XRT7250 datasheet(166/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

147

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 initiate the generation

of DS3 frames, asynchronous with respect to any ex-

ternal signal. The XRT7250 will pulse the TxFrame

output pin "High" whenever it is processing the last

nibble, within a given outbound DS3 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 55).

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 6 Operation

Figure 54 presents an illustration of the Transmit Pay-

load Data Input Interface block (within the XRT7250)

being interfaced to the Terminal Equipment, for Mode

6 Operation.

Mode 6, Operation of the Terminal Equipment

In Figure 54 both the Terminal Equipment and the

XRT7250 will be driven by an external 11.184MHz

clock signal. The Teriminal Equipment will receive

the 11.184MHz clock signal via the

DS3_Nib_Clock_In input pin. The XRT7250 will out-

put the 11.184MHz clock signal via the TxNibClk out-

put pin.

FIGURE 54. 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 DS3 Framer

DS3_Data_Out[3:0]

DS3_Nib_Clock_In

Tx_Start_of_Frame

TxNib[3:0]

TxNibClk

TxNibFrame

NibInt

VCC

44.736MHz Clock Source

TxInClk

11.184MHz

XRT7250 Datasheet(PDF) 166 Page - Exar Corporation

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