Multipurpose Clock Generator (MCG)
MCF51CN128 Reference Manual, Rev. 6
6-20
Freescale Semiconductor
b) Loop until OSCINIT (bit 1) in MCGSC is 1, indicating the crystal selected by the EREFS bit
has been initialized.
c) Because RANGE = 1, set DIV32 (bit 4) in MCGC3 to allow access to the proper RDIV bits
while in an FLL external mode.
d) MCGC1 = 0x98 (%10011000)
– CLKS (bits 7 and 6) set to %10 to select external reference clock as system clock source
– RDIV (bits 5-3) set to %011, or divide-by-256 because 8MHz / 256 = 31.25 kHz that is in
the 31.25 kHz to 39.0625 kHz range required by the FLL
– IREFS (bit 2) cleared to 0, selecting the external reference clock
e) Loop until IREFST (bit 4) in MCGSC is 0, indicating the external reference is the current
source for the reference clock
f) Loop until CLKST (bits 3 and 2) in MCGSC is %10, indicating that the external reference
clock is selected to feed MCGOUT
2. Then, FBE must transition directly to PBE mode or first through BLPE mode and then to PBE
mode:
a) BLPE: If a transition through BLPE mode is desired, first set LP (bit 3) in MCGC2 to 1.
b) BLPE/PBE: MCGC3 = 0x58 (%01011000)
– PLLS (bit 6) set to 1, selects the PLL. At this time, with an RDIV value of %011, the FLL
reference divider of 256 is switched to the PLL reference divider of 8 (see
),
resulting in a reference frequency of 8 MHz/ 8 = 1 MHz. In BLPE mode,changing the PLLS
bit only prepares the MCG for PLL usage in PBE mode
– DIV32 (bit 4) remains set at 1. Because the MCG is in a PLL mode, the DIV32 bit is ignored.
Keeping it set at 1 makes transitions back into an FLL external mode easier.
– VDIV (bits 3-0) set to %1000, or multiply-by-32 because 1 MHz reference * 32= 32MHz.
In BLPE mode, the configuration of the VDIV bits does not matter because the PLL is
disabled. Changing them only sets up the multiply value for PLL usage in PBE mode
c) BLPE: If transitioning through BLPE mode, clear LP (bit 3) in MCGC2 to 0 here to switch to
PBE mode
d) PBE: Loop until PLLST (bit 5) in MCGSC is set, indicating that the current source for the
PLLS clock is the PLL
e) PBE: Then loop until LOCK (bit 6) in MCGSC is set, indicating that the PLL has acquired lock
3. Lastly, PBE mode transitions into PEE mode:
a) MCGC1 = 0x18 (%00011000)
– CLKS (bits7 and 6) in MCGSC1 set to %00 to select the output of the PLL as the system
clock source
– Loop until CLKST (bits 3 and 2) in MCGSC are %11, indicating that the PLL output is
selected to feed MCGOUT in the current clock mode
