XRT72L56 Datasheet, PDF (204/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56 SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
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PRELIMINARY
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.
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 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.
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
• 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
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