Draft 6/5/00
A-8
Application Information
Copyright 2000 by LSI Logic Corporation. All rights reserved.
Keep REXT as close to the REXT and GND pins as possible to reduce
noise pickup into the transmitter.
Because the TP output is a current source, capacitive and inductive
loading can reduce the output voltage from the ideal level. Thus, in actual
application, it might be necessary to adjust the value of the output
current to compensate for external loading. One way to adjust the TP
output level is to change the value of the external resistor connected to
REXT.
A better way to adjust the TP output level is to use the Transmit Level
Adjust register bits (TLVL[3:0]) accessed through the MI serial port
Configuration 1 register. These four bits can adjust the output level by
14% to +16% in 2% steps as described in
Table 2.6
.
A.5 Cable Selection
The L80223 can drive two different cable types:
100 Ohm unshielded twisted-pair, Category 5, or
150 Ohm shielded twisted-pair.
The L80223 must be properly configured for the type of cable to meet
the return loss specifications in IEEE 802.3. This configuration requires
appropriately setting the Cable Type Select (CABLE) bit in the MI serial
port Configuration 1 register and setting the value of some external
resistors, as described in
Table A.3
.
The CABLE bit sets the output current level for the cable type. RTERM
in
Table A.3
is the value of the termination resistors needed to meet the
level and return loss requirements. The value for RTERM on the TPO
outputs is for the two external termination resistors connected from V
DD
Table A.3
Cable Configuration
Cable Type
CABLE
Bit
RTERM (Ohms)
TPO
TPI
100 Ohm UTP, Cat. 5
UTP
50
100
150 Ohm STP
STP
75
150