MC9S08DN60_08 Datasheet, PDF (145/356 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 8 Multi-Purpose Clock Generator (S08MCGV1)
8.4.3 Bus Frequency Divider
The BDIV bits can be changed at anytime and the actual switch to the new frequency will occur
immediately.
8.4.4 Low Power Bit Usage
The low power bit (LP) is provided to allow the FLL or PLL to be disabled and thus conserve power when
these systems are not being used. However, in some applications it may be desirable to enable the FLL or
PLL and allow it to lock for maximum accuracy before switching to an engaged mode. Do this by writing
the LP bit to 0.
8.4.5 Internal Reference Clock
When IRCLKEN is set the internal reference clock signal will be presented as MCGIRCLK, which can be
used as an additional clock source. The MCGIRCLK frequency can be re-targeted by trimming the period
of the internal reference clock. This can be done by writing a new value to the TRIM bits in the MCGTRM
register. Writing a larger value will decrease the MCGIRCLK frequency, and writing a smaller value to
the MCGTRM register will increase the MCGIRCLK frequency. The TRIM bits will effect the MCGOUT
frequency if the MCG is in FLL engaged internal (FEI), FLL bypassed internal (FBI), or bypassed low
power internal (BLPI) mode. The TRIM and FTRIM value is initialized by POR but is not affected by other
resets.
Until MCGIRCLK is trimmed, programming low reference divider (RDIV) factors may result in
MCGOUT frequencies that exceed the maximum chip-level frequency and violate the chip-level clock
timing specifications (see the Device Overview chapter).
If IREFSTEN and IRCLKEN bits are both set, the internal reference clock will keep running during stop
mode in order to provide a fast recovery upon exiting stop.
8.4.6 External Reference Clock
The MCG module can support an external reference clock with frequencies between 31.25 kHz to 5 MHz
in FEE and FBE modes, 1 MHz to 16 MHz in PEE and PBE modes, and 0 to 40 MHz in BLPE mode.
When ERCLKEN is set, the external reference clock signal will be presented as MCGERCLK, which can
be used as an additional clock source. When IREFS = 1, the external reference clock will not be used by
the FLL or PLL and will only be used as MCGERCLK. In these modes, the frequency can be equal to the
maximum frequency the chip-level timing specifications will support (see the Device Overview chapter).
If EREFSTEN and ERCLKEN bits are both set or the MCG is in FEE, FBE, PEE, PBE or BLPE mode,
the external reference clock will keep running during stop mode in order to provide a fast recovery upon
exiting stop.
If CME bit is written to 1, the clock monitor is enabled. If the external reference falls below a certain
frequency (f_{loc_high} or f_{loc_low} depending on the RANGE bit in the MCGC2), the MCU will reset. The LOC
bit in the System Reset Status (SRS) register will be set to indicate the error.
MC9S08DN60 Series Data Sheet, Rev 3
Freescale Semiconductor
145