XRT72L58 Datasheet, PDF (205/486 Pages) Exar Corporation – EIGHT CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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EIGHT CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
PRELIMINARY
XRT72L58
REV. P1.1.2
subsequent "0" from the payload portion of the incoming
LAPD message.
Figure 63 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 63. 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 message. If
this message is written into the Transmit LAPD Mes-
sage 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 mes-
sage to the Remote 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:
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.
186