80221 datasheet(15/91 Pages) LSI

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80220/80221

4-15

MD400159/E

3.4.3 Encoder Bypass

The 4B5B encoder can be bypassed by setting the bypass

encoder/decoder bit in the MI serial port Configuration 1

decoder, 5B code words are passed directly from the

controller interface to the scrambler without any of the

alterations described in the 4B5B Encoder section. Setting

this bit automatically places the device in the FBI mode as

described in the Controller Interface section.

3.4 DECODER

3.4.1 4B5B Decoder - 100 Mbps

Since the TP input data is 4B5B encoded on the transmit

side, it must also be decoded by the 4B5B decoder on the

receive side.

The mapping of the 5B nibbles to the 4B

code words is specified in IEEE 802.3 and shown in Table

2. The 4B45 decoder on the 80220/80221 takes the 5B

code words from the descrambler, converts them into 4B

nibbles per Table 2, and sends the 4B nibbles to the

controller interface. The 4B5B decoder also strips off the

SSD delimiter (a.k.a. /J/K/ symbols) and replaces them

with two 4B Data 5 nibbles (a.k.a /5/ symbol), and strips off

the ESD delimiter (a.k.a /T/R/ symbols) and replaces it

with two 4B Data 0 nibbles (a.k.a /I/ symbol), per IEEE

802.3 specifications and shown in Figure 2.

The 4B5B decoder detects SSD, ESD and, codeword

errors in the incoming data stream as specified in IEEE

802.3. These errors are indicated by asserting RX_ER

output while the errors are being transmitted across

RXD[3:0], and they are also indicated in the serial port by

setting SSD, ESD, and codeword error bits in the MI serial

port Status Output register.

3.4.2 Manchester Decoder - 10 Mbps

In Manchester coded data, the first half of the data bit

contains the complement of the data, and the second half

of the data bit contains the true data. The Manchester

decoder in the 80220/80221 converts the Manchester

encoded data stream from the TP receiver into NRZ data

for the controller interface by decoding the data and

stripping off the SOI pulse. Since the clock and data

recovery block has already separated the clock and data

from the TP receiver, the Manchester decoding process to

NRZ data is inherently performed by that block.

3.4.3 Decoder Bypass

The 4B5B decoder can be bypassed by setting the bypass

encoder/decoder bit in the MI serial port Configuration 1

decoder, (1) 5B code words are passed directly to the

controller interface from the descrambler without any of

the alterations described in the 4B5B Decoder section,

and (2) CRS is continuously asserted whenever the device

is in the Link Pass state. Setting this bit automatically

places the device in the FBI mode as described in the

Controller Interface section.

3.5 CLOCK AND DATA RECOVERY

3.5.1 Clock Recovery - 100 Mbps

Clock recovery is done with a PLL. If there is no valid data

present on the TP inputs, the PLL is locked to the 25 MHz

TX_CLK. When valid data is detected on the TP inputs

with the squelch circuit and when the adaptive equalizer

has settled, the PLL input is switched to the incoming data

on the TP input. The PLL then recovers a clock by locking

onto the transitions of the incoming signal from the twisted

pair wire. The recovered clock frequency is a 25 MHz

nibble clock, and that clock is outputted on the controller

interface signal RX_CLK.

3.5.2 Data Recovery - 100 Mbps

Data recovery is performed by latching in data from the TP

receiver with the recovered clock extracted by the PLL.

The data is then converted from a single bit stream into

nibble wide data word according to the format shown in

Figure 3.

3.5.3 Clock Recovery - 10 Mbps

The clock recovery process for 10 Mbps mode is identical

to the 100 Mbps mode except, (1) the recovered clock

frequency is 2.5 MHz nibble clock, (2) the PLL is switched

from TX_CLK to the TP input when the squelch indicates

valid data, (3) The PLL takes up to 12 transitions (bit times)

to lock onto the preamble, so some of the preamble data

symbols are lost, but the clock recovery block recovers

enough preamble symbols to pass at least 6 nibbles of

preamble to the receive controller interface as shown in

Figure 3.

3.5.4 Data Recovery - 10 Mbps

The data recovery process for 10 Mbps mode is identical

to the 100 Mbps mode. As mentioned in the Manchester

Decoder section, the data recovery process inherently

performs decoding of Manchester encoded data from the

TP inputs.

3.6 SCRAMBLER

3.6.1 100 Mbps

100Base-TX requires scrambling to reduce the radiated

emissions on the twisted pair. The 80220/80221 scram-

bler takes the encoded data from the 4B5B encoder,

scrambles it per the IEEE 802.3 specifications, and sends

it to the TP transmitter.

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