XRT72L56 Datasheet, PDF (242/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56 SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
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PRELIMINARY
of the LAPD Message frame. The LAPD Transmitter
performs this bit-stuffing procedure in order to prevent
the user data from mimicking the Flag Sequence oc-
tet (0x7E) or the ABORT sequence. Therefore, in or-
der to recover the user data to its original content (pri-
or to the bit-stuffing), the LAPD Receiver will remove
the "0" that immediately follows a string of 5 consecu-
tive 1s.
Writing the Incoming LAPD Message into the Re-
ceive LAPD Message Buffer
The LAPD receiver will obtain the LAPD Message
frame from the incoming DS3 data-stream. In addi-
tion to processing the framing overhead octets, per-
forming error checking (via FCS) and removing the
stuffed 0s from the user payload data. The LAPD Re-
ceiver will also write the payload portion of the LAPD
Frame into the Receive LAPD Message buffer at loca-
tions 0xDE through 0x135 in on-chip RAM.
Therefore, the local µP/µC must read this location
when it wishes to process this newly received LAPD
Message.
Figure 86 presents a flow chart depicting how the
LAPD Receiver works.
FIGURE 86. FLOW CHART DEPICTING THE FUNCTIONALITY OF THE LAPD RECEIVER
SSTTAARRTT
ENABLE THE LAPD RECEIVER
This is done by writing the value “0xFC into the
RxLAPD Control Register (Address = 0x18)
LAPD Receiver begins reading in the DL bits
from each inbound DS3 frame
DDoeoses
ththeeLLAAPPDD
RReceecievivererddeteetcetct66
NO
coconnsesceucutitvivee
ZZereoros _s
??
11
YES
AABBOORRTTSSeqequuenencece
DDooeses
thteheLLAAPPDD
RReceecievievrerdedteetcetct77
cocnosnesceuctuitvieve
ZZereoros _s
YES
??
NO
FFlalgagSSeqequuenencece
LLAAPPDDRReceecievivererisisreraedadininggininaaLLAAPPDD
MMesessasgageeFFrarmame,e,coconntatianinininggaaPPMMDDLL
MMesessasgaeg.e.
DDoeoses
NO
thteheLLAAPPDD
RReceecievivererddeteetcetct66
coconnsesceucutitvivee
ZZereorsos
??
YES
EEnnddoof _{fMMesessasgagee(E(EOOMM) )}
VVEERRIFIFYYTTHHEEFFCCSSVVAALLUUEE
RRepeoprotrtrerseuslutlstsininthteheRRxLxLAAPPDD
SStattautus _{sRRegegisitsetre.r...}
“U“Un-ns-tsutuffffcocnontetnetnstsofofRReceecievievded
MMesessasgage”e”
GGeneenreartaete“R“ReceecievievdedLLAAPPDD
InIntetrerruruppt”t”
DDoeoses
ththeeLLAAPPDD
YES
RReceecievivererddeteetcetct77
coconnsesceucutitvivee
ZZereoros _s
??
NO
WWrirtieteRReceecievivededPPMMDDLLMMesessasgagee
initnotothteheRReceecievieveLLAAPPDDMMesessasgaege
BBufuffefrer(A(Adddrdersessesses0x0DxDEE- _{-0x01x3153)5)}
EExxeceucuteteRReceecieviveeLLAAPPDD
InIntetrerruruppt _{tSSerevrvicieceRRoouutitninee}
11
NOTES:
1. The white (e.g., unshaded) boxes reflect tasks that
the user’s system must perform in order to config-
ure the LAPD Receiver to receive LAPD Messages.
2. A brief description of the steps that must exist
within the Receive LAPD Interrupt Service routine
exists in Section 3.3.6.
4.3.4 The Receive Overhead Data Output Inter-
face
Figure 87 presents a simple illustration of the Receive
Overhead Data Output Interface block within the
XRT72L56.
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