commit cd7d411332cbcdb74adb43b3f62f0d7c8f0d3404
Author: portersky <24420859+portersky@users.noreply.github.com>
Date:   Sun Jun 14 19:48:37 2026 +0200

    Inital commit

diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..c9a7e1a
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,14 @@
+[*]
+end_of_line  = LF
+charset      = utf-8
+indent_style = space
+indent_size  = 4
+insert_final_newline = true
+
+[*.{yml,json,lua,md,html}]
+charset      = utf-8
+indent_style = space
+indent_size  = 2
+
+[Makefile]
+indent_style = tab
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..e7ff504
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,60 @@
+name: CI
+
+on:
+  push:
+    branches: [trunk]
+  pull_request:
+    branches: [trunk]
+
+jobs:
+  macos:
+    name: macOS
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Ninja
+        run: brew install ninja
+
+      - name: Install gcovr
+        run: pip install gcovr
+
+      - name: Configure with coverage
+        run: cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON
+
+      - name: Build and generate coverage
+        run: ninja -C build-cov coverage
+
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-macos
+          path: build-cov/coverage/
+
+  windows-clang:
+    name: Windows / Clang
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: ilammy/msvc-dev-cmd@v1
+
+      - name: Install gcovr
+        run: pip install gcovr
+
+      - name: Configure with coverage
+        shell: bash
+        run: |
+          cmake -S . -B build-cov -G Ninja \
+            -DENABLE_COVERAGE=ON \
+            -DCMAKE_C_COMPILER="C:/Program Files/LLVM/bin/clang.exe" \
+            -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang++.exe"
+
+      - name: Build and generate coverage
+        run: ninja -C build-cov coverage
+
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-windows-clang
+          path: build-cov/coverage/
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..07103c1
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,16 @@
+# CMake
+build
+build-*
+cmake-build-*
+coverage
+
+# Editors
+.vscode
+.idea
+.ccls
+.ccls-cache
+.cache
+compile_commands.json
+
+# macOS
+.DS_Store
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..0ce1e2b
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,256 @@
+# AGENTS.md
+
+## Project Overview
+
+`celrs` is a C23 project for interfacing with ELRS TX modules via serial
+USB using the CRSF (Crossfire Serial) protocol. Wired for test-driven
+development using Unity and CMock.
+
+## Build System
+
+- **Generator:** Ninja
+- **CMake minimum:** 3.21
+- **C standard:** C23
+
+### Commands
+
+Configure (default):
+```sh
+cmake -S . -B build -G Ninja
+```
+
+Configure with coverage:
+```sh
+cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON
+```
+
+Build:
+```sh
+ninja -C build
+```
+
+Run tests (full Unity output, colored):
+```sh
+ninja -C build check
+```
+
+Run tests (CTest summary only):
+```sh
+ninja -C build test
+```
+
+Run tests and generate coverage HTML:
+```sh
+ninja -C build-cov coverage
+```
+
+Run (Linux/macOS):
+```sh
+./build/main /dev/ttyUSB0
+```
+
+Run (Windows):
+```sh
+./build/main.exe COM3
+```
+
+### Dependencies
+
+Dependencies are managed via custom `Find*.cmake` scripts in `deps/`.
+These scripts use `FetchContent` under the hood to download and build
+libraries automatically.
+
+To add a new dependency:
+
+1. Add the corresponding `Find<name>.cmake` to `deps/`
+2. Add `find_package(<name> REQUIRED)` to `CMakeLists.txt`
+3. Link with `<name>::<name>` in `target_link_libraries()`
+
+### CMake Module Path
+
+`deps/` is added to `CMAKE_MODULE_PATH` so `find_package()` resolves
+to the custom scripts instead of system-installed packages.
+
+## Coding Conventions
+
+- **Language:** C23
+- **Trailing return type** for function signatures (e.g. `auto fn() -> void`)
+- **4-space indentation**
+- `auto` for obvious types (e.g. `auto main(...) -> int`)
+- **East const** (e.g. `char const*` not `const char*`)
+- **Naming:** `snake_case` for variables, functions, and types
+- **Naming:** `SCREAMING_SNAKE_CASE` only for macros and constants
+- Include order:
+  1. C standard library headers (`<stdlib.h>`, `<string.h>`, etc.)
