LAN9730 datasheet(77/222 Pages) MICROCHIP

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DS00001946A-page 77

LAN9730/LAN9730i

4.6.1.1

4B/5B Encoding

The transmit data passes from the MII block to the 4B/5B encoder. This block encodes the data from 4-bit nibbles to 5-

bit symbols (known as code-groups) according to Table 4-54. Each 4-bit data-nibble is mapped to 16 of the 32 possible

code-groups. The remaining 16 code-groups are either used for control information or are not valid.

The first 16 code-groups are referred to by the hexadecimal values of their corresponding data nibbles, 0 through F. The

remaining code-groups are given letter designations with slashes on either side. For example, an IDLE code-group is /

I/, a transmit error code-group is /H/, etc.

The encoding process may be bypassed by clearing bit 6 of register 31. When the encoding is bypassed, the 5th transmit

data bit is equivalent to TX_ER.

TABLE 4-54:

4B/5B CODE TABLE

Code Group

Sym

Receiver Interpretation

Transmitter Interpretation

11110

0000

DATA

0000

DATA

01001

0001

10100

0010

10101

0011

01010

0100

01011

0101

01110

0110

01111

0111

10010

1000

10011

1001

10110

1010

10111

1011

11010

1100

11011

1101

11100

1110

11101

1111

11111

IDLE

Sent after /T/R until TX_EN

11000

First nibble of SSD, translated to “0101”

following IDLE, else RX_ER

Sent for rising TX_EN

10001

Second nibble of SSD, translated to

“0101” following J, else RX_ER

Sent for rising TX_EN

01101

First nibble of ESD, causes de-assertion of

CRS if followed by /R/, else assertion of

RX_ER

Sent for falling TX_EN

00111

Second nibble of ESD, causes deasser-

tion of CRS if following /T/, else assertion

of RX_ER

Sent for falling TX_EN

00100

Transmit Error Symbol

Sent for rising TX_ER

00110