Clock Generator Module (CGMC)
Technical Data
MC68HC908GP32
•
MC68HC08GP32
—
Rev. 6
114
Clock Generator Module (CGMC)
MOTOROLA
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. 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:
Current N Value
P
0
1
2
3
R
round R
MAX
f
VCL KDES
f
RCL K
--------------------------
integer
f
VCLKDES
f
RCLK
--------------------------
–
×
=
N
round
R
f
VCLKDES
×
f
RCL K
-------------------------------------
=
0
N
<
N
max
≤
N
max
N
<
N
max
2
×
≤
N
max
2
×
N
<
N
max
4
×
≤
N
max
4
×
N
<
N
max
8
×
≤
N
round
R
f
VC LKD ES
×
f
RCLK
2
P
×
-------------------------------------
=
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..