XR72L52 Datasheet, PDF (186/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L52
REV. 1.0.1
xr
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
FIGURE 51. FLOW CHART DEPICT HOW TO USE THE LAPD TRANSMITTER
START
LAPD Transm itter inserts Fram e Header
octets in front of the user payload.
Write the Header in 0x86 to 0x89
Write in Data Link Information
This is accom plished by writing the inform ation
to be transm itted via the LAPD Transm itter to
locarions, 0x8A thru 0xDB in the Fram er
address space.
Enable the LAPD Transmitter for
T ran sm issio n
This is done by writing the value 00000xx1b
into the Tx DS3 LAPD Configuration Register.
(xx dictates the LAPD Message Length)
Initiate Transmission of LAPD Message
This is done by writing the value 000010x0b
into the Tx DS3 LAPD Status/Interrupt
Register.
(x indictates that the user can choose to
enable/disable the LAPD Message Transfer
Com plete Interrupt)
LAPD Transm itter com putes the 16 bit FCS
(a CRC-16 value) over the Header and
Payload bytes and inserts it into the LAPD
Message, following the user payload.
LAPD Transm itter appends a Flag Sequence
Trailer octet to the end of the LAPD
Message after the 16 bit FCS.
Are 5
Consecutive
"Ones" detected
between the
YES
start and end flag
sequence
?
NO
Insert a "0" after
the string of 5
consecutive "1's"
Is
NO
Message
Transm ission
Com plete
?
YES
END
Generate Interrupt LAPD Transm itter will
continue to transm it Flag Sequence octets.
The Mechanics of Transmitting a New LAPD Message
As mentioned above, after the LAPD Transmitter has been enabled, and commanded to transmit the message,
residing in the Transmit LAPD Message buffer, it will continue to transmit this message at one-second inter-
vals. If another (e.g., different) PMDL message is to be transmitted to the Remote LAPD Receiver, the new
message will have to be written into the Transmit LAPD Message buffer, via the Microprocessor Interface sec-
tion of the Framer. However, care must be taken when writing in this new message. 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 mes-
sage into the Transmit LAPD Message buffer, the following should be done:
1. Configure the Framer to automatically reset activated 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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