XRT7250 datasheet(258/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

239

Mode 1 Operation of the Terminal Equipment

When the XRT7250 is operating in this mode, it will

function as the source of the 34.368MHz clock signal.

This clock signal will be used as the Terminal Equip-

ment Interface clock by both the XRT7250 IC and the

Terminal Equipment.

The Terminal Equipment will serially output the pay-

load data of the outbound E3 data stream via its

E3_Data_Out pin. The Terminal Equipment will up-

date the data on the E3_Data_Out pin upon the rising

edge of the 34.368 MHz clock signal, at its

E3_Clock_In input pin (as depicted in Figure 100 and

Figure 101).

The XRT7250 will latch the outbound E3 data stream

(from the Terminal Equipment) on the rising edge of

the RxOutClk signal.

The XRT7250 will indicate that it is processing the

last bit, within a given outbound E3 frame, by pulsing

its TxFrame output pin "High" for one bit-period.

When the Terminal Equipment detects this pulse at its

Tx_Start_of_Frame input, it is expected to begin

transmission of the very next outbound E3 frame to

the XRT7250 via the E3_Data_Out (or TxSer pin).

Finally, the XRT7250 will indicate that it is about to

process an overhead bit by pulsing the TxOH_Ind

output pin "High" one bit period prior to its processing

of an OH (Overhead) bit. In Figure 100, the

TxOH_Ind output pin is connected to the

E3_Overhead_Ind input pin of the Terminal Equip-

ment. Whenever the E3_Overhead_Ind pin is pulsed

"High" the Terminal Equipment is expected to not

transmit a E3 payload bit upon the very next clock

edge. Instead, the Terminal Equipment is expected to

delay its transmission of the very next payload bit, by

one clock cycle.

The behavior of the signals, between the XRT7250

and the Terminal Equipment, for E3 Mode 1 operation

is illustrated in Figure 101.

Inserting the A and N bits into the outbound E3

frames via the Transmit Payload Data Input Inter-

face block

The XRT7250 DS3/E3 Framer permits the Terminal

Equipment to insert its own values for the “A” and/or

“N” bits, into the outbound E3 frame, via the Transmit

Payload Data Input Interface block. If the user de-

sires to do this, the XRT7250 Framer IC must be con-

figured to accept the Terminal Equipment’s value for

the “A” and “N” bits, by writing to appropriate data into

the TxASourceSel[1:0] and TxNSourceSel[1:0] bit-

fields, within the TxE3 Configuration Register (Ad-

dress =0x30), as illustrated below.

Configuring the Transmit Payload Data Input In-

terface block to accept the “A Bits” from the Ter-

minal Equipment

If the user wishes to configure the Transmit Payload

Data Input Interface block to accept the “A” bits from

the Terminal Equipment, then the user must write the

value “10” into the TxASourceSel[1:0] bit-fields.

Once the user does this, then any value, which re-

sides on the TxSer input pin, when the “A” bit is being

processed by the Transmit Section will be inserted in-

to the “A” bit-field within the very next outbound E3

frame.

For completeness, the relationship between the con-

tents of the TxASourceSel[1:0] bits and the resulting

source of the “A” bit is listed below.

Bit 6, 5, TxASourceSel[1:0]

These two Read/Write bit-fields combine to specify

the source of the A-bit, within each outbound E3

frame. The relationship between these two bit-fields

and the resulting source of the A Bit is tabulated be-

low.

TXE3 CONFIGURATION REGISTER (ADDRESS = 0X30)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

BIP-4

Enable

TxASourceSel[1:0]

TxNSourceSel[1:0]

Tx AIS

Enable

Tx LOS

Enable

Tx FAS

Source

Select

R/W

XXX

TXASOURCESEL[1:0]

SOURCE OF A BIT

TxE3 Service Bits Register (Address = 0x35)

XRT7250 Datasheet(PDF) 258 Page - Exar Corporation

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