MC68HC908AS60MFN MOTOROLA [Motorola, Inc], MC68HC908AS60MFN Datasheet - Page 178

• Manufacturer Part Number: MC68HC908AS60MFN
• Description: HCMOS Microcontroller Unit
• Manufacturer: MOTOROLA [Motorola, Inc]

Clock Generator Module (CGM)
10.10.3 Choosing a Filter Capacitor
Technical Data
178
Therefore, the slower the reference the longer it takes to make these
corrections. This parameter is also under user control via the choice of
crystal frequency f
Another critical parameter is the external filter capacitor. The PLL
modifies the voltage on the VCO by adding or subtracting charge from
this capacitor. Therefore, the rate at which the voltage changes for a
given frequency error (thus a change in charge) is proportional to the
capacitor size. The size of the capacitor also is related to the stability of
the PLL. If the capacitor is too small, the PLL cannot make small enough
adjustments to the voltage and the system cannot lock. If the capacitor
is too large, the PLL may not be able to adjust the voltage in a
reasonable time. See
Also important is the operating voltage potential applied to V
power supply potential alters the characteristics of the PLL. A fixed value
is best. Variable supplies, such as batteries, are acceptable if they vary
within a known range at very slow speeds. Noise on the power supply is
not acceptable, because it causes small frequency errors which
continually change the acquisition time of the PLL.
Temperature and processing also can affect acquisition time because
the electrical characteristics of the PLL change. The part operates as
specified as long as these influences stay within the specified limits.
External factors, however, can cause drastic changes in the operation of
the PLL. These factors include noise injected into the PLL through the
filter capacitor, filter capacitor leakage, stray impedances on the circuit
board, and even humidity or circuit board contamination.
As described in
external filter capacitor, C
the PLL. The PLL is also dependent on reference frequency and supply
voltage. The value of the capacitor must, therefore, be chosen with
supply potential and reference frequency in mind. For proper operation,
the external filter capacitor must be chosen according to this equation:
Freescale Semiconductor, Inc.
For More Information On This Product,
Clock Generator Module (CGM)
Go to: www.freescale.com
10.10.2 Parametric Influences on Reaction
XCLK
10.10.3 Choosing a Filter
.
F
C
, is critical to the stability and reaction time of
F
=
C
Fact
V
------------ -
f
RDV
DDA
MC68HC908AS60 — Rev. 1.0
Capacitor.
DDA
MOTOROLA
Time, the
. The