XRT7250 datasheet(237/463 Pages) EXAR

237 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

218

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 DS3

frames, via either of the following modes.

• Serial Mode

• Nibble-Parallel Mode

Each of these modes are described in detail, below.

4.3.5.1

Serial Mode Operation

Behavior of the XRT7250

If the XRT7250 has been configured to operate in the

Serial mode, then the XRT7250 will behave as fol-

lows.

Payload Data Output

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

coming DS3 frames via the RxSer output, 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 DS3 frame, via the RxSer out-

put pin.

Interfacing the XRT7250 to the Receive Terminal

Equipment

Figure 93 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. However, be-

cause the rising edge of RxClk to data delay is be-

tween 14ns to 16ns, the Terminal Equipment should

sample the data on the RxSer output pin (or the

DS3_Data_In pin at the Terminal Equipment) upon

the rising edge of RxClk. This will still permit the Ter-

minal Equipment with a RxSer to RxClk set-up time of

approximately 6ns and a hold time of 14 to 16ns. As

the Terminal Equipment samples RxSer with each ris-

ing edge of RxClk it should also be sampling the fol-

lowing signals.

• RxFrame

• RxOHInd

The Need for sampling RxFrame

FIGURE 93. ILLUSTRATION OF THE XRT7250 DS3/E3 FRAMER IC BEING INTERFACED TO THE RECEIVE TERMINAL

EQUIPMENT (SERIAL MODE OPERATION)

Terminal Equipment

(Receive Payload Section)

XRT7250 DS3 Framer

DS3_Data_In

Rx_DS3_Clock_In

Rx_Start_of_Frame

RxClk

RxFrame

RxOHIns

44.736 MHz Clock Signal

RxSer

Rx_DS3_OH_Ind

RxLineClk

44.736 MHz

Clock Source

XRT7250 Datasheet(PDF) 237 Page - Exar Corporation

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