XRT74L73 Datasheet, PDF (320/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
FIGURE 125. FLOW CHART DEPICTING 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 0x86through 0xDD, within the
Framer Address Space.
CONFIGURE THE N-BIT to CARRYLAPD
Messages
This is accomplished by setting “TxNSourceSel[1:0]
= “1, 0”
ENABLE THE LAPD
TRANSMITTER FOR TRANSMISSION
This is accomplished by writing 00000xx1bto the Tx
E3 LAPD Configuration Register.(where xx dictates
LAPD Message Type)
INITIATE TRANSMISSION OF LAPD
MESSAGE
This is accomplished by writing 000010x0bto the Tx
E3 LAPD Status/InterruptRegister. (where x indicates
the user’s choiceto 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.
NOTE: In Figure 125 , the unshaded boxes depict the tasks
that the user must perform. The shaded boxes present the
resulting tasks that the Transmit HDLC Controller block will
perform.
The Mechanics of Transmitting a New LAPD Mes-
sage frame, if the LAPD Transmitter has been
configured to re-transmit the LAPD Message
frame, repeatedly, at One-Second intervals.
If the LAPD Transmitter has been configured to re-
transmit the LAPD Message frame repeatedly at one-
second intervals, then it will do the following (at one-
second intervals).
• Stuff the PMDL Message.
• Read in the stuffed PMDL Message from the Trans-
mit LAPD Message buffer.
• Encapsulate this stuffed PMDL Message into a
LAPD Message frame.
• Transmit this LAPD Message frame to the Remote
Terminal Equipment.
If another (e.g., a different) PMDL Message is to be
transmitted to the Remote Terminal Equipment, this
new message will have to be written into the Transmit
LAPD Message buffer, via the Microprocessor Inter-
face block of the Framer IC. However, care must be
taken when writing this new PMDL message. If this
message is written into the Transmit LAPD Message
buffer at the wrong time (with respect to these One-
second LAPD Message frame transmissions), the us-
er’s action could interfere with these transmissions,
thereby causing the LAPD Transmitter to transmit a
corrupted message to the Remote Terminal Equip-
ment. In order to avoid this problem, while writing the
new message into the Transmit LAPD Message buff-
er, the user should do the following.
1. Configure the Framer to automatically reset acti-
vated interrupts.
The user can do this by writing a “1” into Bit 3 within
the Framer Operating Mode register (Address =
0x00), as depicted below.
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