XRT72L56 Datasheet, PDF (443/486 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
the Inbound E3 frames. In this case, the Inbound
message bits will be carried by either the “GC” or the
“NR” byte-fields within each E3 Frame. The remote
LAPD Transmitter will transmit a LAPD Message to
the Near-End Receiver via either one of these bytes
within each E3 Frame. The LAPD Receiver will re-
ceive and store the information portion of the re-
ceived LAPD frame into the Receive LAPD Message
Buffer, which is located at addresses: 0xDE through
0x135 within the on-chip RAM. The LAPD Receiver
has the following responsibilities.
• Framing to the incoming LAPD Messages
• Filtering out stuffed "0’s" (within the information
payload)
• Storing the Frame Message into the Receive LAPD
Message Buffer
• Perform Frame Check Sequence (FCS) Verification
• Provide status indicators for
FIGURE 200. LAPD MESSAGE FRAME FORMAT
End of Message (EOM)
Flag Sequence Byte detected
Abort Sequence detected
Message Type
C/R Type
The occurrence of FCS Errors
The LAPD receiver's actions are facilitated via the fol-
lowing two registers.
• Rx E3 LAPD Control Register
• Rx E3 LAPD Status Register
Operation of the LAPD Receiver
The LAPD Receiver, once enabled, will begin search-
ing for the boundaries of the incoming LAPD mes-
sage. The LAPD Message Frame boundaries are de-
lineated via the Flag Sequence octets (0x7E), as de-
picted in Figure 200.
Flag Sequence (8 bits)
SAPI (6-bits)
TEI (7 bits)
Control (8-bits)
C/R ^EA
EA
76 or 82 Bytes of Information (Payload)
FCS - MSB
FCS - LSB
Flag Sequence (8-bits)
Where: Flag Sequence = 0x7E
SAPI + CR + EA = 0x3C or 0x3E
TEI + EA = 0x01
Control = 0x03
The 16 bit FCS is calculated using CRC-16, x16 +
x12 + x5 + 1
The local µP (at the remote terminal), while assem-
bling the LAPD Message frame, will insert an addi-
tional byte at the beginning of the information (pay-
load) field. This first byte of the information field indi-
cates the type and size of the message being trans-
ferred. The value of this information field and the
corresponding message type/size follow:
CL Path Identification = 0x38 (76 bytes)
IDLE Signal Identification = 0x34 (76 bytes)
Test Signal Identification = 0x32 (76 bytes)
ITU-T Path Identification = 0x3F (82 bytes)
Enabling and Configuring the LAPD Receiver
Before the LAPD Receiver can begin to receive and
process incoming LAPD Message frames, the user
must do two things.
1. The byte-field within each E3 frame which will be
carrying the comprising octets of the LAPD Mes-
sage frame must be specified and
2. The LAPD Receiver must be enabled.
Each of these steps are discussed in detail below.
1. Specifying which byte-field, within each E3 frame,
will be carrying the LAPD Message frame.
The LAPD Receiver can receive the LAPD Message
frame octets via either the GC-byte-field or the NR-
byte-field, within each incoming E3 frame. The user
makes this selection by writing the appropriate bit to
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