Clock Generator Module (CGM)
Technical Data
MC68HC908AB32
—
Rev. 1.0
138
Clock Generator Module (CGM)
MOTOROLA
and require fast start-up. The following conditions apply when in manual
mode:
•
ACQ is a writable control bit that controls the mode of the filter.
Before turning on the PLL in manual mode, the ACQ bit must be
clear.
•
Before entering tracking mode (ACQ = 1), software must wait a
given time, t
ACQ
), after turning on the PLL by setting PLLON in the
PLL control register (PCTL).
•
Software must wait a given time, t
AL
, after entering tracking mode
before selecting the PLL as the clock source to CGMOUT
(BCS = 1).
•
The LOCK bit is disabled.
•
CPU interrupts from the CGM are disabled.
9.4.2.4 Programming the PLL
The following procedure shows how to program the PLL.
NOTE:
The round function in the following equations means that the real
number should be rounded to the nearest integer number.
1. Choose the desired bus frequency, f
BUSDES
.
2. Calculate the desired VCO frequency (four times the desired bus
frequency).
3. Choose a practical PLL reference frequency, f
RCLK
.
4. Select a VCO frequency multiplier, N.
f
VCLKDES
4
f
BUSDES
×
=
N
round
f
VCLKDES
f
RCLK
----------------------
=
Содержание MC68HC908AB32
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