Hi. The demo project use a softdevice and hence the high frequency clock is generated by the external 16 MHz crystal. BLE needs a high accuracy HF clock and that is why an external HF crystal is required.
The low frequency clock source is set in the function SOFTDEVICE_HANDLER_INIT(). In the demo the clock source is set to NRF_CLOCK_LFCLKSRC_XTAL_20_PPM by default. I.e. the demo is made for a kit with a 32 kHz external crystal with an accuracy of 22 PPM.
