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CS8920A

Crystal LAN ISA Plug-and-Play Ethernet Controller

Cirrus Logic 

CS8920A

Runt Frame: If a frame is received that is

shorter than 64 bytes, the Runt bit (Register 4,

RxEvent, Bit D) is set. If the RuntA bit (Register

5, RxCTL, Bit D) is set, the frame will still be

buffered by CS8920A. If the RuntiE bit (Regis-

ter 3, RxCFG. Bit D) is set, the host is

interrupted.

Extra Data: If a frame is received that is longer

than 1518 bytes, the Extradata bit (Register 4,

RxEvent, Bit E) is set. If the ExtradataA bit

(Register 5, RxCTL, Bit E) is set, the first 1518

bytes of the frame will still be buffered by

CS8920A. If the ExtradataiE bit (Register 3,

RxCFG. Bit E) is set, the host is interrupted.

Dribble Bits and Alignment Error: Under nor-

mal operating conditions, the MAC may detect

up to 7 additional bits after the last full byte of a

receive packet. These bits, known as dribble bits,

are ignored. If dribble bits are detected, the Drib-

blebit bit (Register 4, RxEvent, Bit 7) is set. If

both the Dribblebit bit and CRCerror bit (Regis-

ter 4, RxEvent, Bit C) are set at the same time,

an alignment error has occurred.

Media Access Management

The Ethernet network topology is a single shared

medium with several attached stations. The Eth-

ernet protocol is designed to allow each station

equal access to the network at any given time.

Any node can attempt to gain access to the net-

work by first completing a deferral process

(described below) after the last network activity,

and then transmitting a packet that will be re-

ceived by all other stations. If two nodes

transmit simultaneously, a collision occurs and

the colliding packets are corrupted. Two primary

tasks of the MAC are to avoid network colli-

sions, and then recover from them when they

occur. In addition, when the CS8920A is using

the AUI, the MAC must support the SQE Test

function described in section 7.2.4.6 of the Eth-

ernet standard.

Collision Avoidance: The MAC continually

monitors network traffic by checking for the

presence of carrier activity (carrier activity is in-

dicated by the assertion of the internal Carrier

Sense signal generated by the ENDEC). If car-

rier activity is detected, the network is assumed

busy and the MAC must wait until the current

packet is finished before attempting transmis-

sion. The CS8920A supports two schemes for

determining when to initiate transmission: Two-

Part Deferral, and Simple Deferral. Selection of

the deferral scheme is determined by the 2-part-

Start Monitoring

Network Activity

Yes Network

Active?

No

Start IPG

Timer

Wait

3.2 µs

Yes

IPG

Timer =

6.4 µs?

No

Network No

Active?

Yes

Tx

Frame No

Ready and Not

in Backoff?

No

Network Yes

Active?

Yes

Transmit

Frame

Figure 3.7. Two-Part Deferral

30

DS238PP2

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