XRT73L03 Datasheet, PDF (39/62 Pages) Exar Corporation – 3 CHANNEL E3/DS3/STS-1 LINE INTERFCE UNIT

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PRELIMINARY
3 CHANNEL E3/DS3/STS-1 LINE INTERFACE UNIT XRT73L03
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
Register CR2.
COMMAND REGISTER CR2_(N)
D4
D3
D2
D1
D0
RESERVED
ENDECDIS(n)
ALOSDIS(n)
DLOSDIS(n)
REQEN(n)
X
X
X
X
0
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
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