CS89712 datasheet(51/170 Pages) CIRRUS

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CS89712

DS502PP2

2.26.5.5

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 (TxCMD register

bit 9) is clear, the MAC will attempt to transmit 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 (TxEvent regis-

ter bit F) and discards the packet. If the Onecoll bit

is set, the MAC discards the packet without at-

tempting any retransmission.

2.26.5.6

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 (Tx-

Event bit 9). The Ethernet port does not initiate

backoff or attempt to retransmit the frame. For ad-

ditional information about Late Collisions, see Sec-

tion 2.26.3.1, “Out-of-Window (Late) Collision” in

this section.

2.26.5.7

Backoff

After the MAC has completed transmitting the jam

sequence, it must wait, or "Back off", before at-

tempting to transmit again. The amount of time it

must wait is determined by one of two Backoff al-

gorithms:

the

Standard

Backoff

algorithm

(ISO/IEC 4.2.3.2.5) or the Modified Backoff algo-

rithm. The algorithm used is selected by the Mod-

BackoffE bit (LineCTL register bit B).

2.26.5.8

Standard Backoff

The Standard Backoff algorithm, also called the

"Truncated Binary Exponential Backoff", is de-

scribed by the equation:

≤ 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 retransmission attempts.

2.26.5.9

Modified Backoff

The Modified Backoff is described 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 retransmission attempts.

The advantage of the Modified Backoff algorithm

over the Standard Backoff algorithm is that it re-

duces the possibility of multiple collisions on the

first three retries. The disadvantage is that it ex-

tends the maximum time needed to gain access to

the network for the first three retries.

The software may choose to disable the Backoff al-

gorithm altogether by setting the DisableBackoff

bit (TestCTL register bit B). When disabled, the

Ethernet port only waits the 9.6 µs IPG time before

starting transmission.

2.27 Encoder/Decoder (ENDEC)

The Ethernet port’s integrated encoder/decoder

(ENDEC) circuit is compliant with the relevant

portions of section 7 of the Ethernet standard

(ISO/IEC 8802-3, 1993). Its primary functions in-

clude: Manchester encoding of transmit data; in-

forming the MAC when valid receive data is

present (Carrier Detection); and, recovering the

clock and NRZ data from incoming Manchester-

encoded data.

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