How to deal with a long time's Clock drift?

[mahone]

Hi,

I use to the nrf51422 with nordic's dev/ev kit.I want to build a project and the purpose is to get the sensor's date and use the ant protocol to deliver the date to another chip every 5 hours.The rest of the time the chip will go to sleep.But recently I test two board's clock drift,I find the clock will drift 1s every 10min.And my test is simple,I use the way: while(1) {
nrf_gpio_pin_set(Pin); nrf_delay_ms(1000); nrf_gpio_pin_clear(Pin); nrf_delay_ms(1000); } I don't know is there any solution can solve this problem? Thanks!

[Stefan Birnir Sverrisson]

Hi Mahone

Thanks Mike for providing a lot of good information. I would however like to add a few things.

For your original test with: while(1) { nrf_gpio_pin_set(Pin); nrf_delay_ms(1000); nrf_gpio_pin_clear(Pin); nrf_delay_ms(1000); }

then you are testing the accuracy of the internal 16MHz RC clock source. Internal 16MHz RC is the clock source for the CPU unless you explicitly start the 16MHz external crystal. The tolerance of the internal 16MHz RC oscillator can be up to plus/minus 5% and is specified in nRF51822 PS v2.0, table 21.

If you would like to do nothing for 5 hours except keep track of time, RTC is much better choice in terms of current consumption, as Mike has already mentioned. The internal 32kHz RC oscillator has default tolerance of plus/minus 2%, but 250ppm when calibrated. The drawback is that calibration adds ~10uA to the current consumption. So the best choice for you would be either to use the internal RTC with a 32kHz external crystal, or as per Mikes suggestion, to use an external RTC. The current consumption is 3.1uA when 32kHz crystal and internal RTC is enabled. If you use external RTC, you could let nRF51 enter System Off and then wake up again on a GPIO level change from the external RTC. System Off consumes 0.6uA with no RAM retention.

If you have softdevice enabled and are using application timers, you could use an application timer to keep track of time, which uses the RTC1 peripheral in the background.

More on the nRF51 internal 32kHz clock sources is here and devzone.nordicsemi.com/.../how-to-minimize-current-consumption-for-ble-application-on-nrf51822

[Mike]

What about the power-consumption when using the application timer? Isn't the usage of the application timer bound to a higher sleep level?

Does application timer not have any jitter? So the accuracy would be the accuracy of the RTC clock source?