LAN8700I datasheet(16/74 Pages) MICROCHIP

16 / 74 page

LAN8700/LAN8700i

DS00002260A-page 16

 2007-2016 Microchip Technology Inc.

4.2.4

NRZI AND MLT3 ENCODING

The scrambler block passes the 5-bit wide parallel data to the NRZI converter where it becomes a serial 125MHz NRZI

data stream. The NRZI is encoded to MLT-3. MLT3 is a tri-level code where a change in the logic level represents a code

bit “1” and the logic output remaining at the same level represents a code bit “0”.

4.2.5

100M TRANSMIT DRIVER

The MLT3 data is then passed to the analog transmitter, which drives the differential MLT-3 signal, on outputs TXP and

TXN, to the twisted pair media across a 1:1 ratio isolation transformer. The 10Base-T and 100Base-TX signals pass

through the same transformer so that common “magnetics” can be used for both. The transmitter drives into the 100



impedance of the CAT-5 cable. Cable termination and impedance matching require external components.

4.2.6

100M PHASE LOCK LOOP (PLL)

The 100M PLL locks onto reference clock and generates the 125MHz clock used to drive the 125 MHz logic and the

100Base-Tx Transmitter.

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.

FIGURE 4-2:

RECEIVE DATA PATH

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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