CS8920 datasheet(9/66 Pages) CIRRUS

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AN84REV1

CS8920 Technical Reference Manual

2.0 Hardware Design

This section gives design guidance for both

embedded and adapter card designs,

including design considerations such as

choosing transformers, and laying out the

board.

2.1 Ethernet Hardware Design for

Embedded Systems and

Motherboards

This section describes the hardware design

of a four-layer, 10BASE-T solution

intended for use on PC motherboards, or in

other embedded applications. The goal of

this design is use minimal board space and

minimal material cost. Therefore, a number

of features (BootPROM, AUI, 10BASE-2)

are not supported in this particular PCB

design. An example of this circuit is

included in the CS8900 Technical

Reference Manual, and is implemented in

an ISA form factor. This same circuit can

be implemented directly on the processor

PCB.

2.1.1 General Description

The small footprint, high performance and

low cost of the CS8920 Ethernet solution,

makes the CS8920 an ideal choice for

embedded systems such as personal

computer (PC) mother boards. The very

high level of integration in the CS8920

results in a very low component count

Ethernet design. This makes it possible to

have a complete solution fit in an area of

1.75 square inches. The full Plug & Play

support, Wake-Up frame and Advanced

Power Management make the CS8920 ideal

for design of new generation of Green PC

platforms. Features like full duplex and

Auto-Negotiation (N-Way) make designs

using CS8920 suitable for LANs that are

getting upgraded with full duplex and

switching hubs. Since the analog filters are

integrated in to the CS8920, a card can

more easily made compliant with FCC part

15 class (B).

2.1.2 Board Design Considerations

2.1.2.1 Crystal Oscillator

The CS8920, in this reference design, uses

a 20.000 MHz crystal oscillator. The

crystal has a maximum load capacitance of

18 pF. The rest of the oscillator circuitry is

built internal to the CS8920. Please note

that the crystal must be placed very close to

XTL1 and XTL2 pins of the CS8920.

This crystal oscillator can be eliminated if

there is an accurate clock signal (20.00

MHz +/- 0.01% and 45-55 duty cycle)

available in the system.

2.1.2.2 ISA Bus Interface

The CS8920 has a direct ISA bus interface.

Note that the ISA bus interface is simple

enough to allow the CS8920 to interface

with variety of microprocessors directly or

with the help of simple programmable logic

such as a PAL or a GAL.

This reference design actually has the form

factor of an ISA adapter card. In this

design, all the ISA bus connections from

the CS8920 are directly routed to the ISA

connector. The pin-out of the CS8920 is

such that if the CS8920 is placed as shown

in Figure 2.1.1a, there will be no cross-over

of the ISA signals.

The CS8920 can be accessed in I/O mode

or memory mode. In memory mode, the

CS8920 can be in the conventional or

upper memory of the PC (the lower 1 Mega

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