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参数资料
| 型号： | MPC990/D |
| 厂商： | FREESCALE SEMICONDUCTOR INC |
| 元件分类： | 时钟产生/分配 |
| 英文描述： | OTHER CLOCK GENERATOR, PQFP52 |
| 封装： | TQFP-52 |
| 文件页数： | 7/7页 |
| 文件大小： | 324K |
| 代理商： | MPC990/D |
MPC990
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
269
APPLICATIONS INFORMATION
Using the On–Board Crystal Oscillator
The MPC990 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 MPC990 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 the optimum frequency accuracy a
series resonant crystal should be used (see specification table
below). Unfortunately most of 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 MPC990 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.
The MPC990 is a clock driver which was designed to
generate outputs with programmable frequency relationships
and not a synthesizer with a fixed input frequency. As a result
the crystal input frequency is a function of the desired output
frequency. For a design which utilizes the external feedback to
the PLL the selection of the crystal frequency is straight forward;
simply chose a crystal which is equal in frequency to the fed
back signal.
Table 14. Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental at Cut
Resonance
Series Resonance*
Frequency Tolerance
±75 ppm at 25°C
Frequency/Temperature Stability
±150 pm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5–7 pF
Equivalent Series Resistance (ESR)
50 to 80
Max
Correlation Drive Level
100
W
Aging
5 ppm/Yr (First 3 Years)
* See accompanying text for series versus parallel resonant discus-
sion.
Power Supply Filtering
The MPC990 provides a separate power supply for the
internal PLL of the device. The purpose of this design technique
is to allow the user to filter externally generated system noise
from the internal, relatively sensitive analog PLL.
Figure 4 illustrates a suggested power supply filter using an
LC filter network. The inductor value should be chosen to
maximize the AC filter impedance while maintaining a low DC
resistance. An inductor with a maximum DC series resistance
of 5
W should be used. The parallel capacitor combination on
the VCCA pin ensures that a low impedance path to ground
exists for frequencies well above the bandwidth of the PLL.
Figure 4. Power Supply Filter
VCCA
VCC
MPC990
0.01 F
22 F
0.01 F
3.3 V
RS ≤ 5
2
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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