LFCLK accuracy in smart beacon kit

[oceanide]

Dear all,

I have a question on the LFCLK accuracy in the smart beacon kit, which is pca20006. When I see the specification of LFCLK crystal 32.768kHz, this crystal has +-20ppm.

From smart beacon reference source v1.0.1, the ble_stack_init function is initialized as below: This means that LFCLK accuracy has +-20ppm.

SOFTDEVICE_HANDLER_INIT(NRF_CLOCK_LFCLKSRC_XTAL_20_PPM, true);

But, smart beacon reference source v1.1.0, the LFCK accuacry is changed from 20 to 50ppm.

SOFTDEVICE_HANDLER_INIT(NRF_CLOCK_LFCLKSRC_XTAL_50_PPM, true);

I think that the LFCLK accuracy should be initialized with 20ppm. I want to know why the LFCLK accuracy is changed in the latest smart beacon reference source even though the hardware is not changed.

Thanks in advance

oceanide

[Locky]

Likely for greater timing error margin with Smart phones. Alot of todays smartphones don't have the most accurate LFCLKs. Pretty sure the iPhone 6 doesn't even properly follow the SIG's BLE timing

[Asbjørn]

The change is done in order to have slightly better tolerances towards central devices that behaves outside or at the edge of the timing specification defined by Bluetooth SIG. If you configure to have a slightly worse crystal than you have the softdevice will compensate for the worse tolerance by extending the listening window slightly. So you'll pay in the sense of slightly more current consumption. It's less important if the beacon is only used a beacon, if it's used during connection to configure the beacon or do a DFU, this could come in to play.