CS8920 datasheet(32/66 Pages) CIRRUS

32 / 66 page

AN84REV1

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.

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