MC68HC908GR4 MOTOROLA [Motorola, Inc], MC68HC908GR4 Datasheet - Page 103

• Manufacturer Part Number: MC68HC908GR4
• Description: Microcontrollers
• Manufacturer: MOTOROLA [Motorola, Inc]

MC68HC908GR8 — Rev 4.0
MOTOROLA
The operating range of the VCO is programmable for a wide range of
frequencies and for maximum immunity to external noise, including
supply and CGM/XFC noise. The VCO frequency is bound to a range
from roughly one-half to twice the center-of-range frequency, f
Modulating the voltage on the CGM/XFC pin changes the frequency
within this range. By design, f
frequency, f
factor, E, or (L 2
CGMRCLK is the PLL reference clock, a buffered version of CGMXCLK.
CGMRCLK runs at a frequency, f
programmable modulo reference divider, which divides f
factor, R. The divider’s output is the final reference clock, CGMRDV,
running at a frequency, f
(30 kHz–100 kHz), always set R = 1 for specified performance. With an
external high-frequency clock source, use R to divide the external
frequency to between 30 kHz and 100 kHz.
The VCO’s output clock, CGMVCLK, running at a frequency, f
fed back through a programmable prescale divider and a programmable
modulo divider. The prescaler divides the VCO clock by a power-of-two
factor P and the modulo divider reduces the VCO clock by a factor, N.
The dividers’ output is the VCO feedback clock, CGMVDV, running at a
frequency, f
information.)
The phase detector then compares the VCO feedback clock, CGMVDV,
with the final reference clock, CGMRDV. A correction pulse is generated
based on the phase difference between the two signals. The loop filter
then slightly alters the DC voltage on the external capacitor connected
to CGM/XFC based on the width and direction of the correction pulse.
The filter can make fast or slow corrections depending on its mode,
described in
capacitor and the reference frequency determine the speed of the
corrections and the stability of the PLL.
The lock detector compares the frequencies of the VCO feedback clock,
CGMVDV, and the final reference clock, CGMRDV. Therefore, the
speed of the lock detector is directly proportional to the final reference
Freescale Semiconductor, Inc.
For More Information On This Product,
Clock Generator Module (CGMC)
NOM
VDV
Acquisition and Tracking
Go to: www.freescale.com
, (38.4 kHz) times a linear factor, L, and a power-of-two
= f
E
VCLK
)f
NOM
/(N
RDV
.
VRS
= f
2
P
RCLK
). (See
is equal to the nominal center-of-range
RCLK
/R. With an external crystal
Modes. The value of the external
, and is fed to the PLL through a
Programming the PLL
Clock Generator Module (CGMC)
Functional Description
RCLK
Technical Data
for more
by a
VCLK
VRS
.
, is
103