+  2. *(blank line)*
+  3. OS-specific headers (Windows API, POSIX, etc.)
+  4. *(blank line)*
+  5. Local/project headers
+
+## Shell Scripts
+
+- Always use `#!/bin/sh` shebang for shell scripts
+- Scripts must be POSIX compliant (no bashisms)
+- When providing commands to users:
+  - Windows/PowerShell: use `` ` `` for line continuation
+  - Unix/Linux/macOS: use `\` for line continuation
+
+## Commit Messages
+
+- Follow the 50/72 rule:
+  - Subject line: max 50 characters
+  - Body lines: wrapped at 72 characters
+- Use conventional commit prefixes (`feat:`, `fix:`, `docs:`, `chore:`,
+  etc.)
+- Separate subject from body with a blank line
+- Do **not** add yourself as a co-author (`Co-Authored-By:` trailers are
+  forbidden)
+
+Example:
+
+```
+feat: add CRSF CRC8 calculation
+
+Implement CRC8-CCITT (poly 0x07) for CRSF frame validation.
+Added unit tests for empty, single-byte, and known-value cases.
+```
+
+## Documentation (Markdown)
+
+- Wrap normal text and lists at **max 80 columns** (for readability in
+  terminals and editors).
+- **Exceptions**: Tables and code blocks (```` ``` ````) can exceed 80
+  columns when formatting requires it (e.g. trees, alignment).
+- Use standard Markdown: `**bold**`, `` `inline code` ``, `##` headings,
+  `-` or numbered lists, fenced code blocks with language hints
+  (```` ```c ````, ```` ```sh ````).
+- Keep examples concise, up-to-date, and self-documenting.
+- Do not use em dashes (`—`). Use a colon or rewrite the sentence.
+- Each shell command gets its own fenced code block; do **not** combine
+  multiple commands into one block. Precede each block with a short
+  plain-text label describing what the command does:
+
+  Setup build:
+  ```sh
+  cmake -S . -B build -G Ninja
+  ```
+
+- This file (`AGENTS.md`) follows its own rules.
+
+## Source Layout
+
+```text
+celrs/
+  crsf.h / crsf.c      CRSF protocol: CRC8, frame parse/build
+  serial.h / serial.c  Serial port abstraction (Win/POSIX stubs)
+  logger.h / logger.c  Level-filtering logger
+  log_write.h/.c       stdout log sink
+main.c                 Entry point — demo heartbeat + read
+tests/
+  test_crsf.c          CRSF CRC, parse, build tests
+  test_serial.c        Serial open/close/stub tests
+  test_logger.c        Logger level-filtering tests
+deps/
+  FindUnity.cmake      Fetches Unity v2.6.1 via ZIP
+  FindCMock.cmake      Fetches CMock v2.6.0 via ZIP
+```
+
+## TDD Workflow
+
+This project follows Red-Green-Refactor. All changes to testable source
+files under `celrs/` should be test-driven: write a failing test first,
+then implement.
+
+### Adding a new module
+
+1. Create `celrs/<module>.h` with the public prototype.
+2. Create `tests/test_<module>.c`. Set CMock expectations for any
+  dependency calls, then assert the result.
+3. Register the test in `tests/CMakeLists.txt`:
+
+```cmake
+add_executable(test_module test_module.c)
+target_include_directories(test_module PRIVATE "${CMAKE_SOURCE_DIR}")
+target_link_libraries(test_module PRIVATE celrs_module Unity::Unity CMock::CMock)
+target_compile_features(test_module PRIVATE c_std_23)
+cmock_generate_mock(test_module "${CMAKE_SOURCE_DIR}/celrs/dep.h")
+add_test(NAME test_module COMMAND test_module)
+list(APPEND TEST_TARGETS test_module)
+```
+
+4. Stub `celrs/<module>.c` with a dummy return, confirm RED, implement,
+  confirm GREEN:
+
+```sh
+ninja -C build check
+```
+
+### Mocking a dependency
+
+Use `cmock_generate_mock` in the test target to generate a mock from a
+header. Include `Mock<name>.h` in the test and use the generated API:
+
+```c
+#include "Mockdep.h"
+
+void setUp(void)    { Mockdep_Init(); }
+void tearDown(void) { Mockdep_Verify(); Mockdep_Destroy(); }
+
+void test_something(void) {
+    dep_fn_ExpectAndReturn(arg, expected);
+    TEST_ASSERT_TRUE(module_do_thing());
+}
+```
+
+## Behavioral Guidelines
+
+Reduce common LLM coding mistakes. Bias toward caution over speed.
