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Multipurpose Clock Generator (MCG)
MCF51CN128 Reference Manual, Rev. 6
Freescale Semiconductor
6-13
6.4.2
MCG Mode State Diagram
shows the MCG’s mode state diagram. The arrows indicate the permitted mode transitions.
PLL Bypassed
External (PBE)
• MCGC1[IREFS] = 0
• MCGC1[CLKS] = 10
• MCGC1[RDIV] is programmed
to divide the reference clock to
be within the range of 1 to
2 MHz.
• MCGC2[LP] = 0
• MCGC3[PLLS] = 1
MCGOUT is derived from the external reference clock; the PLL is
operational, but its output clock is not used. This mode is useful to allow
the PLL to acquire its target frequency while MCGOUT is driven from the
external reference clock.
MCGOUT is derived from the external reference clock. The external
reference clock that is enabled can be produced by an external crystal,
ceramic resonator, or another external clock source connected to the
required crystal oscillator (XOSC). The PLL clock frequency locks to a
multiplication factor, as specified by MCGC3[VDIV], times the external
reference frequency, as specified by MCGC1[RDIV], MCGC2[RANGE],
and MCGC3[DIV32]. If the BDM is enabled, MCGLCLK is derived from
the DCO (open-loop mode) divided by two. If the BDM is not enabled, the
FLL is disabled in a low-power state.
Bypassed Low
Power Internal
(BLPI)
• MCGC1[IREFS] = 1
• MCGC1[CLKS] = 01
• MCGC2[LP] = 1 (and the BDM
is disabled)
MCGOUT is derived from the internal reference clock.
The PLL and FLL are disabled, and MCGLCLK is not available for BDC
communications. If the BDM becomes enabled, the mode switches to
one of the bypassed internal modes as determined by the state of
MCGC3[PLLS].
Bypassed Low
Power External
(BLPE)
• MCGC1[IREFS] = 0
• MCGC1[CLKS] = 10
• MCGC2[LP] = 1 (and the BDM
is disabled)
MCGOUT is derived from the external reference clock. The external
reference clock that is enabled can be produced by an external crystal,
ceramic resonator, or another external clock source connected to the
required crystal oscillator (XOSC).
The PLL and FLL are disabled, and MCGLCLK is not available for BDC
communications. If the BDM becomes enabled, the mode switches to
one of the bypassed external modes as determined by the state of
MCGC3[PLLS].
Stop
—
Entered when the MCU enters a Stop state. The FLL and PLL are
disabled, and all MCG clock signals are static except in the following
cases:
MCGIRCLK is active in Stop mode when all the following conditions
become true:
• MCGC1[IRCLKEN] = 1
• MCGC1[IREFSTEN] = 1
MCGERCLK is active in Stop mode when all the following conditions
become true:
• MCGC2[ERCLKEN] = 1
• MCGC2[EREFSTEN] = 1
Table 6-10. MCG Modes of Operation (continued)
Mode
Related field values
Description