PXD20RM Datasheet, PDF (250/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

The reference clock will be always the FXOSC. The Frequency Meter returns a precise value of CK_32K,
CK_FIRC or CK_SIRC according to the CLKSEL1 bit value. The measure starts when the SFM (Start
Frequency Measure) bit in the CMU_CSR is set to ‘1’. The measurement duration is given by the
CMU_MDR register in numbers of clock cycles of the selected clock source with a width of 20 bits. The
SFM bit is reset to ‘0’ by the hardware once the frequency measurement is done and the count is loaded
in the CMU_FDR. The frequency FRC can be derived from the value loaded in the CMU_FDR register as
follows:
FRC = (FOSC * MD) / n,
Eqn. 8-2
Where n is the value in the CMU_FDR register and MD is the value in the CMU_MDR.
Frequency Meter by default evaluates CK_FIRC, but the software can swap to CK_SIRC or CK_SXOSC
by programming the CLKSEL bits in the CMU_CSR register.The CKON bits indicate which is the actual
clock at the output of the multiplexer MUX1.
8.7 Clock Monitor Unit (CMU)
8.7.1 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.
8-54
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
Freescale Semiconductor