LAN83C183 Datasheet, PDF (24/113 Pages) SMSC Corporation – FAST ETHERNET PHYSICAL LAYER DEVICE

1.2.7 Twisted-Pair Transmitters
This section describes the operation of the 10 and 100 Mbits/s TP transmitters.
1.2.7.1 100 MBITS/S TP TRANSMITTER
The TP transmitter consists of an MLT3 encoder, waveform generator, and line driver.
The MLT3 encoder converts the NRZI data from the scrambler into a three-level code
required by IEEE 802.3. MLT3 coding uses three levels, converting ones to
transitions between the three levels, and zeros 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 MLT3 three level encoded waveform and uses an
array of switched current sources to control the shape of the twisted-pair output
signal. The waveform generator consists of switched current sources, a clock
generator, filter, and logic. The switched current sources control the rise and fall time
as well as signal level to meet IEEE 802.3 requirements. The output of the switched
current sources goes through a second order low-pass filter that “smooths” the
current output and removes any high-frequency components. In this way, the
waveform generator preshapes the output waveform transmitted onto the twisted-pair
cable such that the waveform meets 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 greater than 100 meters of category 5 unshielded twisted-pair cable or 150-
ohm shielded twisted-pair cable.
1.2.7.2 10 MBITS/S TP TRANSMITTER
Even though the 10 Mbits/s transmitter operation is much different than that of 100
Mbits/s, it also consists of a waveform generator and line driver (see Figure 1.1).
The waveform generator, which consists of a ROM, DAC, clock generator, and filter,
shapes the output transmit pulse. The DAC generates a stair-stepped representation
of the desired output waveform. The stairstepped DAC output then is passed through
a low-pass filter to “smooth” the DAC output and remove any high-frequency
components. The DAC values are determined from the data at the ROM addresses.
The data is chosen to shape the pulse to the desired template. The clock generator
clocks the data into the DAC at high speed. 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 shown in
Figure 1.4 and Table 1.5. 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 greater than 100 meters of category 3/4/5 100-ohm unshielded twisted-pair
cable or 150-ohm shielded twisted-pair cable without any external filters.
During the idle period, no output signals are transmitted on the TP outputs except for
link pulses.
SMSC DS – LAN83C183
24
Rev. 12/14/2000