DP83916 datasheet(3/96 Pages) NSC

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10 Functional Description

The SONIC-16

(Figure 1-1 ) consists of an encoderdecoder

(ENDEC) unit media access control (MAC) unit separate

receive and transmit FIFOs a system buffer management

engine and a user programmable system bus interface unit

on a single chip SONIC-16 is highly pipelined providing

maximum system level performance This section provides

a functional overview of SONIC-16

11 IEEE 8023 ENDEC UNIT

The ENDEC (EncoderDecoder) unit is the interface be-

tween the Ethernet transceiver and the MAC unit It pro-

vides the Manchester data encoding and decoding func-

tions for IEEE 8023 EthernetThin-Ethernet type local area

networks The ENDEC operations of SONIC-16 are identical

to 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 into

Manchester data and sends the converted data differentially

to the transceiver Conversely during reception an analog

PLL decodes the Manchester data to NRZ format and re-

ceive clock The ENDEC unit is a functionally complete

Manchester encoderdecoder incorporating a balanced

driver and receiver on-board crystal oscillator collision sig-

nal translator and a diagnostic loopback The features in-

clude

Compatible with Ethernet I and II IEEE 8023 10BASE5

and 10BASE2

10Mbs Manchester encodingdecoding with receive

clock recovery

Requires no precision components

Loopback capability for diagnostics

Externally selectable half or full step modes of operation

at transmit output

Squelch circuitry at the receive and collision inputs reject

noise

Connects to the transceiver (AUI) cable via external

pulse transformer

111 ENDEC Operation

The primary function of the ENDEC unit

(Figure 1-2 ) 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 the

necessary special signals (eg

collision detect

carrier

sense and clocks) to the MAC unit

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 on the Coaxial Transceiver

Interface (eg National’s DP8392) transmit pair To perform

this operation the NRZ bit stream from the MAC unit is

passed through the Manchester encoder block of the EN-

DEC unit Once the bit stream is encoded it is transmitted

out differentially to the transmit differential pair through the

transmit driver

Manchester Decoder

During reception from the network

the differential receive data from the transceiver (eg the

DP8392) is converted 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 once received by the differential

receiver is passed to the phase locked loop (PLL) decoder

block The PLL decodes the data and generates a data re-

ceive clock and a NRZ serial data stream to the MAC unit

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 EN-

DEC’s receive differential pair The MAC unit is also provid-

ed with a collision detection signal (COL) that informs the

MAC unit that a collision is taking place somewhere on the

TLF11722 – 2

FIGURE 1-1 SONIC-16 Block Diagram

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