DP83847 datasheet(17/60 Pages) NSC

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The bypass option for the functional blocks within the

100BASE-TX transmitter provides flexibility for applications

where data conversion is not always required. The

DP83847 implements the 100BASE-TX transmit state

machine diagram as specified in the IEEE 802.3u Stan-

dard, Clause 24.

3.2.1 Code-group Encoding and Injection

The code-group encoder converts 4-bit (4B) nibble data

generated by the MAC into 5-bit (5B) code-groups for

transmission. This conversion is required to allow control

data to be combined with packet data code-groups. Refer

to Table 5: 4B5B Code-Group Encoding/Decoding for 4B to

5B code-group mapping details.

The code-group encoder substitutes the first 8-bits of the

MAC preamble with a J/K code-group pair (11000 10001)

upon transmission. The code-group 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 Transmit Enable signal

from the MAC, the code-group encoder injects the T/R

code-group pair (01101 00111) indicating the end of frame.

After the T/R code-group pair, the code-group encoder

continuously injects IDLEs into the transmit data stream

until the next transmit packet is detected (reassertion of

Transmit Enable).

3.2.2 Scrambler

The scrambler is required to control the radiated emissions

at the media connector and on the twisted pair cable (for

100BASE-TX applications). By scrambling the data, the

total energy launched onto the cable is randomly distrib-

uted over a wide frequency range. Without the scrambler,

energy levels at the PMD and on the cable could peak

beyond FCC limitations at frequencies related to repeating

5B sequences (i.e., continuous transmission of IDLEs).

The scrambler is configured as a closed loop linear feed-

back shift register (LFSR) with an 11-bit polynomial. The

output of the closed loop LFSR is X-ORd with the serial

Figure 4. 100BASE-TX Transmit Block Diagram

4B5B CODE-

GROUP ENCODER

& INJECTOR

SCRAMBLER

NRZ TO NRZI

ENCODER

5B PARALLEL

TO SERIAL

TX_CLK

TXD[3:0] /

TX_ER

100BASE-TX

LOOPBACK

MUX

BINARY TO

MLT-3 /

COMMON

DRIVER

FROM PGM

BP_4B5B

BP_SCR

MUX

DIV BY 5

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