80221 datasheet(14/91 Pages) LSI

14 / 91 page

80220/80221

MD400159/E

3.4 ENCODER

3.4.1 4B5B Encoder - 100 Mbps

100Base-TX requires that the data be 4B5B encoded.

4B5B coding converts the 4-Bit data nibbles into 5-Bit date

code words. The mapping of the 4B nibbles to the 5B code

words is specified in IEEE 802.3 and shown in Table 2.

The 4B5B encoder on the 80220/80221 takes 4B nibbles

from the controller interface, converts them into 5B words

according to Table 2, and sends the 5B words to the

scrambler. The 4B5B encoder also substitutes the first 8

bits of the preamble with the SSD delimiters (a.k.a. /J/K/

symbols) and adds an ESD delimiter (a.k.a /T/R/ symbols)

to the end of every packet, as defined in IEEE 802.3 and

shown in Figure 2. The 4B5B encoder also fills the period

between packets, called the idle period, with the a continu-

ous stream of idle symbols, as shown in Figure 2.

3.4.2 Manchester Encoder - 10 Mbps

The Manchester encoding process combines clock and

NRZ data such that the first half of the data bit contains the

complement of the data, and the second half of the data bit

contains the true data, as specified in IEEE 802.3. This

guarantees that a transition always occurs in the middle of

the bit cell. The Manchester encoder on the 80220/80221

converts the 10 Mbps NRZ data from the controller inter-

face into a Manchester Encoded data stream for the TP

transmitter and adds a start of idle pulse (SOI) at the end

of the packet as specified in IEEE 802.3 and shown in

Figure 2. The Manchester encoding process is only done

on actual packet data, and the idle period between packets

is not Manchester encoded and filled with link pulses.

Table 2. 4B/5B Symbol Mapping

Symbol

Description 5B Code

4B Code

Name

Data 0

11110

0000

Data 1

01001

0001

Data 2

10100

0010

Data 3

10101

0011

Data 4

01010

0100

Data 5

01011

0101

Data 6

01110

0110

Data 7

01111

0111

Data 8

10010

1000

Data 9

10011

1001

Data A

10110

1010

Data B

10111

1011

Data C

11010

1100

Data D

11011

1101

Data E

11100

1110

Data F

11101

1111

Idle

11111

0000

SSD #1

11000

0101

SSD #2

10001

0101

ESD #1

01101

0000

ESD #2

00111

0000

Halt

00100

Undefined

---

Invalid

All

0000*

codes

others*

* These 5B codes are not used. For decoder, these 5B

codes are decoded to 4B 0000. For encoder, 4B 0000 is

encoded to 5B 11110, as shown in symbol Data 0.

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