LXT971A 3.3V Dual-Speed Fast Ethernet Transceiver
Table 11. 4B/5B Coding (Continued)
Code Type
4B Code
3210
Name
5B Code
43210
Interpretation
undefined
undefined
undefined
undefined
undefined
INVALID
undefined
undefined
undefined
undefined
undefined
undefined
1.
2.
3.
4.
H
4
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
00100
00000
00001
00010
00011
00101
00110
01000
01100
10000
11001
Transmit Error. Used to force signaling
errors
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
Invalid
The /I/ (Idle) code group is sent continuously between frames.
The /J/ and /K/ (SSD) code groups are always sent in pairs; /K/ follows /J/.
The /T/ and /R/ (ESD) code groups are always sent in pairs; /R/ follows /T/.
An /H/ (Error) code group is used to signal an error condition.
3.7.3.2
PMA Sublayer
3.7.3.2.1
Link
In 100 Mbps mode, the LXT971A establishes a link whenever the scrambler becomes locked and
remains locked for approximately 50 ms. Whenever the scrambler loses lock (receiving less than
12 consecutive idle symbols during a 2 ms window), the link is taken down. This provides a very
robust link, essentially filtering out any small noise hits that may otherwise disrupt the link.
Furthermore, 100 Mbps idle patterns will not bring up a 10 Mbps link.
The LXT971A reports link failure via the MII status bits (Register bits 1.2 and 17.10) and interrupt
functions. Link failure causes the LXT971A to re-negotiate if auto-negotiation is enabled.
3.7.3.2.2
Link Failure Override
The LXT971A normally transmits data packets only if it detects the link is up. Setting Register bit
16.14 = 1 overrides this function, allowing the LXT971A 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
LXT971A automatically transmits FLP bursts if the link is down.
3.7.3.2.3
Carrier Sense
For 100BASE-TX and 100FX 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:
Datasheet
Document #: 249414
Revision #: 002
Rev. Date: August 7, 2002
41
INTEL [ INTEL CORPORATION ]
