MC908GR16ACFA FREESCALE [Freescale Semiconductor, Inc], MC908GR16ACFA Datasheet - Page 61

• Manufacturer Part Number: MC908GR16ACFA
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [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
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
N, the range multiplier, must be an integer.
In cases where desired bus frequency has some tolerance, choose f
either by other module requirements (such as modules which are clocked by CGMXCLK), cost
requirements, or ideally, as high as the specified range allows. See
Specifications. After choosing N, the actual bus frequency can be determined using equation in 2
above.
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
RCLK
f
f
VCLK
NOM
BUS
VRS
Specifications.), after turning on the PLL by setting PLLON in the PLL
MC68HC908GR16A Data Sheet, Rev. 1.0
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
.
VCLK
Definition
RCLK
BUSDES
, and the reference frequency, f
)
RCLK
RCLK
. Typically, the reference crystal is 1–8
. For stability and lock time reduction,
Chapter 20 Electrical
RCLK
to a value determined
ACQ
Functional Description
(See
RCLK
4.8
, is:
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