+For trivial tasks, use judgment.
+
+### Think Before Coding
+
+- State assumptions explicitly. If uncertain, ask.
+- If multiple interpretations exist, present them, don't pick silently.
+- If something is unclear, stop. Name what's confusing. Ask.
+
+### Simplicity First
+
+Minimum code that solves the problem. Nothing speculative.
+
+- No features beyond what was asked.
+- No abstractions for single-use code.
+- If you write 200 lines and it could be 50, rewrite it.
+
+### Surgical Changes
+
+Touch only what you must. Clean up only your own mess.
+
+- Don't "improve" adjacent code, comments, or formatting.
+- Don't refactor things that aren't broken.
+- Match existing style, even if you'd do it differently.
+- Remove imports/variables/functions that YOUR changes made unused.
+- Don't remove pre-existing dead code unless asked.
+
+### Goal-Driven Execution
+
+Transform tasks into verifiable goals:
+
+- "Add validation" means: write tests for invalid inputs, then pass.
+- "Fix the bug" means: write a test that reproduces it, then pass.
+- "Refactor X" means: ensure tests pass before and after.
+
+## Platform Support
+
+The project supports Windows, Linux, macOS, Emscripten, and Android via
+`Platform.cmake` and `Flags.cmake` in `deps/`.
+
+## CRSF Protocol Notes
+
+CRSF is the Crossfire Serial Protocol used by ELRS. Key points:
+
+- Frame header is always `0xC8`
+- CRC8-CCITT with polynomial `0x07`, init `0x00`
+- CRC is computed over: destination + source + type + size + payload
+- Standard baud rate for ELRS CRSF is 400000 bps
+- TX modules expose CRSF over USB serial (appears as COM port on Windows,
+  /dev/ttyUSB* on Linux)
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..43c994c
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1 @@
+@AGENTS.md
diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..d56e3c4
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,25 @@
+cmake_minimum_required(VERSION 3.21)
+project(celrs VERSION 0.1.0)
+
+# CMake configuration
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/deps")
+
+# Platform flags
+include(Platform)
+include(Flags)
+
+include(Coverage)
+
+# Core Library
+add_subdirectory(celrs)
+
+# Tools
+add_subdirectory(tools)
+
+# Testing
+enable_testing()
+add_subdirectory(tests)
+
+# IDE configuration
+include(IDE)
diff --git a/LICENSE b/LICENSE
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index 0000000..4a61a86
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 PorterSky
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..b98fc8c
--- /dev/null
+++ b/README.md
@@ -0,0 +1,147 @@
+# celrs
+
+A C23 project for interfacing with ELRS TX modules (e.g., BAYCK Nano Dual
+Band) via serial USB using the CRSF (Crossfire Serial) protocol.
+
+Built on the same TDD foundation as [ctdd](https://github.com/PorterSky/ctdd)
+using [Unity](https://github.com/ThrowTheSwitch/Unity) and
+[CMock](https://github.com/ThrowTheSwitch/CMock).
+
+All dependencies are fetched automatically via CMake `FetchContent` — no
+manual installation required beyond the tools listed below.
+
+## Requirements
+
+| Tool         | Purpose                                               |
+| ------------ | ----------------------------------------------------- |
+| CMake ≥ 3.21 | Build system                                          |
+| Ninja        | Build backend                                         |
+| C23 compiler | GCC 14+, Clang 18+                                    |
+| Ruby ≥ 3.0   | CMock mock generation                                 |
+| gcovr ≥ 6.0  | Coverage reports — optional (`uv tool install gcovr`) |
+
+## Build
+
+Configure:
+```sh
+cmake -S . -B build -G Ninja
+```
+
+Build:
+```sh
+ninja -C build
+```
+
+## Test
+
+Full Unity output with colors:
+```sh
+ninja -C build check
+```
+
+CTest summary only:
+```sh
+ninja -C build test
+```
+
+`check` builds all suites and runs CTest with `--output-on-failure`,
+so assertion-level detail appears on any failure without running
+binaries by hand.
