AM79C874VC datasheet(20/60 Pages) AMD

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Am79C874

P R E L I M INARY

MLT-3

This block is responsible for converting the NRZI data

stream from the PDX block to the MLT-3 encoded data

stream. The effect of MLT-3 is the reduction of energy

on the copper media (TX or FX cable) in the critical fre-

quency range of 1 MHz to 100 MHz. The receive sec-

tion of this block is responsible for equalizing and

amplifying the received data stream and link detection.

The adaptive equalizer compensates for the amplitude

and phase distortion due to the cable.

MLT-3 is a tri-level signal. All transitions are between

0 V and +1 V or 0 V and -1 V. A transition has a logical

value of 1 and a lack of a transition has a logical value

of 0. The benefit of MLT-3 is that it reduces the maxi-

mum frequency over the data line. The bit rate of TX

data is 125 Mbps. The maximum frequency (using

NRZI) is half of 62.5 MHz. MLT-3 reduces the maximum

frequency to 31.25 MHz.

A data signal stream following MLT-3 rules is illustrated

in Figure 2. The data stream is 1010101.

Figure 2.

MLT-3 Waveform

The TX± drivers convert the NRZI serial output to

MLT-3 format. The RX± receivers convert the received

MLT-3 signals to NRZI. The transmit and receive sig-

nals will be compliant with IEEE 802.3u, Section 25.

The required signals (MLT-3) are described in detail in

ANSI X3.263:1995 TP-PMD Revision 2.2 (1995).

The NetPHY-1LP device provides on-chip filtering. Ex-

ternal filters are not required for either the transmit or

receive signals. The traces from the transformer to the

NetPHY-1LP device should have a controlled imped-

ance as a differential pair of 100 ohms. The same is

true between the transformer and the RJ-45 connector.

The TX

pins can be connected to the media via either

a 1:1 transformer or a 1.25:1 transformer. The 1.25:1

ratio provides a 20% transmit power savings over the

1:1 ratio. Refer to Figure 3.

Adaptive Equalizer

The NetPHY-1LP device is designed to accommodate

a maximum cable length of 140 meters UTP CAT-5 ca-

ble. 140 meters of UTP CAT-5 cable has an attenuation

of 31 dB at 100 MHz. The typical attenuation of a 100

meter cable is 21 dB. The worst case attenuation is

around 24-26 dB defined by TP-PMD.

The amplitude and phase distortion from the cable will

cause intersymbol interference (ISI) which makes clock

and data recovery impossible. The adaptive equalizer

is made by closely matching the inverse transfer func-

tion of the twist-pair cable. This is a variable equalizer

that changes its equalizer frequency response in ac-

cordance to cable length. The cable length is estimated

based on comparisons of incoming signal strength

against some of the known cable characteristics. The

equalizer has a monotonical frequency response, and

tunes itself automatically for any cable length to com-

pensate for the amplitude and phase distortion incurred

from the cable.

Baseline Wander Compensation

The 100BASE-TX data stream is not always DC bal-

anced. The transformer blocks the DC component of

the incoming signal, thus the DC offset of the differen-

tial receive inputs can wander. The shift in the signal

levels, coupled with non-zero rise and fall times of the

serial stream can cause pulse-width distortion. This

creates jitter and a possible increase in error rates.

Therefore, a DC restoration circuit is needed to com-

pensate for the attenuation of the DC component.

The NetPHY-1LP device implements a patent-

pending DC restoration circuit. Unlike the traditional im-

plementation, it does not need the feedback informa-

tion from the slicer and clock recovery circuit. This not

only simplifies the system/circuit design, but also elim-

inates any random/systematic offset on the receive

path. In 10BASE-T and 100Base-FX modes, the base-

line wander correction circuit is not required and there-

fore will be bypassed.

MLT-3

1010101

8 ns

22235I-4

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