Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support

Datasheet

4.2.24B/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 4.1. 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 TXER.

Note that encoding can be bypassed only when the MAC interface is configured to operate in MII mode.

Table 4.1 4B/5B Code Table
CODE

 

 

 

RECEIVER

 

 

TRANSMITTER

 

GROUPSYM

 

INTERPRETATION

 

 

INTERPRETATION

 

 

 

 

 

 

 

 

 

 

 

 

11110

0

0

 

0000

 

DATA

0

 

0000

 

DATA

 

 

 

 

 

 

 

 

 

 

 

 

01001

1

1

 

0001

 

 

1

 

0001

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10100

2

2

 

0010

 

 

2

 

0010

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10101

3

3

 

0011

 

 

3

 

0011

 

 

 

 

 

 

 

 

 

 

 

 

 

 

01010

4

4

 

0100

 

 

4

 

0100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

01011

5

5

 

0101

 

 

5

 

0101

 

 

 

 

 

 

 

 

 

 

 

 

 

 

01110

6

6

 

0110

 

 

6

 

0110

 

 

 

 

 

 

 

 

 

 

 

 

 

 

01111

7

7

 

0111

 

 

7

 

0111

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10010

8

8

 

1000

 

 

8

 

1000

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10011

9

9

 

1001

 

 

9

 

1001

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10110

A

A

 

1010

 

 

A

 

1010

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10111

B

B

 

1011

 

 

B

 

1011

 

 

 

 

 

 

 

 

 

 

 

 

 

 

11010

C

C

 

1100

 

 

C

 

1100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

11011

D

D

 

1101

 

 

D

 

1101

 

 

 

 

 

 

 

 

 

 

 

 

 

 

11100

E

E

 

1110

 

 

E

 

1110

 

 

 

 

 

 

 

 

 

 

 

 

 

 

11101

F

F

 

1111

 

 

F

 

1111

 

 

 

 

 

 

 

 

 

 

 

 

 

11111

I

IDLE

 

 

 

 

Sent after /T/R until TXEN

 

 

 

 

 

 

11000

J

First nibble of SSD, translated to “0101”

Sent for rising TXEN

 

 

 

following IDLE, else RXER

 

 

 

 

 

 

 

 

 

 

 

10001

K

Second nibble of SSD, translated to

Sent for rising TXEN

 

 

 

“0101” following J, else RXER

 

 

 

 

 

 

 

 

 

 

 

01101

T

First nibble of ESD, causes de-assertion

Sent for falling TXEN

 

 

 

of CRS if followed by /R/, else assertion

 

 

 

 

 

 

 

of RXER

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SMSC LAN8720/LAN8720i

19

Revision 1.0 (05-28-09)

 

DATASHEET