4.16.6.4 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 changing 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 t

he 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).

Figure 4-62. Spread Spectrum Mechanism
When enabling the spread spectrum function, several restrictions appear on the FRACR value that lead to modifying FRACR by one unit on very specific configurations. Observe the following sequence to ensure proper operation of the PLL:
  1. Calculate the PLL core frequency (fCOREPLLCK) parameters (MUL and FRACR) as described in Divider and Phase Lock Loop Programming.
  2. Calculate the spread spectrum parameters (STEP and NSTEP) as described above.
  3. Depending on the FRACR, STEP and NSTEP values, the following four cases must be considered to verify whether FRACR needs adjustment or not:
    1. If (FRACR = 222 - 1), configure FRACR = FRACR – 1.
    2. If (FRACR > NSTEP x STEP x 64), no change is required on FRACR.
    3. If (FRACR + k x 222 = p x STEP x 64)(1), configure FRACR = FRACR + 1.
    4. If (FRACR + 1 + k x 222 = p x STEP x 64)(1), configure FRACR = FRACR - 1.
  4. Program the PLL according to the FRACR value found in step 3.
  5. Program STEP and NSTEP.
  6. Enable the PLL.
  7. Enable the spread spectrum(2).
Note:
  1. k is zero or a positive integer, and p is an integer in the [1,NSTEP] range.
  2. The spread spectrum can be enabled only if the PLL is already running.