PXD20RM Datasheet, PDF (243/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

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.
IRCOSC_CLK
16 MHz
FXOSC_CLK
4–16 M
FMPLL_0
64 MHz
CK 0 (reference)
CK FXOSC _CMU
CK PLL
OLR
FLL
FXOSC valid (on AND stable)/off
FMPLL_0 valid (on AND locked)/off
MC_CGM
Loss of crystal
FMPLL_0 freq.
out of range
FCU
Figure 8-33. CMU component interaction
8.6.2 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
• Event generation for various failures detected inside monitoring unit
Freescale Semiconductor
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
8-47