34.6.4 Spread Spectrum

Spread spectrum is obtained by slightly modifying the PLL target frequency. Two parameters are used to configure the spread spectrum:

• STEP—the frequency step
• NSTEP—the number of times the STEP will be applied

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. Prior to change the PLL frequency, the spread spectrum must be disabled and a period of 2 x NSTEP cycles of the PLL source clock must elapse.

When enabled, the spread spectrum logic modifies the fractional part of the PLL. The fractional factor applied to the PLL is in the following range: FRACR - (64 x STEP x NSTEP) up to FRACR.

Starting from the base frequency of the PLL configured in PMC_PLL_CTRL1 (MUL, FRACR), the spread spectrum mechanism decreases the PLL frequency, and when the minimum is reached, the PLL frequency is increased up to the value configured through the PMC_PLL_CTRL1 register (the PLL frequency never overpasses that value).