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參數(shù)資料
| 型號： | MPC9239FN |
| 廠商： | MOTOROLA INC |
| 元件分類： | 時鐘產(chǎn)生/分配 |
| 英文描述： | 900 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封裝： | PLASTIC, LCC-28 |
| 文件頁數(shù)： | 9/9頁 |
| 文件大?。?/td> | 136K |
| 代理商： | MPC9239FN |
4
MPC9239
MOTOROLA ADVANCED CLOCK DRIVERS DEVICE DATA
425
6 is the low impedance connections between VCC and GND for
the bypass capacitors. Combining good quality general pur-
pose chip capacitors with good PCB layout techniques will
produce effective capacitor resonances at frequencies ade-
quate to supply the instantaneous switching current for the
MPC9239 outputs. It is imperative that low inductance chip
capacitors are used; it is equally important that the board layout
does not introduce back all of the inductance saved by using the
leadless capacitors. Thin interconnect traces between the
capacitor and the power plane should be avoided and multiple
large vias should be used to tie the capacitors to the buried
power planes. Fat interconnect and large vias will help to
minimize layout induced inductance and thus maximize the
series resonant point of the bypass capacitors. Note the dotted
lines circling the crystal oscillator connection to the device. The
oscillator is a series resonant circuit and the voltage amplitude
across the crystal is relatively small. It is imperative that no
actively switching signals cross under the crystal as crosstalk
energy coupled to these lines could significantly impact the jitter
of the device. Special attention should be paid to the layout of
the crystal to ensure a stable, jitter free interface between the
crystal and the on–board oscillator. Although the MPC9239 has
several design features to minimize the susceptibility to power
supply noise (isolated power and grounds and fully differential
PLL), there still may be applications in which overall perfor-
mance is being degraded due to system power supply noise.
The power supply filter and bypass schemes discussed in this
section should be adequate to eliminate power supply noise
related problems in most designs.
Figure 6. PCB Board Layout Recommendation for
the PLCC28 Package
C2
1
CF
Xtal
C1
= VCC
= GND
= Via
Using the On–Board Crystal Oscillator
The MPC9239 features a fully integrated on–board crystal
oscillator to minimize system implementation costs. The
oscillator is a series resonant, multivibrator type design as
opposed to the more common parallel resonant oscillator
design. The series resonant design provides better stability and
eliminates the need for large on chip capacitors. The oscillator
is totally 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 MPC9239 as possible to avoid any board level
parasitics. To facilitate co–location surface mount crystals are
recommended, but not required. Because the series resonant
design is affected by capacitive loading on the xtal terminals
loading variation introduced by crystals from different vendors
could be a potential issue. For crystals with a higher shunt
capacitance it may be required to place a resistance across the
terminals to suppress the third harmonic. Although typically not
required it is a good idea to layout the PCB with the provision
of adding this external resistor. The resistor value will typically
be between 500 and 1K
.
The oscillator circuit is a series resonant circuit and thus for
optimum performance a series resonant crystal should be used.
Unfortunately most crystals are characterized in a parallel
resonant mode. Fortunately there is no physical difference
between a series resonant and a parallel resonant crystal. The
difference is purely in the way the devices are characterized. As
a result a parallel resonant crystal can be used with the
MPC9239 with only a minor error in the desired frequency. A
parallel resonant mode crystal used in a series resonant circuit
will exhibit a frequency of oscillation a few hundred ppm lower
than specified, a few hundred ppm translates to kHz inaccura-
cies. In a general computer application this level of inaccuracy
is immaterial. Table 12 below specifies the performance
requirements of the crystals to be used with the MPC9239.
Table 12. Recommended Crystal Specifications
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Series Resonance*
Frequency Tolerance
±75ppm at 25°C
Frequency/Temperature Stability
±150pm 0 to 70°C
Operating Range
0 to 70
°C
Shunt Capacitance
5–7pF
Equivalent Series Resistance (ESR)
50 to 80
Correlation Drive Level
100
W
Aging
5ppm/Yr (First 3 Years)
* See accompanying text for series versus parallel resonant discus-
sion.
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