MC68HC908AZ60 Datasheet, PDF (153/480 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Clock Generator Module (CGM)
Acquisition/Lock Time Specifications
is too large, the PLL may not be able to adjust the voltage in a
reasonable time. See Choosing a Filter Capacitor on page 151.
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.
Choosing a Filter
Capacitor
As described in Parametric Influences on Reaction Time on page 150,
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:
C_F
=
C_f
a
ct


V---f--_{rD-d--D-v--A-}
For acceptable values of C_fact, (see Electrical Specifications on page
434). 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.
This equation does not always yield a commonly available capacitor
size, so round to the nearest available size. If the value is between two
different sizes, choose the higher value for better stability. Choosing the
lower size may seem attractive for acquisition time improvement, but the
25-cgm
MOTOROLA
Clock Generator Module (CGM)
MC68HC908AZ60 — Rev 2.0
151