XRT7250 datasheet(163/463 Pages) EXAR

163 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

144

3. Interface the XRT7250, to the Terminal Equip-

ment, as illustrated in Figure 50.

NOTE: The XRT7250 Framer IC cannot support the Framer

Local Loop-back Mode of operation. The user must config-

ure 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 Operation, please see Section 6.0.

4.2.1.5

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 DS3 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 DS3 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).

NOTE: In this case, the Terminal Equipment should pulse

the TxFrameRef input signal (of the XRT7250 Framer IC)

coincident with it applying the first payload nibble, within a

given outbound DS3 frame. Hence, the duration of this

pulse should be one nibble-period of the DS3 signal (see

Figure 53).

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 53).

NOTE: The TxNibClk signal, from the XRT7250 operates

nominally at 11.184 MHz (e.g., 44.736 MHz divided by 4).

However, for reasons described below, TxNibClk effectively

operates at a "Low"er clock frequency. The Transmit Pay-

load Data Input Interface is only used to accept the payload

data, which is intended to be carried by outbound DS3

frames. The Transmit Payload Data Input Interface is not

designed to accommodate the entire DS3 data stream.

The DS3 Frame consists of 4704 payload bits or 1176

nibbles. Therefore, the XRT7250 will supply 1176 Tx-

NibClk pulses between the rising edges of two con-

secutive TxNibFrame pulses. The DS3 Frame repeti-

tion rate is 9.398kHz. Hence, 1176 TxNibClk pulses

for each DS3 frame period amounts to TxNibClk run-

ning at approximately 11.052 MHz. The method by

which the 1176 TxNibClk pulses are distributed

throughout the DS3 frame period is presented below.

Nominally, the Transmit Section within the XRT7250

will generate a TxNibClk pulse for every 4 RxOutClk

(or TxInClk) periods. However, in 14 cases (within a

DS3 frame period), the Transmit Payload Data Input

Interface will allow 5 TxInClk periods to occur be-

tween two consecutive TxNibClk pulses.

Interfacing the Transmit Payload Data Input Inter-

face block of the XRT7250 to the Terminal Equip-

ment for Mode 5 Operation

Figure 52 presents an illustration of the Transmit Pay-

load Data Input Interface block (within the XRT7250)

being interfaced to the Terminal Equipment, for Mode

5 Operation.

XRT7250 Datasheet(PDF) 163 Page - Exar Corporation

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