CS8900A datasheet(37/128 Pages) CIRRUS

37 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

ed by more than 2 to 7 ms and no more than 25 to

150 ms (see Section 7.4 on page 113 for 10BASE-

T timing).

The state of the link segment is reported in the

LinkOK bit (Register 14, LineST, Bit 7). If the

HC0E bit (Register 15, SelfCTL, Bit D) is clear, it

is also indicated by the output of the LINKLED

pin. If the link is "good", the LinkOK bit is set and

the LINKLED pin is driven low. If the link is "bad"

the LinkOK bit is clear and the LINKLED pin is

high. To disable this feature, the host must set the

DisableLT bit (Register 19, TestCTL, Bit 7). If

DisableLT is set, the CS8900A will transmit and

receive packets independent of the link segment.

3.11.5 Receive Polarity Detection and Correction

The CS8900A automatically checks the polarity of

the receive half of the twisted pair cable. If the po-

larity is correct, the PolarityOK bit (Register 14,

LineST, bit C) is set. If the polarity is reversed, the

PolarityOK bit is clear. If the PolarityDis bit (Reg-

ister 13, LineCTL, Bit C) is clear, the CS8900A au-

tomatically corrects a reversal. If the PolarityDis

bit is set, the CS8900A does not correct a reversal.

The PolarityOK bit and the PolarityDis bit are in-

dependent.

To detect a reversed pair, the receiver examines re-

ceived link pulses and the End-of-Frame (EOF) se-

quence of incoming packets. If it detects at least

one reversed link pulse and at least four frames in a

row with negative polarity after the EOF, the re-

ceive pair is considered reversed. Any data re-

ceived before the correction of the reversal is

ignored.

3.11.6 Collision Detection

If half-duplex operation is selected (Register 19,

Bit E, FDX), the CS8900A detects a 10BASE-T

collision whenever the receiver and transmitter are

active simultaneously. When a collision is present,

the Collision Detection circuit informs the MAC by

asserting

the

internal

Collision

signal

(see

Section 3.9 on page 28 for collision handling).

3.12 Attachment Unit Interface (AUI)

The CS8900A Attachment Unit Interface (AUI)

provides a direct interface to external 10BASE2,

10BASE5, and 10BASE-FL Ethernet transceivers.

It is fully compliant with Section 7 of the Ethernet

standard (ISO/IEC 8802-3), and as such, is capable

of driving a full 50-meter AUI cable.

The AUI consists of three pairs of signals: Data Out

(DO+/DO-), Data In (DI+/DI-), and Collision In

(CI+/CI-). To select the AUI, the host should set

the AUI bit (Register 13, LineCTL, Bit 8). The AUI

can also be selected automatically as described in

the previous section (Section 3.10.4 on page 35).

Figure 15 provides a block diagram of the AUI.

(For a connection diagram, see Section 7.6 on

page 121).

3.12.1 AUI Transmitter

The AUI transmitter is a differential driver de-

signed to drive a 78

Ω cable. It accepts data from

the ENDEC and transmits it directly on the

DO+/DO- pins. After transmission has started, the

CS8900A expects to see the packet "looped-back"

(or echoed) to the receiver, causing the Carrier

Sense signal to be asserted. This Carrier Sense

presence indicates that the transmit signal is getting

through to the transceiver. If the Carrier Sense sig-

nal remains deasserted throughout the transmis-

DI+

DI-

DO+

DO-

ENDEC

CL+

CL-

Collision

Detect

AUICol (to MAC)

AUIRX

AUISQL

AUITX

AUI

- +

Figure 15. AUI

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