Datasheet

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DP83865
4.0 Functional Description (Continued)
4.7.5 100BASE-T 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 52 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).
4.7.6 Parallel-to-Serial Converter
The 5-bit (5B) code-groups are then converted to a serial
data stream at 125 MHz.
4.7.7 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 such as frequencies related to
repeating 5B sequences (e.g., 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-ORed with the serial
NRZ data from the serializer block. The result is a scram-
bled data stream with sufficient randomization to decrease
radiated emissions at certain frequencies by as much as 20
Figure 6. 10BASE-T/100BASE-TX Transmit Block Diagram
4B/5B ENCODER
AND
INJECTION LOGIC
MDI +/−
TX_CLK
100BASE-T
TXD[3:0] / TX_ER
FROM PGM
PARALLEL
TO SERIAL
SCRAMBLER
NRZ-TO-NRZI
BINARY-TO-MLT
DIVIDER
100BASE-X
LOOPBACK
NRZ TO
MANCHESTER
DECODER
LINK PULSE
GENERATOR
10, 100, 1000
MUX/DAC/DRIVER
10BASE-T
TXD[3:0] / TX_ER