DS3172 datasheet(112/232 Pages) DALLAS

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10.12.3 Detailed Description

The receiver performs clock and data recovery from an alternate mark inversion (AMI) coded signal or a B3ZS- or

HDB3-coded AMI signal and monitors for loss of the incoming signal. The transmitter drives standard pulse-shape

waveforms onto 75

W coaxial cable. Refer to Figure 10-32 for a detailed functional block diagram of the DS3/E3

LIU. The jitter attenuator can be mapped into the receiver data path, mapped into the transmitter data path, or be

disabled. The DS3/E3 LIU conforms to the telecommunications standards listed in Table 4-1. Figure 1-1 shows the

external components required for proper operation.

Figure 10-32. DS3/E3 LIU Block Diagram

Analog

Local

Loopback

Clock &

Data

Recovery

RXPn

RXNn

TXPn

TXNn

Power

Supply

squelch

Driver

Monitor

VDD

VSS

Automatic

Gain

Control

Adaptive

Equalizer

ALOS

CLKC

Clock Rate

Adapter

CLKB

CLKA

TO B3ZS/HDB3

DECODER

FROM B3ZS/HDB3

ENCODER

FROM DS3/E3

LINE

TO DS3/E3 LINE

10.12.4 Transmitter

10.12.4.1 Transmit Clock

The clock used in the LIU Transmitter is typically based on either the CLAD clock or TCLKI, selected by the

CLADC bit in PORT.CR3.

10.12.4.2 Waveshaping, Line Build-Out, Line Driver

The waveshaping block converts the transmit clock, positive data, and negative data signals into a single AMI

signal with the waveshape required for interfacing to DS3/E3 lines. Table 18-6 through Table 18-8 and Figure 18-9

(AC Timing section) show the waveform template specifications and test parameters.

Because DS3 signals must meet the waveform templates at the cross-connect through any cable length from 0 to

450ft, the waveshaping circuitry includes a selectable LBO feature. For cable lengths of 225ft or greater, the TLBO

configuration bit (PORT.CR2.TLBO) should be low. When TLBO is low, output pulses are driven onto the coaxial

cable without any preattenuation. For cable lengths less than 225ft, TLBO should be high to enable the LBO

circuitry. When TLBO is high, pulses are preattenuated by the LBO circuitry before being driven onto the coaxial

cable. The LBO circuitry provides attenuation that mimics the attenuation of 225ft of coaxial cable.

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