XRT7250 datasheet(366/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

347

the TxNibClk output clock signal. The XRT7250 will

indicate that it is processing the last nibble, within a

given E3 frame, by pulsing its TxNibFrame output pin

"High" for one TxNibClk clock period. When the Ter-

minal Equipment detects a pulse at its

Tx_Start_of_Frame input pin, it is expected to trans-

mit the first nibble, of the very next Outbound E3

frame to the XRT7250 via the E3_Data_Out[3:0] (or

TxNib[3:0] pins).

Finally, for the Nibble-Parallel Mode operation, the

XRT7250 will pulse the TxOHInd output pin “High” for

a total of 14 nibble periods (e.g., for the 7 overhead

bytes, within each of the E3, ITU-T G.832 frames). At

the beginning of an E3 frame, the XRT7250 will pulse

the TxOHInd output pin “High” for 4 nibble periods.

These four nibbles represent the “FA1” and “FA2”

bytes within each E3 frame. Throughout the remain-

der of the E3 framing period, the XRT7250 will pulse

the TxOHInd output pin 5 times. The width (or dura-

tion) of each of these pulses will be two nibbles.

Clearly, each of these 5 pulses corresponds to the

five remaining overhead bytes, within the E3, ITU-T

G.832 framing structure.

The behavior of the signals between the XRT7250

and the Terminal Equipment for E3 Mode 4 Operation

is illustrated in Figure 164.

How to configure the XRT7250 into Mode 4

1. Set the NibIntf input pin "High".

2. Set the TimRefSel[1:0] bit-fields (within the

Framer Operating Mode Register) to "00" as illus-

trated below.

FIGURE 164. BEHAVIOR OF THE TERMINAL INTERFACE SIGNALS BETWEEN THE XRT7250 AND THE TERMINAL EQUIP-

MENT

(MODE 4 OPERATION)

Terminal Equipment Signals

RxOutClk

Tx_Start_of_Frame

E3_Nib_Clock_In

E3_Data_Out[3:0]

Payload Nibble [1059]

Overhead Nibble [0]

XRT7250 Transmit Payload Data I/F Signals

E3 Frame Number N

E3 Frame Number N + 1

Note: TxNibFrame pulses high to denote

E3 Frame Boundary.

RxOutClk

TxNibFrame

TxNibClk

TxNib[3:0]

Nibble [1059]

Overhead Nibble [0]

E3_Overhead_Ind

TxOH_Ind

TxOH_Ind pulses high for 4 Nibble periods

FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT2

BIT 1

BIT 0

Local Loopback

DS3/E3*

Internal LOS

Enable

RESET

Interrupt

Enable Reset

Frame Format

TimRefSel[1:0]

XRT7250 Datasheet(PDF) 366 Page - Exar Corporation

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