CS8900A datasheet(32/128 Pages) CIRRUS

32 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

the final 3.2 µs). The MAC then begins transmis-

sion 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 contin-

ues 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 shortened

due to a temporary loss of carrier. Figure 10 dia-

grams the two-part deferral process.

3.9.5.3 Simple Deferral

In the simple deferral scheme, the IPG timer is

started whenever Carrier Sense is deasserted. 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 begins 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. Fig-

ure 11 diagrams the simple deferral process.

3.9.5.4 Collision Resolution

If a collision is detected while the CS8900A is

transmitting, the MAC responds 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):

3.9.5.5 Normal Collisions

If a collision is detected before the end of the pre-

amble and SFD, the MAC finishes the preamble

and SFD, transmits the jam sequence (32-bit pat-

tern of all 0’s), and then initiates Backoff. If a col-

lision is detected after the transmission of the

preamble and SFD but before 512 bit times, the

MAC immediately terminates transmission, trans-

mits the jam sequence, and then initiates Backoff.

In either case, if the Onecoll bit (Register 9, TxC-

MD, Bit 9) is clear, the MAC will attempt to trans-

mit a packet a total of 16 times (the initial attempt

plus 15 retransmissions) due to normal collisions.

On the 16th collision, it sets the 16coll bit

(Register 8, TxEvent, Bit F) and discards the pack-

et. If the Onecoll bit is set, the MAC discards the

packet without attempting any retransmission.

Transmit

Frame

Start Monitoring

Network Activity

IPG

Timer =

6.4

µs?

Network

Active?

Network

Active?

Start IPG

Timer

Network

Active?

Yes

Wait

3.2

µs

Yes

Frame

Ready

and Not

in Backoff?

Figure 10. Two-Part Deferral

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