Crystal LAN CS8920, Техническое справочное руководство, Страница: 32 / 67

AN84REV1 32
CS8920 Technical Reference Manual
2.2.3 Routing of the digital signals
Most of the digital signals from the
CS8920 go to the ISA bus connector.
Route these signals directly to the
connector. Isolate the digital signals from
analog signals.
2.2.4 Routing of the analog signals
2.2.4.1 Routing of the clock signals:
Place the 20.000 MHz crystal within one
inch of XTL1 (pin #135) and XTL2 (pin
#136) pins of the CS8920. The 20.000
MHz crystal traces should be short, have no
via, and run on the component side.
2.2.4.2 Biasing resistor at RES pin of the
CS8920
A 4.99 K Ohm resistor is connected
between pins RES (pin #131) and AVSS3
(pin #132) of the CS8920. This resistor
biases internal analog circuits of the
CS8920, and should be placed as close as
possible to RES pin (pin #131) of the
CS8920.
2.2.4.3 Routing of the 10BASE-T signals
Four signals are used for 10BASE-T
communication, two differential transmit
signals and two differential receive signals.
An isolation transformer is placed between
the transmit and receive traces and a RJ-45
(modular phone jack) connector. The
isolation transformer should be placed as
close as possible to the RJ-45 connector.
Both transmit and receive signal traces
should be routed so they are parallel and of
equal length. The signal traces should be
on the component side and should have
direct and short paths. The widths of the
receive signal traces should at least be 25
mil. while widths of the transmit signal
traces should be at least 100 mil. This will
provide a good impedance matching for the
transmit and receive circuitry inside the
CS8920. A ground trace should be run
parallel to the transmit traces. Also, a
ground plane should run underneath the
transmit and receive traces on the solder
side of a two layered PCB. Please refer to
the Figures 2.2.12 and 2.2.13 for
illustration of the above guide lines.
2.2.4.4 Routing of the AUI signals
The CS8920 has three pairs of differential
signals connecting it to an Auxiliary Unit
Interface (AUI). An isolation transformer
separates the three signal pairs and the AUI
connector (a 15 pin sub-D connector). The
isolation transformer should be placed as
close as possible to the AUI connector.
Signal traces of each differential pair
should be in parallel with equal length and
impedance. Thus minimizing differential
noise due to impedance mis-match. Place
the AUI signal traces on the component
side.