SCC Asynchronous HDLC Mode and IrDA
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
25-3
25.5
Receiver Transparency Decoding
The asynchronous HDLC receiver decodes characters according to RFC 1549. To recover the original
data, it examines incoming data bytes and performs the transparency algorithm in the following ways:
•
It discards characters whose corresponding bit is set in the Rx control character map. This character
is assumed to have been inserted in the character stream by an intermediate device and is not part
of the original frame.
•
It reverses the transmission transparency sequence by discarding a received control-escape
character (0x7D) and exclusive-ORing the following byte with 0x20 before performing the CRC
calculation and writing the byte into memory.
Figure 25-2
shows the algorithm because some cases are not covered by RFC 1549.
Figure 25-2. Receive Flowchart
XOR_NEXT
?
Rx CHAR
CHAR < 0x20
?
XOR_NEXT=1
CHAR=CTRL ESC
?
CHAR=Closing Flag
?
False
True
False
True
False
True
True
False
False
Write CHAR to
Buffer
Exit
End of Frame
True
Mapped
?
Abort
Exit
True
CHAR ª 0x20
XOR_NEXT=0
CHAR=Closing Flag
?
False
Summary of Contents for PowerQUICC MPC870
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