Datasheet

 2013-2015 Microchip Technology Inc. DS00001987A-page 25
LAN8740A/LAN8740Ai
3.3 HP Auto-MDIX Support
HP Auto-MDIX facilitates the use of CAT-3 (10BASE-T) or CAT-5 (100BASE-TX) media UTP interconnect cable without
consideration of interface wiring scheme. If a user plugs in either a direct connect LAN cable, or a cross-over patch
cable, as shown in Figure 3-4, the device’s Auto-MDIX transceiver is capable of configuring the TXP/TXN and RXP/RXN
pins for correct transceiver operation.
The internal logic of the device detects the TX and RX pins of the connecting device. Since the RX and TX line pairs
are interchangeable, special PCB design considerations are needed to accommodate the symmetrical magnetics and
termination of an Auto-MDIX design.
The Auto-MDIX function can be disabled via the AMDIXCTRL bit in the Special Control/Status Indications Register.
Note: When operating in 10BASE-T or 100BASE-TX manual modes, the Auto-MDIX crossover time can be
extended via the Extend Manual 10/100 Auto-MDIX Crossover Time bit of the EDPD NLP/Crossover
Time/EEE Configuration Register. Refer to Section 4.2.12, "EDPD NLP/Crossover Time/EEE Configura-
tion Register" for additional information.
FIGURE 3-4: DIRECT CABLE CONNECTION VS. CROSS-OVER CABLE CONNECTION
1
2
3
4
5
6
7
8
TXP
TXN
RXP
Not Used
Not Used
RXN
Not Used
Not Used
1
2
3
4
5
6
7
8
TXP
TXN
RXP
Not Used
Not Used
RXN
Not Used
Not Used
Direct Connect Cable
RJ-45 8-pin straight-through
for 10BASE-T/100BASE-TX
signaling
1
2
3
4
5
6
7
8
TXP
TXN
RXP
Not Used
Not Used
RXN
Not Used
Not Used
1
2
3
4
5
6
7
8
TXP
TXN
RXP
Not Used
Not Used
RXN
Not Used
Not Used
Cross-Over Cable
RJ-45 8-pin cross-over for
10BASE-T/100BASE-TX
signaling