4.2.2.2 Base Line Wander Compensation
Figure 9. 100BASE-TX BLW Event
The DP83848I is completely ANSI TP-PMD compliant and PMD Standard as well as the IEEE 802.3 100BASE-TX
includes Base Line Wander (BLW) compensation. The Standard for both voltage thresholds and timing parame-
BLW compensation block can successfully recover the TP- ters.
PMD defined “killer” pattern.
Note that the reception of normal 10BASE-T link pulses
BLW can generally be defined as the change in the aver- and fast link pulses per IEEE 802.3u Auto-Negotiation by
age DC content, relatively short period over time, of an AC the 100BASE-TX receiver do not cause the DP83848I to
coupled digital transmission over a given transmission assert signal detect.
medium. (i.e., copper wire).
BLW results from the interaction between the low fre-
quency components of a transmitted bit stream and the fre- 4.2.4 MLT-3 to NRZI Decoder
quency response of the AC coupling component(s) within
The DP83848I decodes the MLT-3 information from the
the transmission system. If the low frequency content of
Digital Adaptive Equalizer block to binary NRZI data.
the digital bit stream goes below the low frequency pole of
the AC coupling transformers then the droop characteris-
tics of the transformers will dominate resulting in potentially
serious BLW.
4.2.5 NRZI to NRZ
The digital oscilloscope plot provided in Figure 9 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 µs. Left uncompensated, events such as this
can cause packet loss.
In a typical application, the NRZI to NRZ decoder is
required in order to present NRZ formatted data to the
descrambler.
4.2.6 Serial to Parallel
4.2.3 Signal Detect
The 100BASE-TX receiver includes a Serial to Parallel
converter which supplies 5-bit wide data symbols to the
PCS Rx state machine.
The signal detect function of the DP83848I is incorporated
to meet the specifications mandated by the ANSI FDDI TP-
29
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