MC68HC908GP20 Datasheet, PDF (116/406 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

P, the power of two multiplier, and N, the range multiplier, are
integers.
In cases where desired bus frequency has some tolerance,
choose f_RCLK to a value determined either by other module
requirements (such as modules which are clocked by CGMXCLK),
cost requirements, or ideally, as high as the specified range
allows. See Section 23. Preliminary Electrical Specifications.
Choose the reference divider, R = 1. After choosing N and P, the
actual bus frequency can be determined using equation in 2
above.
When the tolerance on the bus frequency is tight, choose f_RCLK to
an integer divisor of f_BUSDES, and R = 1. If f_RCLK cannot meet this
requirement, use the following equation to solve for R with
practical choices of f_RCLK, and choose the f_RCLK that gives the
lowest R.
R
=
round
R_M
AX
×






f--_{V----fC--R--L--C-K--L-D--K--E----S- }
–
i
nte
ge
r



-f-^V----f^C--_R--^L--_C-^K--_L-^D--_K--^E----^S- _{}



4. Select a VCO frequency multiplier, N.
N
=
ro
un

d

R------×_{-----ff--VR----CC---L-L--K-K---D----E---S--}
Reduce N/R to the lowest possible R.
5. If N is < N_max, use P = 0. If N > N_max, choose P using this table:
Current N Value
P
0 < N ≤ Nmax
0
Nmax < N ≤ Nmax × 2
1
Nmax × 2 < N ≤ Nmax × 4
2
Nmax × 4 < N ≤ Nmax × 8
3
Advance Information
116
MC68HC908GP20 — Rev 2.1
Freescale Semiconductor