CS8920A Ver la hoja de datos (PDF) - Cirrus Logic

Número de pieza

componentes Descripción

Lista de partido

CS8920A

Crystal LAN ISA Plug-and-Play Ethernet Controller

Cirrus Logic 

CS8920A

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

Start Monitoring

Network Activity

Yes Network

Active?

No

Wait

9.6 µs

Tx

Frame

No

Ready and Not

in Backoff?

No

Network Yes

Active?

Yes

Transmit

Frame

Figure 3.8. Simple Deferral

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

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 CS8920A is transmitting, the MAC re-

sponds in one of three ways depending on

whether it is a normal collision (within the first

512 bits of transmission) or a late collision (after

the first 512 bits of transmission):

Normal Collisions: If a collision is detected be-

fore the end of the preamble and SFD, the MAC

finishes the preamble and SFD, transmits the jam

sequence (32-bit pattern of all 0’s), and then in-

itiates Backoff. If a collision is detected after the

transmission of the preamble and SFD but before

512 bit times, the MAC immediately terminates

transmission, transmits the jam sequence, and

then initiates Backoff. In either case, if the

Onecoll bit (Register 9, TxCMD, Bit 9) is clear,

the MAC will attempt to transmit a packet a total

of 16 times (the initial attempt plus 15 retrans-

missions) due to normal collisions. On the 16th

collision, it sets the 16coll bit (Register 8,

TxEvent, Bit F) and discards the packet. If the

DS238PP2

31

Share Link: