Clock Description
MPC5606S Microcontroller Reference Manual, Rev. 7
224 Freescale Semiconductor
• 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,
that is, 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 to 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, resets the device, or generates the interrupt
according to the system needs.
It can also monitor an external crystal oscillator clock, the speed of which must be greater than the internal
RC clock divided by a division factor given by the RCDIV[1:0] field of the CMU_CSR register, and when
enabled, generates a system clock transition request or an interrupt.
The second task of the CMU is to provide a frequency meter, which allows measuring the frequency of
one clock source as compared to 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.
Figure 8-28. CMU component interaction
8.10.2 Main features
• RC oscillator frequency measurement
• External oscillator clock monitoring with respect to CK_FIRC/n clock
• PLL clock frequency monitoring with respect to CK_FIRC/4 clock
CMU
CK 0 (reference)
CK FXOSC
CK PLL
OLR
FLL
IRCOSC_CLK
16 MHz
FXOSC_CLK
4–16 MHz
FMPLL_0
64 MHz
FXOSC valid (on AND stable)/off
FMPLL_0 valid (on AND locked)/off
MC_CGM
FCU
Loss of crystal
FMPLL_0 freq.
out of range
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