LAN8740A Datasheet, PDF (20/136 Pages) Microchip Technology – Small Footprint MII/RMII 10/100 Energy Efficient Ethernet Transceiver with HP Auto-MDIX and flexPWR® Technology

LAN8740A/LAN8740Ai
3.1.2.1 100M Receive Input
The MLT-3 from the cable is fed into the transceiver (on inputs RXP and RXN) via a 1:1 ratio transformer. The ADC
samples the incoming differential signal at a rate of 125M samples per second. Using a 64-level quanitizer, it generates
6 digital bits to represent each sample. The DSP adjusts the gain of the ADC according to the observed signal levels
such that the full dynamic range of the ADC can be used.
3.1.2.2 Equalizer, Baseline Wander Correction and Clock and Data Recovery
The 6 bits from the ADC are fed into the DSP block. The equalizer in the DSP section compensates for phase and ampli-
tude distortion caused by the physical channel consisting of magnetics, connectors, and CAT- 5 cable. The equalizer
can restore the signal for any good-quality CAT-5 cable between 1 m and 100 m.
If the DC content of the signal is such that the low-frequency components fall below the low frequency pole of the iso-
lation transformer, then the droop characteristics of the transformer will become significant and Baseline Wander (BLW)
on the received signal will result. To prevent corruption of the received data, the transceiver corrects for BLW and can
receive the ANSI X3.263-1995 FDDI TP-PMD defined “killer packet” with no bit errors.
The 100M PLL generates multiple phases of the 125 MHz clock. A multiplexer, controlled by the timing unit of the DSP,
selects the optimum phase for sampling the data. This is used as the received recovered clock. This clock is used to
extract the serial data from the received signal.
3.1.2.3 NRZI and MLT-3 Decoding
The DSP generates the MLT-3 recovered levels that are fed to the MLT-3 converter. The MLT-3 is then converted to an
NRZI data stream.
3.1.2.4 Descrambling
The descrambler performs an inverse function to the scrambler in the transmitter and also performs the Serial In Parallel
Out (SIPO) conversion of the data.
During reception of IDLE (/I/) symbols. the descrambler synchronizes its descrambler key to the incoming stream. Once
synchronization is achieved, the descrambler locks on this key and is able to descramble incoming data.
Special logic in the descrambler ensures synchronization with the remote transceiver by searching for IDLE symbols
within a window of 4000 bytes (40 µs). This window ensures that a maximum packet size of 1514 bytes, allowed by the
IEEE 802.3 standard, can be received with no interference. If no IDLE-symbols are detected within this time-period,
receive operation is aborted and the descrambler re-starts the synchronization process.
3.1.2.5 Alignment
The de-scrambled signal is then aligned into 5-bit code-groups by recognizing the /J/K/ Start-of-Stream Delimiter (SSD)
pair at the start of a packet. Once the code-word alignment is determined, it is stored and utilized until the next start of
frame.
3.1.2.6 5B/4B Decoding
The 5-bit code-groups are translated into 4-bit data nibbles according to the 4B/5B table. The translated data is pre-
sented on the RXD[3:0] signal lines. The SSD, /J/K/, is translated to “0101 0101” as the first 2 nibbles of the MAC pre-
amble. Reception of the SSD causes the transceiver to assert the receive data valid signal, indicating that valid data is
available on the RXD bus. Successive valid code-groups are translated to data nibbles. Reception of either the End of
Stream Delimiter (ESD) consisting of the /T/R/ symbols, or at least two /I/ symbols causes the transceiver to de-assert
the carrier sense and receive data valid signals.
Note: These symbols are not translated into data.
DS00001987A-page 20
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