XRT7250 datasheet(265/463 Pages) EXAR

265 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

246

If the XRT7250 has been configured to operate in this

mode, then the XRT7250 will behave as follows.

A. Looped Timing (Uses the RxLineClk as the

Timing Reference)

In this mode, the Transmit Section of the XRT7250

will use the RxLineClk signal as its timing reference.

When the XRT7250 is operating in the Nibble-Mode,

it will internally divide the RxLineClk signal, by a fac-

tor of four (4) and will output this signal via the TxNib-

Clk output pin.

B. Nibble-Parallel Mode

The XRT7250 will accept the E3 payload data, from

the Terminal Equipment in a nibble-parallel manner,

via the TxNib[3:0] input pins. The Transmit Terminal

Equipment Input Interface block will latch this data in-

to its circuitry, on the rising edge of the TxNibClk out-

put signal.

C. Delineation of the outbound E3 frames

The XRT7250 will pulse the TxNibFrame output pin

"High" for one bit-period coincident with the XRT7250

processing the last nibble of a given E3 frame.

D. Sampling of payload data, from the Terminal

Equipment

In Mode 4, the XRT7250 will sample the data, at the

TxNib[3:0] input pins, on the third rising edge of the

RxOutClk clock signal, following a pulse in the TxNib-

Clk signal (see Figure 107).

NOTE: The TxNibClk signal, from the XRT7250 operates

nominally at 8.592 MHz (e.g., 34.368 MHz divided by 4).

The E3 Frame consists of 1536 bits or 384 nibbles.

Therefore, the XRT7250 will supply 384 TxNibClk

pulses between the rising edges of two consecutive

TxNibFrame pulses. The E3 Frame repetition rate is

22.375kHz. Hence, 384 TxNibClk pulses for each E3

frame period amounts to TxNibClk running at approx-

imately 8.592 MHz. The method by which the 384

TxNibClk pulses are distributed throughout the E3

frame period is presented below.

Nominally, the Transmit Section within the XRT7250

will generate a TxNibClk pulse for every 4 RxOutClk

(or TxInClk) periods.

Interfacing the Transmit Payload Data Input Inter-

face block of the XRT7250 to the Terminal Equip-

ment for Mode 4 Operation

Figure 106 presents an illustration of the Transmit

Payload Data Input Interface block (within the

XRT7250) being interfaced to the Terminal Equip-

ment, for Mode 4 Operation.

Mode 4 Operation of the Terminal Equipment

When the XRT7250 is operating in this mode, it will

function as the source of the 8.592MHz (e.g., the

34.368MHz clock signal divided by 4) clock signal,

that will be used as the Terminal Equipment Interface

clock by both the XRT7250 and the Terminal Equip-

ment.

The Terminal Equipment will output the payload data

of the outbound E3 data stream via its

E3_Data_Out[3:0] pins on the rising edge of the

8.592MHz clock signal at the E3_Nib_Clock_In input

pin.

The XRT7250 will latch the outbound E3 data stream

(from the Terminal Equipment) on the rising edge of

the TxNibClk output clock signal. The XRT7250 will

FIGURE 106. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA

INPUT INTERFACE BLOCK OF THE XRT7250 FOR MODE 4 (NIBBLE-PARALLEL/LOOP-TIMED) OPERATION

Terminal Equipment

XRT7250 E3 Framer

E3_Data_Out[3:0]

E3_Nib_Clock_In

Tx_Start_of_Frame

TxNib[3:0]

TxNibClk

TxNibFrame

NibInt

VCC

RxLineClk

34.368MHz

8.592 MHz

TxOH_Ind

E3_Overhead_Ind

XRT7250 Datasheet(PDF) 265 Page - Exar Corporation

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