XRT7250 datasheet(285/463 Pages) EXAR

285 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

266

the user should have enabled the LAPD Transmitter.

The only remaining to do is initiate the transmission of

this message. This process is initiated by writing a

“1” to Bit 3 (Tx DL Start) within the Tx E3 LAPD Sta-

tus and Interrupt Register (Address = 0x34), as de-

picted below.

)

A “0” to “1” transition in Bit 3 (Tx DL Start) in this reg-

ister, initiates the transmission of LAPD Message

frames. At this point, the LAPD Transmitter will begin

to search through the PMDL message, which is resid-

ing within the Transmit LAPD Message buffer. If the

LAPD Transmitter finds any string of five (5) consecu-

tive “1’s” in the PMDL Message then the LAPD Trans-

mitter will insert a “0” immediately following these

strings of consecutive “1’s”. This procedure is known

as stuffing. The purpose of PMDL Message stuffing

is to insure that the user’s PMDL Message does not

contain strings of data that mimic the Flag Sequence

octet (e.g., six consecutive “1’s”) or the ABORT Se-

quence octet (e.g., seven consecutive “1’s”). After-

wards, the LAPD Transmitter will begin to encapsu-

late the PMDL Message, residing in the Transmit

LAPD Message buffer, into a LAPD Message frame.

Finally, the LAPD Transmitter will fragment the out-

bound LAPD Message frame into bits and will begin

to transport these bits via the N bit-field within each

outbound E3 frame.

While the LAPD Transmitter is transmitting this LAPD

Message frame, the TxDL Busy bit-field (Bit 2) within

the Tx E3 LAPD Status and Interrupt Register, will be

set to “1”. This bit-field allows the user to poll the sta-

tus of the LAPD Transmitter. Once the LAPD Trans-

mitter has completed the transmission of the LAPD

Message, then this bit-field will toggle back to “0”.

The user can configure the LAPD Transmitter to inter-

rupt the local Microprocessor/Microcontroller upon

completion of transmission of the LAPD Message

frame, by setting bit-field “1” (TxLAPD Interrupt En-

able) within the Tx E3 LAPD Status and Interrupt reg-

ister (Address = 0x34). to “1” as depicted below.

)

‘The purpose of t his interrupt is to let the Micropro-

cessor/Microcontroller know that the LAPD Transmit-

ter is available and ready to transmit a LAPD Mes-

sage frame (which contains a new PMDL Message)

to the remote terminal equipment. Bit 0 (Tx LAPD In-

terrupt Status) within the Tx E3 LAPD Status and In-

terrupt Register will reflect the status for the Transmit

LAPD Interrupt.

NOTE: This bit-field will be reset upon reading this register.

Summary of Operating the LAPD Transmitter

Once the user has invoked the TxDL Start command,

the LAPD Transmitter will do the following.

• Generate the four octets of the LAPD Message

frame header (e.g., the Flag Sequence, SAPI, TEI,

Control, etc.,) and insert them into the header byte

positions within the LAPD Message frame.

• It will read in the contents of the Transmit LAPD

Message buffer (e.g., the PMDL Message data)

and insert it into the Information Payload portion of

the LAPD Message frame.

TXE3 LAPD STATUS AND INTERRUPT REGISTER (ADDRESS = 0X34)

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

Not Used

TxDL Start

TxDL Busy

TxLAPD

Interrupt

Enable

TxLAPD

Interrupt

Status

R/W

RUR

000

TXE3 LAPD STATUS AND INTERRUPT REGISTER (ADDRESS = 0X34)

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

Not Used

TxDL Start

TxDL Busy

TxLAPD

Interrupt

Enable

TxLAPD

Interrupt

Status

R/W

RUR

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