Routing Of The Digital Signals; Routing Of The Analog Signals - Cirrus Logic CRYSTAL LAN CS890 Technical Reference Manual

2.4.3 Routing of the digital signals

Most of the digital signals from the
CS8900 go to the ISA bus connector.
Route these signals directly to the connec-
tor. Isolate the digital signals from analog
signals.

2.4.4 Routing of the analog signals

Routing of the clock signals:
20.000 MHz crystal within one inch of
XTL1 (pin #97) and XTL2 (pin #98) pins
of the CS8900. The 20.000 MHz crystal
traces should be short, have no via, and run
on the component side.
Biasing resistor at RES pin of the CS8900:
A 4.99 K Ohm resistor is connected be-
tween pins RES (pin #93) and AVSS3 (pin
#94) of the CS8900. This resistor biases
internal analog circuits of the CS8900, and
should be placed as close as possible to
RES pin (pin #93) of the CS8900.
Routing of the 10BASE-T signals:
signals are used for 10BASE-T communi-
cation, two differential transmit signals and
two differential receive signals. An isola-
tion transformer is placed between the
transmit and receive traces and a RJ-45
(modular phone jack) connector. The iso-
lation transformer should be placed as close
AN83REV1
CS8900 Technical Reference Manual
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 sig-
nal 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 CS8900. A
Place the
ground trace should be run parallel to the
transmit traces.
should run underneath the transmit and re-
ceive traces on the solder side of a two lay-
ered PCB.
2.4.12 and 2.4.13 for illustration of the
above guide lines.
Routing of the AUI signals: The CS8900
has three pairs of differential signals con-
necting it to an Auxiliary Unit Interface
(AUI). An isolation transformer separates
the three signal pairs and the AUI connec-
tor (a 15 pin sub-D connector). The isola-
tion transformer should be placed as close
Four
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 sig-
nal traces on the component side.
Also, a ground plane
Please refer to the Figures
45