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Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
SMSC LAN8720/LAN8720i 19 Revision 1.0 (05-28-09)
DATASHEET
4.2.2 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 4.1. 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 5
th
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
GROUP SYM
RECEIVER
INTERPRETATION
TRANSMITTER
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”
following IDLE, else RXER
Sent for rising TXEN
10001 K Second nibble of SSD, translated to
“0101” following J, else RXER
Sent for rising TXEN
01101 T First nibble of ESD, causes de-assertion
of CRS if followed by /R/, else assertion
of RXER
Sent for falling TXEN