XRT7250 datasheet(270/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

251

B. Nibble-Parallel Mode

The XRT7250 will accept the E3 payload data, from

the Terminal Equipment, in a parallel manner, via the

TxNib[3:0] input pins. The Transmit Terminal Equip-

ment Input Interface will latch this data into its circuit-

ry, on the rising edge of the TxNibClk output signal.

C. Delineation of outbound E3 Frames

The Transmit Section will use the TxInClk input signal

as its timing reference and will initiate the generation

of E3 frames, asynchronous with respect to any ex-

ternal signal. The XRT7250 will pulse the TxFrame

output pin "High" whenever it is processing the last

bit, within a given outbound E3 frame.

D. Sampling of payload data, from the Terminal

Equipment

In Mode 6, the XRT7250 will sample the data, at the

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

TxInClk clock signal, following a pulse in the TxNibClk

signal (see Figure 111).

NOTE: The TxNibClk signal, from the XRT7250 operates

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

Interfacing the Transmit Payload Data Input Inter-

face block of the XRT7250 to the Terminal Equip-

ment for Mode 6 Operation

Figure 110 presents an illustration of the Transmit

Payload Data Input Interface block (within the

XRT7250) being interfaced to the Terminal Equip-

ment, for Mode 6 Operation.

Mode 6 Operation of the Terminal Equipment

In Figure 110 both the Terminal Equipment and the

XRT7250 will be driven by an external 8.592MHz

clock signal. The Terminal Equipment will receive the

8.592MHz clock signal via the E3_Nib_Clock_In input

pin. The XRT7250 will output the 8.592MHz clock

signal via the TxNibClk output pin.

The Terminal Equipment will serially output the data

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

the signal at the E3_Clock_In input pin. The

XRT7250 will latch the data, residing on the Tx-

Nib[3:0] input pins, on the rising edge of the TxNibClk

signal.

In this case the XRT7250 has the responsibility of

providing the framing reference signal by pulsing the

TxFrame output pin (and in turn the

Tx_Start_of_Frame input pin of the Terminal Equip-

ment) "High" for one bit-period, coincident with the

last bit within a given E3 frame.

Finally, the XRT7250 will always internally generate

the Overhead bits, when it is operating in both the E3

and Nibble-parallel modes. The XRT7250 will pull the

TxOHInd input pin "Low".

The behavior of the signals between the XRT7250

and the Terminal Equipment for E3 Mode 6 Operation

is illustrated in Figure 111.

FIGURE 110. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT PAYLOAD DATA

INPUT INTERFACE BLOCK OF THE XRT7250 FOR MODE 6 (NIBBLE-PARALLEL/LOCAL-TIMED/FRAME-MASTER)

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

34.368MHz Clock Source

TxInClk

8.592MHz

TxOH_Ind

E3_Overhead_Ind

XRT7250 Datasheet(PDF) 270 Page - Exar Corporation

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