28.5.3 Main Clock Source Selection

The source of MAINCK can be selected from the following:

• the main RC oscillator
• the main crystal oscillator
• an external clock signal provided on the XIN input (Bypass mode of the main crystal oscillator)

The advantage of the main RC oscillator is its fast start-up time. By default, this oscillator is selected to start the system and it must be selected prior to entering ULP1 mode.

The advantage of the main crystal oscillator is its high level of accuracy.

The selection of the oscillator is made by configuring CKGR_MOR.MOSCSEL. The switchover of the MAINCK source is glitch-free, thus the switchover can be performed even if MCK is fed by MAINCK. PMC_SR.MOSCSELS indicates when the switch sequence is done.

Setting PMC_IMR.MOSCSELS triggers an interrupt to the processor.