XRT72L56 Datasheet, PDF (406/486 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
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
either the “GC” or the “NR” byte-field of each Out-
bound E3 frame (depending upon which byte has
been selected to carry the PMDL channel).
Conversely, if the user writes a “1” into this bit-field,
then the LAPD Transmitter will be disabled. The
Transmit E3 Framer block will insert the contents of
the Tx GC Byte Register into the “GC” byte-field for
each Outbound E3 frame. Likewise, the Transmit E3
Framer block will also insert the contents of the Tx
NR Byte Register into the NR” byte-field for each Out-
bound 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 should have specified the type of LAPD
Message that is to be transmitted. The user should
have also specified whether the LAPD Transmitter will
transport the LAPD Message frame octets via the
GC-byte field or via the NR-byte field of each Out-
bound E3 frame. Finally the LAPD Transmitter should
have been enabled. Then initiate the transmission of
this message by writing a “1” to Bit 3 (Tx DL Start)
within the Tx E3 LAPD Status and Interrupt Register
(Address = 0x34), as depicted 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 (TxDL Start) in this regis-
ter, initiates the transmission of LAPD Message
frames. At this point, the LAPD Transmitter will begin
to search thorugh 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
Transmitter 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 Sequence octet (e.g., seven consecutive
“1’s”). Afterwards, the LAPD Transmitter will begin to
encapsulate the PMDL Message, residing in the
Transmit LAPD Message buffer, into a LAPD Mes-
sage frame. Finally, the LAPD Transmitter will frag-
ment the Outbound LAPD Message frame into octets
and will begin to transport these octets via the GC or
the NR byte-fields (depending upon the user’s selec-
tion) of 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 Enable)
within the Tx E3 LAPD Status and Interrupt register
(Address = 0x34). to “1” as depicted below.
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