+
+## Coverage
+
+Configure with coverage instrumentation:
+```sh
+cmake -S . -B build-cov -G Ninja -DENABLE_COVERAGE=ON
+```
+
+Generate the HTML report:
+```sh
+ninja -C build-cov coverage
+```
+
+Open `build-cov/coverage/index.html` in a browser to view results.
+
+Only `celrs/` source files are measured. Unity, CMock, and generated
+mock files are excluded. Requires GCC or Clang with gcov support, and
+`gcovr` on `PATH`.
+
+> **Windows note:** requires GCC (e.g. `scoop install gcc`) or a Clang
+> build that includes compiler-rt. A custom Clang without compiler-rt
+> will fail at link time.
+
+## Run
+
+Connect your ELRS TX module via USB, then run:
+
+Windows:
+```sh
+./build/main.exe COM3
+```
+
+Linux / macOS:
+```sh
+./build/main /dev/ttyUSB0
+```
+
+## Architecture
+
+```
+celrs/
+  crsf.h / crsf.c      CRSF protocol: CRC8, frame parse/build
+  serial.h / serial.c  Serial port abstraction (Win/POSIX)
+  logger.h / logger.c  Level-filtering logger
+  log_write.h/.c       stdout log sink
+main.c                 Entry point — demo heartbeat + read
+tests/
+  test_crsf.c          CRSF CRC, parse, build tests
+  test_serial.c        Serial open/close/stub tests
+  test_logger.c        Logger level-filtering tests
+deps/
+  FindUnity.cmake      Fetches Unity v2.6.1 via ZIP
+  FindCMock.cmake      Fetches CMock v2.6.0 via ZIP
+```
+
+## CRSF Protocol
+
+CRSF (Crossfire Serial Protocol) is the serial protocol used by ELRS for
+communication between ground station and TX/RX modules.
+
+### Frame format
+
+```
++------+------+------+------+-------+-------+
+| 0xC8 | dest | src  | type | size  | ...   |  CRC  |
++------+------+------+------+-------+-------+
+  1 byte  1B     1B     1B     1B     N B     1 byte
+```
+
+- **Header:** Always `0xC8`
+- **Destination:** Target device address
+- **Source:** Sender device address
+- **Type:** Frame type (heartbeat, RC channels, telemetry, etc.)
+- **Size:** Payload length in bytes
+- **Payload:** Frame-specific data
+- **CRC:** CRC8-CCITT over dest+src+type+size+payload
+
+### Common device addresses
+
+| Address | Device                        |
+| ------- | ----------------------------- |
+| 0x00    | FC Broadcast                  |
+| 0x10    | Flight Controller             |
+| 0x80    | TBS Ground Station            |
+| 0xEA    | Custom Module                 |
+| 0xDD    | RC Device                     |
+
+### Common frame types
+
+| Type   | Name                        |
+| ------ | --------------------------- |
+| 0x01   | RC Channels Packed          |
+| 0x02   | Packet Link Telemetry       |
+| 0x03   | Heartbeat                   |
+| 0x08   | Device Info                 |
+| 0x09   | Parameter List              |
+| 0x17   | MSP Read                    |
+| 0x18   | MSP Write                   |
diff --git a/celrs/CMakeLists.txt b/celrs/CMakeLists.txt
new file mode 100644
index 0000000..30397f1
--- /dev/null
+++ b/celrs/CMakeLists.txt
@@ -0,0 +1,17 @@
