XRT7250 datasheet(444/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

425

Operation of the Receive Payload Data Output In-

terface block

The Receive Payload Data Output Interface permits

the user to read out the payload data of Inbound E3

frames, via either of the following modes.

• Serial Mode

• Nibble-Parallel Mode

Each of these modes are described in detail, below.

6.3.5.1

Serial Mode Operation Behavior of the

XRT7250

If the XRT7250 has been configured to operate in this

mode, then the XRT7250 will behave as follows.

Payload Data Output

The XRT7250 will output the payload data, of the in-

coming E3 frames, upon the rising edge of RxClk.

Delineation of Inbound DS3 Frames

The XRT7250 will pulse the RxFrame output pin

"High" for one bit-period, coincident with it driving the

first bit within a given E3 frame, via the RxSer output

pin.

Interfacing the XRT7250 to the Receive Terminal

Equipment

Figure 209 presents a simple illustration as how the

user should interface the XRT7250 to that terminal

equipment which processes Receive Direction pay-

load data.

Required Operation of the Terminal Equipment

The XRT7250 will update the data on the RxSer out-

put pin, upon the rising edge of RxClk. Hence, the

Terminal Equipment should sample the data on the

RxSer output pin (or the E3_Data_In pin at the Termi-

nal Equipment) upon the rising edge of RxClk. As the

Terminal Equipment samples RxSer with each rising

edge of RxClk it should also be sampling the follow-

ing signals.

• RxFrame

• RxOHInd

The Need for sampling RxFrame

The XRT7250 will pulse the RxFrame output pin

"High" coincident with it driving the very first bit of a

given E3 frame onto the RxSer output pin. If knowl-

edge of the E3 Frame Boundaries is important for the

operation of the Terminal Equipment, then this is a

very important signal for it to sample.

The Need for sampling RxOHInd

The XRT7250 will indicate that it is currently driving

an overhead bit onto the RxSer output pin, by pulsing

the RxOHInd output pin "High". If the Terminal Equip-

ment samples this signal "High", then it should know

that the bit, that it is currently sampling via the RxSer

pin is an overhead bit and should not be processed.

FIGURE 209. ILLUSTRATION OF THE RECEIVE PAYLOAD DATA OUTPUT INTERFACE BLOCK (OF THE XRT7250 DS3/

E3 FRAMER IC) BEING INTERFACED TO THE RECEIVE TERMINAL EQUIPMENT (SERIAL MODE OPERATION)

Terminal Equipment

(Receive Payload Section)

XRT7250 E3 Framer

E3_Data_In

Rx_E3_Clock_In

Rx_Start_of_Frame

RxClk

RxFrame

RxOHInd

34.368 MHz Clock Signal

RxSer

Rx_E3_OH_Ind

RxLineClk

34.368 MHz

Clock Source

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