XRT72L13 Datasheet, PDF (279/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

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PRELIMINARY
XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
0
0
0
0
X
0
X
1
Bit 0 - TxLAPD Enable
This bit-field allow the user to enable or disable the
LAPD Transmitter in accordance with Table 20 .
TABLE 20: RELATIONSHIP BETWEEN TXLAPD MSG LENGTH AND THE LAPD MESSAGE SIZE
TXLAPD ENABLE
0
1
RESULTING ACTION OF THE LAPD TRANSMITTER
The LAPD Transmitter is disabled and the DL bits, in the DS3 frame,
are transmitted as all "1"s.
The LAPD Transmitter is enabled and is transmitting a continuous
stream of Flag Sequence octets (0x7E).
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
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, 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
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Not Used
Tx DL
Start
Tx DL
Busy
TxLAPD
Interrupt
Enable
TxLAPD
Interrupt
Status
R/O
R/O
R/O
R/O
R/W
RO
R/W
RUR
0
0
0
0
1
X
X
X
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 In-
terrupt 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
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" com-
mand, 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 81 ).
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