DP83815 datasheet(21/108 Pages) NSC

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3.0 Functional Description (Continued)

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Figure 3-6 100BASE-TX Transmit Block Diagram

3.9.1 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 3-1 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 de-assertion 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 (re-assertion of

Transmit Enable).

3.9.2 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

distributed 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

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

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

serial NRZ data from the code-group encoder. The result is

a scrambled data stream with sufficient randomization to

decrease radiated emissions at certain frequencies by as

much as 20 dB.

FROM CGM

BP_4B5B

BP_SCR

4B5B CODE-

MUX

5B PARALLEL

SCRAMBLER

MUX

NRZ TO NRZI

BINARY

TD +/-

100BASE-TX

GROUP ENABLER

TXD(3:0)/TXER

TXCLK

TO SERIAL

ENCODER

TO MLT-3/

COMMON

DRIVER

LOOPBACK

DP83815 Datasheet(PDF) 21 Page - National Semiconductor (TI)

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