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 startup time. By default, this oscillator is selected to start the system and it must be selected prior to entering Wait mode.

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

The selection of the oscillator is made with bit CKGR_MOR.MOSCSEL. The switchover of the MAINCK source is glitch-free, so there is no need to run MCK out of SLCK, PLLACK or UPLLCK in order to change the selection. PMC_SR.MOSCSELS indicates when the switch sequence is done.

Setting PMC_IMR.MOSCSELS triggers an interrupt to the processor.

MAINCK Switching Sequence

When switching the Main Clock MAINCK source from the Main Crystal oscillator to the Main RC oscillator it is mandatory to follow the below steps:

Start the Main RC oscillator and keep MAINCK on the Main Crystal Oscillator (this step is optional at startup as it is the default configuration)
Switch MAINCK to the Main RC oscillator and keep the Main Crystal Oscillator on
Switch off the Main Crystal Oscillator is a third separate step