+add_library(celrs_crsf STATIC crsf.c)
+target_include_directories(celrs_crsf PUBLIC "${CMAKE_SOURCE_DIR}")
+target_compile_features(celrs_crsf PRIVATE c_std_23)
+
+add_library(celrs_serial STATIC serial.c)
+target_include_directories(celrs_serial PUBLIC "${CMAKE_SOURCE_DIR}")
+target_compile_features(celrs_serial PRIVATE c_std_23)
+
+# Level-filtering logger — calls log_write(); symbol resolved by the final binary
+add_library(celrs_logger STATIC logger.c)
+target_include_directories(celrs_logger PUBLIC "${CMAKE_SOURCE_DIR}")
+target_compile_features(celrs_logger PRIVATE c_std_23)
+
+# Real log_write implementation — linked into production binaries only
+add_library(celrs_log_write STATIC log_write.c)
+target_include_directories(celrs_log_write PUBLIC "${CMAKE_SOURCE_DIR}")
+target_compile_features(celrs_log_write PRIVATE c_std_23)
diff --git a/celrs/crsf.c b/celrs/crsf.c
new file mode 100644
index 0000000..fb60be4
--- /dev/null
+++ b/celrs/crsf.c
@@ -0,0 +1,74 @@
+#include "celrs/crsf.h"
+#include <string.h>
+
+/* CRC8-CCITT with polynomial 0x07, init 0x00 (used by CRSF/ELRS) */
+uint8_t cel_crsf_crc(uint8_t const* data, size_t len) {
+    uint8_t crc = 0x00;
+    for (size_t i = 0; i < len; i++) {
+        crc ^= data[i];
+        for (int j = 0; j < 8; j++) {
+            crc = (crc & 0x80) ? (crc << 1) ^ 0x07 : (crc << 1);
+        }
+    }
+    return crc;
+}
+
+int cel_crsf_frame_validate(cel_crsf_frame const* frame) {
+    /* Rebuild the data that was CRC'd: dest + src + type + size + payload */
+    uint8_t data[260];
+    size_t offset = 0;
+    data[offset++] = frame->destination;
+    data[offset++] = frame->source;
+    data[offset++] = frame->type;
+    data[offset++] = frame->size;
+    memcpy(data + offset, frame->payload, frame->size);
+    offset += frame->size;
+
+    uint8_t calc_crc = cel_crsf_crc(data, offset);
+    return calc_crc == frame->crc ? 0 : -1;
+}
+
+int cel_crsf_frame_parse(cel_crsf_frame* frame, uint8_t const* buf, size_t len) {
+    if (frame == NULL || buf == NULL) return -1;
+    /* Minimum: header(1) + dest(1) + src(1) + type(1) + size(1) = 5 bytes */
+    if (len < 5) return -1;
+
+    /* Verify header */
+    if (buf[0] != CEL_CRSF_FRAME_HEADER) return -1;
+
+    frame->destination = buf[1];
+    frame->source      = buf[2];
+    frame->type        = buf[3];
+    frame->size        = buf[4];
+
+    uint8_t size = buf[4];
+    /* Total: header(1) + dest(1) + src(1) + type(1) + size(1) + payload(N) + crc(1) */
+    size_t total = 6 + size;
+    if (len < total) return -1;
+
+    memcpy(frame->payload, buf + 5, size);
+    frame->crc = buf[5 + size];
+
+    return cel_crsf_frame_validate(frame);
+}
+
+size_t cel_crsf_frame_build(uint8_t* dst, uint8_t destination, uint8_t source,
+                            uint8_t type, uint8_t const* payload, uint8_t size) {
+    if (dst == NULL) return 0;
+
+    dst[0] = CEL_CRSF_FRAME_HEADER;
+    dst[1] = destination;
