XRT7250 datasheet(238/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

219

The XRT7250 will pulse the RxFrame output pin

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

given DS3 frame onto the RxSer output pin. If knowl-

edge of the DS3 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.

The Behavior of the Signals between the Receive

Payload Data Output Interface block and the Ter-

minal Equipment

The behavior of the signals between the XRT7250

and the Terminal Equipment for DS3 Serial Mode Op-

eration is illustrated in Figure 94.

4.3.5.2

Nibble-Parallel Mode Operation

Behavior of the XRT7250

If the XRT7250 has been configured to operate in the

Nibble-Parallel Mode, then the XRT7250 will behave

as follows.

Payload Data Output

The XRT7250 will output the payload data of the in-

coming DS3 frames, via the RxNib[3:0] output pins,

upon the falling edge of RxClk.

NOTES:

1. In this case, RxClk will function as the Nibble Clock

signal between the XRT7250 the Terminal Equip-

ment. The XRT7250 will pulse the RxClk output

signal "High" 1176 times, for each inbound DS3

frame.

2. Unlike Serial Mode operation, the duty cycle of

RxClk, in Nibble-Parallel Mode operation is approx-

imately 25%.

Delineation of Inbound DS3 Frames

The XRT7250 will pulse the RxFrame output pin

"High" for one nibble-period coincident with it driving

FIGURE 94. AN ILLUSTRATION OF THE BEHAVIOR OF THE SIGNALS BETWEEN THE RECEIVE PAYLOAD DATA OUTPUT

INTERFACE BLOCK OF THE XRT7250 AND THE TERMINAL EQUIPMENT (SERIAL MODE OPERATION)

Terminal Equipment Signals

DS3_Clock_In

DS3_Data_In

Rx_Start_of_Frame

DS3_Overhead_Ind

XRT7250 Receive Payload Data I/F Signals

RxClk

RxSer

RxFrame

RxOH_Ind

Payload[4702]

Payload[4703]

X-Bit

Payload[0]

Payload[4702]

Payload[4703]

X-Bit

Payload[0]

Note: X-Bit will not be processed by the

Transmit Payload Data Input Interface.

DS3 Frame Number N

DS3 Frame Number N + 1

Note: RxFrame pulses high to denote

DS3 Frame Boundary.

Note: RxOH_Ind pulses high to

denote Overhead Data

(e.g., the X-bit).

XRT7250 Datasheet(PDF) 238 Page - Exar Corporation

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