DM9161 datasheet(15/47 Pages) ETC

15 / 47 page

DM9161

10/100 Mbps Fast Ethernet Physical Layer Single Chip Transceiver

Final

Version: DM9161-DS-F02

May 10,2002

The block diagram in figure 7-3 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 Encoder

- NRZI to MLT-3

- MLT-3 Driver

7.2.1.1 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, see reference Table 7-1. 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 DM9161 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.

7.2.1.2 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.

7.2.1.3 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

7.2.1.4 NRZ to NRZI Encoder

Since 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.

7.2.1.5 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.

7.2.1.6 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’s primary winding, resulting in a

minimal current MLT-3 signal. Refer to figure 7-4 for the

block diagram of the MLT-3 converter.

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