XRT7250 datasheet(159/463 Pages) EXAR

159 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

140

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 DS3 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

DS3 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 DS3

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 48 presents an illustration of the Transmit Pay-

load Data Input Interface block (within the XRT7250)

being interfaced to the Terminal Equipment, for Mode

3 operation.

Mode 3 Operation of the Terminal Equipment

In Figure 48, both the Terminal Equipment and the

XRT7250 are driven by an external 44.736MHz clock

signal. This clock signal is connected to the

DS3_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 DS3_Data_Out output pin, upon the

rising edge of the signal at the DS3_Clock_In input

pin. Similarly, the XRT7250 will latch the data, resid-

ing on the TxSer input pin, on the rising edge of TxIn-

Clk.

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 DS3

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 DS3

frame on the TxSer input pin.

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

very next DS3 frame will be generated.

The Terminal Equipment is expected to respond ap-

propriately by providing the XRT7250 with the first bit

of the new DS3 frame, upon demand. Hence, in this

mode, the XRT7250 is referred to as the Frame Mas-

FIGURE 48. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA

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

Terminal Equipment

XRT7250 DS3 Framer

DS3_Data_Out

DS3_Clock_In

Tx_Start_of_Frame

DS3_Overhead_Ind

TxSer

TxInClk

TxFrame

TxOH_Ind

NibInt

44.736 MHz Clock

Source

XRT7250 Datasheet(PDF) 159 Page - Exar Corporation

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