MC68HC708MP16 Datasheet, PDF (127/398 Pages) Freescale Semiconductor, Inc – Microcontrollers

Clock Generator Module (CGM)
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 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 acceptable values of C_FACT, see Table 21-10. CGM
Acquisition/Lock Time Specifications. 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.
8.10.4 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:
• Correct selection of filter capacitor, C_F (See 8.10.3 Choosing a
Filter Capacitor.)
• Room temperature operation
• Negligible external leakage on CGMXFC
• Negligible noise
MC68HC708MP16 — Rev. 3.1
Freescale Semiconductor
Clock Generator Module (CGM)
Technical Data
127