AM79C874VC AMD (ADVANCED MICRO DEVICES), AM79C874VC Datasheet - Page 21

• Manufacturer Part Number: AM79C874VC
• Manufacturer: AMD (ADVANCED MICRO DEVICES)

Specifications of AM79C874VC

• Lead Free Status / RoHS Status: Not Compliant

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 2. The data stream is 1010101.
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.
22235K
MLT-3
1
8 ns
Figure 2. MLT-3 Waveform
0
1
0
1
D A T A
0
22236G-4
1
S H E E T
Am79C874
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.
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