MC68HC08AS32 Datasheet, PDF (117/394 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Clock Generator Module (CGM)
Acquisition/Lock Time Specifications
8.10.2 Parametric Influences on Reaction Time
Acquisition and lock times are designed to be as short as possible while
still providing the highest possible stability. These reaction times are not
constant, however. Many factors directly and indirectly affect the
acquisition time.
The most critical parameter which affects the PLL reaction times is the
reference frequency, f_RDV. This frequency is the input to the phase
detector and controls how often the PLL makes corrections. For stability,
the corrections must be small compared to the desired frequency, so
several corrections are required to reduce the frequency error.
Therefore, the slower the reference the longer it takes to make these
corrections. This parameter is also under user control via the choice of
an external crystal frequency, f_XCLK.
Another critical parameter is the external filter capacitor. The PLL
modifies the voltage on the VCO by adding or subtracting charge from
this capacitor. Therefore, the rate at which the voltage changes for a
given frequency error (thus change in charge) is proportional to the
capacitor size. The size of the capacitor also is related 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 8.10.3 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.
MC68HC08AS32 — Rev. 3.0
MOTOROLA
Clock Generator Module (CGM)
Advance Information
117