Purpose of external 32.768kHz clock

A 32kHz clock is required to use the low power modes on the CC2540/41 chips. There is an internal RC oscillator, which for BLE timing purposes is not accurate enough. Hence we either use an accurate external 32kHz clock or use a 32kHz crystal within the spec (as shown in the Datasheet).

the reason for using an external clock is to reduce the BOM cost of having an extra crystal. This step is only useful in boards that have other components which already have an accurate 32kHz clock that can be shipped and used to drive the 32kHz clock pins.

i believe one does not have to set anything in the SW, as the stack by default is looking for an external clock, be it crystal or an external 32kHz clock source.