XRT72L56 Datasheet, PDF (301/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
Frame Check Sequence Bytes
The 16 bit FCS (Frame Check Sequence) is calculat-
ed over the LAPD Message Header and Information
Payload bytes, by using the CRC-16 polynomial, x¹⁶
+ x¹² + x⁵ + 1.
Operation of the LAPD Transmitter
If the user wishes to transmit a message via the
LAPD Transmitter, the information portion (or the
body) of the message must be written into the Trans-
mit LAPD Message Buffer, which is located at 0x86
through 0xDD in on-chip RAM via the Microprocessor
Interface. Afterwards, the user must do five things:
1. Configure the source of the “N” bit (within each
outbound E3 frame, to be the LAPD Transmitter.
2. Specify the length of LAPD message to be trans-
mitted.
3. Specify whether the LAPD Transmitter should
transmit this LAPD Message frame only once, or
an indefinite number of times at One-Second
intervals.
4. Enable the LAPD Transmitter.
5. Initiate the Transmission of the PMDL Message.
Each of these steps will be discussed in detail.
STEP 1 - Configure the source of the “N” bit (with-
in each outbound E3 frame, to be the LAPD Trans-
mitter.
This is accomplished by writing the appropriate data
into the TxNSourceSel[1:0] bit-fields, within the TxE3
Configuration Register, as illustrated below.
TXE3 CONFIGURATION REGISTER (ADDRESS = 0X30)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
Tx
BIP-4
Enable
TxASourceSel[1:0]
TxNSourceSel[1:0]
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
BIT 2
Tx AIS
Enable
R/W
0
BIT 1
Tx LOS
Enable
R/W
0
BIT 0
Tx FAS
Source
Select
R/W
0
Setting TxNSourceSel[1:0] to “10” configures the
Transmit E3 Framer block to use the LAPD Transmit-
ter as the data source for the “N” bits. Hence, the “N”
bit, (within each outbound E3 frame) is now carrying
LAPD Messages to the remote terminal equipment.
STEP 2 - Specify the type of LAPD Message frame
to be Transmitted (within the Transmit LAPD Mes-
sage Buffer)
The user must write in a specific octet value into the
first octet position within the Transmit LAPD Buffer
(e.g., at Address Location 0x86 within the Framer IC).
This octet is referred to as the LAPD Message Frame
ID octet. The value of this octet must correspond to
the type of LAPD Message frame that is desired to be
transmitted. This octet will ultimately be used by the
Remote Terminal Equipment in order to help it identify
the type of LAPD message frame that it is receiving.
Table 54 lists these octets and the corresponding
LAPD Message types.
STEP 3 - Write the PMDL Message into the re-
maining part of the Transmit LAPD Message Buff-
er.
The user must now write in his/her PMDL Message
into the remaining portion of the Transmit LAPD Mes-
sage buffer (e.g., addresses 0x87 through 0x135
within the Framer IC).
STEP 4 - Specifying the Length of the LAPD Mes-
sage
One of two different sizes of LAPD Messages can be
transmitted. This can be accomplished by writing the
appropriate data to bit 1 within the Tx E3 LAPD Con-
figuration Register. The bit-format of this register is
presented below.
TRANSMIT E3 LAPD CONFIGURATION REGISTER (ADDRESS = 0X33)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Not Used
Auto
Retransmit
Not Used
TxLAPD
Msg Length
TxLAPD
Enable
R/O
R/O
R/O
R/O
R/W
R/O
R/W
R/W
0
0
0
0
X
0
X
X
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