PXD10RM Datasheet, PDF (218/1332 Pages) Freescale Semiconductor, Inc – PXD10 Microcontroller

8.9.7 Recommendations
To avoid any unpredictable behavior of the PLL clock, it is recommended to respect the following
guidelines:
• The PLL VCO frequency should reside in the range 256 MHz to 512 MHz. Care is required when
programming the multiplication and division factors to respect this requirement.
• The user must change the multiplication, division factors only when the PLL output clock is not
selected as system clock. MOD_PERIOD, INC_STEP, SPREAD_SEL bits should be modified
before activating the FM modulated mode. Then strobe has to be generated 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 PLL is in power down mode.
• Use progressive clock switching.
8.10 Clock Monitor Unit (CMU)
8.10.1 Introduction
The Clock Monitor Unit (CMU), also referred to as Clock Quality Checker or Clock Fault Detector, serves
two purposes. The main task is to permanently supervise the integrity of the device’s system clock sources,
i.e., crystal oscillator FXOSC, FIRC, and FMPLL0. If FMPLL0 leaves an upper or lower frequency
boundary or the crystal oscillator fails it can detect and forward this kind of event towards the mode and
clock management unit. The clock management unit in turn can then switch to a safe mode where it uses
a safe fallback clock source such as an on-chip RC oscillator, reset the device or generate the interrupt
according to the system needs.
It can also monitor external crystal oscillator clock which must be greater than the internal RC clock
divided by a division factor given by RCDIV[1:0] of CMU_CSR register and generates a system clock
transition request or an interrupt when enabled.
The second task of the CMU is to provide a frequency meter, which allows to measure the frequency of
one clock source vs. a reference clock. This is useful to allow the calibration of the on-chip RC
oscillator(s), as well as being able to correct/calculate the time deviation of a counter which is clocked by
the RC oscillator.
NOTE
The CMU does not monitor SXOSC, SIRC, or FMPLL_1.
8-40
PXD10 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
Freescale Semiconductor