AM79C873 datasheet(14/44 Pages) AMD

14 / 44 page

Am79C873

PRELIMINARY

Figure 3.

100BASE Transmitter Functional Block Diagram

The block diagram in Figure 3 provides an overview of

the functional blocks contained in the transmit section.

The transmitter section contains the following func-

tional blocks:

s 4B5B Encoder

s Scrambler

s Parallel-to-Serial Converter

s NRZ-to-NRZI Converter

s PECL Driver (For FX Operation)

s NRZI to MLT-3 (For TX Operation)

s MLT-3 Driver (For TX Operation)

4B5B Encoder

The 4B5B encoder converts 4-bit (4B) nibble data gen-

erated by the MAC Reconciliation Layer into a 5-bit

(5B) code group for transmission (see Table 1). This

conversion is required for control and packet data to be

combined in code groups. The 4B5B encoder substi-

tutes the ﬁrst 8 bits of the MAC preamble with a J/K

code-group pair (11000 10001) upon transmit.

The 4B5B encoder continues to replace subsequent

4B preamble and data nibbles with corresponding 5B

code-groups. At the end of the transmit packet, upon

the deassertion of the Transmit Enable signal from the

MAC Reconciliation layer, the 4B5B encoder injects the

T/R code-group pair (01101 00111) indicating end of

frame. After the T/R code-group pair, the 4B5B encoder

continuously injects IDLEs into the transmit data

stream until Transmit Enable is asserted and the next

transmit packet is detected.

The NetPHY-1 device includes a Bypass 4B5B conver-

sion option within the 100BASE-TX transmitter for sup-

port of applications like 100 Mbps repeaters which do

not require 4B5B conversion.

MII

Signals

MII

Interface/

Control

4B/5B

Encoder

Rise/Fall

Time

CTL

TX CGM

LED

Driver

Carrier

Sense

10BASE-T

Module

LED1-4#

25M OSCI

RXI

10TXD

100TXD

PECL

Driver

FXTD

NRZ

NRZI

NRZI to

MLT-3

Driver

Parallel

to Serial

Scrambler

Collision

Detection

Auto-

Negotiation

22164A-5

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