MC68HC708AS48 FREESCALE [Freescale Semiconductor, Inc], MC68HC708AS48 Datasheet - Page 105

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MC68HC708AS48

Manufacturer Part Number
MC68HC708AS48
Description
Advance Information
Manufacturer
FREESCALE [Freescale Semiconductor, Inc]
Datasheet
8.4.2.4 Programming the PLL
MC68HC708AS48
MOTOROLA
NOTE:
Rev. 4.0
Use this procedure to program the PLL:
The round function means that the result is rounded to the nearest
integer.
1. Choose the desired bus frequency, f
2. Calculate the desired VCO frequency, f
3. Using a reference frequency, f
4. Calculate the VCO frequency, f
Software must wait a given time, t
before selecting the PLL as the clock source to CGMOUT
(BCS = 1).
The LOCK bit is disabled.
CPU interrupts from the CGM are disabled.
calculate the VCO frequency multiplier, N.
Clock Generator Module (CGM)
Example: f
Example: f
Example:
Example: f
N
f
VCLKDES
VCLKDES
f
=
VCLK
VCLK
round
N
= 8
=
= 4
BUSDES
= 4
=
N f
RCLK
32 MHz
------------------- -
f
--------------------- -
4 MHz
VCLK
4 MHz = 32 MHz
VCLKDES
f
8 MHz = 32 MHz
f
AL
RCLK
BUSDES
, equal to the crystal frequency,
= 8 MHz
RCLK
.
, after entering tracking mode
BUSDES
Clock Generator Module (CGM)
VCLKDES
=
8
.
.
Functional Description
Advance Information
105

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