Draft 6/5/00
2-16
Functional Description
Copyright 2000 by LSI Logic Corporation. All rights reserved.
2.2.3.3 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 2.2.3.1,
“4B5B Encoder (100 Mbits/s),” page 2-14
. Setting the bit automatically
places the device in the FBI mode as described in the
subsection entitled
“FBI Selection”
on
page 2-13
.
2.2.4 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.
2.2.4.1 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 2.4
, 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 2.2
).
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].
2.2.4.2 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