MC908GZ60CFUE Datasheet, PDF (77/352 Pages) Freescale Semiconductor, Inc – Standard features, Features of the CPU08

Functional Description
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 (See 4.8
Acquisition/Lock Time Specifications.), 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.
4.3.6 Programming the PLL
Use the following procedure to program the PLL. For reference, the variables used and their meaning are
shown in Table 4-1.
Table 4-1. Variable Definitions
Variable
f_BUSDES
f_VCLKDES
f_RCLK
f_VCLK
f_BUS
f_NOM
f_VRS
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
Programmed VCO center frequency
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).
f_VCLKDES = 4 x f_BUSDES
3. Choose a practical PLL (crystal) reference frequency, f_RCLK. Typically, the reference crystal is 1–8
MHz.
Frequency errors to the PLL are corrected at a rate of f_RCLK.
For stability and lock time reduction, this rate must be as fast as possible. The VCO frequency must
be an integer multiple of this rate. The relationship between the VCO frequency, f_VCLK, and the
reference frequency, f_RCLK, is:
f_VCLK = (N) (f_RCLK)
N, the range multiplier, must be an integer.
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
Freescale Semiconductor
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