LAN8700 datasheet(21/73 Pages) SMSC

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±15kV ESD Protected MII/RMII Fast-Ethernet PHY with HP Auto-MDIX and SMSC flexPWRTM in a Small Footprint

Datasheet

SMSC LAN8700/LAN8700I

Revision 1.0 (02-09-07)

DATASHEET

Figure 4.2 Receive Data Path

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 amplitude 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 isolation 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.

MAC

A/D

Converter

MLT-3

Converter

NRZI

Converter

4B/5B

Decoder

Magnetics

CAT-5

RJ45

100M

PLL

MII 25Mhz by 4 bits

RMII 50Mhz by 2 bits

RX_CLK

(for MII only)

25MHz by

5 bits

NRZI

MLT-3

6 bit Data

Descrambler

and SIPO

125 Mbps Serial

DSP: Timing

recovery, Equalizer

and BLW Correction

MLT-3

MII/RMII

25MHz

by 4 bits

Ext Ref_CLK (for RMII only)

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