CS8920A datasheet(32/144 Pages) CIRRUS

32 / 144 page

Onecoll bit is set, the MAC discards the packet

without attempting any re-transmission.

Late Collisions: If a collision is detected after

the first 512 bits have been transmitted, the

MAC immediately terminates transmission,

transmits the jam sequence, discards the packet,

and sets the Out-of-window bit (Register 8,

TxEvent, Bit 9). The CS8920A does not initiate

backoff or attempt to re-transmit the frame. For

additional information about Late Collisions, see

Out-of-Window Error in this section.

Backoff: After the MAC has completed transmit-

ting the jam sequence, it must wait, or "Back

off", before attempting to transmit again. The

amount of time it must wait is determined by

one of two Backoff algorithms: the Standard

Backoff algorithm (ISO/IEC 4.2.3.2.5) or the

Modified Backoff algorithm. The host selects

which algorithm through the ModBackoffE bit

(Register 13, LineCTL, Bit B).

Standard Backoff: The Standard Backoff algo-

rithm, also called the "Truncated Binary

Exponential Backoff", is described by the equa-

tion:

≤ r ≤ 2k

where r (a random integer) is the number of slot

times the MAC must wait (1 slot time = 512 bit

times), and k is the smaller of n or 10, where n

is the number of re-transmission attempts.

Modified Backoff: The Modified Backoff is de-

scribed by the equation:

≤ r ≤ 2k

where r (a random integer) is the number of slot

times the MAC must wait, and k is 3 for n <3

and k is the smaller of n or 10 for n>3, where n

is the number of re-transmission attempts.

The advantage of the Modified Backoff algo-

rithm over the Standard Backoff algorithm is that

it reduces the possibility of multiple collisions

on the first three re-tries. The disadvantage is

that it extends the maximum time needed to gain

access to the network for the first three re-tries.

The host may choose to disable the Backoff al-

gorithm altogether by setting the DisableBackoff

bit (Register 19, TestCTL, Bit B). When dis-

abled, the CS8920A only waits the 9.6

µs IPG

time before starting transmission.

Encoder

Carrier

Detector

Decoder

& PLL

MUX

RXSQL

AUISQL

AUIRX

AUITX

AUICol

Clock

Carrier Sense

RX CLK

RX NRZ

TXCLK

TX NRZ

TEN

Port Select

Collision

MAC

ENDEC

10BASE-T

Transceiver

AUI

Figure 3.9. ENDEC

CS8920A

DS238PP2

CS8920A Datasheet(PDF) 32 Page - Cirrus Logic

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