XRT7250 datasheet(388/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

369

If the user writes a “0” into this bit-field, then the

LAPD Transmitter will be enabled, and the LAPD

Transmitter will immediately begin to transmit a con-

tinuous stream of Flag Sequence octets (0x7E), via

either the “GC” or the “NR” byte-field of each Out-

bound E3 frame (depending upon which byte has

been selected to carry the PMDL channel).

Conversely, if the user writes a “1” into this bit-field,

then the LAPD Transmitter will be disabled. The

Transmit E3 Framer block will insert the contents of

the Tx GC Byte Register into the “GC” byte-field for

each Outbound E3 frame. Likewise, the Transmit E3

Framer block will also insert the contents of the Tx

NR Byte Register into the NR” byte-field for each Out-

bound E3 frame. No transmission of PMDL data will

occur.

STEP 7 - Initiate the Transmission

At this point, the user should have written the PMDL

message into the on-chip Transmit LAPD Message

buffer and should have specified the type of LAPD

Message that is to be transmitted. The user should

have also specified whether the LAPD Transmitter will

transport the LAPD Message frame octets via the

GC-byte field or via the NR-byte field of each Out-

bound E3 frame. Finally the LAPD Transmitter should

have been enabled. Then initiate the transmission of

this message by writing a “1” to Bit 3 (Tx DL Start)

within the Tx E3 LAPD Status and Interrupt Register

(Address = 0x34), as depicted below.

)

A “0” to “1” transition in Bit 3 (TxDL Start) in this regis-

ter, initiates the transmission of LAPD Message

frames. At this point, the LAPD Transmitter will begin

to search thorugh 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

Transmitter 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 Sequence octet (e.g., seven consecutive

“1’s”). Afterwards, the LAPD Transmitter will begin to

encapsulate the PMDL Message, residing in the

Transmit LAPD Message buffer, into a LAPD Mes-

sage frame. Finally, the LAPD Transmitter will frag-

ment the Outbound LAPD Message frame into octets

and will begin to transport these octets via the GC or

the NR byte-fields (depending upon the user’s selec-

tion) of 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 Enable)

within the Tx E3 LAPD Status and Interrupt register

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

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

0000

000

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