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PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
199
The Mechanics of Transmitting a New LAPD
Message
As mentioned above, after the LAPD Transmitter has
been enabled and commanded to transmit the mes-
sage residing in the “Transmit LAPD Message” buff-
er; it will continue to transmit this message at one-
second intervals. If the user wishes to transmit anoth-
er (e.g., different) PMDL message to the “Far End”
LAPD Receiver, he/she will have to write this “new”
message into the “Transmit LAPD Message” buffer,
via the Microprocessor Interface section of the UNI.
However, the user must be careful when writing in
this new message. If he/she writes this message into
the “Transmit LAPD Message” buffer at the “wrong
time” (with respect to these “one-second” transmis-
sions), the user’s action could interfere with these
transmissions; thereby causing the LAPD Transmitter
to transmit a “corrupted” message to the “Far-End”
LAPD Receiver. In order to avoid this problem, while
writing the new message into the “Transmit LAPD
Message” buffer the user should do the following:
Configure the UNI to automatically reset acti-
vated interrupts
The user can do this by writing a “1” into Bit 5 of the
UNI I/O Control Register, as depicted below.
1.
This action will prevent the LAPD Transmitter from
generating its own “one-second” interrupts.
2.
Enable the “One-Second” Interrupt
F
IGURE
52. F
LOW
C
HART
DEPICT
HOW
TO
USE
THE
LAPD T
RANSMITTER
START
WRITE IN DATA LINK INFORMATION
The user accomplishes this by writing the
information that he/she wishes to transmit
(via the LAPD Transmitter) to locations 86h
through DDh, within the UNI Address Space.
ENABLE THE LAPD
TRANSMITTER FOR TRANSMISSION
This is accomplished by writing 00000xx1b
to the Tx DS3 LAPD Configuration Register
(where xx dictates LAPD Message Type).
INITIATE TRANSMISSION OF LAPD
MESSAGE
This is accomplished by writing 000010x0b
to the Tx DS3 LAPD Status/Interrupt
Register (where x indicates the user’s choice
to enable/disable “LAPD Message Transfer
Complete” Interrupt).
LAPD Transmitter inserts Frame Header
octets in front of the user payload.
LAPD Transmitter computes the 16 bit FCS
(a CRC-16 value) and inserts it into the LAPD
Message, following the user payload
LAPD Transmitter appends a Flag Sequence
Trailer octet to the end of the LAPD Message
(after the 16 bit FCS).
Is
5 consecutive
“1s” detected
Is
Message
Transmission
Complete
Insert a “0” after the
string of 5 consecutive
“1s”
END
Generate Interrupt
LAPD Transmitter will
continue to transmit
Flag Sequence octets.
Yes
No
Yes
No
Address = 01h, UNI I/O Control Register
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
LOC Enable
Test PMON
IntEn Reset
AMI
Unipolar
TxClk Inv
RxClk Inv
Reframe
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
1
0
1
0
0
0
0
0