MC68HC908AS32A Datasheet, PDF (102/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Clock Generator Module (CGM)
5.3.2.4 Programming the PLL
Use this 9-step procedure to program the PLL. Table 5-1 lists the variables used and their meaning.
Please also reference Figure 5-2
.
Table 5-1. Variable Definitions
Variable
f_BUSDES
f_VCLKDES
f_CGMRCLK
f_CGMVCLK
f_BUS
f_NOM
f_CGMVRS
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---^C----^L---^K---^D---^E----^S---
f_CGMRCLK
Example: N = ^{3----2-----M-----H----z--} = 8
4 MHz
4. Calculate the VCO frequency, f_CGMVCLK.
f_CGMVCLK = N × f_CGMRCLK
Example: f_CGMVCLK = 8 × 4 MHz = 32 MHz
5. Calculate the bus frequency, f_BUS, and compare f_BUS with f_BUSDES.
fBus = -f--C----G-----M---4--V----C----L---K---
Example:
fBus =
3----2-----M-----H----z--
4
=
8 MHz
6. If the calculated f_Bus is not within the tolerance limits of your application, select another f_BUSDES or
another f_RCLK.
MC68HC908AS32A Data Sheet, Rev. 2.0
102
Freescale Semiconductor