pw_hdlc

High-Level Data Link Control (HDLC) is a data link layer protocol intended for serial communication between devices. HDLC 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 on devices.

Why use the pw_hdlc module?

  • Enables the transmission of RPCs and other data between devices over serial.

  • Detects corruption and data loss.

  • Light-weight, simple, and easy to use.

  • Supports streaming to transport without buffering, since the length is not encoded.

Try it out!

For an example of how to use HDLC with pw_rpc, see the RPC over HDLC example project.

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.

API Usage

There are two primary functions of the pw_hdlc module:

  • 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.

Encoder

The Encoder API provides a single function that encodes data as an HDLC unnumbered information frame.

C++

Status hdlc::WriteUIFrame(uint64_t address, ConstByteSpan data, stream::Writer &writer)

Writes a span of data to a pw::stream::Writer and returns the status. This implementation uses the pw_checksum module to compute the CRC-32 frame check sequence.

#include "pw_hdlc/encoder.h"
#include "pw_hdlc/sys_io_stream.h"

int main() {
  pw::stream::SysIoWriter serial_writer;
  Status status = WriteUIFrame(123 /* address */,
                               data,
                               serial_writer);
  if (!status.ok()) {
    PW_LOG_INFO("Writing frame failed! %s", status.str());
  }
}

Python

The encode module supports encoding HDLC frames.

pw_hdlc.encode.ui_frame(address: int, data: bytes)bytes

Encodes an HDLC UI-frame with a CRC-32 frame check sequence.

import serial
from pw_hdlc import encode

ser = serial.Serial()
ser.write(encode.ui_frame(b'your data here!'))

Decoder

The decoder class unescapes received bytes and adds them to a buffer. Complete, valid HDLC frames are yielded as they are received.

C++

class pw::hdlc::Decoder
pw::Result<Frame> Process(std::byte b)

Parses a single byte of an HDLC stream. Returns a Result with the complete frame if the byte completes a frame. The status is the following:

  • OK - A frame was successfully decoded. The Result contains the Frame, which is invalidated by the next Process call.

  • UNAVAILABLE - No frame is available.

  • RESOURCE_EXHAUSTED - A frame completed, but it was too large to fit in the decoder’s buffer.

  • DATA_LOSS - A frame completed, but it was invalid. The frame was incomplete or the frame check sequence verification failed.

void Process(pw::ConstByteSpan data, F &&callback, Args&&... args)

Processes a span of data and calls the provided callback with each frame or error.

This example demonstrates reading individual bytes from pw::sys_io and decoding HDLC frames:

#include "pw_hdlc/decoder.h"
#include "pw_sys_io/sys_io.h"

int main() {
  std::byte data;
  while (true) {
    if (!pw::sys_io::ReadByte(&data).ok()) {
      // Log serial reading error
    }
    Result<Frame> decoded_frame = decoder.Process(data);

    if (decoded_frame.ok()) {
      // Handle the decoded frame
    }
  }
}

Python

class pw_hdlc.decode.FrameDecoder

Decodes one or more HDLC frames from a stream of data.

__init__()

Initialize self. See help(type(self)) for accurate signature.

process(data: bytes)Iterator[pw_hdlc.decode.Frame]

Decodes and yields HDLC frames, including corrupt frames.

The ok() method on Frame indicates whether it is valid or represents a frame parsing error.

Yields

Frames, which may be valid (frame.ok()) or corrupt (!frame.ok())

process_valid_frames(data: bytes)Iterator[pw_hdlc.decode.Frame]

Decodes and yields valid HDLC frames, logging any errors.

Below is an example using the decoder class to decode data read from serial:

import serial
from pw_hdlc import decode

ser = serial.Serial()
decoder = decode.FrameDecoder()

while True:
    for frame in decoder.process_valid_frames(ser.read()):
        # Handle the decoded frame

Additional features

pw::stream::SysIoWriter

The SysIoWriter C++ class implements the Writer interface with pw::sys_io. This Writer may be used by the C++ encoder to send HDLC frames over serial.

HdlcRpcClient

class pw_hdlc.rpc.HdlcRpcClient(read: Callable[[], bytes], paths_or_modules: Union[Iterable[Union[str, pathlib.Path, module]], pw_protobuf_compiler.python_protos.Library], channels: Iterable[pw_rpc.descriptors.Channel], output: Callable[[bytes], Any] = <function write_to_file>, client_impl: Optional[pw_rpc.client.ClientImpl] = None)

An RPC client configured to run over HDLC.

__init__(read: Callable[[], bytes], paths_or_modules: Union[Iterable[Union[str, pathlib.Path, module]], pw_protobuf_compiler.python_protos.Library], channels: Iterable[pw_rpc.descriptors.Channel], output: Callable[[bytes], Any] = <function write_to_file>, client_impl: Optional[pw_rpc.client.ClientImpl] = None)

Creates an RPC client configured to communicate using HDLC.

Parameters
  • read – Function that reads bytes; e.g serial_device.read.

  • paths_or_modules – paths to .proto files or proto modules

  • channel – RPC channels to use for output

  • output – where to write “stdout” output from the device

rpcs(channel_id: Optional[int] = None)Any

Returns object for accessing services on the specified channel.

This skips some intermediate layers to make it simpler to invoke RPCs from an HdlcRpcClient. If only one channel is in use, the channel ID is not necessary.

Roadmap

  • 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.

  • Higher performance - We plan to improve the overall performance of the decoder and encoder implementations by using SIMD/NEON.

Compatibility

C++17