LAN8700 Datasheet, PDF (17/74 Pages) SMSC Corporation – ±15kV ESD Protected MII/RMII Fast-Ethernet PHY with HP Auto-MDIX and SMSC flexPWR TM in a Small Footprint

LAN8700/LAN8700i
4.3.2 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 1m and 150m.
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 PHY 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 125MHz 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.
4.3.3 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.
4.3.4 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 PHY by searching for IDLE symbols within a
window of 4000 bytes (40us). 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.
The descrambler can be bypassed by setting bit 0 of register 31.
4.3.5 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.
4.3.6 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 PHY to assert the RX_DV 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 PHY to de-assert carrier sense and RX_DV.
These symbols are not translated into data.
The decoding process may be bypassed by clearing bit 6 of register 31. When the decoding is bypassed the 5^th receive
data bit is driven out on RX_ER/RXD4. Decoding may be bypassed only when the MAC interface is in MII mode.
4.3.7 RECEIVE DATA VALID SIGNAL
The Receive Data Valid signal (RX_DV) indicates that recovered and decoded nibbles are being presented on the
RXD[3:0] outputs synchronous to RX_CLK. RX_DV becomes active after the /J/K/ delimiter has been recognized and
RXD is aligned to nibble boundaries. It remains active until either the /T/R/ delimiter is recognized or link test indicates
failure or SIGDET becomes false.
RX_DV is asserted when the first nibble of translated /J/K/ is ready for transfer over the Media Independent Interface
(MII mode).
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DS00002260A-page 17