XRT7250 datasheet(171/463 Pages) EXAR

171 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

152

Method 1 Operation of the Terminal Equipment

If the Terminal Equipment intends to insert any over-

head data into the outbound DS3 data stream, (via

the Transmit Overhead Data Input Interface), then it is

expected 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

DS3_OH_Clock_In signal).

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

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

DS3_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 DS3 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 21relates the number of rising clock edges (in

the TxOHClk signal, since TxOHFrame was sampled

"High") to the DS3 Overhead Bit, that is being pro-

cessed.

FIGURE 57. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT OVERHEAD DATA

INPUT INTERFACE (METHOD 1)

Terminal Equipment

XRT7250 DS3 Framer

DS3_OH_Out]

DS3_OH_Clock_In

Tx_Start_of_Frame

TxOHClk

TxOHFrame

TxOHIns

44.736 MHz Clock Source

TxInClk

TxOH

Insert_OH

RxLineClk

44.736 MHz

Clock Source

XRT7250 Datasheet(PDF) 171 Page - Exar Corporation

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