mc68hc908qz16 Freescale Semiconductor, Inc, mc68hc908qz16 Datasheet - Page 63

• Manufacturer Part Number: mc68hc908qz16
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

The following conditions apply when in manual mode:
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
Freescale Semiconductor
1. Choose the desired bus frequency, f
2. Calculate the desired VCO frequency (four times the desired bus frequency).
3. Choose a practical PLL (crystal) reference frequency, f
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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
Acquisition/Lock Time
control register (PCTL).
Software must wait a given time, t
clock source to CGMOUT (BCS = 1).
The LOCK bit is disabled.
CPU interrupts from the CGM are disabled.
MHz.
Frequency errors to the PLL are corrected at a rate of f
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
reference frequency, f
N, the range multiplier, must be an integer.
Table
The round function in the following equations means that the real number
should be rounded to the nearest integer number.
4-1.
f
Variable
f
VCLKDES
BUSDES
f
f
f
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
RCLK
f
f
VCLK
NOM
BUS
VRS
RCLK
Specifications.), after turning on the PLL by setting PLLON in the PLL
, is:
Table 4-1. Variable Definitions
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
AL
f
VCLKDES
, after entering tracking mode before selecting the PLL as the
f
BUSDES
VCLK
= (N) (f
NOTE
= 4 x f
.
Definition
RCLK
BUSDES
)
RCLK
RCLK
. Typically, the reference crystal is 1–8
.
ACQ
Functional Description
(See
VCLK
4.8
, and the
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