VT6103 datasheet(27/46 Pages) VIA

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VT6103 - Fast Ethernet 10 / 100 PHY / Transceiver

Revision 1.1 April 15, 2002

-21-

Functional Description

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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 data code

words. The mapping of the 4B nibbles to the 5B code words

is specified in IEEE 802.3.

The 4B5B encoder takes 4B

nibbles and converts to 5B words according to IEE802.3, and

sends the 5B words to the scrambler. The 4B5B encoder also

substitutes the first 8 bits of the preamble with the SSD

delimiters (/J/K symbols) and adds an ESD delimiter (/T/R

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 a continuous stream of idle

symbols, as shown in IEE802.3.

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 cell. The Manchester encoder in the VT6103 converts the

10 Mbps 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. Manchester encoding of the NRZ

data occurs only when TXEN is asserted.

Decoder

4B5B Encoder – 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 5B nibbles to 4B code words is

specified in IEEE 802.3. The 4B5B decoder takes the 5B

code words from the descrambler, converts them into 4B

nibbles. The 4B5B decoder also strips off the SSD delimiter

(/J/K symbols) and replaces them with two 4B Data 5 nibbles

(/5 symbol), and strips off the ESD delimiter (/T/R symbols)

and replaces it with two 4B Data 0 nibbles (/I symbol), per

IEEE 802.3 specifications. The 4B5B decoder detects SSD,

ESD and, codeword errors in the incoming data stream as

specified in IEEE 802.3.

These errors are indicated by

asserting RXER output while the errors are being transmitted

across RXD[3:0], and they are also indicated in the MI

MI Status Output register.

Manchester Encoder – 10 Mbps

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

Manchester encoded data stream from the TP receiver into

NRZ data for the controller interface by decoding the data and

stripping off the SOI pulse. Since the clock and data recovery

block has already separated the clock and data from the TP

receiver, the Manchester decoding process to NRZ data is

inherently performed by that block.

VT6103 Datasheet(PDF) 27 Page - VIA Technologies, Inc.

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