LAN9220 datasheet(60/142 Pages) MICROCHIP

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LAN9220

DS00002417A-page 60

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4.3

100Base-TX Receive

The receive data path is shown in Figure 4-2. Detailed descriptions are given below.

4.3.1

100M RECEIVE INPUT

The MLT-3 from the cable is fed into the PHY (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.

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 SSD, /J/K/, is translated

to “0101 0101” as the first 2 nibbles of the MAC preamble. Reception of the SSD causes the PHY to assert the internal

RX_DV signal, indicating that valid data is available on the Internal RXD bus. Successive valid code-groups are trans-

lated 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 the internal carrier sense and RX_DV.

These symbols are not translated into data.

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