MC68HC08AS32 Datasheet, PDF (118/394 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Clock Generator Module (CGM)
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.
8.10.3 Choosing a Filter Capacitor
As described in 8.10.2 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, f_RDV, and
supply voltage, V_DD. The value of the capacitor, therefore, must 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. Refer to 8.4.2 Phase-Locked Loop Circuit
(PLL) for the value of f_RDV and 21.10 CGM Component Information for
the value of C_FACT.
C_F
=
C
FACT


V--f--_{R-D--D-D--V-A-}
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
PLL can become unstable. Also, always choose a capacitor with a tight
tolerance (±20% or better) and low dissipation.
Advance Information
118
Clock Generator Module (CGM)
MC68HC08AS32 — Rev. 3.0
MOTOROLA