Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 725

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.
XOR_NEXT=1
End of Frame
Freescale Semiconductor
Rx CHAR
True
CHAR < 0x20
?
False
True
XOR_NEXT
?
False
True
CHAR=CTRL ESC
?
False
True
CHAR=Closing Flag
?
False
Write CHAR to
Buffer
Exit
Figure 25-2. Receive Flowchart
MPC885 PowerQUICC Family Reference Manual, Rev. 2
SCC Asynchronous HDLC Mode and IrDA
Mapped
?
False
True
CHAR=Closing Flag
?
False
CHAR ª 0x20
XOR_NEXT=0
Abort
True
Exit
25-3