LAN91C111_11 Datasheet, PDF (27/133 Pages) SMSC Corporation – 10/100 Non-PCI Ethernet Single Chip MAC + PHY

10/100 Non-PCI Ethernet Single Chip MAC + PHY
Datasheet
7.7.1
7.7.2
7.7.3
MII Disable
The internal PHY MII interface can be disabled by setting the MII disable bit in the MI serial port Control
register. When the MII is disabled, the MII inputs are ignored, the MII outputs are placed in high
impedance state, and the TP output is high impedance.
Encoder
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/ symbols) 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.
Manchester Encoder - 10 Mbps
The Manchester encoding process combines clock and NRZ data such that the first half of the data
bit contains the complement of the data, and the second half of the data bit contains the true data, as
specified in IEEE 802.3. This guarantees 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.
Decoder
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.
SYMBOL NAME
0
1
2
3
4
5
6
SMSC LAN91C111 REV C
Table 7.1 4B/5B Symbol Mapping
DESCRIPTION
Data 0
Data 1
Data 2
Data 3
Data 4
Data 5
Data 6
5B CODE
11110
01001
10100
10101
01010
01011
01110
27
DATASHEET
4B CODE
0000
0001
0010
0011
0100
0101
0110
Revision 1.92 (06-27-11)