XRT7250 datasheet(184/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

165

Prior to executing step 2 (Enabling the LAPD Trans-

mitter), the LAPD Transmitter will be disabled and the

Transmit DS3 Framer block will be setting each of the

DL bits (within the outbound DS3 data stream) to 1.

After the user executes this step, the LAPD Transmit-

ter will begin transmitting the flag sequence octet

(0x7E) via the DL bits.

NOTE: Upon power up or reset, the LAPD Transmitter is

disabled. Therefore, the user must set this bit to "1" in order

to enable the LAPD Transmitter.

STEP 3 - Initiate the Transmission

At this point, the LAPD Transmitter is ready to begin

transmission. The user has written the information

portion of the PMDL message into the on-chip Trans-

mit LAPD Message buffer. Further, the user has

specified the type of LAPD message that he/she

wishes to transmit, and has enabled the LAPD Trans-

mitter. The only thing remaining to do is to initiate the

transmission of this message. The user initiates this

process by writing a "1" to Bit 3 of the Tx DS3 LAPD

Status/Interrupt Register (TxDL Start). The bit format

of this register is presented below.

A "0" to "1" transition of Bit 3 (TxDL Start) in this reg-

ister, initiates the transmission of the data link mes-

sage. While the LAPD transmitter is transmitting the

message, the 'TxDL Busy' (bit 2) bit will be set to 1.

This bit-field allows the user to poll the status of the

LAPD Transmitter. Once the message transfer is

completed, this bit-field will toggle back to '0'.

The user can configure the LAPD Transmitter to inter-

rupt the µC/µP upon completion of transmission of

the LAPD Message, by setting bit-field "1" (TxLAPD

Interrupt Enable) of the Tx DS3 LAPD Status/Inter-

rupt register to 1. The purpose of this interrupt is to let

the local µC/µP know that the LAPD Transmitter is

available and ready to transmit a new message. Bit 0

will reflect the interrupt status for the LAPD Transmit-

ter.

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

Details Associated with the Transmission of a

PMDL Message

Once the user has invoked the TxDL Start command,

the LAPD Transmitter will do the following.

• Generate the four octets of LAPD frame header

(e.g., Flag Sequence, SAPI, TEI, Control, etc.) and

insert it into the LAPD Message, prior to the user's

information (see the LAPD Message Frame Format

in Figure 62).

• Compute the 16 bit Frame Check Sum (FCS) of the

LAPD Message Frame (e.g., of the LAPD Message

header and information payload) and append this

value to the LAPD Message.

• Append a trailer Flag Sequence octet to the end of

the message LAPD (following the 16 bit FCS

value).

• Serialize the composite LAPD message and begin

inserting the LAPD message into the DL bit fields of

each outgoing DS3 Frame.

• Complete the transmission of the frame overhead,

payload, FCS value, and trailer Flag Sequence

octet via the Transmit DS3 Framer.

Once the LAPD Transmitter has completed its trans-

mission of the LAPD Message, the Framer will gener-

ate an interrupt to the local µC/µP (if enabled). After-

wards, the LAPD Transmitter will proceed to retrans-

mit the LAPD Message, repeatedly at one second in-

tervals. During Idle periods (e.g., in between these

transmission of the LAPD Message), the LAPD

Transmitter will be sending a continuous stream of

Flag Sequence Bytes. The LAPD Transmitter will

continue this behavior until the user has disabled the

LAPD Transmitter by writing a "0" to bit 0 (TxLAPD

Enable) within the Tx DS3 LAPD Configuration Reg-

ister. If the LAPD Transmitter is idle, then it will con-

tinuously send the Flag Sequence octets (via the DL

bits of each outbound DS3 Frame) to the remote ter-

minal equipment.

NOTE: In order to prevent the user's data (e.g., the payload

portion of the LAPD Message Frame) from mimicking the

Flag Sequence byte, the LAPD Transmitter will insert a "0"

into the LAPD data stream immediately following the detec-

tion of five (5) consecutive 1s (this stuffing occurs only while

the information payload is being transmitted). The 'remote'

LAPD Receiver (see Section 4.3.3.2) will have the responsi-

bility of detecting the 5 consecutive 1s and removing the

TRANSMIT DS3 LAPD STATUS/INTERRUPT REGISTER (ADDRESS = 0X34)

BIT 7BIT 6BIT 5BIT 4

BIT 3BIT2BIT 1BIT 0

Not Used

Tx DL

Start

Tx DL

Busy

TxLAPD

Interrupt

Enable

TxLAPD

Interrupt

Status

R/O

R/W

RUR

000

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