XRT74L73 Datasheet, PDF (318/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
TRANSMIT E3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Not Used
Auto
Retransmit
Not Used
TxLAPD
Msg Length
TxLAPD
Enable
R/O
R/O
R/O
E/O
R/W
R/O
R/W
R/W
0
0
0
0
X
0
X
1
If the user writes a “0” into this bit-field, then the
LAPD Transmitter will be enabled, and the LAPD
Transmitter will immediately begin to transmit a con-
tinuous stream of Flag Sequence octets (0x7E), via
the “N” bit-field of each outbound E3 frame.
Conversely, if the user writes a “1” into this bit-field,
then the LAPD Transmitter will be disabled. The
Transmit E3 Framer block will automatically insert a
“1” into the “N” bit-field, within each outbound E3
frame. No transmission of PMDL data will occur.
STEP 7 - Initiate the Transmission
At this point, the user should have written the PMDL
message into the on-chip Transmit LAPD Message
buffer and the type of LAPD Message that is desired
to be transmitted should have been specified. Finally,
the user should have enabled the LAPD Transmitter.
The only remaining to do is initiate the transmission of
this message. This process is initiated by writing a
“1” to Bit 3 (Tx DL Start) within the Tx E3 LAPD Sta-
tus and Interrupt Register (Address = 0x34), as de-
picted below.
)
TXE3 LAPD STATUS AND INTERRUPT REGISTER (ADDRESS = 0X34)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Not Used
TxDL Start TxDL Busy
TxLAPD
Interrupt
Enable
TxLAPD
Interrupt
Status
RO
RO
RO
RO
R/W
RO
R/W
RUR
0
0
0
0
0
0
0
0
A “0” to “1” transition in Bit 3 (Tx DL Start) in this reg-
ister, initiates the transmission of LAPD Message
frames. At this point, the LAPD Transmitter will begin
to search through the PMDL message, which is resid-
ing within the Transmit LAPD Message buffer. If the
LAPD Transmitter finds any string of five (5) consecu-
tive “1’s” in the PMDL Message then the LAPD Trans-
mitter will insert a “0” immediately following these
strings of consecutive “1’s”. This procedure is known
as stuffing. The purpose of PMDL Message stuffing
is to insure that the user’s PMDL Message does not
contain strings of data that mimic the Flag Sequence
octet (e.g., six consecutive “1’s”) or the ABORT Se-
quence octet (e.g., seven consecutive “1’s”). After-
wards, the LAPD Transmitter will begin to encapsu-
late the PMDL Message, residing in the Transmit
LAPD Message buffer, into a LAPD Message frame.
Finally, the LAPD Transmitter will fragment the out-
bound LAPD Message frame into bits and will begin
to transport these bits via the N bit-field within each
outbound E3 frame.
While the LAPD Transmitter is transmitting this LAPD
Message frame, the TxDL Busy bit-field (Bit 2) within
the Tx E3 LAPD Status and Interrupt Register, will be
set to “1”. This bit-field allows the user to poll the sta-
tus of the LAPD Transmitter. Once the LAPD Trans-
mitter has completed the transmission of the LAPD
Message, then this bit-field will toggle back to “0”.
The user can configure the LAPD Transmitter to inter-
rupt the local Microprocessor/Microcontroller upon
completion of transmission of the LAPD Message
frame, by setting bit-field “1” (TxLAPD Interrupt En-
able) within the Tx E3 LAPD Status and Interrupt reg-
ister (Address = 0x34). to “1” as depicted below.
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