Draft 6/5/00
Transmitter Droop
A-9
Copyright 2000 by LSI Logic Corporation. All rights reserved.
to TPO .Each value for RTERM on the TPI inputs is for the sum of the
four series resistors across TPI ,as shown in
Figure A.1
–
Figure A.3
.
These resistors should be 1% tolerance. Also note that some output level
adjustment may be necessary due to parasitics as described in
Section
A.4, “TP Transmit Output Current Set,” page A-7
.
IEEE 802.3 specifies that 10BASE-T and 100BASE-TX operate over
twisted-pair cable lengths of between 0–100 meters. The squelch levels
can be reduced by 4.5 dB if the Receive Input Level Adjust bit (RLVL0)
is set in the MI serial port Configuration 1 register. This allows the
L80223 to operate with up to 150 meters of twisted-pair cable. The
equalizer is already designed to accommodate between 0–125 meters of
cable.
A.6 Transmitter Droop
The IEEE 802.3 specification has a transmit output droop requirement for
100BASE-TX. Because the L80223 TP output is a current source, it has
no perceptible droop by itself. However, the inductance of the transformer
added to the device transmitter output as shown in
Figure A.1
through
Figure A.3
causes droop to appear at the transmit interface to the TP
wire. If the transformer connected to the L80223 outputs meets the
requirements of
Table A.1
, the transmit interface to the TP cable then
meets the IEEE 802.3 droop requirements.
A.7 Automatic JAM
The L80223 has an automatic JAM generation feature that automatically
transmits a JAM packet when receive activity is detected. This feature is
primarily designed to give the user a means to easily implement
half-duplex flow control. In a typical application, a watermark signal from
a system FIFO or memory is be tied directly to the JAMn pin. When the
system FIFO is nearly full and more data is being received, the device
automatically transmits a JAM packet and creates a collision, which
causes the far end device to back off, allowing time for the system FIFO
to empty itself.