XRT74L74 Datasheet, PDF (230/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L74
REV. P1.1.1
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
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 this step is executed, the LAPD Transmitter will
begin transmitting the flag sequence octet (0x7E) via
the DL bits.
NOTE: Upon power up or reset, the LAPD Transmitter is
disabled. Therefore, this bit must be set to a "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 is wished to
be transmitted, and has enabled the LAPD Transmit-
ter. The only thing remaining to do is to initiate the
transmission of this message. This process is initiat-
ed by writing a "1" to Bit 3 of the Tx DS3 LAPD Sta-
tus/Interrupt Register (TxDL Start). The bit format of
this register is presented below.
TRANSMIT DS3 LAPD STATUS/INTERRUPT REGISTER (ADDRESS = 0X34)
BIT 7
BIT 6
BIT 5
Not Used
BIT 4
R/O
R/O
R/O
R/O
0
0
0
0
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 LAPD Transmitter can be configured to interrupt
the µC/µP upon completion of transmission of the
LAPD Message, by setting bit-field 1 (TxLAPD Inter-
rupt 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 avail-
able and ready to transmit a new message. Bit 0 will
reflect 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,
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 69).
• 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).
BIT 3
BIT2
BIT 1
BIT 0
Tx DL
Start
R/W
1
Tx DL
Busy
RO
X
TxLAPD
Interrupt
Enable
R/W
X
TxLAPD
Interrupt
Status
RUR
X
• 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 5.3.3.2) will have the respon-
sibility of detecting the 5 consecutive 1s and removing the
subsequent "0" from the payload portion of the incoming
LAPD message.
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