LAN91C111 datasheet(28/125 Pages) MICROCHIP

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LAN91C111

DS00002276A-page 28

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7.7.7.4

Transmit Rise and Fall Time Adjust

The transmit output rise and fall time can be adjusted with the two transmit rise/fall time adjust bits in the PHY Ml serial

port Configuration 1. The adjustment range is -0.25ns to +0.5ns in 0.25ns steps.

7.7.7.5

STP (150 Ohm) Cable Mode

The transmitter can be configured to drive 150 Ohm shielded twisted pair cable. The STP mode can be selected by

appropriately setting the cable type select bit in the PHY MI serial port Configuration 1 register. When STP mode is

enabled, the output current is automatically adjusted to comply with IEEE 802.3 levels.

7.7.7.6

Transmit Disable

The TP transmitter can be disabled by setting the transmit disable bit in the PHY Ml serial port Configuration 1 register.

When the transmit disable bit is set, the TP transmitter is forced into the idle state, no data is transmitted, no link pulses

are transmitted, and internal loopback is disabled.

7.7.7.7

Transmit Powerdown

The TP transmitter can be powered down by setting the transmit powerdown bit in the PHY Ml serial port Configuration

1 register. When the transmit powerdown bit is set, the TP transmitter is powered down, the TP transmit outputs are

high impedance, and the rest of the LAN91C111 operates normally.

7.7.8

TWISTED PAIR RECEIVER

7.7.8.1

Receiver - 100 Mbps

The TX receiver detects input signals from the twisted pair input and converts it to a digital data bit stream ready for dock

and data recovery. The receiver can reliably detect data from a 100BASE-TX transmitter that has been passed through

0-100 meters of 100-Ohm category 5 UTP or 150 Ohm STP.

The TX receiver consists of an adaptive equalizer, baseline wander correction circuit, comparators, and MLT-3 decoder.

The TP inputs first go to an adaptive equalizer. The adaptive equalizer compensates for the low pass characteristic of

the cable, and it has the ability to adapt and compensate for 0-100 meters of category 5, 100 Ohm UTP or 150 Ohm

STP twisted pair cable. The baseline wander correction circuit restores the DC component of the input waveform that

was removed by external transformers. The comparators convert the equalized signal back to digital levels and are used

to qualify the data with the squelch circuit. The MLT-3 decoder takes the three level MLT-3 digital data from the compar-

ators and converts it to back to normal digital data to be used for dock and data recovery.

7.7.8.2

Receiver - 10 Mbps

The 10 Mbps receiver is able to detect input signals from the twisted pair cable that are within the template shown in

Figure 7-5. The inputs are biased by internal resistors. The TP inputs pass through a low pass filter designed to eliminate

any high frequency noise on the input. The output of the receive filter goes to two different types of comparators, squelch

and zero crossing. The squelch comparator determines whether the signal is valid, and the zero crossing comparator

1000

1.00

1001

0.98

1010

0.96

1011

0.94

1100

0.92

1101

0.90

1110

0.88

1111

0.86

TABLE 7-2:

TRANSMIT LEVEL ADJUST (CONTINUED)

TLVL[3:0]

Gain

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