Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
3.3
HP Auto-MDIX Support
HP Auto-MDIX facilitates the use of CAT-3 (10BASE-T) or CAT-5 (100BASE-T) 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.
RJ-45 8-pin straight-through
for 10BASE-T/100BASE-TX
signaling
RJ-45 8-pin cross-over for
10BASE-T/100BASE-TX
signaling
TXP
TXN
TXP
TXP
TXN
TXP
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
1
2
3
4
5
6
7
8
TXN
TXN
2
3
4
5
6
7
8
RXP
RXP
RXP
RXP
Not Used
Not Used
RXN
Not Used
Not Used
RXN
Not Used
Not Used
RXN
Not Used
Not Used
RXN
Not Used
Not Used
Not Used
Not Used
Not Used
Not Used
Not Used
Not Used
Direct Connect Cable
Cross-Over Cable
Figure 3.4 Direct Cable Connection vs. Cross-over Cable Connection
Revision 1.4 (08-23-12)
26
SMSC LAN8720A/LAN8720Ai
DATASHEET
SMSC [ SMSC CORPORATION ]
