XRT73LC00IV datasheet(27/53 Pages) EXAR

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XRT73LC00

E3/DS3/STS-1 LINE INTERFACE UNIT

PRELIMINARY

REV. P1.0.1

The XRT73LC00 Transmit Clock Duty Cycle Adjust

circuit alleviates the need to supply a signal with a

50% duty cycle to the TCLK input pin.

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 XRT73LC00 is configured to operate in the

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

Encoder block operates 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 (“000”). If the B3ZS Encoder

finds an occurrence of three consecutive zeros, 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 and “V” represents

a bipolar Violation (e.g., a bipolar pulse that violates

the Alternating Polarity requirements of the AMI line

code).

The B3ZS Encoder decides whether to substitute

with either a "00V" or a "B0V" pattern to insure that

an odd number of bipolar pulses exist between any

two consecutive violation pulses.

Figure 14 illustrates the B3ZS Encoder at work with

two separate strings of three (or more) consecutive

zeros.

2.3.2

HDB3 Encoding

If the XRT73LC00 is configured to operate in the E3

Mode, then the HDB3/B3ZS Encoder block operates

in the HDB3 Mode. When the Encoder is operating in

this mode, it parses through and searches the Trans-

mit Data Stream from the Transmit Logic Block for the

occurrence of four (4) consecutive zeros (“0000”). If

the HDB3 Encoder finds an occurrence of four con-

secutive zeros, then it substitutes these four “0’s” with

either a “000V” or a “B00V” pattern to insure that an

odd number of bipolar pulses exist between any two

consecutive violation pulses.

Figure 15 illustrates the HDB3 Encoder at work with

two separate strings of four (or more) consecutive ze-

ros.

FIGURE 14. AN EXAMPLE OF B3ZS ENCODING

TClk

TPOS

SR data

Encoded

PDATA

Encoded

NDATA

000

Line signal

FIGURE 15. AN EXAMPLE OF HDB3 ENCODING

TClk

TPOS

SR data

Encoded

PDATA

Encoded

NDATA

100

00000

Line signal

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