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SMSC LAN91C111 32/16/8-Bit Three-In-One Fast Ethernet Controller
Revision 1.0 (08-14-08) 44 SMSC AN 9.6
APPLICATION NOTE
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.
7.2.3 Cable Selection
The LAN91C111 can drive two different cable types:
100 Ohm unshielded twisted-pair, Category 5, or
150 Ohm shielded twisted-pair.
The LAN91C111 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 7.2.
Table 7.2 - Table 7.2 - Cable Configurations
The CABLE bit sets the output current level for the cable type. RTERM in Table ý7.2 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
to TPO. Each
value for RTERM on the TPI inputs is for the sum of the four series resistors across TPI.
These resistors should be 1% tolerance. Also note that some output level adjustment may be
necessary due to parasitic as described in Section 7.2.2 - TP Transmit Output Current Set.
IEEE 802.3 specifies that 10BASE-T and 100BASE-TX operate over twisted-pair cable lengths of
between 0–100 meters. The squelch levels can be reduced by 4.5 dB if the Receive Input Level Adjust
bit (RLVL0) is set in the MI serial port Configuration 1 register. This allows the LAN91C111 to operate
with up to 150 meters of twisted-pair cable. The equalizer is designed to accommodate between 0–125
meters of cable.
7.2.4 Transmitter Droop
The IEEE 802.3 specification has a transmit output droop requirement for 100BASE-TX. Because the
LAN91C111 TP output is a current source, it has no perceptible droop by itself. However, the
inductance of the transformer added to the device transmitter output causes droop to appear at the
transmit interface to the TP wire. If the transformer connected to the LAN91C111 outputs meets the
requirements of Table 7.1 - TP Transformer Specification the transmit interface to the TP cable then
meets the IEEE 802.3 droop requirements.
7.3 MII Management Functions
The MII is a nibble-wide packet data interface defined in the IEEE Specification 802.3. The MII
interface encompasses the signals that physically transport the management information across the
MII, a frame format, and a protocol specification for exchanging management frames, and a register
set that can be read and written using these frames. MII management refers to the ability of a
management entity to communicate with PHY via the MII serial management interface (MI) for the
purpose of displaying, selecting, and/or controlling different PHY options. The host manipulates the
MAC to drive the MII management serial interface. By manipulating the MAC's registers (MDOE,
MCLK, MDI, and MDO bits in the Management Interface Register), MII management frames are
generated on the management interface for reading or writing information from/to the PHY registers.
CABLE TYPE CABLE BIT
RTERM (OHMS)
TPO TPI
100 Ohm UTP, Cat. 5 UTP 50 100
150 Ohm STP STP 75 150