Clock Generator Module (CGM)
Acquisition/Lock Time Specifications
Obviously, the acquisition and lock times can vary according to how large the
frequency error is and may be shorter or longer in many cases.
4.8.2 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 reaction times of the PLL is the
reference frequency, fRDV. 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 crystal frequency, fXCLK
.
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
4.8.3 Choosing a Filter Capacitor.
Also important is the operating voltage potential applied to VDDA. 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.
4.8.3 Choosing a Filter Capacitor
As described in 4.8.2 Parametric Influences on Reaction Time, the external filter
capacitor, CF, 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.
MC68HC908MR32 • MC68HC908MR16 — Rev. 6.0
MOTOROLA Clock Generator Module (CGM)
Data Sheet
75
FREESCALE [ Freescale ]
