XRT74L73 Datasheet, PDF (459/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FIGURE 209. FLOW CHART DEPICTING THE FUNCTIONALITY OF THE LAPD RECEIVER (CONTINUED)
From Figure 184
Generate the “Receive LAPD Message frame” interrupt.
Compute “Frame Check Sequence (FCS)” value of incoming
LAPD Message Frame.
Compare “locally-computed” FCS value with that contained
within the newly received LAPD Message frames.
A
End
Do the
two FCS values
Yes
Match?
No
FCS Error Detected
Assert the “Rx FCS Error” bit-field within the “Rx E3
LAPD Status Register (Address = 0x19).
6.3.4 The Receive Overhead Data Output Inter-
face
Figure 210 presents a simple illustration of the Re-
ceive Overhead Data Output Interface block within
the XRT74L73.
FIGURE 210. THE RECEIVE OVERHEAD OUTPUT INTERFACE BLOCK
RxOHFrame
RxOH
RxOHClk
RxOHEnable
Receive
Overhead
Output Interface
Block
From Receive E3
Framer Block
The E3, ITU-T G.832 frame consists of 537 bytes. Of
these bytes, 530 bytes are payload bits and the re-
maining 7 bytes are overhead bytes. The XRT74L73
has been designed to handle and process both the
payload type and overhead type bytes for each E3
frame.
Within the Receive Section of the XRT74L73, the Re-
ceive Payload Data Output Interface block has been
designed to handle the payload bits. Likewise, the
Receive Overhead Data Output Interface block has
been designed to handle and process the overhead
bits.
The Receive Overhead Data Output Interface block
unconditionally outputs the contents of all overhead
bits. The XRT74L73 does not offer the user a means
to shut off this transmission of data. However, the
Receive Overhead Output Interface block does pro-
vide the user with the appropriate output signals for
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