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TinyProto is a lightweight, portable HDLC-based data link layer protocol for embedded systems and microcontrollers. Communicate reliably over UART, SPI, I2C or any byte stream — with automatic retransmission, CRC error detection, and zero dynamic memory allocation.

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Table of Contents

• Introduction
• Protocol Modes
• Architecture
• Key Features
• Supported Platforms
• Quick Start
  • C API
  • C++ API
  • Python
• Building
• Testing
• Installation
• Tools
• API Reference
• License

Introduction

TinyProto is a Layer 2 (Data Link) protocol implementation based on RFC 1662 and the HDLC specification. It provides reliable, framed communication between two or more devices over any byte-oriented transport (UART, SPI, I2C, USB CDC, TCP sockets, etc.).

The library is designed for resource-constrained environments — from 8-bit AVR microcontrollers with just 60 bytes of SRAM to full Linux/Windows desktop systems. All memory is statically allocated; no malloc is ever called.

TinyProto is not an application-layer protocol. It can be used as a transport for higher-level protocols such as Protocol Buffers, MessagePack, JSON, or any custom framing you need.

Protocol Modes

TinyProto supports three protocol layers, from simplest to most capable:

Layer	API Prefix	Description	Use Case
Light	tiny_light_*	SLIP-like framing with optional CRC. No acknowledgments.	Simple, low-overhead streaming
HDLC	hdlc_*	Low-level HDLC framing (flag bytes, byte stuffing, CRC).	Custom protocol building blocks
Full-Duplex (FD)	tiny_fd_*	Complete HDLC implementation with sliding window, ACK/NAK, retransmission	Reliable bidirectional communication

Full-Duplex Modes (FD)

The FD protocol supports two HDLC operating modes:

ABM — Asynchronous Balanced Mode (Peer-to-Peer)

┌────────────┐                        ┌────────────┐
│  Device A  │ ◄──── UART/SPI ──────► │  Device B  │
│   (peer)   │      bidirectional     │   (peer)   │
└────────────┘                        └────────────┘

• Both devices are equal peers — either side can initiate communication
• Connection established via SABM/UA exchange
• Supports hot plug/unplug with automatic reconnection
• Ideal for: point-to-point links between two MCUs, MCU ↔ PC communication

NRM — Normal Response Mode (Multi-drop / RS-485)

┌────────────┐      ┌────────────┐      ┌────────────┐
│  Primary   │─────►│ Secondary1 │      │ Secondary2 │
│  (master)  │◄─────│  addr = 1  │      │  addr = 2  │
│            │─────────────────────────►│            │
│            │◄─────────────────────────│            │
└────────────┘      └────────────┘      └────────────┘

• One primary station controls communication with multiple secondaries
• Primary polls each secondary in round-robin order using P/F (Poll/Final) bit
• Secondaries only transmit when polled
• Connection established via SNRM/UA exchange
• Supports full sliding window for each secondary (up to 7 outstanding frames)
• Ideal for: RS-485 buses, multi-drop serial networks, sensor networks

Frame Types

TinyProto implements all standard HDLC frame types:

Frame	Type	Description
I	Information	Carries user data with sequence numbers
RR	Supervisory	Receiver Ready — acknowledges received frames
RNR	Supervisory	Receiver Not Ready — flow control
REJ	Supervisory	Reject — requests retransmission from sequence N
SREJ	Supervisory	Selective Reject — requests retransmission of specific frame
SABM	Unnumbered	Set ABM — initiates peer-to-peer connection
SNRM	Unnumbered	Set NRM — initiates primary/secondary connection
DISC	Unnumbered	Disconnect — terminates connection
UA	Unnumbered	Unnumbered Acknowledge — confirms mode setting
DM	Unnumbered	Disconnected Mode — rejects commands when disconnected
FRMR	Unnumbered	Frame Reject — reports protocol errors
UI	Unnumbered	Unnumbered Information — connectionless data
RSET	Unnumbered	Reset — resets sequence counters

Architecture

┌─────────────────────────────────────────────────┐
│                   Application                   │
├──────────────┬──────────────┬───────────────────┤
│  C++ API     │   C API      │   Python API      │
│  IFd / Hdlc  │  tiny_fd_*   │   tinyproto.*     │
│  Light       │  hdlc_*      │                   │
├──────────────┴──────────────┴───────────────────┤
│            Full-Duplex Protocol (FD)            │
│    ┌─────────────┬────────────┬───────────┐     │
│    │ I-frame     │  S-frame   │  U-frame  │     │
│    │ queue       │  handler   │  handler  │     │
│    └─────────────┴────────────┴───────────┘     │
├─────────────────────────────────────────────────┤
│             HDLC Low-Level Framing              │
│          (byte stuffing, CRC, flags)            │
├─────────────────────────────────────────────────┤
│           Hardware Abstraction Layer            │
│         (timers, mutexes, serial I/O)           │
├─────────────────────────────────────────────────┤
│     UART  │  SPI  │  I2C  │  USB  │  TCP  │ ...│
└─────────────────────────────────────────────────┘

