LFCLKSRC crystal PPM accuracy

[Matt Barr]

Our application includes a 32.768 kHz crystal oscillator with a nominal +/- 20 PPM accuracy. When I factor in aging over 10 years and worst case variation due to temperature it could theoretically get to -250 PPM.

There are a variety of enumerated NRF_CLOCK_LFCLKSRC_XTAL_x_PPM initializers, and I'm not sure if I am best served by a nominal 20 PPM, a more typical 150 PPM, or a pessimistic 250 PPM initializer.

How is the specified PPM accuracy used by the softdevice, and what would be the correct choice in this situation?

Thanks and Best Regards, Matt

[Matt Barr]

I notice that nRF51-DK projects typically use 20 PPM:

SOFTDEVICE_HANDLER_INIT(NRF_CLOCK_LFCLKSRC_XTAL_20_PPM, false);

The ant examples use 50 PPM:

err_code = sd_softdevice_enable(NRF_CLOCK_LFCLKSRC_XTAL_50_PPM, softdevice_assert_callback);

[Stefan Birnir Sverrisson]

Hi

BLE requires 500 ppm in order to be within specification and in order to work. You should sum up all of the accuracy parameters of the crystal to get the total ppm accuracy. If nominal ppm, temperature variation and aging add up to worst case 500ppm or less, that is sufficient for BLE.

ANT has 50ppm reqirement so if you intend to use ANT as well, you should have a higher accuracy crystal.

[Matt Barr]

Thank you for the answer Stefan, I appreciate the information on PPM requirements.