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).
4.2 100BASE-TX RECEIVER
The 100BASE-TX receiver consists of several functional
blocks which convert the scrambled MLT-3 125 Mb/s serial
data stream to synchronous 4-bit nibble data that is pro-
vided to the MII. Because the 100BASE-TX TP-PMD is
integrated, the differential input pins, RD±, can be directly
routed from the AC coupling magnetics.
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
See Figure 7 for a block diagram of the 100BASE-TX
receive function. This provides an overview of each func-
tional block within the 100BASE-TX receive section.
decrease radiated emissions at certain frequencies by as The Receive section consists of the following functional
much as 20 dB. The DP83848I uses the PHY_ID (pins blocks:
PHYAD [4:0]) to set a unique seed value.
— Analog Front End
— Digital Signal Processor
— Signal Detect
4.1.2 NRZ to NRZI Encoder
— MLT-3 to Binary Decoder
After the transmit data stream has been serialized and
scrambled, the data must be NRZI encoded in order to
— NRZI to NRZ Decoder
— Serial to Parallel
— Descrambler
comply with the TP-PMD standard for 100BASE-TX trans-
mission over Category-5 Unshielded twisted pair cable.
— Code Group Alignment
— 4B/5B Decoder
4.1.3 Binary to MLT-3 Convertor
— Link Integrity Monitor
— Bad SSD Detection
The Binary to MLT-3 conversion is accomplished by con-
verting the serial binary data stream output from the NRZI
encoder into two binary data streams with alternately
phased logic one events. These two binary streams are
then fed to the twisted pair output driver which converts the
voltage to current and alternately drives either side of the
transmit transformer primary winding, resulting in a MLT-3
signal.
4.2.1 Analog Front End
In addition to the Digital Equalization and Gain Control, the
DP83848I includes Analog Equalization and Gain Control
in the Analog Front End. The Analog Equalization reduces
the amount of Digital Equalization required in the DSP.
The 100BASE-TX MLT-3 signal sourced by the PMD Out-
put Pair common driver is slew rate controlled. This should
be considered when selecting AC coupling magnetics to
ensure TP-PMD Standard compliant transition times (3 ns
< Tr < 5 ns).
4.2.2 Digital Signal Processor
The Digital Signal Processor includes Adaptive Equaliza-
tion with Gain Control and Base Line Wander Compensa-
tion.
The 100BASE-TX transmit TP-PMD function within the
DP83848I is capable of sourcing only MLT-3 encoded data.
Binary output from the PMD Output Pair is not possible in
100 Mb/s mode.
www.national.com
26
TI [ TEXAS INSTRUMENTS ]
