LAN91C111 datasheet(23/125 Pages) MICROCHIP

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DS00002276A-page 23

LAN91C111

7.7.2

ENCODER

7.7.2.1

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. The 4B5B encoder on the

LAN91C111 takes 4B nibbles from the controller interface, converts them into 5B words 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/ sym-

bols) and adds an ESD delimiter (a.k.a. MR/ symbols) to the end of every packet, as defined in IEEE 802.3. The 4B5B

encoder also fills the period between packets, called the idle period, with the continuous stream of idle symbols.

7.7.2.2

Manchester Encoder - 10 Mbps

The Manchester encoding process combines clock and NRZ data such that the first half of the data bit contains the com-

plement of the data, and the second half of the data bit contains the true data, as specified in IEEE 802.3. This ensures

that a transition always occurs in the middle of the bit call. The Manchester encoder on the LAN91C111 converts the

10Mbps NRZ data from the controller interface 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. 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.

7.7.3

DECODER

7.7.3.1

4B5B Decoder - 100 Mbps

Since the TP input data is 4B5B encoded on the transmit side, it must also be decoded by the 4B5B decoder on the

receive side. The mapping of the 5B nibbles to the 4B code words is specified in IEEE 802.3. The 4B5B decoder on the

LAN91C111 takes the 5B code words from the descrambler, converts them into 4B nibbles per Table 2, and sends the

4B nibbles to the controller interface. The 4B5B decoder also strips off the SSD delimiter (a.k.a. /J/K/ symbols) and

replaces them with two 4B Data 5 nibbles (a.k.a. /5/ symbol), and strips off the ESD delimiter (a.k.a. /T/R/ symbols) and

replaces it with two 4B Data 0 nibbles (a.k.a. /I/symbol), per IEEE 802.3 specifications and shown in Figure 7-3.

TABLE 7-1:

4B/5B SYMBOL MAPPING

Symbol Name

Description

5B Code

4B Code

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

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