SMSC LAN91C111 32/16/8-Bit Three-In-One Fast Ethernet Controller

6.6.1Conclusion

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

7.1Transmit / 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.

7.1.1Transmit 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 VDD 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

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

 

 

 

 

 

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.

7.1.2Receive 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.

Revision 1.0 (08-14-08)

42

SMSC AN 9.6

APPLICATION NOTE