Fast Ethernet 10/100 Quad Transceivers — LXT974/LXT975
The LXT974/975 does not support fiber connections at 10 Mbps.
2.6.2.1
Preamble Handling
In 10BASE-T Mode, the LXT974/975 strips the entire preamble off of received packets. CRS is
asserted a few bit times after carrier is detected. RX_DV is held Low for the duration of the
preamble.
When RX_DV is asserted, the very first two nibbles driven by the LXT974/975 are the SFD “5D”
hex followed by the body of the packet. In 10T loopback, the LXT974/975 loops back whatever
the MAC transmits to it, including the preamble.
2.6.2.2
Link Test
In 10 Mbps mode, the LXT974/975 always transmit link pulses. If the link test function is enabled,
it monitors the connection for link pulses. Once link pulses are detected, data transmission are
enabled and remain enabled as long as either the link pulses or data transmission continues. If the
link pulses stop, the data transmission is disabled.
If the link test function is disabled, the LXT974/975 transmits to the connection regardless of
detected link pulses. The link test function can be disabled by setting bit 19.8 = 1 or by setting
AUTOENA to disable auto-negotiation and setting CFG_1 input High.
2.6.2.3
2.6.2.4
2.6.2.5
Link Failure
Link failure occurs if Link Test is enabled and link pulses or packets stop being received. If this
condition occurs, the LXT974/975 returns to the auto-negotiation phase if auto-negotiation is
enabled.
SQE (Heartbeat)
By default, the SQE (heartbeat) function is disabled on the LXT974/975. To enable this function,
set bit 19.10 =1. When this function is enabled, the LXT974/975 asserts its COL output after each
transmit packet. See Figure 32 on page 58 for SQE timing parameters.
Jabber
If MAC transmission exceeds the jabber timer, the LXT974/975 disables the transmit and loopback
functions and enables the COL pin. See Figure 33 on page 59 for jabber timing parameters.
The LXT974/975 automatically exits jabber mode after the unjab time has expired. This function
can be disabled by setting bit 19.9 = 1.
2.7
LED Functions
The LXT974/975 provides three programmable LEDs per port. Refer to Table 51 on page 68 for
LED programming details. The LXT974/975 also provides a serial LED output.
Datasheet
37
INTEL [ INTEL ]