+    dst[2] = source;
+    dst[3] = type;
+    dst[4] = size;
+
+    if (payload != NULL && size > 0) {
+        memcpy(dst + 5, payload, size);
+    }
+
+    /* CRC over dest + src + type + size + payload */
+    uint8_t crc = cel_crsf_crc(dst + 1, 3 + 1 + size);
+    dst[5 + size] = crc;
+
+    return 1 + 3 + 1 + size + 1; /* header + 3 fields + size byte + payload + crc */
+}
diff --git a/celrs/crsf.h b/celrs/crsf.h
new file mode 100644
index 0000000..d0ba3e8
--- /dev/null
+++ b/celrs/crsf.h
@@ -0,0 +1,65 @@
+#pragma once
+#include <stdint.h>
+#include <stddef.h>
+
+/* CRSF frame header byte */
+#define CEL_CRSF_FRAME_HEADER 0xC8
+
+/* CRSF device addresses */
+#define CEL_CRSF_ADDRESS_FC_BROADCAST      0x00
+#define CEL_CRSF_ADDRESS_FC                0x10
+#define CEL_CRSF_ADDRESS_TBS_GROUND_STATION 0x80
+#define CEL_CRSF_ADDRESS_CUSTOM_MODULE     0xEA
+#define CEL_CRSF_ADDRESS_RC_DEVICE         0xDD
+#define CEL_CRSF_ADDRESS_GPS               0xEC
+#define CEL_CRSF_ADDRESS_FLIGHT_CONTROLLER 0xED
+
+/* CRSF frame types */
+typedef enum {
+    CEL_CRSF_FRAMETYPE_PACKET_LINK_TELEMETRY    = 0x02,
+    CEL_CRSF_FRAMETYPE_RC_CHANNELS_PACKED       = 0x01,
+    CEL_CRSF_FRAMETYPE_GPS                      = 0x02,
+    CEL_CRSF_FRAMETYPE_HEARTBEAT                = 0x03,
+    CEL_CRSF_FRAMETYPE_VERSION                  = 0x04,
+    CEL_CRSF_FRAMETYPE_PARAMETER_SETTINGS_ENTRY = 0x05,
+    CEL_CRSF_FRAMETYPE_PARAMETER_READ           = 0x06,
+    CEL_CRSF_FRAMETYPE_PARAMETER_WRITE          = 0x07,
+    CEL_CRSF_FRAMETYPE_DEVICE_INFO              = 0x08,
+    CEL_CRSF_FRAMETYPE_PARAMETER_LIST           = 0x09,
+    CEL_CRSF_FRAMETYPE_RC_CHANNELS_RAW          = 0x16,
+    CEL_CRSF_FRAMETYPE_MSP_READ                 = 0x17,
+    CEL_CRSF_FRAMETYPE_MSP_WRITE                = 0x18,
+    CEL_CRSF_FRAMETYPE_CURR_VOLTAGE_TEMP        = 0x1E,
+    CEL_CRSF_FRAMETYPE_BATTERY_SENSOR           = 0x1F,
+    CEL_CRSF_FRAMETYPE_COMPRESSED_SENSORS       = 0x28,
+    CEL_CRSF_FRAMETYPE_ARM                      = 0x0D,
+    CEL_CRSF_FRAMETYPE_SETTING                  = 0x9E,
+    CEL_CRSF_FRAMETYPE_SUPERBOX                 = 0xA0,
+    CEL_CRSF_FRAMETYPE_DEVICE_SUPERBOX          = 0xA1,
+} cel_crsf_frame_type;
+
+/* Parsed CRSF frame */
+typedef struct {
+    uint8_t  destination;
+    uint8_t  source;
+    uint8_t  type;
+    uint8_t  size;
+    uint8_t  payload[255];
+    uint8_t  crc;
+} cel_crsf_frame;
+
+/* CRC8 calculation over CRSF frame data (CCITT poly 0x07) */
+uint8_t cel_crsf_crc(uint8_t const* data, size_t len);
+
+/* Validate CRC of a CRSF frame (header already stripped, starts at dest addr) */
+int cel_crsf_frame_validate(cel_crsf_frame const* frame);
+
+/* Parse a raw buffer into a cel_crsf_frame. Returns 0 on success, -1 on error.
+   buf should start with 0xC8 header. */
+int cel_crsf_frame_parse(cel_crsf_frame* frame, uint8_t const* buf, size_t len);
+
+/* Build a CRSF frame into dst buffer. Returns total bytes written.
