XRT72L50 Datasheet, PDF (181/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
The LAPD Transmitter can be configured to interrupt the µC/µP upon completion of transmission of the LAPD
Message by setting Bit 1 (TxLAPD Interrupt Enable) of the Tx DS3 LAPD Status/Interrupt 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 reflects the interrupt status for the LAPD Transmitter.
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 and written the LAPD Transmitter does the following.
• Depending on the message type, 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,
(at the end of 76 or 82 bytes).
• Append a trailer Flag Sequence octet to the end of the message LAPD following the 16 bit FCS value.
• Serialize the composite LAPD message. Between the two 0x7E flags, ZeroStuff any consecutive five “Ones”
by inserting an extra “0”. This insures that any occurrence of 0x7E in the payload does not serve as a
terminating flag sequence. Insert the Zero Stuffed 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 transmission of the LAPD Message, the Framer generates an
interrupt to the local µC/µP (if enabled). Afterwards, the LAPD Transmitter proceeds to retransmit the LAPD
Message repeatedly at one second intervals. 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 continues 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 Register. If the LAPD Transmitter is idle, then it
will continuously send the Flag Sequence octets (via the DL bits of each outbound DS3 Frame) to the remote
terminal 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
detection of five (5) consecutive 1s (this stuffing occurs for all bits between the two flag sequence bytes 0x7E). The
'remote' LAPD Receiver (see Section 4.3.3.2) will have the responsibility of detecting the 5 consecutive 1s and
removing the subsequent "0" from the payload portion of the incoming LAPD message.
Figure 51 presents a flow chart depicting the procedure (in white boxes) that the user should use in order to
transmit a LAPD message. This figure also indicates (via the shaded boxes) what the LAPD Transmitter
circuitry will do before and during message transmission.
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