MC68HC08AZ60 Datasheet, PDF (113/452 Pages) Motorola, Inc – Advance Information

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Functional Description
Programming the
PLL
Use this 9-step procedure to program the PLL. The table below lists the
variables used and their meaning.
Variable
f_BUSDES
f_VCLKDES
f_RCLK
f_VCLK
f_BUS
f_NOM
f_VRS
Table 2. Variable Definitions
Definition
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
1. Choose the desired bus frequency, f_BUSDES.
Example: f_BUSDES = 8 MHz
2. Calculate the desired VCO frequency, f_VCLKDES.
f_VCLKDES = 4 × f_BUSDES
Example: f_VCLKDES = 4 × 8 MHz = 32 MHz
3. Using a reference frequency, f_RCLK, equal to the crystal frequency,
calculate the VCO frequency multiplier, N. Round the result to the
nearest integer.
N = ^f--V--f-^C-_R---^L-_C-^K--_L-^D---_K^E---^S--
Example: N = ^{3--4--2---M--M---H--H---z-z--} = 8
4. Calculate the VCO frequency, f_VCLK.
f_VCLK = N × f_RCLK
Example: f_VCLK = 8 × 4 MHz = 32 MHz
5. Calculate the bus frequency, f_BUS, and compare f_BUS with
f_BUSDES.
9-cgm
MOTOROLA
MC68HC08AZ60 — Rev 1.0
Clock Generator Module (CGM)
113
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