XRT7250 datasheet(155/463 Pages) EXAR

155 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

136

Figure 44 presents an illustration of the Transmit Pay-

load Data Input Interface block (within the XRT7250)

being interfaced to the Terminal Equipment, for Mode

1 operation.

Mode 1, Operation of the Terminal Equipment

When the XRT7250 is operating in this mode, it will

function as the source of the 44.736MHz clock signal

(via the RxOutClk signal). This clock signal will be

used as the Terminal Equipment Interface clock by

both the XRT7250 IC and the Terminal Equipment.

The Terminal Equipment will serially output the pay-

load data of the outbound DS3 data stream via its

DS3_Data_Out pin. The Terminal Equipment will up-

date the data on the DS3_Data_Out pin upon the ris-

ing edge of the 44.736 MHz clock signal, at its

DS3_Clock_In input pin (as depicted in Figure 44 and

Figure 45).

The XRT7250 will latch the outbound DS3 data

stream (from the Terminal Equipment) on the rising

edge of the RxOutClk signal.

The XRT7250 will indicate that it is processing the

last bit, within a given outbound DS3 frame, by puls-

ing its TxFrame output pin "High" for one bit-period.

When the Terminal Equipment detects this pulse at its

Tx_Start_of_Frame input, it is expected to begin

transmission of the very next outbound DS3 frame to

the XRT7250 via the DS3_Data_Out (or TxSer pin).

Finally, the XRT7250 will indicate that it is about to

process an overhead bit by pulsing the TxOH_Ind

output pin "High" one bit period prior to its processing

of an OH (Overhead) bit. In Figure 44, the TxOH_Ind

output pin is connected to the DS3_Overhead_Ind in-

put pin of the Terminal Equipment. Whenever the

DS3_Overhead_Ind pin is pulsed "High" the Terminal

Equipment is expected to not transmit a DS3 payload

bit upon the very next clock edge. Instead, the Termi-

nal Equipment is expected to delay its transmission of

the very next payload bit, by one clock cycle.

The behavior of the signals, between the XRT7250

and the Terminal Equipment, for DS3 Mode 1 opera-

tion is illustrated in Figure 45.

FIGURE 44. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA

INPUT INTERFACE BLOCK (OF THE XRT7250) FOR MODE 1(SERIAL/LOOP-TIMING) OPERATION

Terminal Equipment

XRT7250 DS3 Framer

DS3_Data_Out

DS3_Clock_In

Tx_Start_of_Frame

DS3_Overhead_Ind

TxSer

RxOutClk

TxFrame

TxOH_Ind

NibInt

44.736 MHz

XRT7250 Datasheet(PDF) 155 Page - Exar Corporation

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