LAN83C183 Datasheet, PDF (22/113 Pages) SMSC Corporation – FAST ETHERNET PHYSICAL LAYER DEVICE

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 process guarantees that a
transition always occurs in the middle of the bit cell. The Manchester encoder on the
device converts the 10 Mbits/s NRZI data from the Ethernet controller interface into
a single 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 and shown in Figure 1.2. The
Manchester encoding process is only done on actual packet data; during the idle
period between packets, no signal is transmitted except for periodic link pulses.
1.2.3.3 ENCODER BYPASS
Setting the Bypass Encoder/Decoder bit (BYP_ENC) in the MI serial port
Configuration 1 register bypasses the 4B5B encoder. When this bit is set, 5B code
words are passed directly from the controller interface to the scrambler without any
of the alterations described in Section 1.2.3.1, “4B5B Encoder (100 Mbits/s),”
page 1-20. Setting the bit automatically places the device in the FBI mode as
described in the subsection entitled “FBI Selection” on page 1-19.
1.2.4 Decoder
This section describes the 4B5B decoder, used in 100 Mbits/s operation, which
converts 5B encoded data to 4B nibbles. It also describes the Manchester Decoder,
used in 10BASE-T operation.
1.2.4.1 4B5B DECODER (100 MBITS/S)
Because the TP input data is 4B5B encoded on the transmit side, the 4B5B decoder
must decode it on the receive side. The mapping of the 5B codewords to the 4B
nibbles is specified in IEEE 802.3. The 4B5B decoder takes the 5B codewords from
the descrambler, converts them into 4B nibbles according to Table 1.4, and sends
the 4B nibbles to the receive Ethernet controller.
The 4B5B decoder also strips off the SSD delimiter (/J/K/ symbols), and replaces it
with two 4B Data 5 nibbles (/5/ symbol). It also strips off the ESD delimiter (/T/R/
symbols), and replaces it with two 4B Data 0 nibbles
(/I/ symbol), per IEEE 802.3 specifications (see Figure 1.2).
The 4B5B decoder detects SSD, ESD, and codeword errors in the incoming data
stream as specified in IEEE 802.3. To indicate these errors, the device asserts the
RX_ER output as well as the SSD, ESD, and CWRD bits in the MI serial port Status
Output register while the errors are being transmitted across RXD[3:0].
1.2.4.2 MANCHESTER DECODER (10 MBITS/S)
In Manchester coded data, 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. The Manchester
Decoder converts the single data stream from the TP receiver into non-return to zero
(NRZ) data for the controller interface. To do this, it decodes the data and strips off
the SOI pulse. Because the Clock and Data Recovery block has already separated
the clock and data from the TP receiver, that block inherently performs the the
Manchester decoding.
1.2.4.3 DECODER BYPASS
Setting the Bypass Encoder/Decoder bit (BYP_ENC) in the MI serial port
Configuration 1 register bypasses the 4B5B decoder. When this bit is set, 5B code
words are passed directly to the controller interface from the descrambler without any
of the alterations described in Section 1.2.4, “Decoder,” page 1-22. Additionally, the
SMSC DS – LAN83C183
22
Rev. 12/14/2000