Key Features

• Reliable Delivery — Sliding window (1–7 frames), automatic retransmission, REJ and SREJ recovery
• Error Detection — 8-bit checksum, FCS-16 (CCITT), or FCS-32 (CCITT)
• Two HDLC Modes — ABM (peer-to-peer) and NRM (primary/secondary multi-drop)
• Hot Plug/Unplug — Automatic connection recovery in ABM mode
• Zero Dynamic Allocation — All buffers are statically sized; fully deterministic memory usage
• Tiny Footprint — From 60 bytes SRAM, 1.3 KiB flash (features are compile-time configurable)
• Large Frames — Payload up to 32 KiB (configurable)
• Full Logging — Built-in protocol debug logging with CSV export
• Multi-Language — C, C++, and Python APIs
• Portable — Works on any platform with a C99 compiler

Supported Platforms

TinyProto runs anywhere a C99/C++11 compiler is available:

Platform	MCU Examples	Transport
Arduino	Uno, Mega, Zero, Due, Nano	UART, SPI
ESP32	ESP32, ESP32-S2, ESP32-C3	UART, SPI
ARM Cortex	STM32, nRF52, Teensy, SAMD	UART, SPI, I2C
AVR	ATmega328, ATmega2560	UART
Linux	Any	UART, TCP, pipes
Windows	Any	COM ports, TCP
RISC-V	ESP32-C3, GD32V	UART, SPI

Custom Platform Support

If your platform isn't listed, implement the HAL abstraction (timing + mutex functions):

1. Add TINY_CUSTOM_PLATFORM to your compiler defines
2. Implement HAL functions and call tiny_hal_init()
3. Optionally add CONFIG_TINYHAL_THREAD_SUPPORT for multi-threaded environments

See hal/linux/linux_hal.inl and hal/esp32/esp32_hal.inl for reference implementations, or use the template at tools/hal_template_functions/platform_hal.c.

Quick Start

C API

ABM mode (peer-to-peer):

#include "proto/fd/tiny_fd.h"

// Callback when a complete message is received
void on_frame_received(void *udata, uint8_t addr, uint8_t *buf, int len)
{
    printf("Received %d bytes\n", len);
}

// Allocate protocol buffer (use tiny_fd_buffer_size_by_mtu() to calculate)
uint8_t buffer[tiny_fd_buffer_size_by_mtu(64, 4)];

tiny_fd_handle_t handle;
tiny_fd_init_t init = {
    .pdata        = NULL,
    .on_read_cb   = on_frame_received,
    .buffer       = buffer,
    .buffer_size  = sizeof(buffer),
    .window_frames = 4,
    .send_timeout  = 1000,
    .retry_timeout = 200,
    .retries       = 3,
    .crc_type      = HDLC_CRC_16,
    .mode          = TINY_FD_MODE_ABM,
};

tiny_fd_init(&handle, &init);

// In your main loop:
// 1. Feed received bytes to the protocol
tiny_fd_on_rx_data(handle, rx_bytes, rx_len);

// 2. Get bytes to transmit
uint8_t tx_buf[64];
int tx_len = tiny_fd_get_tx_data(handle, tx_buf, sizeof(tx_buf), 0);
if (tx_len > 0) {
    serial_write(tx_buf, tx_len);
}

// 3. Send application data
tiny_fd_send_packet(handle, "Hello", 6, 1000);

NRM mode (multi-drop primary):

tiny_fd_init_t init = {
    // ... same as above, plus:
    .mode         = TINY_FD_MODE_NRM,
    .peers_count  = 3,           // support up to 3 secondaries
    .addr         = TINY_FD_PRIMARY_ADDR,  // this is the primary station
};

NRM mode (secondary station):

tiny_fd_init_t init = {
    // ... same as above, plus:
    .mode         = TINY_FD_MODE_NRM,
    .addr         = 1,           // unique secondary address (1-63)
};

C++ API

Full-duplex with static buffer (Arduino-friendly):

#include "TinyProtocolFd.h"

