Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Functional Description
10.4.2.4 Programming the PLL
Use this 9-step procedure to program the PLL. Table 10-1 lists the
variables used and their meaning.
Table 10-1. Variable Definitions
Variable
Definition
f
Desired bus clock frequency
Desired VCO clock frequency
Chosen reference crystal frequency
Calculated VCO clock frequency
Calculated bus clock frequency
Nominal VCO center frequency
Shifted VCO center frequency
BUSDES
f
VCLKDES
f
RCLK
f
VCLK
f
BUS
f
NOM
f
VRS
1. Choose the desired bus frequency, fBUSDES
Example: fBUSDES = 8 MHz
.
2. Calculate the desired VCO frequency, fVCLKDES
VCLKDES = 4 × fBUSDES
Example: fVCLKDES = 4 × 8 MHz = 32 MHz
.
f
3. Using a reference frequency, fRCLK, equal to the crystal frequency,
calculate the VCO frequency multiplier, N. Round the result to the
nearest integer.
fVCLKDES
N = ----------------------
f
RCLK
32 MHz
Example: N = -------------------- = 8
4 MHz
4. Calculate the VCO frequency, fVCLK
.
fVCLK = N × fRCLK
Example: fVCLK = 8 × 4 MHz = 32 MHz
MC68HC908AS60 — Rev. 1.0
Technical Data
Clock Generator Module (CGM)
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