2 4b/5b encoding, Table 4.1 4b/5b code table, 4b/5b encoding – SMSC FlexPWR LAN8720 User Manual
Page 19
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
. 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