RE: second revision nRF51422 + S210: RUN mode consumption

Stefan Birnir Sverrisson — Tue, 25 Mar 2014 10:14:38 GMT

The low frequency 32kHz clock is required by RTC and WDT timers as table 28 states. It is not required by other peripherals. The S210 softdevice (ANT stack) uses RTC timer so it needs a low frequency clock source (LFCLK) to run it.
Yes, the S210 ANT stack requires +-50ppm. Using +-25ppm 16MHz crystal and synthesize the LFCLK to that should give +-33ppm synthesized LFCLK, as you suggested, which is sufficient for ANT. Ole Morten’s comments are perhaps more valid for BLE than ANT since the ANT stack can not use the internal RC as that provides only 250ppm accuracy. So for ANT, you are left with two options for the LFCLK, either to have external 32kHz crystal with +-50ppm accuracy or better, or you select the synthesized clock and consume the extra current (~1mA).
As mentioned before, the synthesized 32kHz clock will add around 1mA of extra current compared to when using external 32kHz crystal. This is because it will require both 16MHz crystal and 1.2V internal regulator to be enabled. As you have more peripheral activity on the chip, the current consumption difference between 32kHz crystal and synthesized clock will decrease as the peripherals will anyway require the 16MHz clock and the internal 1.2V regulator. With your 16Hz broadcast frequency and having SPI enabled periodically to receive data for broadcast, I assume that you will need to utilize 16MHz clock source + 1.2V regulator less than 10% so the current consumption increase of using the 32kHz synthesized clock instead of crystal would still be close to 1mA