80220 Datasheet, PDF (14/91 Pages) LSI Computer Systems – 100BASE-TX/10BASE-T Ethernet Media Interface Adapter

80220/80221
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
0
Data 0
11110
0000
1
Data 1
01001
0001
2
Data 2
10100
0010
3
Data 3
10101
0011
4
Data 4
01010
0100
5
Data 5
01011
0101
6
Data 6
01110
0110
7
Data 7
01111
0111
8
Data 8
10010
1000
9
Data 9
10011
1001
A
Data A 10110
1010
B
Data B 10111
1011
C
Data C 11010
1100
D
Data D 11011
1101
E
Data E 11100
1110
F
Data F
11101
1111
I
Idle
11111
0000
J
SSD #1 11000
0101
K
SSD #2 10001
0101
T
ESD #1 01101
0000
R
ESD #2 00111
0000
H
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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MD400159/E