PXS20RM Datasheet, PDF (905/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Frequency-Modulated Phase-Locked Loop (FMPLL)
to enable the new settings. If STRB_BYP is set to 1 then MOD_PERIOD, INC_STEP and
SPREAD_SEL can be modified only when FMPLL is in power down mode.
• Use progressive clock switching for the system clock.
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
27-9