MC908GR16ACFA FREESCALE [Freescale Semiconductor, Inc], MC908GR16ACFA Datasheet - Page 73

• Manufacturer Part Number: MC908GR16ACFA
• Description: M68HC08 Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Acquisition/Lock Time Specifications
The most critical parameter which affects the reaction times of the PLL is the reference frequency, f
.
RCLK
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 under user control via the choice of crystal frequency f
. (See
XCLK
4.3.3 PLL Circuits
and
4.3.6 Programming the
PLL.)
Another critical parameter is the external filter network. The PLL modifies the voltage on the VCO by
adding or subtracting charge from capacitors in this network. Therefore, the rate at which the voltage
changes for a given frequency error (thus change in charge) is proportional to the capacitance. 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.)
Also important is the operating voltage potential applied to V
. The power supply potential alters the
DDA
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
As described in
4.8.2 Parametric Influences on Reaction
Time, the external filter network is critical to the
stability and reaction time of the PLL. The PLL is also dependent on reference frequency and supply
voltage.
Figure 4-9
shows two types of filter circuits. In low-cost applications, where stability and reaction time of
the PLL are not critical, the three component filter network shown in
Figure 4-9
(B) can be replaced by a
single capacitor, C
, as shown in shown in
Figure 4-9
(A). Refer to
Table 4-5
for recommended filter
F
components at various reference frequencies. For reference frequencies between the values listed in the
table, extrapolate to the nearest common capacitor value. In general, a slightly larger capacitor provides
more stability at the expense of increased lock time.
CGMXFC
CGMXFC
R
F1
C
F2
C
F
C
F1
V
V
SSA
SSA
(A)
(B)
Figure 4-9. PLL Filter
MC68HC908GR16A Data Sheet, Rev. 1.0
Freescale Semiconductor
73