XRT72L50 Datasheet, PDF (285/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
Prior to the transmission of any data via the LAPD Transmitter, the LAPD Transmitter must be enabled. This is
accomplished by writing a "1" to bit 0 (TxLAPD Enable) of the Tx E3 LAPD Configuration Register, as depicted
below.
Transmit E3 LAPD Configuration Register (Address = 0x33)
BIT 7
RO
0
BIT 6
BIT 5
Not Used
RO
RO
0
0
BIT 4
RO
0
BIT 3
Auto
Retransmit
R/W
X
BIT2
Not Used
RO
0
BIT 1
TxLAPD
Msg Length
R/W
X
BIT 0
TxLAPD
Enable
R/W
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 continuous 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 Status and
Interrupt Register (Address = 0x34), 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
0
0
BIT 1
TxLAPD
Interrupt
Enable
R/W
0
BIT 0
TxLAPD
Interrupt
Status
RUR
0
A “0” to “1” transition in Bit 3 (Tx DL Start) in this register, initiates the transmission of LAPD Message frames.
At this point, the LAPD Transmitter will begin to search through the PMDL message, which is residing within
the Transmit LAPD Message buffer. It will compute and append to the 2 byte FCS value If the LAPD
Transmitter finds any string of five (5) consecutive “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 Message frame. Finally, the LAPD Transmitter will fragment
the outbound 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 status of
the LAPD Transmitter. Once the LAPD Transmitter has completed the transmission of the LAPD Message,
then this bit-field will toggle back to “0”.
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