Clock Generator Module (CGM)
Acquisition/Lock Time Specifications
MC68HC908AZ60A — Rev 2.0 Technical Data
MOTOROLA Clock Generator Module (CGM) 193
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.
10.10.3 Choosing a Filter Capacitor
As described in Parametric Influences on Reaction Time on page
192, 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:
For acceptable values of C
fact
, (see Electrical Specifications on page
530). 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 may become unstable. Also, always choose a capacitor with a tight
tolerance (±20% or better) and low dissipation.
10.10.4 Reaction Time Calculation
The actual acquisition and lock times can be calculated using the
equations below. These equations yield nominal values under the
following conditions:
C
F
C
fact
V
DDA
fCGMRDV
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=
