MC68HC08AS32 Datasheet, PDF (106/280 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

to the stability of the PLL. If the capacitor is too small, the PLL cannot make small
enough adjustments to the voltage and the system cannot lock. If the capacitor is
too large, the PLL may not be able to adjust the voltage in a reasonable time. (See
5.9.3 Choosing a Filter Capacitor.)
Also important is the operating voltage potential applied to the PLL analog portion
potential (V_DDA/V_DDAREF). Typically, V_DDA/V_DDAREF is at the same potential as
V_DD. 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.
5.9.3 Choosing a Filter Capacitor
As described in 5.9.2 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 5.3.2 Phase-Locked Loop Circuit
(PLL) for the value of f_RDV and 17.9 CGM Component Information for the value
of C_FACT.
C_F
=
C_F
A
C
T


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.
Data Sheet
106
MC68HC08AS32 — Rev. 4.1
Freescale Semiconductor