XRT73L03IV datasheet(39/62 Pages) EXAR

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3 CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT

XRT73L03

PRELIMINARY

REV. P1.0.13

ceive a line signal with an overall cable length of 300

feet or greater. Conversely, turn OFF the Receive

Equalizer if the Receive Section of a given channel is

going to receive a line signal over a cable length of

less than 300 feet.

NOTES:

1. If the Receive Equalizer block is turned ON when it

is receiving a line signal over short cable length the

received line signal may be over-equalized, which

could degrade performance by increasing the

amount of jitter that exists in the recovered data

and clock signals or by creating bit-errors.

2. The Receive Equalizer has been designed to

counter the frequency-dependent cable loss that a

line signal experiences as it travels from the trans-

mitting terminal to the receiving terminal. However,

the Receive Equalizer was not designed to counter

flat loss where all of the Fourier frequency compo-

nents within the line signal are subject to the same

amount of attenuation. Flat loss is handled by the

AGC block.

Disable the Receive Equalizer block by doing either of

the following:

a. Operating in the Hardware Mode

Set the REQEN(n) input pin "Low".

b. Operating in the HOST Mode

Write a "0" to the REQEN(n) bit-field in Command

3.3

CLOCK RECOVERY PLL

The purpose of the Clock Recovery PLL is to track

the incoming Dual-Rail data stream and to derive and

generate a recovered clock signal.

It is important to note that the Clock Recovery PLL re-

quires a line rate clock signal at the ExClk input pin.

The Clock Recovery PLL operates in one of two

modes:

• The Training Mode

• The Data/Clock Recovery Mode

3.3.1

The Training Mode

If a given channel in the XRT73L03 is not receiving a

line signal via the RTIP and RRing input pins, or if the

frequency difference between the line signal and that

applied via the ExClk input pin exceeds 0.5%, the

channel operates in the Training Mode. When the

channel is operating in the Training Mode, it does the

following:

a. Declare a Loss of Lock indication by toggling its

respective RLOL(n) output pin “High".

b. Output a clock signal via the RxClk(n) output pin

which is derived from the signal applied to the

EXClk(n) input pin.

3.3.2

The Data/Clock Recovery Mode

If the frequency difference between the line signal

and that applied via the ExClk input pin is less than

0.5%, the channel operates in the Data/Clock Recov-

ery mode. In this mode, the Clock Recovery PLL

locks onto the line signal via the RTIP and RRing in-

put pins.

3.4

THE HDB3/B3ZS DECODER

The Remote Transmitting Terminal typically encodes

the line signal into some sort of Zero Suppression

Line Code (e.g., HDB3 for E3 and B3ZS for DS3 and

STS-1). The purpose of this encoding activity was to

aid in the Clock Recovery process of this data from

the Near-End Receiving Terminal. However, once the

data has made it across the E3, DS3 or STS-1 Trans-

port Medium and has been recovered by the Clock

Recovery PLL, it is now necessary to restore the orig-

inal content of the data. Hence, the purpose of the

HDB3/B3ZS Decoding block is to restore the data

transmitted over the E3, DS3 or STS-1 line to its orig-

inal content prior to Zero Suppression Coding.

3.4.1

B3ZS Decoding DS3/STS-1 Applications

If the XRT73L03 is configured to operate in the DS3

or STS-1 Modes, then the HDB3/B3ZS Decoding

Blocks perform B3ZS Decoding. When the Decoders

are operating in this mode, each of the Decoders

parses through its respective incoming Dual-Rail data

and checks for the occurrence of either a "00V" or a

"B0V" pattern. If the B3ZS Decoder detects this par-

ticular pattern, it substitutes these bits with a "000"

pattern.

NOTE: If the B3ZS Decoder detects any bipolar violations

that is not in accordance with theB3ZS Line Code format or

if the B3ZS Decoder detects a string of 3 or more consecu-

tive "0’s" in the incoming line signal, the B3ZS Decoder

COMMAND REGISTER CR2_(N)

RESERVED

ENDECDIS(n)

ALOSDIS(n)

DLOSDIS(n)

REQEN(n)

XRT73L03IV Datasheet(PDF) 39 Page - Exar Corporation

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