XRT7250 datasheet(275/463 Pages) EXAR

275 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

256

Method 1 Operation of the Terminal Equipment

If the Terminal Equipment intends to insert any over-

head data into the outbound E3 data stream, (via the

Transmit Overhead Data Input Interface), then it is ex-

pected to do the following.

1. To sample the state of the TxOHFrame signal

(e.g., the Tx_Start_of_Frame input signal) on the

rising edge of the TxOHClk (e.g., the

E3_OH_Clock_In signal).

2. To keep track of the number of rising clock edges

that have occurred, via the TxOHClk (e.g., the

E3_OH_Clock_In signal) since the last time the

TxOHFrame signal was sampled "High". By

doing this the Terminal Equipment will be able to

keep track of which overhead bit is being pro-

cessed by the Transmit Overhead Data Input

Interface block at any given time. When the Ter-

minal Equipment knows which overhead bit is

being processed, at a given TxOHClk period, it

will know when to insert a desired overhead bit

value into the outbound E3 data stream. From

this, the Terminal Equipment will know when it

should assert the TxOHIns input pin and place

the appropriate value on the TxOH input pin (of

the XRT7250).

Table 49 relates the number of rising clock edges (in

the TxOHClk signal, since TxOHFrame was sampled

"High") to the E3 Overhead Bit, that is being pro-

cessed.

FIGURE 113. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT OVERHEAD DATA

INPUT INTERFACE (METHOD 1)

Terminal Equipment

XRT7250 E3 Framer

E3_OH_Out]

E3_OH_Clock_In

Tx_Start_of_Frame

TxOHClk

TxOHFrame

TxOHIns

34.368 MHz Clock Source

TxInClk

TxOH

Insert_OH

RxLineClk

34.368 MHz

Clock Source

XRT7250 Datasheet(PDF) 275 Page - Exar Corporation

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