MC68HC08AZ16 Datasheet, PDF (122/527 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 121
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 120, 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 the following
equation:
27-cgm
MOTOROLA
C_F = C_{FACT}^V--f--_R-^D--_D-^D--_V-^A-_
For the value of V_DDA, the voltage potential at which the MCU is operating
should be used. 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
Clock Generator Module (CGM)
MC68HC08AZ32
121