The application can request the SoftDevice to guarantee that the HFCLK source is set to the external crystal and that it is ramped up and stable before the start of the timeslot.
If the application requests the SoftDevice to have the external high-frequency crystal ready by the start of the timeslot, the SoftDevice will handle all the enabling and disabling of the crystal. The application does not need to disable the crystal at the end of the timeslot. The SoftDevice will disable the crystal after the end of the timeslot unless the SoftDevice needs to use it within a short period of time after the end of the timeslot. In that case, the SoftDevice will leave the crystal running.
If the application does not request the SoftDevice to have the external high-frequency crystal ready by the start of the timeslot, then the application must not use the RADIO during the timeslot and must take into consideration that the HFCLK source is inaccurate during the timeslot unless the application itself makes sure that the crystal is ramped up and ready at the start of the timeslot. If the application starts the crystal before or during the timeslot, it is the responsibility of the application to disable it again.