XRT7250 datasheet(267/463 Pages) EXAR

267 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

248

3. Interface the XRT7250, to the Terminal Equip-

ment, as illustrated in Figure 106.

NOTE: The XRT7250 Framer IC cannot support the Framer

Local Loop-back Mode of operation, while operating in

Mode 4. The user must configure the XRT7250 Framer IC

into any of the following modes prior to configuring the

Framer Local Loop-back Mode operation.

• Mode 2 - Serial/Local-Timed/Frame-Slave Mode

• Mode 3 - Serial/Local-Timed/Frame-Master Mode

• Mode 5 - Nibble-Parallel/Local-Timed/Frame-Slave

Mode

• Mode 6 - Nibble-Parallel/Local-Timed/Frame-Mas-

ter Mode.

For more detailed information on the Framer Local

Loop-back Mode, please see Section 6.0.

5.2.1.5

Mode 5 - The Nibble-Parallel/Local-

Timed/Frame-Slave Interface Mode Behavior of

the XRT7250

If the XRT7250 has been configured to operate in this

mode, then the XRT7250 will function as follows:

A. Local-Timed - Uses the TxInClk signal as the

Timing Reference

In this mode, the Transmit Section of the XRT7250

will use the TxInClk signal at its timing reference.

Further, the chip will internally divide the TxInClk

clock signal by a factor of 4 and will output this divid-

ed clock signal via the TxNibClk output pin. The

Transmit Terminal Equipment Input Interface block

(within the XRT7250) will use the rising edge of the

TxNibClk signal, to latch the data, residing on the Tx-

Nib[3:0] into its circuitry.

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 use the TxFrameRef

input signal as its Framing Reference (e.g., the Trans-

mit Section of the XRT7250 initiates frame generation

upon the rising edge of the TxFrameRef signal).

D. Sampling of payload data, from the Terminal

Equipment

In Mode 5, 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 108).

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 5 Operation

Figure 108 presents an illustration of the Transmit

Payload Data Input Interface block (within the

XRT7250) being interfaced to the Terminal Equip-

ment, for Mode 5 Operation.

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