Design & roadmap#

pw_hdlc: Simple, robust, and efficient serial communication

pw_hdlc implements a subset of the High-Level Data Link Control (HDLC) protocol. HDLC is a data link layer protocol intended for serial communication between devices and is standardized as ISO/IEC 13239:2002.


The pw_hdlc module provides a simple, robust frame-oriented transport that uses a subset of the HDLC protocol. pw_hdlc supports sending between embedded devices or the host. It can be used with pw_rpc to enable remote procedure calls (RPCs) on embedded devices.

pw_hdlc has two primary functions:

  • Encoding data by constructing a frame with the escaped payload bytes and frame check sequence.

  • Decoding data by unescaping the received bytes, verifying the frame check sequence, and returning successfully decoded frames.

Design considerations#

  • pw_hdlc only supports unnumbered information frames.

  • It uses special escaped bytes to mark the beginning and end of a frame.

  • Frame data is stored in a buffer until the end-of-frame delimiter byte is read.

Protocol Description#


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


7D 5E


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 pw_hdlc needs 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.


  • Expanded protocol support - pw_hdlc currently only supports unnumbered information frames. Support for different frame types and extended control fields may be added in the future.

More pw_hdlc docs#

Get started & guides

How to set up and use pw_hdlc

API reference

Reference details about the pw_hdlc API


Design details about pw_hdlc

Code size analysis

The code size impact of pw_hdlc

RPC over HDLC example

A step-by-step example of sending RPCs over HDLC

Experimental async router

An experimental asynchronous HDLC router using pw_channel