XRT72L50 Datasheet, PDF (286/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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The user can configure the LAPD Transmitter to interrupt 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 7
RO
0
BIT 6
BIT 5
Not Used
RO
RO
0
0
BIT 4
RO
0
BIT 3
TxDL Start
BIT 2
TxDL Busy
R/W
RO
X
X
BIT 1
TxLAPD
Interrupt
Enable
R/W
1
BIT 0
TxLAPD
Interrupt
Status
RUR
X
‘The purpose of t his interrupt is to let the Microprocessor/Microcontroller know that the LAPD Transmitter is
available and ready to transmit a LAPD Message frame (which contains a new PMDL Message) to the remote
terminal equipment. Bit 0 (Tx LAPD Interrupt Status) within the Tx E3 LAPD Status and Interrupt 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.
• 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 N bit-field of each outgoing E3
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 frame, the Framer will
generate an Interrupt to the MIcroprocessor/Microcontroller (if enabled). Afterwards, the LAPD Transmitter will
either halt its transmission of LAPD Message frames or will proceed to retransmit the LAPD Message frame,
repeatedly at one-second intervals. In between these transmissions of the LAPD Message frames, 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 “1” into bit 3 (No Data Link) within
the Tx E3 Configuration register.
NOTE: In order to prevent the user’s data (e.g., the PMDL Message within the LAPD Message frame) from mimicking the
Flag Sequence byte or an ABORT Sequence, the LAPD Transmitter will parse through the PMDL Message data
and insert a “0” into this data, immediately following the detection of five (5) consecutive “1’s” (this stuffing occurs
while the PMDL message data is being read in from the Transmit LAPD Message frame. The Remote LAPD
Receive (See Section 5.3.3 will have the responsibility of checking the newly received PMDL messages for a
string of five (5) consecutive “1’s” and removing the subsequent “0” from the payload portion of the incoming LAPD
Message.
Figure 106 presents a flow chart diagram. Figure 106 depicts the procedure (in white boxes) that the user
should use in order to transmit a PMDL message via the LAPD Transmitter, when the LAPD Transmitter is
configured to retransmit the LAPD Message frame, repeatedly at One-Second intervals. This figure also
indicates (via the Shaded boxes) what the LAPD Transmitter circuitry will do before and during message
transmission.
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