Which HF clock will be chosen by BLE stack?

[nordicdev]

Hi, In my code, I have not started the HF clock. I call the ble_stack_init where "SOFTDEVICE_HANDLER_INIT(NRF_CLOCK_LFCLKSRC_XTAL_20_PPM, NULL);" is called. Which HF clock will be chosen at what PPM by the micro if I don't deliberately start the HF clock like this:

	NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;

	NRF_CLOCK->TASKS_HFCLKSTART = 1;


     while(NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);

[Stian Røed Hafskjold]

The code in your question does not really start the HF clock. It starts the HF crystal. The HF clock is started automatically whenever it is requested by a peripheral (e.g. radio). If the crystal is not running, the internal RC will provide the clock source.

The Softdevice requires an external crystal because the internal RC is not accurate enough for the BLE spec. infocenter.nordicsemi.com/.../ext_reqs.html

The Softdevice starts the crystal automatically and makes sure that it's stable before any radio transmission.

You can see a plot of the SD advertising here: devzone.nordicsemi.com/.../ The second field "ramp" is when the crystal is starting up. And the standby after this is to make sure that it is completely stable.

The maximum PPM allowed for the HF crystal is given here, infocenter.nordicsemi.com/.../clock.html, and is 40 ppm for BLE.

[Stian Røed Hafskjold]

For 30µs the HFINT should be good enough (internal HF clock, not the crystal). Remember that the app_timer library for instance uses the LFCLK (RTC peripheral) which is only at 32kHz. You should use the TIMER peripheral which is using the HFCLK. The timer starts the HFINT automatically, and there's no need to start the crystal. Just let the SD start the crystal by itself when it needs it, and the TIMER can run on the HFINT without conflicting.