dp83846a National Semiconductor Corporation, dp83846a Datasheet - Page 17

• Manufacturer Part Number: dp83846a
• Description: Dsphyter ? Single 10/100 Ethernet Transceiver
• Manufacturer: National Semiconductor Corporation

The bypass option for the functional blocks within the
100BASE-TX transmitter provides flexibility for applications
where data conversion is not always required. The
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 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
FROM PGM
BP_SCR
BP_4B5B
Figure 5. 100BASE-TX Transmit Block Diagram
100BASE-TX
LOOPBACK
DIV BY 5
TX_CLK
GROUP ENCODER
5B PARALLEL
TXD[3:0] /
4B5B CODE-
& INJECTOR
TO SERIAL
SCRAMBLER
TX_ER
17
NRZ TO NRZI
BINARY TO
ENCODER
DP83846A implements the 100BASE-TX transmit state
machine diagram as specified in the IEEE 802.3u Stan-
dard, Clause 24.
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
NRZ data from the code-group encoder. The result is a
scrambled data stream with sufficient randomization to
MUX
COMMON
DRIVER
MLT-3 /
TD
MUX
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