Datasheet

2.0 Functional Description (Continued)
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The code-group encoder converts 4 bit (4B) nibble data
generated by the MAC into 5 bit (5B) code-groups for trans-
mission. This conversion is required to allow control data to
be combined with packet data code-groups. Refer to Table
2 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 transmit. 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 or Repeater, 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).
The DP83843 also incorporates a special injection function
which allows for fixed transmission of special repeating pat-
terns for testing purposes. These special patterns are not
delimited with Start of Stream Delimiter (SSD) or End of
Stream Delimiter (ESD) code-groups and should not be
enabled during normal network connectivity.
These patterns, selectable via bits [8:7] of PCRS (address
16h), include:
8=0, 7=0: Normal operation (injection disabled)
8=0, 7=1: Transmit repeating FEFI pattern
8=1, 7=0: Transmit repeating 1.28 µs period squarewave
8=1, 7=1: Transmit repeating 160 ns period squarewave
Note that these patterns will be routed through the transmit
scrambler and become scrambled (and therefore poten-
tially less useful) unless the scrambler is bypassed via bit
12 of LBR (address 17h). It should be noted that if the
scrambler is bypassed by forcing the
FXEN pin (and subse-
quently resetting the device) the TPTD+/− outputs will
become disabled and the test pattern data will be routed to
the FXTD/AUITD+/− outputs. Additionally, the test patterns
will not be generated if the DP83843 is in symbol mode.
2.2.1 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 combined with the NRZ
5B data from the code-group encoder via an X-OR logic
function. The result is a scrambled data stream with suffi-
cient randomization to decrease radiated emissions at cer-
tain frequencies by as much as 20 dB. The DP83843 uses
the PHYID as determined by the PHYAD [4:0] pins to set a
unique seed value for the scrambler so that the total energy
produced by a multi-PHY application (i.e. repeater) distrib-
utes the energy out of phase across the spectrum and
helps to reduce overall electro-magnetic radiation.
The scrambler is automatically bypassed when the
DP83843 is placed in
FXEN mode via hardware or, alterna-
tively, controlled by bit 12 of LBR (address 17h) via soft-
ware.
2.2.2 NRZ to NRZI Encoder
After the transmit data stream has been scrambled and
serialized, the data must be NRZI encoded in order to com-
ply with the TP-PMD standard for 100BASE-TX transmis-
sion over Category-5 unshielded twisted pair cable. There
is no ability to bypass this block within the DP83843.
2.2.3 Binary to MLT-3 Convertor / Common Driver
The Binary to MLT-3 conversion is accomplished by con-
verting the serial binary datastream 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
these streams to current sources and alternately drives
either side of the transmit transformer primary winding
resulting in a minimal current (20 mA max) MLT-3 signal.
Refer to Figure 4 .
Figure 1. Binary to MLT-3 conversion
D
Q
Q
CP
binary_in
binary_plus
binary_minus
binary_in
binary_plus
binary_minus
COMMON
DRIVER
MLT-3
differential MLT-3