10/100 Non-PCI Ethernet Single Chip MAC + PHY

Datasheet

10Mbps operation is similar to the 100Mbps TX operation except, (1) there is no scrambler/descrambler, (2) the encoder/decoder is Manchester instead of 4B5B, (3) the data rate is 10Mbps instead of 100Mbps, and (4) the twisted pair symbol data is two level Manchester instead of ternary MLT-3.

The Management Interface, (hereafter referred to as the MI serial port), is a two pin bi-directional link through which configuration inputs can be set and status outputs can be read. Each block plus the operating modes are described in more detail in the following sections.

7.7.1MII 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.

7.7.2Encoder4B5B 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.

7.7.3Decoder4B5B 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.

Table 7.1 4B/5B Symbol Mapping
SYMBOL NAME

DESCRIPTION

 

5B CODE4B CODE

 

 

 

 

 

0

Data 0

 

11110

0000

 

 

 

 

 

1

Data 1

 

01001

0001

 

 

 

 

 

2

Data 2

 

10100

0010

 

 

 

 

 

SMSC LAN91C111 REV C

 

27

Revision 1.91 (08-18-08)

DATASHEET