MPC5604B/C Microcontroller Reference Manual, Rev. 8
Freescale Semiconductor 111
or the crystal oscillator fails it can detect and forward these kind of events towards the MC_ME and
MC_CGM. The clock management unit in turn can then switch to a SAFE mode where it uses the default
safe clock source (FIRC), reset the device or generate the interrupt according to the system needs.
It can also monitor the external crystal oscillator clock, which must be greater than the internal RC clock
divided by a division factor given by CMU_CSR[RCDIV], 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.
6.8.2 Main features
• FIRC, SIRC, SXOSC oscillator frequency measurement using FXOSC as reference clock
• External oscillator clock monitoring with respect to FIRC_clk/n clock
• FMPLL clock frequency monitoring for a high and low frequency range with FIRC as reference
clock
• Event generation for various failures detected inside monitoring unit
6.8.3 Block diagram
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