DP83936AVF datasheet(14/104 Pages) NSC

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30 Functional Description (Continued)

32 IEEE 8023 ENCODERDECODER (ENDEC) UNIT

The EncoderDecoder (ENDEC) unit is the interface be-

tween either the Twisted Pair Interface Module or the Ether-

net transceiver and the Media Access Control (MAC) unit

Providing the Manchester data encoding and decoding

functions for IEEE 8023 Ethernet Thin-Ethernet or Twisted

Pair types of local area networks the ENDEC operations of

SONIC-T are identical to those of the DP83910A CMOS

Serial Network Interface device During transmission the

ENDEC unit combines non-return-zero (NRZ) data from the

MAC section and clock pulses to produce Manchester data

and sends the converted data differentially to the transceiv-

er Conversely during reception an analog Phase Lock

Loop (PLL) decodes the Manchester data into both NRZ

formatted data and a receive clock The SONIC-T ENDEC

unit is a functionally complete Manchester encoderdecod-

er incorporating a balanced driver and receiver an on-board

crystal oscillator a collision signal translator and a diagnos-

tic loopback The features include

Compatibility with Ethernet I and II

IEEE 8023

10BASE5 10BASE2 and 10BASE-T

10Mbs Manchester encodingdecoding with receive

clock recovery

No precision components requirement

Loopback capability for diagnostics

Squelch circuitry at the receive and collision inputs reject

noise

Connection to the transceiver (Attachment Unit Inter-

face) cable via external pulse transformer

321 ENDEC Operation

The primary function of the ENDEC unit

(Figure 3-5) is to

perform the encoding and decoding necessary for compati-

bility between the differential pair Manchester encoded data

of the transceiver and the Non-Return-to-Zero (NRZ) serial

data of the MAC unit data line In addition to encoding and

decoding the data stream the ENDEC also supplies all of

the special signals (eg collision detect carrier sense and

clocks) necessary to the MAC unit The signals provided to

the MAC unit from the on-chip ENDEC are also provided as

outputs to the user

Manchester Encoder and Differential Output Driver

During transmission to the network the ENDEC unit trans-

lates the NRZ serial data from the MAC unit into differential

pair Manchester encoded data To perform this operation

the NRZ bit stream from the MAC unit is passed through the

Manchester encoder block of the ENDEC unit Once the bit

stream is encoded it is transmitted out differentially to the

transmit differential pair through the transmit driver

The SONIC-T Controller is compatible with the IEEE 8023

‘‘full-step’’ standard That is the Transmita and Transmitb

differential outputs are at equal voltages while they are idle

at the primary of the isolation transformer at the network

interface This voltage relationship provides a zero differen-

tial voltage for operation with transformer coupled loads

(See Section 81 for network interfacing considerations)

Manchester Decoder

During reception from the network

the differential receive data from the transceiver is convert-

ed from Manchester encoded data into NRZ serial data and

a receive clock which are sent to the receive data and

clock inputs of the MAC unit To perform this operation the

signal is passed to the PLL decoder block once it is re-

ceived from the differential receiver The PLL decodes the

data and generates a data receive clock and a NRZ serial

data stream to the MAC unit

Data typically becomes valid from the decoder within 6 bit

times and the decoder detects the end of a frame when no

more mid-bit transitions are detected (See Section 81 for

network interfacing considerations)

Special Signals

In addition to performing the Manchester

encoding and decoding function the ENDEC unit provides

control and clocking signals to the MAC unit The ENDEC

sends a carrier sense (CRS) signal that indicates to the

MAC unit that data is present from the network on the

ENDEC’s receive differential pair When the ENDEC’s colli-

sion receiver detects a 10 MHz signal on the differential

collision input pair the ENDEC unit provides the MAC unit

with a collision detection signal (COL) COL indicates that a

collision is taking place somewhere on the network

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