CS8900-CQ View Datasheet(PDF) - Cirrus Logic

Part Name

Description

MFG CO.

CS8900-CQ

Highly-Integrated ISA Ethernet Controller

Cirrus Logic 

CS8900

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 CS8900 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-

DefDis bit (Register 13, LineCTL, Bit D). If the

2-partDefDis bit is clear, the MAC uses a two-

part deferral process defined in section 4.2.3.2.1

of the Ethernet standard (ISO/IEC 8802-3,

1993). If the 2-partDefDis bit is set, the MAC

uses a simplified deferral scheme. Both schemes

are described below:

Two-Part Deferral: In the two-part deferral proc-

ess, the 9.6 µs Inter Packet Gap (IPG) timer is

started whenever the internal Carrier Sense sig-

nal is de-asserted. If activity is detected during

the first 6.4 µs of the IPG timer, the timer is re-

set and then restarted once the activity has

stopped. If there is no activity during the first 6.4

µs of the IPG timer, the IPG timer is allowed to

time out (even if network activity is detected

during the final 3.2 µs). The MAC then begins

transmission if a transmit packet is ready and if

it is not in Backoff (Backoff is described later in

this section). If no transmit packet is pending,

the MAC continues to monitor the network. If

activity is detected before a transmit frame is

ready, the MAC defers to the transmitting station

and resumes monitoring the network.

The two-part deferral scheme was developed to

prevent the possibility of the IPG being short-

ened due to a temporary loss of carrier. Figure

3.7 diagrams the two-part deferral process.

Simple Deferral: In the simple deferral scheme,

the IPG timer is started whenever Carrier Sense

28

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

is de-asserted. Once the IPG timer is finished

(after 9.6 µs), if a transmit frame is pending and

if the MAC is not in Backoff, transmission be-

gins (even if network activity is detected during

the 9.6 µs IPG). If no transmit packet is pending,

the MAC continues to monitor the network. If

activity is detected before a transmit frame is

ready, the MAC defers to the transmitting station

and resumes monitoring the network. Figure 3.8

diagrams the simple deferral process.

Collision Resolution: If a collision is detected

while the CS8900 is transmitting, the MAC re-

DS150PP2

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