10/100 Non-PCI Ethernet Single Chip MAC + PHY

Datasheet

If 25 consecutive descrambled idle pattern 1's are not detected within the 1ms interval, the descrambler goes out of synchronization and restarts the synchronization process.

If the descrambler is in the unsynchronized state, the descrambler loss of synchronization detect bit is set in the Ml serial port Status Output register to indicate this condition. Once this bit is set, it will stay set until the descrambler achieves synchronization.

10 Mbps

A descrambler is not used in 10 Mbps mode.

Descrambler Bypass

The descrambler can be bypassed by setting the bypass scrambler/descrambler bit in the PHY MI serial port Configuration 1 register. When this bit is set, the data bypasses the descrambler and goes directly from the TP receiver to the 4B5B decoder.

7.7.7Twisted Pair TransmitterTransmitter - 100 Mbps

The TX transmitter consists of MLT-3 encoder, waveform generator and line driver.

The MLT-3 encoder converts the NRZ data from the scrambler into a three level MLT-3 code required by IEEE 802.3. MLT-3 coding uses three levels and converts 1's to transitions between the three levels, and converts 0's to no transitions or changes in level.

The purpose of the waveform generator is to shape the transmit output pulse. The waveform generator takes the MLT-3 three level encoded waveform and uses an array of switched current sources to control the rise/fall time and level of the signal at the Output. The output of the switched current sources then goes through a low pass filter in order to "smooth" the current output and remove any high frequency components. In this way, the waveform generator preshapes the output waveform transmitted onto the twisted pair cable to meet the pulse template requirements outlined in IEEE 802.3. The waveform generator eliminates the need for any external filters on the TP transmit output.

The line driver converts the shaped and smoothed waveform to a current output that can drive 100 meters of category 5 unshielded twisted pair cable or 150 Ohm shielded twisted pair cable.

Transmitter - 10 Mbps

The transmitter operation in 10 Mbps mode is much different than the 100 Mbps transmitter. Even so, the transmitter still consists of a waveform generator and line driver.

The purpose of the waveform generator is to shape the output transmit pulse. The waveform generator consists of a ROM, DAC, dock generator, and filter. The DAC generates a stair-stepped representation of the desired output waveform. The stairstepped DAC output then goes through a low pass filter in order to "smooth' the DAC output and remove any high frequency components. The DAC values are determined from the ROM outputs; the ROM contents are chosen to shape the pulse to the desired template and are clocked into the DAC at high speed by the clock generator. In this way, the waveform generator preshapes the output waveform to be transmitted onto the twisted pair cable to meet the pulse template requirements outlined in IEEE 802.3 Clause 14 and also shown in Figure 7.4. The waveshaper replaces and eliminates external filters on the TP transmit output.

The line driver converts the shaped and smoothed waveform to a current output that can drive 100 meters of category 3/4/5 100 Ohm unshielded twisted pair cable or 150 Ohm shielded twisted pair cable tied directly to the TP output pins without any external filters. During the idle period, no output signal is transmitted on the TP outputs (except link pulse).

Revision 1.91 (08-18-08)

30

SMSC LAN91C111 REV C

DATASHEET