
Clock Generator Module (CGM)
Technical Data
MC68HC08AZ32A — Rev 1.0
126
Clock Generator Module (CGM)
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MOTOROLA
5. Calculate the bus frequency, f
BUS
, and compare f
BUS
with
f
BUSDES
.
Example:
6. If the calculated f
bus
is not within the tolerance limits of your
application, select another f
BUSDES
or another f
RCLK
.
7. Using the value 4.9152 MHz for f
NOM
, calculate the VCO linear
range multiplier, L. The linear range multiplier controls the
frequency range of the PLL.
Example:
8. Calculate the VCO center-of-range frequency, f
CGMVRS
. The
center-of-range frequency is the midpoint between the minimum
and maximum frequencies attainable by the PLL.
f
CGMVRS
= L
×
f
NOM
Example: f
CGMVRS
= 7
×
4.9152 MHz = 34.4 MHz
NOTE:
For proper operation,
.
Exceeding the recommended maximum bus frequency or VCO
frequency can crash the MCU.
9. Program the PLL registers accordingly:
a. In the upper four bits of the PLL programming register (PPG),
program the binary equivalent of N.
b. In the lower four bits of the PLL programming register (PPG),
program the binary equivalent of L.
f
BUS
f
4
-----------------------
=
f
BUS
32 MHz
4
--------------------
=
8 MHz
=
L
round
f
f
NOM
-----------------------
=
L
32 MHz
4.9152 MHz
-------------------------------
=
7
=
f
CGMVRS
f
CGMVCLK
–
f
NOM
----------------
≤
F
Freescale Semiconductor, Inc.
n
.