SMSC LAN91C111 32/16/8-Bit Three-In-One Fast Ethernet Controller
Revision 1.0 (08-14-08) 42 SMSC AN 9.6
APPLICATION NOTE
6.6.1 Conclusion
Given the flexibility of the LAN91C111 and with some creative software, the LAN91C111 can be
connected to controllers other than Little Endian types.
7 Physical Layer and Magnetics
7.1 Transmit / Receive Interface
The interface between the Twisted Pair outputs & the Twist Pair inputs on the LAN91C111 and the
twisted pair cable is typically transformer coupled and terminated with the appropriate resistors.
7.1.1 Transmit Interface
The transformer for the transmitter should have a winding ratio of 1:1 with a center tap on the primary
winding tied to VDD. The specifications for the transformer are shown in Table ý7.1 - TP Transformer
Specification.
The transmit output must be terminated with two external termination resistors to meet the output
impedance and return loss requirements of IEEE 802.3. These two external resistors must be
connected between V
DD
and each of the TPO± outputs. Their value should be chosen to provide the
correct termination impedance for the transmitter inside the LAN91C111. The value of the two external
termination resistors depends on the type of cable the device drives. Refer to Section ý7.2.3 - Cable
Selection for more details.
Table 7.1 - TP Transformer Specification
To minimize common mode output noise and to aid in meeting radiated emissions requirements, it may
be necessary to add a common mode choke on the transmit outputs. Common mode bundle
termination may required and can be achieved by connecting the unused pairs in the RJ45 connector
to chassis ground through 75-ohm resistors and a 1000 pF capacitor. A common mode AC ground
return path can be designed by connecting the center tap with a 75W resistor and capacitor to chassis
ground.
To minimize noise pickup into the transmit path in a system or on a PCB, the loading on TPO± should
be minimized and both outputs should always be loaded equally.
7.1.2 Receive Interface
Receive data is typically transformer coupled into the receive inputs on TPI± and terminated with
external resistors.
The transformer for the receiver should have a winding ratio of 1:1. The specifications for this
transformer are shown in Table 7.1.
PARAMETER SPECIFICATION
TRANSMIT RECEIVE
TURNS RATIO
1:1 CT 1:1
Inductance, ( HMin) 350 350
Leakage Inductance, (H) 0.05–0.15 0.0–0.2
Capacitance, (pF Max) 15 15
DC Resistance, (Ω Max) 0.4 0.4