DM9621 datasheet(50/66 Pages) DAVICOM

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DM9621

USB2.0 to Fast Ethernet Controller

Preliminary

Version: DM9621 -14-DS-P02

February 23, 2011

7.2.2 100Base-TX Operation

The block diagram in figure 3 provides an overview of

the functional blocks contained in the transmit

section.

The transmitter section contains the following

functional blocks:

- 4B5B Encoder

- Scrambler

- Parallel to Serial Converter

- NRZ to NRZI Converter

- NRZI to MLT-3

- MLT-3 Driver

7.2.3 4B5B Encoder

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

generated by the MAC Reconciliation Layer into a

5-bit (5B) code group for transmission, reference

Table 1. This conversion is required for control and

packet data to be combined in code groups. The

4B5B encoder substitutes 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 DM9621 includes a Bypass 4B5B conversion

option within the 100Base-TX Transmitter for support

of applications like 100 Mbps repeaters which do not

require 4B5B conversion.

7.2.4 Scrambler

The scrambler is required to control the radiated

emissions (EMI) by spreading the transmit energy

across the frequency spectrum at the media

connector and on the twisted pair cable in

100Base-TX operation.

By scrambling the data, the total energy presented to

the cable is randomly distributed over a wide

frequency range. Without the scrambler, energy

levels on the cable could peak beyond FCC

limitations at frequencies related to repeated 5B

sequences like continuous transmission of IDLE

symbols. The scrambler output is combined with the

NRZ 5B data from the code-group encoder via an

XOR logic function. The result is a scrambled data

stream with sufficient randomization to decrease

radiated emissions at critical frequencies.

7.2.5 Parallel to Serial Converter

The Parallel to Serial Converter receives parallel 5B

scrambled data from the scrambler and serializes it

(converts it from a parallel to a serial data stream).

The serialized data stream is then presented to the

NRZ to NRZI Encoder block

7.2.6 NRZ to NRZI Encoder

After the transmit data stream has been scrambled

and serialized, the data must be NRZI encoded for

compatibility with the TP-PMD standard for

100Base-TX transmission over Category-5

unshielded twisted pair cable.

7.2.7 MLT-3 Converter

The MLT-3 conversion is accomplished by converting

the data stream output from the NRZI encoder into

two binary data streams with alternately phased logic

one events.

7.2.8 MLT-3 Driver

The two binary data streams created at the MLT-3

converter are fed to the twisted pair output driver

which converts these streams to current sources and

alternately drives either side of the transmit

transformer primary winding resulting in a minimal

current MLT-3 signal. Refer to figure 4 for the block

diagram of the MLT-3 converter.

DM9621 Datasheet(PDF) 50 Page - Davicom Semiconductor, Inc.

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