MC68HC08AZ60 Datasheet, PDF (129/452 Pages) Motorola, Inc – Advance Information

Freescale Semiconductor, Inc.
Clock Generator Module (CGM)
Acquisition/Lock Time Specifications
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
128, 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
408). 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.
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:
• Correct selection of filter capacitor, C_F (see Choosing a Filter
Capacitor on page 129).
• Room temperature operation
• Negligible external leakage on CGMXFC
• Negligible noise
25-cgm
MOTOROLA
MC68HC08AZ60 — Rev 1.0
Clock Generator Module (CGM)
129
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