80220 Datasheet, PDF (16/91 Pages) LSI Computer Systems – 100BASE-TX/10BASE-T Ethernet Media Interface Adapter

80220/80221
3.6.2 10 Mpbs
A scrambler is not used in 10 Mbps mode.
3.6.3 Scrambler Bypass
The scrambler can be bypassed by setting the bypass
scrambler/descrambler bit in the MI serial port Configura-
tion 1 register. When this bit is set, the 5B data bypasses
the scrambler and goes directly from the 4B5B encoder to
the twisted pair transmitter.
3.7 DESCRAMBLER
3.7.1 100 Mbps
The 80220/80221 descrambler takes the scrambled data
from the data recovery block, descrambles it per the IEEE
802.3 specifications, aligns the data on the correct 5B
word boundaries, and sends it to the 4B5B decoder.
The algorithm for synchronization of the descrambler is
the same as the algorithm outlined in the IEEE 802.3
specification. Once the descrambler is synchronized, it
will maintain synchronization as long as enough
descrambled idle pattern 1's are detected within a given
interval. To stay in synchronization, the descrambler
needs to detect at least 25 consecutive descrambled idle
pattern 1's in a 1 mS interval. If 25 consecutive descrambled
idle pattern 1's are not detected within the 1 mS 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
MI serial port Status Output register to indicate this condi-
tion. Once this bit is set, it will stay set until the descrambler
achieves synchronization.
3.7.2 10 Mpbs
A descrambler is not used in 10 Mbps mode.
3.7.3 Descrambler Bypass
The descrambler can be bypassed by setting the bypass
scrambler/descrambler bit in the MI serial port Configura-
tion 1 register. When this bit is set, the data bypasses the
descrambler and goes directly from the TP receiver to the
4B5B decoder.
3.8 TWISTED PAIR TRANSMITTER
3.8.1 Transmitter - 100 Mbps
The transmitter consists of a 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 gen-
erator preshapes the output waveform transmitted onto
the twisted pair cable to meet the pulse template require-
ments 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 wave-
form to a current output that can drive 100 meters of
category 5 unshielded twisted pair cable or 150 Ohm
shielded twisted pair cable.
3.8.2 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, clock 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 transmit-
ted onto the twisted pair cable to meet the pulse template
requirements outlined in IEEE 802.3 Clause 14 and also
shown in Figure 4. The waveshaper replaces and elimi-
nates external filters on the TP transmit output.
The line driver converts the shaped and smoothed wave-
form 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).
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MD400159/E