Clock Generator Module (CGM)
Functional Description
MC68HC908AB32
—
Rev. 1.0
Technical Data
MOTOROLA
Clock Generator Module (CGM)
139
5. Calculate and verify the adequacy of the VCO and bus
frequencies f
VCLK
and f
BUS
.
6. Select a VCO linear range multiplier, L.
where f
NOM
= 4.9152MHz
7. Calculate and verify the adequacy of the VCO programmed
center-of-range frequency f
VRS
.
f
VRS
= (L)f
NOM
8. Verify the choice of N and L by comparing f
VCLK
to f
VRS
and
f
VCLKDES
. For proper operation, f
VCLK
must be within the
application’s tolerance of f
VCLKDES
, and f
VRS
must be as close as
possible to f
VCLK
.
NOTE:
Exceeding the recommended maximum bus frequency or VCO
frequency can cause the MCU to “crash”.
9. Program the PLL registers accordingly:
a. In the upper 4 bits of the PLL programming register (PPG),
program the binary equivalent of N.
b. In the lower 4 bits of the PLL programming register (PPG),
program the binary equivalent of L.
9.4.2.5 Special Programming Exceptions
The programming method described in
does not account for two possible exceptions — a value of zero for N or
L is meaningless when used in the equations given. To account for these
exceptions:
f
VCLK
N
f
RCLK
×
=
f
BUS
f
VCLK
(
)
4
⁄
=
L
round
f
VCLK
f
NOM
-------------
=
Содержание MC68HC908AB32
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