80221 datasheet(16/91 Pages) LSI

16 / 91 page

80220/80221

MD400159/E

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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