CS8920 datasheet(10/66 Pages) CIRRUS

10 / 66 page

AN84REV1

CS8920 Technical Reference Manual

bytes of address space) or in extended

memory address space. Address decoders

for I/O as well as memory mode are on chip

for the CS8920.

2.1.2.4 EEPROM

A 128 word (128 X16 bit) EEPROM

(location U3) is used in the reference

design to interface with the CS8920. This

EEPROM holds the IEEE assigned

Ethernet MAC (physical) address for the

board (see Section 3.3.1). The EEPROM

also holds other configuration information

for the CS8920. The EEPROM also holds

the Plug & Play resource information. The

last few bytes of the EEPROM are used to

store information about the hardware

configuration and software requirements.

Since the CS8920 has full Plug & Play

support, it requires an EEPROM to

configure itself after a reset.

Please refer to the CS8920 data sheet for

information about programming the

EEPROM. Please refer to the Section 3.0

of this document for information about

EEPROM internal word assignments.

2.1.2.5 LEDs

Many embedded systems do not require

LEDs for Ethernet traffic. Therefore this

reference design does not implement any

LEDs. However, the CS8920 has direct

drives for the four LEDs. Please refer to

the data sheet for the CS8920 for a

description of the LED functions available.

2.1.2.6 10BASE-T Interface

The 10BASE-T interface for the CS8920 is

straight forward. Please refer to Figure

2.1.4 for connections and components of

this circuit. Transmit and receive signal

lines from the CS8920 are connected to an

isolation transformer at location T1. This

isolation transformer has a 1:1 ratio

between the primary and the secondary

windings on the receive side, and a 1:

√2

(1:1.41) ratio between the primary and the

secondary windings for the transmit lines.

Resistor R1 provides termination for the

receive lines. Resistors R2 and R3 are in

series with the differential pair of transmit

lines for impedance matching.

2.1.2.7 10BASE-2 and AUI Interfaces

As many embedded systems require only a

10BASE-T interface, this reference design

implements only the 10BASE-T interface.

However, should a user require a 10BASE-

2 or AUI interface, the CS8920 provides a

direct interface to the AUI. Please refer to

the CS8900 Technical Reference Manual

or the CS8920 datasheet for details about

the AUI interface.

2.1.3 Logic Schematics

Figures 2.1.2 and 2.1.4 detail the logic

schematics for the various circuits used in

the reference design.

2.1.4 Component Placement and Signal

Routing

Please refer to the Section 2.2 of this

document for more details on the placement

of components on the board. It is important

to provide very clean and adequate +5 V

and ground connections to the CS8920.

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