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參數(shù)資料
| 型號： | MPC992FA |
| 廠商： | MOTOROLA INC |
| 元件分類： | 時鐘產(chǎn)生/分配 |
| 英文描述： | 375 MHz, PROC SPECIFIC CLOCK GENERATOR, PQFP32 |
| 封裝： | PLASTIC, TQFP-32 |
| 文件頁數(shù)： | 5/6頁 |
| 文件大?。?/td> | 89K |
| 代理商： | MPC992FA |
MPC992
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
268
APPLICATIONS INFORMATION
Using the On–Board Crystal Oscillator
The MPC992 features an on–board crystal oscillator to allow
for seed clock generation as well as final distribution. The
on–board oscillator is completely self contained so that the only
external component required is the crystal. As the oscillator is
somewhat sensitive to loading on its inputs the user is advised
to mount the crystal as close to the MPC992 as possible to avoid
any board level parasitics. To facilitate co–location surface
mount crystals are recommended, but not required.
The oscillator circuit is a series resonant circuit as opposed
to the more common parallel resonant circuit, this eliminates the
need for large on–board capacitors. Because the design is a
series resonant design, for optimum frequency accuracy a
series resonant crystal should be used (see specification table
below). Unfortunately most off the shelf crystals are character-
ized in a parallel resonant mode. However a parallel resonant
crystal is physically no different than a series resonant crystal,
a parallel resonant crystal is simply a crystal which has been
characterized in its parallel resonant mode. Therefore in the
majority of cases a parallel specified crystal can be used with
the MPC992 with just a minor frequency error due to the actual
series resonant frequency of the parallel resonant specified
crystal. Typically a parallel specified crystal used in a series
resonant mode will exhibit an oscillatory frequency a few
hundred ppm lower than the specified value. For most
processor implementations a few hundred ppm translates into
kHz inaccuracies, a level which does not represent a major
issue.
Figure 2 shows an optional series capacitor in the crystal
oscillator interface. The on–board oscillator introduces a small
phase shift in the overall loop which causes the oscillator to
operate at a frequency slightly slower than the specified crystal.
The series capacitor is used to compensate the loop and allow
the oscillator to function at the specified crystal frequency. If a
100ppm type error is not important, the capacitor can be left off
the PCB. For more detailed information, order Motorola
Application Note AN1579/D.
Figure 2. Recommended Crystal Interface
XTAL2
CTUNE
(Optional)
XTAL1
MPC992
Table 4. Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Series Resonance*
Frequency Tolerance
±75ppm at 25°C
Frequency/Temperature Stability
±150ppm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5–7pF
Equivalent Series Resistance (ESR)
50 to 80
max
Correlation Drive Level
100
W
Aging
5ppm/Yr (First 3 Years)
Power Supply Filtering
The MPC992 is a mixed analog/digital product and as such
it exhibits some sensitivities that would not necessarily be seen
on a fully digital product. Analog circuitry is naturally susceptible
to random noise, especially if this noise is seen on the power
supply pins. The MPC992 provides separate power supplies for
the digital circuitry (VCCI) and the internal PLL (VCCA) of the
device. The purpose of this design technique is to try and isolate
the high switching noise digital outputs from the relatively
sensitive internal analog phase–locked loop. In a controlled
environment such as an evaluation board this level of isolation
is sufficient. However, in a digital system environment where it
is more difficult to minimize noise on the power supplies a
second level of isolation may be required. The simplest form of
isolation is a power supply filter on the VCCA pin for the
MPC992.
Figure 3 illustrates a typical power supply filter scheme. The
MPC992 is most susceptible to noise with spectral content in
the 10kHz to 1MHz range. Therefore the filter should be
designed to target this range. The key parameter that needs to
be met in the final filter design is the DC voltage drop that will
be seen between the VCC supply and the VCCA pin of the
MPC992. From the data sheet the IVCCA current (the current
sourced through the VCCA pin) is typically 15mA (20mA
maximum), assuming that a minimum of 3.0V must be
maintained on the VCCA pin very little DC voltage drop can be
tolerated when a 3.3V VCC supply is used. The resistor shown
in Figure 3 must have a resistance of 10–15
to meet the
voltage drop criteria. The RC filter pictured will provide a
broadband filter with approximately 100:1 attenuation for noise
whose spectral content is above 20KHz. As the noise frequency
crosses the series resonant point of an individual capacitor it’s
overall impedance begins to look inductive and thus increases
with increasing frequency. The parallel capacitor combination
shown ensures that a low impedance path to ground exists for
frequencies well above the bandwidth of the PLL.
2
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