The PLL has two modes of operation: normal and power-down. The modes are programmed using the PLLPWR bit in the PLLFREQ0 register (see Section 4.2.19).
Normal: The PLL oscillates based on the values in the PLLFREQ0 and PLLFREQ1 registers and drives the output.
Power-down: Most of the PLL internal circuitry is disabled and the PLL does not drive the output.