MOTOROLA
Chapter 26. SCC Asynchronous HDLC Mode and IrDA
26-3
Part V. The Communications Processor Module
updates RxBD[Data Length] with the total frame length, including the CRC bytes. The
controller sets RxBD[L], writes the frame status bits, and clears RxBD[E] (if RxBD[CM]
is zero). It then sets SCCE[RXF], which indicates that a frame was received and is in
memory. The controller then waits for the start of the next frame, which may or may not
have an opening ßag.
26.4 Transmitter Transparency Encoding
The asynchronous HDLC transmitter encodes characters according to RFC 1549, a de facto
standard of the Internet Engineering Task Force (IETF). It examines outgoing bytes and
performs the transparency algorithm for the following conditions:
¥
The byte is a ßag (0x7E for PPP, 0xC0 or 0xC1 for IrLAP)
¥
The byte is a control-escape character (0x7D)
¥
The byte value is between 0x00 and 0x1F and the corresponding bit in the Tx control
character table is set
When a condition applies, a two-byte sequence is sent instead of the byte. The sequence
consists of the control-escape character (0x7D) followed by the original byte
exclusive-ORed with 0x20.
26.5 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 26-2 shows the algorithm because some cases are not covered by RFC 1549.
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