pw_hdlc: Design

Lightweight, simple, and easy serial communication

pw_hdlc implements a subset of the HDLC protocol.

Protocol Description

Frames

The HDLC implementation in pw_hdlc supports only HDLC unnumbered information frames. These frames are encoded as follows:

_________________________________________
| | | |                          |    | |...
| | | |                          |    | |... [More frames]
|_|_|_|__________________________|____|_|...
 F A C       Payload              FCS  F

 F = flag byte (0x7e, the ~ character)
 A = address field
 C = control field
 FCS = frame check sequence (CRC-32)

Encoding and sending data

This module first writes an initial frame delimiter byte (0x7E) to indicate the beginning of the frame. Before sending any of the payload data through serial, the special bytes are escaped:

Unescaped Special Bytes	Escaped Special Bytes
7E	7D 5E
7D	7D 5D

The bytes of the payload are escaped and written in a single pass. The frame check sequence is calculated, escaped, and written after. After this, a final frame delimiter byte (0x7E) is written to mark the end of the frame.

Decoding received bytes

Frames may be received in multiple parts, so we need to store the received data in a buffer until the ending frame delimiter (0x7E) is read. When the pw_hdlc decoder receives data, it unescapes it and adds it to a buffer. When the frame is complete, it calculates and verifies the frame check sequence and does the following:

• If correctly verified, the decoder returns the decoded frame.

• If the checksum verification fails, the frame is discarded and an error is reported.