
Draft 6/5/00
2-44
Functional Description
Copyright 2000 by LSI Logic Corporation. All rights reserved.
2.5.2 Full/Half Duplex Indication
Full Duplex detection can be monitored through the Duplex Detect bit
(DPLX_DET) in the MI serial port Status Output register.
The device can also be programmed such that the Full-Duplex indication
appears on the PLED1n pin. To do this, appropriately set the
Programmable LED Output Select bits in the MI serial port Configuration
2 register as described in
Table 2.9
. When the PLED1n pin is
programmed to be a Full-Duplex detect output, it is asserted LOW when
the device is configured for Full Duplex operation. The PLED1 output has
both pullup and pulldown driver transistors and a weak pullup resistor, so
it can drive an LED from either V
DD
or GND and can also drive a digital
input.
2.5.3 Loopback
2.5.3.1 Internal CRS Loopback
TX_EN is internally looped back onto CRS during every transmit packet.
This internal CRS loopback is disabled during collision, in Full-Duplex
mode, in the Link Fail State, and when the Transmit Disable bit
(XMT_DIS) is set in the MI serial port Configuration 1 register. In 10
Mbits/s mode, internal CRS loopback is also disabled when jabber is
detected.
2.5.3.2 Diagnostic Loopback
Setting the loopback bit (LPBK) in the MI serial port Control register
selects the diagnostic loopback mode. When diagnostic loopback is
enabled, the TXD[3:0] data is looped back onto RXD[3:0], TX_EN is
looped back onto CRS, RX_DV operates normally, the TP receive and
transmit paths are disabled, the transmit link pulses are halted, and the
Half/Full Duplex modes do not change. Diagnostic loopback cannot be
enabled when in the FBI mode (see
Section 2.2.2.2, “FBI Interface,”
page 2-12
).