LXT972A Datasheet, PDF (34/70 Pages) Intel Corporation – 3.3V Dual-Speed Fast Ethernet Transceiver Datasheet

LXT972A — 3.3V Dual-Speed Fast Ethernet Transceiver Datasheet
3.7.3.2
3.7.3.3
PMA Sublayer
Link
In 100Mbps mode, the LXT972A establishes a link whenever the scrambler becomes locked and
remains locked for approximately 50ms. Whenever the scrambler loses lock (receiving less than 12
consecutive idle symbols during a 2ms window), the link are taken down. This provides a very
robust link, essentially filtering out any small noise hits that may otherwise disrupt the link.
Furthermore, 100M idle patterns will not bring up a 10M link.
The LXT972A reports link failure via the MII status bits (1.2 and 17.10) and interrupt functions. If
auto-negotiation is enabled, link failure causes the LXT972A to re-negotiate.
Link Failure Override
The LXT972A normally transmits data packets only if it detects the link is up. Setting bit 16.14 = 1
overrides this function, allowing the LXT972A to transmit data packets even when the link is
down. This feature is provided as a diagnostic tool. Note that auto-negotiation must be disabled to
transmit data packets in the absence of link. If auto-negotiation is enabled, the LXT972A
automatically transmits FLP bursts if the link is down.
Carrier Sense
For 100TX links, a start-of-stream delimiter (SSD) or /J/K symbol pair causes assertion of carrier
sense (CRS). An end-of-stream delimiter (ESD) or /T/R symbol pair causes de-assertion of CRS.
The PMA layer also de-asserts CRS if IDLE symbols are received without /T/R; however, in this
case RX_ER is asserted for one clock cycle when CRS is de-asserted.
Usage of CRS for Interframe Gap (IFG) timing is not recommended for the following reasons:
• De-assertion time for CRS is slightly longer than assertion time. This causes IFG intervals to
appear somewhat shorter to the MAC than it actually is on the wire.
• CRS de-assertion is not aligned with TX_EN de-assertion on transmit loopbacks in half-
duplex mode.
Receive Data Valid
The LXT972A asserts RX_DV to indicate that the received data maps to valid symbols. However,
RXD outputs zeros until the received data is decoded and available for transfer to the controller.
Twisted-Pair PMD Sublayer
The twisted-pair Physical Medium Dependent (PMD) layer provides the signal scrambling and
descrambling, line coding and decoding (MLT-3 for 100TX, Manchester for 10T), as well as
receiving, polarity correction, and baseline wander correction functions.
Scrambler/Descrambler
The purpose of the scrambler is to spread the signal power spectrum and further reduce EMI using
an 11-bit, data-independent polynomial. The receiver automatically decodes the polynomial
whenever IDLE symbols are received.
34
Datasheet