XRT73L03IV datasheet(31/62 Pages) EXAR

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

XRT73L03

PRELIMINARY

REV. P1.0.13

1. In this mode the Transmit Logic Block ignores the

TNData input pin.

2. If the Transmit Section of a given channel is config-

ured to accept Single-Rail data from the Terminal

Equipment, the B3ZS/HDB3 Encoder must be

enabled.

Figure 11 illustrates the behavior of the TPData and

TxClk(n) signals when the Transmit Logic Block has

been configured to accept Single-Rail data from the

Terminal Equipment.

2.2

THE TRANSMIT CLOCK DUTY CYCLE ADJUST CIR-

CUITRY

The on-chip Pulse-Shaping circuitry in the Transmit

Section of each Channel of the XRT73L03 generates

pulses of the appropriate shapes and width to meet

the applicable pulse template requirements. The

widths of these output pulses are defined by the width

of the half-period pulses in the TxClk(n) signal.

However, if the widths of the pulses in the TxClk(n)

clock signal are allowed to vary significantly, this

could jeopardize the chip's ability to generate Trans-

mit Output pulses of the appropriate width, thereby

not meeting the Pulse Template requirement specifi-

cation. Consequently, the chip's ability to generate

compliant pulses could depend upon the duty cycle of

the clock signal applied to the TxClk(n) input pin.

The Transmit Clock Duty Cycle Adjust Circuitry ac-

cepts clock pulses via the TxClk(n) input pin at duty

cycles ranging from 30% to 70% and converts them

to a 50% duty cycle.

2.3

THE HDB3/B3ZS ENCODER BLOCK

The purpose of the HDB3/B3ZS Encoder Block is to

aid in the Clock Recovery process at the Remote Ter-

minal Equipment by ensuring an upper limit on the

number of consecutive zeros that can exist in the line

signal.

2.3.1

B3ZS Encoding

If the XRT73L03 has been configured to operate in

the DS3 or SONET STS-1 Modes, the HDB3/B3ZS

Encoder blocks operate in the B3ZS Mode. When the

Encoder is operating in this mode it parses through

and searches the Transmit Binary Data Stream from

the Transmit Logic Block for the occurrence of three

(3) consecutive zeros (e.g., "000"). If the B3ZS En-

coder finds an occurrence of three consecutive zeros,

then it substitutes these three "0’s" with either a "00V"

or a "B0V" pattern.

"B" represents a Bipolar pulse that is compliant with

the Alternating Polarity requirements of the AMI (Al-

ternate Mark Inversion) line code.

"V" represents a Bipolar Violation (e.g., a Bipolar

pulse that violates the Alternating Polarity require-

ments of the AMI line code).

The B3ZS Encoder decides whether to substitute

with either the "00V" or the "B0V" pattern in order to

insure that an odd number of Bipolar pulses exist be-

tween any two consecutive violation pulses.

Figure 12 illustrates the B3ZS Encoder at work with

two separate strings of three or more consecutive ze-

ros.

FIGURE 11. THE BEHAVIOR OF THE TPDATA AND TXCLK INPUT SIGNALS WHILE THE TRANSMIT LOGIC BLOCK IS

ACCEPTING SINGLE-RAIL DATA FROM THE TERMINAL EQUIPMENT

TxClk

TPData

Data

XRT73L03IV Datasheet(PDF) 31 Page - Exar Corporation

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