MC9S08DZ128 Datasheet, PDF (189/458 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 8 Multi-Purpose Clock Generator (S08MCGV2)
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 Table 8-3),
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) still 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 in order to select the output of the PLL as the
system clock source
b) 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
– Now, With an RDIV of divide-by-8, a BDIV of divide-by-1, and a VDIV of multiply-by-32,
MCGOUT = [(8 MHz / 8) * 32] / 1 = 32 MHz, and the bus frequency is MCGOUT / 2, or
16 MHz
MC9S08DZ128 Series Data Sheet, Rev. 1
Freescale Semiconductor
189