Datasheet

Clock Generator Module (CGMC)
Functional Description
MC68HC908GR8 — Rev 4.0 Technical Data
MOTOROLA Clock Generator Module (CGMC) 107
allows. See 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.
4. Select a VCO frequency multiplier, N.
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:
Then recalculate N:
6. Calculate and verify the adequacy of the VCO and bus
frequencies f
VCLK
and f
BUS
.
7. Select the VCO’s power-of-two range multiplier E, according to
Current N Value P
0
1
2
3
R round R
MAX
f
VCLKDES
f
RCLK
--------------------------



integer
f
VCLKDES
f
RCLK
--------------------------



–



×
=
N round
Rf
VCLKDES
×
f
RCLK
-------------------------------------



=
0N<N
max
≤
N
max
N<N
max
2×≤
N
max
2× N<N
max
4×≤
N
max
4× N<N
max
8×≤
N round
Rf
VCLKDES
×
f
RCLK
2
P
×
-------------------------------------



=
f
VCLK
2
P
NR⁄×()f
RCLK
×=
f
BUS
f
VCLK
()4⁄=
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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