XRT7250 datasheet(224/463 Pages) EXAR

224 / 463 page

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

205

The DS3 frame consists of 4760 bits. Of these bits,

4704 bits are payload bits and the remaining 56 bits

are overhead bits. The XRT7250 has been designed

to handle and process both the payload type and

overhead type bits for each DS3 frame.

The Receive Payload Data Output Interface block,

within the Receive Section of the XRT7250, has

been designed to handle the payload bits. Likewise,

the Receive Overhead Data Output Interface block

has been designed to handle and process the over-

head bits.

The Receive Overhead Data Output Interface block

unconditionally outputs the contents of all overhead

bits within the incoming DS3 data stream. The

XRT7250 does not offer the user a means to shut off

this transmission of data. However, the Receive

Overhead Output Interface block does provide the us-

er with the appropriate output signals for external Da-

ta Link Layer equipment to sample and process these

overhead bits, via the following two methods.

• Method 1- Using the RxOHClk clock signal.

• Method 2 - Using the RxClk and RxOHEnable out-

put signals.

Each of these methods are described below.

4.3.4.1

Method 1 - Using the RxOHClk Clock

signal

The Receive Overhead Data Output Interface block

consists of four (4) signals. Of these four signals, the

following three signals are to be used when sampling

the DS3 overhead bits via Method 1.

• RxOH

• RxOHClk

• RxOHFrame

Each of these signals are listed and described below

in Table 41.

FIGURE 87. A SIMPLE ILLUSTRATION OF THE RECEIVE OVERHEAD OUTPUT INTERFACE BLOCK

Receive Overhead

Output Interface

Block

Receive Overhead

Output Interface

Block

From Receive

DS3 Framer Block

RxOHFrame

RxOH

RxOHClk

RxOHEnable

XRT7250 Datasheet(PDF) 224 Page - Exar Corporation

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