Chapter 4 Clock Description
MPC5602P Microcontroller Reference Manual, Rev. 4
96
Freescale Semiconductor
4.8.6.4
Powerdown mode
To reduce consumption, the FMPLL can be switched off when not required by programming the registers
ME_x_MC on the ME module.
4.8.7
Recommendations
To avoid any unpredictable behavior of the PLL clock, it is recommended to follow these 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.
4.9
Clock Monitor Unit (CMU)
4.9.1
Overview
The Clock Monitor Unit (CMU) serves three purposes:
•
PLL clock monitoring: detects if PLL leaves an upper or lower frequency boundary
•
XOSC clock monitoring: monitor the XOSC clock, which must be greater than the IRCOSC clock
divided by a division factor given by CMU_CSR[RCDIV]
•
Frequency meter: measure the frequency of the IRCOSC clock versus the reference XOSC clock
frequency
When mismatch occurs in the CMU either with the PLL monitor or the XOSC monitor, the CMU notifies
the RGM, ME and the FCU (Fault Collection Unit) modules. The default behavior is such that a reset
occurs and a status bit is set in the RGM. The user also has the option to change the behavior of the action
by disabling the reset and selecting an alternate action. The alternate action can be either entering safe
mode or generating an interrupt.
Table 4-9. CMU module summary
Module
Monitored clocks
CMU_0
• XOSC integrity supervisor
• FMPLL_0 integrity supervisor
• IRCOSC frequency meter