XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
PRELIMINARY
REV. P1.0.1
216
Bit 0 - TxLAPD Enable
This bit-field allows the user to enable or disable the
LAPD Transmitter in accordance with Table 42 .
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 this step is executed, the LAPD Transmitter will
begin transmitting the flag sequence octet (0x7E) via
the DL bits.
N
OTE
:
Upon power up or reset, the LAPD Transmitter is
disabled. Therefore, this bit must be set to a "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 is wished to
be transmitted, and has enabled the LAPD Transmit-
ter. The only thing remaining to do is to initiate the
transmission of this message. This process is initiat-
ed by writing a "1" to Bit 3 of the Tx DS3 LAPD Sta-
tus/Interrupt Register (TxDL Start). The bit format of
this register is presented below.
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 LAPD Transmitter can be configured to interrupt
the μC/μP upon completion of transmission of the
LAPD Message, by setting bit-field 1 (TxLAPD Inter-
rupt 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 avail-
able and ready to transmit a new message. Bit 0 will
reflect the interrupt status for the LAPD Transmitter.
N
OTE
:
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 69 ).
0
0
0
0
X
0
X
1
TRANSMIT DS3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
T
ABLE
42: R
ELATIONSHIP
BETWEEN
T
X
LAPD M
SG
L
ENGTH
AND
THE
LAPD M
ESSAGE
S
IZE
T
X
LAPD E
NABLE
R
ESULTING
A
CTION
OF
THE
LAPD T
RANSMITTER
0
The LAPD Transmitter is disabled and the DL bits, in the DS3 frame,
are transmitted as all 1s.
1
The LAPD Transmitter is enabled and is transmitting a continuous
stream of Flag Sequence octets (0x7E).
TRANSMIT DS3 LAPD STATUS/INTERRUPT REGISTER (ADDRESS = 0X34)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
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