HCS08 datasheet(199/374 Pages) NXP

199 / 374 page

Multi-Purpose Clock Generator (S08MCGV1)

MC9S08JM60 Series Data Sheet, Rev. 5

Freescale Semiconductor

199

c) MCGC1 = 0xB8 (%10111000)

– CLKS (bits 7 and 6) set to %10 in order to select external reference clock as system clock

source

– RDIV (bits 5-3) set to %111, or divide-by-128 because 4 MHz / 128 = 31.25 kHz which 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

d) Loop until IREFST (bit 4) in MCGSC is 0, indicating the external reference is the current

source for the reference clock

e) Loop until CLKST (bits 3 and 2) in MCGSC are %10, indicating that the external reference

clock is selected to feed MCGOUT

2. Then, FBE must transition either 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: MCGC1 = 0x90 (%10010000)

– RDIV (bits 5-3) set to %010, or divide-by-4 because 4 MHz / 4 = 1 MHz which is in the 1

MHz to 2 MHz range required by the PLL. In BLPE mode, the configuration of the RDIV

does not matter because both the FLL and PLL are disabled. Changing them only sets up the

the dividers for PLL usage in PBE mode

c) BLPE/PBE: MCGC3 = 0x44 (%01000100)

– PLLS (bit 6) set to 1, selects the PLL. In BLPE mode, changing this bit only prepares the

MCG for PLL usage in PBE mode

– VDIV (bits 3-0) set to %0100, or multiply-by-16 because 1 MHz reference * 16 = 16 MHz.

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

d) BLPE: If transitioning through BLPE mode, clear LP (bit 3) in MCGC2 to 0 here to switch to

PBE mode

e) PBE: Loop until PLLST (bit 5) in MCGSC is set, indicating that the current source for the

PLLS clock is the PLL

f) PBE: Then loop until LOCK (bit 6) in MCGSC is set, indicating that the PLL has acquired lock

3. Last, PBE mode transitions into PEE mode:

a) MCGC1 = 0x10 (%00010000)

– CLKS (bits7 and 6) in MCGSC1 set to %00 in order 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

b) Now, With an RDIV of divide-by-4, a BDIV of divide-by-1, and a VDIV of multiply-by-16,

MCGOUT = [(4 MHz / 4) * 16] / 1 = 16 MHz, and the bus frequency is MCGOUT / 2, or 8 MHz

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