MC68HC908MR8 Datasheet, PDF (136/372 Pages) Motorola, Inc – Microcontrollers

Clock Generator Module (CGM)
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 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 8.11.3 Choosing a Filter Capacitor.
Also important is the operating voltage potential applied to V_DDA. The
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.
8.11.3 Choosing a Filter Capacitor
As described in 8.11.2 Parametric Influences on Reaction Time, the
external filter capacitor, C_F, is critical to the stability and reaction time of
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:
CF
=
CFACT



V--f--R-D---D-D----V-A---_{}
For acceptable values of C_FACT, see 8.11 Acquisition/Lock Time
Specifications. For the value of V_DDA, choose the voltage potential at
which the MCU is operating. If the power supply is variable, choose a
value near the middle of the range of possible supply values.
Technical Data
136
Clock Generator Module (CGM)
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor