19.6.2 Spread Spectrum

Spread spectrum is performed by modifying the target frequency of the PLL at its input clock rate. Two parameters are used to configure the spread spectrum:

• STEP—the size of the step that will be applied to the FRACR input of the PLL
• NSTEP—the number of times the STEP will be applied to the FRACR input of the PLL

The spread spectrum can be applied only if the PLL is already enabled and locked. Once the spread spectrum has been enabled, it is no longer possible to modify the target frequency of the PLL. The spread spectrum must first be disabled and (2 x NSTEP) cycles of the source clock of the PLL must be waited.

Starting from the base frequency of the PLL configured in PMC_PLL_CTRL1, the spread spectrum mechanism reduces FRACR by (8 x STEP) units at each rising edge of the source clock of the PLL. If FRACR is too low, MUL is automatically modified to match the new PLL frequency. This subtraction is performed NSTEP times. Then the FSM increments FRACR by (8 x STEP) units until it reaches the initial PLL frequency value (as configured in the PMC_PLL_CTRL1). If FRACR is too high, MUL is automatically modified to match the new PLL frequency.