PXD20RM Datasheet, PDF (258/1628 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. 8-3
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
8-62
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
Freescale Semiconductor