XRT7250 datasheet(263/463 Pages) EXAR

263 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

244

If the XRT7250 has been configured to operate in this

mode, then the XRT7250 will function as follows.

A. Local-Timed - Uses the TxInClk signal as the

Timing Reference

In this mode, the Transmit Section of the XRT7250

will use the TxInClk signal as its timing reference.

B. Serial Mode

The XRT7250 will receive the E3 payload data, in a

serial manner, via the TxSer input pin. The Transmit

Payload Data Input Interface (within the XRT7250)

will latch this data into its circuitry, on the rising edge

of the TxInClk input clock signal.

C. Delineation of outbound DS3 frames (Frame

Master Mode)

The Transmit Section of the XRT7250 will use the Tx-

InClk signal as its timing reference, and will initiate E3

frame generation, asynchronously with respect to any

externally applied signal. The XRT7250 will pulse its

TxFrame output pin "High" whenever its it processing

the very last bit-field within a given E3 frame.

D. Sampling of payload data, from the Terminal

Equipment

In Mode 3, the XRT7250 will sample the data, at the

TxSer input pin, on the rising edge of TxInClk.

Interfacing the Transmit Payload Data Input Inter-

face block of the XRT7250 to the Terminal Equip-

ment for Mode 3 Operation

Figure 104 presents an illustration of the Transmit

Payload Data Input Interface block (within the

XRT7250) being interfaced to the Terminal Equip-

ment, for Mode 3 operation.

Mode 3 Operation of the Terminal Equipment

In Figure 104, both the Terminal Equipment and the

XRT7250 are driven by an external 34.368 MHz clock

signal. This clock signal is connected to the

E3_Clock_In input of the Terminal Equipment and the

TxInClk input pin of the XRT7250.

The Terminal Equipment will serially output the pay-

load data on its E3_Data_Out output pin, upon the

rising edge of the signal at the E3_Clock_In input pin.

Similarly, the XRT7250 will latch the data, residing on

the TxSer input pin, on the rising edge of TxInClk.

The XRT7250 will pulse the TxFrame output pin

"High" for one bit-period, coincident while it is pro-

cessing the last bit-field within a given outbound E3

frame. The Terminal Equipment is expected to moni-

tor the TxFrame signal (from the XRT7250) and to

place the first bit, within the very next outbound E3

frame on the TxSer input pin.

NOTE: In this case, the XRT7250 dictates exactly when the

very next E3 frame will be generated. The Terminal Equip-

ment is expected to respond appropriately by providing the

XRT7250 with the first bit of the new E3 frame, upon

demand. Hence, in this mode, the XRT7250 is referred to

FIGURE 104. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA

INPUT INTERFACE BLOCK OF THE XRT7250 FOR MODE 3 (SERIAL/LOCAL-TIME/FRAME-MASTER) OPERATION

Terminal Equipment

XRT7250 E3 Framer

E3_Data_Out

E3_Clock_In

Tx_Start_of_Frame

E3_Overhead_Ind

TxSer

TxInClk

TxFrame

TxOH_Ind

NibInt

34.368 MHz Clock

Source

XRT7250 Datasheet(PDF) 263 Page - Exar Corporation

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