21.5.5.1.2 PLL Reference selection

The PLL controller supports three independent sources of reference clock:

• DFLL48M: The reference clock is provided by the DFLL48M.
• XOSC: The reference clock is provided by the External Multipurpose Crystal Oscillator (XOSC).
• GCLK: the reference clock is provided by the Generic Clock Controller.

The reference source can be selected by setting the REFSEL bits in the PLLCTRL register. The frequency of the reference must be comprised between 4 MHz and 60 MHz.