AM79C875KC datasheet(19/48 Pages) AMD

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Am79C875

signal on the first NLP. It locks the polarity correction

circuitry after the reception of two consecutive packets.

The state of the polarity correction circuitry is locked as

long as link is established. This function is only avail-

able in 10BASE-T mode.

Far-End Fault Indication (FEFI)

Auto-Negotiation provides a remote fault capability for

dete c tin g an asym me tric lin k fa ilur e. Since

100BASE-FX systems do not use Auto-Negotiation, an

alternative, in-band signaling scheme is used to signal

remote fault conditions. This scheme, Far End Fault In-

dication, relies on the characteristics of the quiescent

state, a continuous IDLE stream. The IDLE stream is a

continuous stream of logic ones followed by one logic

zero, with the pattern repeated at least 3 times.

A Far-End Fault will be signaled under the following

three conditions: (1) When no activity is received from

the link partner, since this can indicate a broken receive

wire, (2) When the clock recovery circuit detects a

receive signal error or PLL lock error, (3) When man-

agement entity sets the transmit Far-end fault bit

(Register 24, bit 7).

The Far-End fault mechanism defaults to enabled in

100BASE-FX mode and disabled in 100BASE-TX and

10BASE-T modes, and may be controlled by software

after reset.

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

frequency range of 1 MHz to 100 MHz. The receive

section 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 maxi-

mum frequency to 31.25 MHz.

A data signal stream following MLT-3 rules is illustrated

in Figure 6. The data stream is 1010101.

MLT-3

1010101

8 ns

22236G-8

Figure 6. 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™ 4LP device provides on-chip filtering.

External filters are not required for either the transmit or

receive signals.

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 7.

Adaptive Equalizer

The NetPHY™ 4LP device is designed for the maxi-

mum of 140 meter UTP-5 cable. A 140-meter UTP-5

cable attenuates the signal by 32 dB at 100 MHz which

far exceed the cable plant attenuation (24-26 dB) de-

fined by TP-PMD.

The amplitude and phase distortion from cable causes

inter-symbol interference (ISI) which makes clock and

data recovery impossible. Adaptive equalization is

done by closely matching the characteristics of the

twisted-pair cable. This is a variable equalizer which

changes equalizer frequency response in accordance

with cable length. The cable length is estimated based

on comparisons of incoming signal strength against

some known cable characteristics. The equalizer tunes

itself automatically to the any cable length to compen-

sate 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 media, with transformer and common

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