XRT74L74 Datasheet, PDF (231/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
XRT74L74
REV. P1.1.1
Figure 70 presents a flow chart depicting the proce-
dure (in white boxes) that the user should use in or-
der to transmit a LAPD message. This figure also in-
dicates (via the shaded boxes) what the LAPD Trans-
mitter circuitry will do before and during message
transmission.
FIGURE 70. FLOW CHART DEPICT HOW TO USE THE LAPD TRANSMITTER
SSTTAARRTT
LAPD Transmitter inserts Frame Header
octets in front of the user payload.
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 0x86
through 0xDD, within the Framer 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 Length)
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 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
Yes
“1s” detected
?
No
Insert a “0” after the
string of 5 consecutive
“1s”
Is
Message
No
Transmission ^Yes
Complete
?
END
Generate Interrupt
LAPD Transmitter will
continue to transmit
Flag Sequence octets.
The Mechanics of Transmitting a New LAPD Mes-
sage
As mentioned above, after the LAPD Transmitter has
been enabled, and commanded to transmit the mes-
sage, residing in the Transmit LAPD Message buffer,
it will continue to transmit this message at one-sec-
ond intervals. If another (e.g., different) PMDL mes-
sage is to be transmitted to the Remote LAPD Re-
ceiver, the new message will have to be written into
the Transmit LAPD Message buffer, via the Micropro-
cessor Interface section of the Framer. However,
care must be taken when writing in this new mes-
sage. If this message is written into the Transmit
LAPD Message buffer at the wrong time (with respect
to these one-second transmissions), the user’s action
could interfere with these transmissions, thereby
causing the LAPD Transmitter to transmit a corrupted
message to the Remote LAPD Receiver. In order to
avoid this problem, while writing the new message in-
to the Transmit LAPD Message buffer, the following
should be done:
1. Configure the Framer to automatically reset acti-
vated interrupts
This can be done by writing a "1" into Bit 3 of the
Framer Operating Mode Register, as depicted below.
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