XRT7250 datasheet(328/463 Pages) EXAR

328 / 463 page

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

309

The E3, ITU-T G.751 frame consists of 1536 bits. Of

these bytes, 1524 bits are payload bits and the re-

maining 12 bits are overhead bits. The XRT7250 has

been designed to handle and process both the pay-

load type and overhead type bits for each E3 frame.

Within the Receive Section of the XRT7250, the Re-

ceive Payload Data Output Interface block has been

designed to handle the payload bits. Likewise, the

Receive Overhead Data Output Interface block has

been designed to handle and process the overhead

bits.

The Receive Overhead Data Output Interface block

unconditionally outputs the contents of all overhead

bits. The XRT7250 does not offer the user a means

to shut off this transmission of data. However, the

Receive Overhead Output Interface block does pro-

vide the user with the appropriate output signals for

external Data 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.

5.3.4.1

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 E3 overhead bits via Method 1.

• RxOH

• RxOHClk

• RxOHFrame

Each of these signals are listed and described below

in Table 63.

Interfacing the Receive Overhead Data Output In-

terface block to the Terminal Equipment (Method

Figure 145 illustrates how one should interface the

Receive Overhead Data Output Interface block to the

Terminal Equipment when using Method 1 to sample

and process the overhead bits from the inbound E3

data stream.

FIGURE 144. A SIMPLE ILLUSTRATION OF THE RECEIVE OVERHEAD OUTPUT INTERFACE BLOCK

Receive Overhead

Output Interface

Block

Receive Overhead

Output Interface

Block

From Receive

E3 Framer Block

RxOHFrame

RxOH

RxOHClk

RxOHEnable

XRT7250 Datasheet(PDF) 328 Page - Exar Corporation

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