MC68HC08AB16A Datasheet, PDF (146/380 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
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.
9.10.3 Choosing a Filter Capacitor
As described in 9.10.2 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 the
following
equation:
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 lower size may seem attractive for acquisition time improvement,
Technical Data
146
MC68HC08AB16A — Rev. 2.0
Clock Generator Module (CGM)
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