XRT7250 datasheet(185/463 Pages) EXAR

185 / 463 page

XRT7250

DS3/E3 FRAMER IC

áç

REV. 1.1.1

166

subsequent "0" from the payload portion of the incoming

LAPD message.

Figure 63 presents a flow chart depicting the proce-

dure (in 'white boxes') that the user should use in or-

der to transmit a LAPD message. This figure also in-

dicates (via the shaded boxes) what the LAPD Trans-

mitter circuitry will do before and during message

transmission.

The Mechanics of Transmitting a New LAPD Mes-

sage

As mentioned above, after the LAPD Transmitter has

been enabled, and commanded to transmit the mes-

sage, residing in the Transmit LAPD Message buffer,

it will continue to transmit this message at one-sec-

ond intervals. If another (e.g., different) PMDL mes-

sage is to be transmitted to the Remote LAPD Re-

ceiver, the new message will have to be written into

the Transmit LAPD Message buffer, via the Micropro-

cessor Interface section of the Framer. However,

care must be taken when writing in this new mes-

sage. If this message is written into the Transmit

LAPD Message buffer at the wrong time (with respect

to these one-second transmissions), the user's action

could interfere with these transmissions, thereby

causing the LAPD Transmitter to transmit a corrupted

message to the Remote LAPD Receiver. In order to

avoid this problem, while writing the new message in-

to the Transmit LAPD Message buffer, the user

should do the following:

1. Configure the Framer to automatically reset acti-

vated interrupts

This can be done by writing a "1" into Bit 3 of the

Framer Operating Mode Register, as depicted below.

FIGURE 63. FLOW CHART DEPICT 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 0x86

through 0xDD, within the Framer Address Space.

ENABLE THE LAPD

TRANSMITTER FOR TRANSMISSION

This is accomplished by writing 00000xx1b

to the Tx DS3 LAPD Configuration Register.

(where xx dictates LAPD Message Length)

INITIATE TRANSMISSION OF LAPD

MESSAGE

This is accomplished by writing 000010x0b

to the Tx DS3 LAPD Status/Interrupt

to enable/disable “LAPD Message Transfer

Complete” Interrupt

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.

Yes

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