dp83846a National Semiconductor Corporation, dp83846a Datasheet - Page 20

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

The 100BASE-TX MLT-3 signal sourced by the TD com-
mon driver output pins 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).
The 100BASE-TX transmit TP-PMD function within the
DP83846A is capable of sourcing only MLT-3 encoded
data. Binary output from the TD outputs is not possible in
100 Mb/s mode.
3.3 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.
See Figure 8 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.
The Receive section consists of the following functional
blocks:
The DP83846A is completely ANSI TP-PMD compliant and
includes Base Line Wander (BLW) compensation. The
BLW compensation block can successfully recover the TP-
PMD defined “killer” pattern and pass it to the digital adap-
tive equalization block.
BLW can generally be defined as the change in the aver-
age DC content, over time, of an AC coupled digital trans-
mission over a given transmission medium. (i.e., copper
wire).
BLW results from the interaction between the low fre-
quency components of a transmitted bit stream and the fre-
Figure 7. 100BASE-TX BLW Event
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— ADC
— Input and BLW Compensation
— Signal Detect
— Digital Adaptive Equalization
— MLT-3 to Binary Decoder
— Clock Recovery Module
— NRZI to NRZ Decoder
— Serial to Parallel
— DESCRAMBLER (bypass option)
— Code Group Alignment
— 4B/5B Decoder (bypass option)
— Link Integrity Monitor
— Bad SSD Detection
The bypass option for the functional blocks within the
100BASE-TX receiver provides flexibility for applications
where data conversion is not always required.
3.3.1 Input and Base Line Wander Compensation
Unlike the DP83223V Twister, the DP83846A requires no
external attenuation circuitry at its receive inputs, RD
accepts TP-PMD compliant waveforms directly, requiring
only a 100 termination plus a simple 1:1 transformer.
quency response of the AC coupling component(s) within
the transmission system. If the low frequency content of the
digital bit stream goes below the low frequency pole of the
AC coupling transformers then the droop characteristics of
the transformers will dominate resulting in potentially seri-
ous BLW.
The digital oscilloscope plot provided in Figure 7 illustrates
the severity of the BLW event that can theoretically be gen-
erated during 100BASE-TX packet transmission. This
event consists of approximately 800 mV of DC offset for a
period of 120 s. Left uncompensated, events such as this
can cause packet loss.
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