XRT7250 datasheet(287/463 Pages) EXAR

287 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

268

NOTE: In Figure 118, the unshaded boxes depict the tasks

that the user must perform. The shaded boxes present the

resulting tasks that the Transmit HDLC Controller block will

perform.

The Mechanics of Transmitting a New LAPD Mes-

sage frame, if the LAPD Transmitter has been

configured to re-transmit the LAPD Message

frame, repeatedly, at One-Second intervals.

If the LAPD Transmitter has been configured to re-

transmit the LAPD Message frame repeatedly at one-

second intervals, then it will do the following (at one-

second intervals).

• Stuff the PMDL Message.

• Read in the stuffed PMDL Message from the Trans-

mit LAPD Message buffer.

• Encapsulate this stuffed PMDL Message into a

LAPD Message frame.

• Transmit this LAPD Message frame to the Remote

Terminal Equipment.

If another (e.g., a different) PMDL Message is to be

transmitted to the Remote Terminal Equipment, this

new message will have to be written into the Transmit

LAPD Message buffer, via the Microprocessor Inter-

face block of the Framer IC. However, care must be

taken when writing this new PMDL message. If this

message is written into the Transmit LAPD Message

buffer at the wrong time (with respect to these One-

second LAPD Message frame transmissions), the us-

er’s action could interfere with these transmissions,

thereby causing the LAPD Transmitter to transmit a

corrupted message to the Remote Terminal Equip-

ment. In order to avoid this problem, while writing the

new message into the Transmit LAPD Message buff-

er, the user should do the following.

1. Configure the Framer to automatically reset acti-

vated interrupts.

The user can do this by writing a “1” into Bit 3 within

the Framer Operating Mode register (Address =

0x00), as depicted below.

FIGURE 118. FLOW CHART DEPICTING HOW TO USE THE LAPD TRANSMITTER

START

WRITE IN DATA LINK INFORMATION

The user accomplishes this by writing the information

that he/she wishes to transmit (via the LAPD

Transmitter) to locations 0x86through 0xDD, within the

Framer Address Space.

ENABLE THE LAPD

TRANSMITTER FOR TRANSMISSION

This is accomplished by writing 00000xx1bto the Tx

E3 LAPD Configuration Register.(where xx dictates

LAPD Message Type)

LAPD Transmitter inserts Frame Header

octets in front of the user payload.

LAPD Transmitter computes the 16 bit FCS

(a CRC-16 value) and inserts it into the LAPD

Message, following the user payload

LAPD Transmitter appends a Flag Sequence

Trailer octet to the end of the LAPD Message

(after the 16 bit FCS).

5 consecutive

“1s” detected

Message

Transmission

Complete

Insert a “0” after the

string of 5 consecutive

“1s”

END

Generate Interrupt

LAPD Transmitter will

continue to transmit

Flag Sequence octets.

INITIATE TRANSMISSION OF LAPD

MESSAGE

This is accomplished by writing 000010x0bto the Tx

E3 LAPD Status/InterruptRegister. (where x indicates

the user’s choiceto enable/disable “LAPD Message

Transfer Complete” Interrupt

Yes

CONFIGURE THE N-BIT to CARRYLAPD

Messages

This is accomplished by setting “TxNSourceSel[1:0]

= “1, 0”

XRT7250 Datasheet(PDF) 287 Page - Exar Corporation

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