ICS1893 Datasheet, PDF (48/152 Pages) Integrated Circuit Systems – 3.3-V 10Base-T/100Base-TX Integrated PHYceiver™

ICS1893 Data Sheet - Release
Chapter 7 Functional Blocks
7.4 Functional Block: 100Base-TX TP-PMD Operations
The ICS1893 supports both 10Base-T and 100Base-TX operations. For 100Base-TX operations, the
TP-PMD module performs stream-cipher scrambling/descrambling and MLT-3 encoding/decoding (3-level,
multi-level transition) in compliance with the ANSI Standard X3.263: 199X FDDI TP-PMD as defined in the
specification for 100Base-TX Twisted-Pair Physical Media Dependent (TP-PMD) Sublayer. The ICS1893’s
TP-PMD also performs DC restoration (that is, baseline wander correction) and adaptive equalization on
the received signals.
Note:
1. For an overview of 100Base-TX operations, see Section 5.5, “100Base-TX Operations”.
2. For more information on the Twisted-Pair Interface, see Section 6.5, “Twisted-Pair Interface”.
7.4.1 100Base-TX Operation: Stream Cipher Scrambler/Descrambler
When the ICS1893 is operating in 100Base-TX mode, it employs a stream cipher scrambler/descrambler
that complies with the ANSI Standard X3.263: 199X FDDI TP-PMD. The purpose of the stream cipher
scrambler is to spread the transmission spectrum to minimize electromagnetic compatibility problems. The
stream cipher descrambler restores the received serial bit stream to its unscrambled form.
The ICS1893 “seeds”(that is, initializes) the Transmit Stream Cipher Shift register by using the ICS1893
PHY address from Table 8-16, which minimizes crosstalk and noise in repeater applications.
The MAC/Repeater Interface bypasses the stream cipher scrambler/descrambler when in the 100M
Symbol Interface mode.
7.4.2 100Base-TX Operation: MLT-3 Encoder/Decoder
When operating in the 100Base-TX mode, the ICS1893 TP-PMD sublayer employs an MLT-3 encoder and
decoder. During data transmission, the TP-PMD encoder converts the NRZI bit stream received from the
PMA sublayer to a 3-level Multi-Level Transition code. The three levels are -1, 0, and +1. The results of
MLT-3 encoding provide a reduction in the transmitted energy over the critical frequency range from 20
MHz to 100 MHz. The TP-PMD MLT-3 decoder converts the received three-level signal back to an NRZI bit
stream.
7.4.3 100Base-TX Operation: DC Restoration
The ICS1893’s 100Base-TX operations uses a stream-cipher scrambler to minimize peak amplitudes in the
frequency spectrum. However, the nature of the stream cipher and MLT-3 encoding is such that long
sequences of consecutive zeros or ones can exist. These unbalanced data patterns produce an
undesirable DC component in the data stream known as ‘baseline wander’.
Baseline wander adversely affects the noise immunity of the receiver, because the ‘baseline’signal moves
or ‘wanders’from its nominal DC value. The ICS1893 uses a unique technique to restore the DC
component ‘lost’by the medium. As a result, the design is very robust, immune to noise and independent
of the data stream.
ICS1893 Rev C 6/6/00
Copyright © 2000, Integrated Circuit Systems, Inc.
All rights reserved.
48
June, 2000