt priority than RTC overflow interrupt, you can do this:

do {
  RAM_counter_copy = RAM_counter;
  total_time = RAM_counter_copy << 24 | RTC_counter;
} while (RAM_counter_copy != RAM_counter);

RE: Suggestions for low power time keeping on nRF51?

Roger Clark — Fri, 20 Jan 2017 19:57:06 GMT

Thanks Torbjørn.

I will try using that method.

Re: Time drift

I think the drift in the crystal is well within the limits of what I need, and I am not using the value to calculate an accumulated value, but only to broadcast an instantaneous indication of flow rate, based on frequency of pulses, which would normally occur at a maximum frequency of 10Hz

This data is encoded into the manufacturer data of the advertising packet, which is broadcast once per second ( to give longer battery life)

RE: Suggestions for low power time keeping on nRF51?

ovrebekk — Fri, 20 Jan 2017 14:46:27 GMT

Hi Roger

You can definitely use the RTC counter value to keep track of the 32kHz clock. The RTC counter is 24 bits, which means it will overflow every 512 seconds when using the smallest prescaler (for maximum accuracy).

When it overflows you can wake up, and increment a counter in RAM.

To read out the current value you just have to concatenate the counter with the current RTC value:

total_time = RAM_counter << 24 | RTC_counter;

The time of each 32kHz clock is ~30.5us, so this will be the limit of your accuracy. If you want to keep track of absolute time you also have to consider the drift of your clock source. As an example a 20ppm 32k crystal would introduce a potential drift of 1.7 seconds per day.

Best regards
Torbjørn