// Static buffer — no heap allocation
tinyproto::Fd<1024> proto;

void onReceive(void *udata, uint8_t addr, tinyproto::IPacket &pkt) {
    // Process received message
    Serial.write(pkt.data(), pkt.size());
}

void setup() {
    Serial.begin(115200);
    proto.setReceiveCallback(onReceive);
    proto.setWindowSize(4);
    proto.enableCrc16();
    proto.begin();
}

void loop() {
    // Feed serial data to protocol
    if (Serial.available()) {
        uint8_t byte = Serial.read();
        proto.run_rx(&byte, 1);
    }
    // Transmit protocol data
    uint8_t tx;
    if (proto.run_tx(&tx, 1) == 1) {
        Serial.write(tx);
    }
}

Dynamic buffer (desktop / powerful MCUs):

#include "TinyProtocolFd.h"

// Dynamic allocation — buffer size determined at runtime
tinyproto::FdD proto(4096);
proto.setWindowSize(7);
proto.enableCrc32();
proto.begin();

// Send data
proto.write("Hello World", 12);

Python

import tinyproto

proto = tinyproto.Hdlc()

def on_read(data):
    print("Received:", data.hex())

proto.on_read = on_read
proto.begin()

# Feed raw bytes from your transport
proto.rx(bytearray([0x7E, 0xFF, 0x3F, 0xF3, 0x39, 0x7E]))

# Queue data and get encoded bytes to transmit
proto.put(bytearray(b"Hello"))
tx_data = proto.tx()

Building

Linux

# Using Make
make

# Using CMake
mkdir build && cd build
cmake -DEXAMPLES=ON ..
make

# Run unit tests
make ARCH=linux unittest
./bld/unit_test

Windows

mkdir build && cd build
cmake -G "Visual Studio 16 2019" -DEXAMPLES=ON ..
# Open the generated .sln file in Visual Studio

ESP32 (IDF)

Place the library in your project's components/ directory, then build normally with idf.py build or make.

AVR

make ARCH=avr

Testing

TinyProto has a comprehensive test suite with 101 tests covering all protocol layers:

# Build and run all tests
make ARCH=linux DEBUG_MODE=y unittest && ./bld/unit_test

# Run specific test groups
./bld/unit_test -g TINY_FD_ABM    # ABM mode tests (28 tests)
./bld/unit_test -g TINY_FD_NRM    # NRM mode tests (15 tests)
./bld/unit_test -g CPP_FD         # C++ FD wrapper tests (10 tests)
./bld/unit_test -g CPP_HDLC       # C++ HDLC wrapper tests (3 tests)
./bld/unit_test -g CPP_LIGHT      # C++ Light wrapper tests (3 tests)
./bld/unit_test -g HDLC           # Low-level HDLC tests
./bld/unit_test -g LIGHT          # Light protocol tests

Installation

Arduino

Option 1 — From source:

1. Download from https://github.com/lexus2k/tinyproto
2. Copy to Arduino/libraries/tinyproto
3. Restart Arduino IDE → Examples → tinyproto

Option 2 — Library Manager:

1. Arduino IDE → Sketch → Include Library → Manage Libraries
2. Search for "tinyproto" and install

ESP32 IDF

cd your_project/components
git clone https://github.com/lexus2k/tinyproto

PlatformIO

Add to platformio.ini:

lib_deps = lexus2k/tinyproto

Python

cd tinyproto
python setup.py install

Tools

tiny_loopback

A serial loopback testing tool for benchmarking and debugging:

# Build
make ARCH=linux

# Test Light protocol
./bld/tiny_loopback -p /dev/ttyUSB0 -t light -g -c 8 -a -r

# Test Full-Duplex protocol
./bld/tiny_loopback -p /dev/ttyUSB0 -t fd -c 8 -w 3 -g -a -r

API Reference

Full API documentation is available at codedocs.xyz/lexus2k/tinyproto.

Key C Functions

Function	Description
tiny_fd_init()	Initialize FD protocol instance
tiny_fd_close()	Close and free FD instance
tiny_fd_on_rx_data()	Feed received bytes to protocol
tiny_fd_get_tx_data()	Get bytes to transmit
tiny_fd_send_packet()	Queue data for transmission
tiny_fd_send_to()	Send to specific peer address (NRM)
tiny_fd_get_status()	Check connection status
tiny_fd_disconnect()	Initiate disconnect

Key C++ Classes

Class	Description
tinyproto::Fd<N>	FD protocol with static N-byte buffer
tinyproto::FdD	FD protocol with dynamic buffer allocation
tinyproto::Hdlc	HDLC low-level framing
tinyproto::Light	Lightweight SLIP-like protocol
tinyproto::IPacket	Packet buffer for data exchange

License

Dual licensed under GPLv3 and Commercial licenses.

Copyright 2016-2025 (C) Alexey Dynda

GNU General Public License: Protocol Library is free software under the GNU Lesser General Public License v3.0 or later. See COPYING for details.

Commercial License: Available for proprietary use. Contact via email on the GitHub account for details.

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