+   dst must have space for at least 5 + size bytes (header, addr, src, type,
+   size byte, payload, crc). */
+size_t cel_crsf_frame_build(uint8_t* dst, uint8_t destination, uint8_t source,
+                            uint8_t type, uint8_t const* payload, uint8_t size);
diff --git a/celrs/log_write.c b/celrs/log_write.c
new file mode 100644
index 0000000..1a7a0a6
--- /dev/null
+++ b/celrs/log_write.c
@@ -0,0 +1,6 @@
+#include "celrs/log_write.h"
+#include <stdio.h>
+
+void cel_log_write(char const* msg) {
+    printf("%s\n", msg);
+}
diff --git a/celrs/log_write.h b/celrs/log_write.h
new file mode 100644
index 0000000..b08d340
--- /dev/null
+++ b/celrs/log_write.h
@@ -0,0 +1,3 @@
+#pragma once
+
+void cel_log_write(char const* msg);
diff --git a/celrs/logger.c b/celrs/logger.c
new file mode 100644
index 0000000..17b62b1
--- /dev/null
+++ b/celrs/logger.c
@@ -0,0 +1,18 @@
+#include "celrs/logger.h"
+#include "celrs/log_write.h"
+#include <stdio.h>
+
+static cel_log_level s_level = CEL_LOG_DEBUG;
+
+void cel_logger_set_level(cel_log_level level) { s_level = level; }
+
+static void emit(char const* prefix, char const* msg) {
+    char buf[512];
+    snprintf(buf, sizeof(buf), "[%s] %s", prefix, msg);
+    cel_log_write(buf);
+}
+
+void cel_log_debug(char const* msg) { if (s_level <= CEL_LOG_DEBUG) emit("DEBUG", msg); }
+void cel_log_info(char const* msg)  { if (s_level <= CEL_LOG_INFO)  emit("INFO",  msg); }
+void cel_log_warn(char const* msg)  { if (s_level <= CEL_LOG_WARN)  emit("WARN",  msg); }
+void cel_log_err(char const* msg)   { if (s_level <= CEL_LOG_ERROR) emit("ERROR", msg); }
diff --git a/celrs/logger.h b/celrs/logger.h
new file mode 100644
index 0000000..9114649
--- /dev/null
+++ b/celrs/logger.h
@@ -0,0 +1,9 @@
+#pragma once
+
+typedef enum { CEL_LOG_DEBUG = 0, CEL_LOG_INFO, CEL_LOG_WARN, CEL_LOG_ERROR, CEL_LOG_NONE } cel_log_level;
+
+void cel_logger_set_level(cel_log_level level);
+void cel_log_debug(char const* msg);
+void cel_log_info(char const* msg);
+void cel_log_warn(char const* msg);
+void cel_log_err(char const* msg);
diff --git a/celrs/serial.c b/celrs/serial.c
new file mode 100644
index 0000000..dcb89de
--- /dev/null
+++ b/celrs/serial.c
@@ -0,0 +1,69 @@
+#ifdef _MSC_VER
+#define _CRT_SECURE_NO_WARNINGS
+#endif
+
+#include "celrs/serial.h"
+
+/*
+ * Platform-agnostic serial port implementation.
+ *
+ * Windows uses Win32 CreateFile/ReadFile/WriteFile.
+ * POSIX uses open/read/write with termios.
+ *
+ * This is a stub implementation that compiles but does nothing.
+ * Real platform-specific code will be added when TDD tests drive it.
+ */
+
+#include <stdlib.h>
+#include <string.h>
+
+struct cel_serial_port {
+    char path[256];
+    int  baud_rate;
+    int  fd;  /* platform-specific handle (HANDLE on Win, int on POSIX) */
+};
+
+cel_serial_port* cel_serial_open(char const* path, int baud_rate) {
+    if (path == NULL) return NULL;
+
+    cel_serial_port* port = (cel_serial_port*)calloc(1, sizeof(cel_serial_port));
+    if (port == NULL) return NULL;
+
+    strncpy(port->path, path, sizeof(port->path) - 1);
+    port->path[sizeof(port->path) - 1] = '\0';
+    port->baud_rate = baud_rate;
+    port->fd = -1;
+
+    /* TODO: platform-specific open (CreateFile on Win, open+termios on POSIX) */
+    (void)baud_rate;
+
+    return port;
+}
+
+void cel_serial_close(cel_serial_port* port) {
+    if (port == NULL) return;
+    /* TODO: platform-specific close */
+    free(port);
+}
+
+size_t cel_serial_read(cel_serial_port* port, unsigned char* buf, size_t len, int timeout_ms) {
+    (void)port;
+    (void)buf;
+    (void)len;
+    (void)timeout_ms;
+    /* TODO: platform-specific read */
+    return 0;
+}
+
+size_t cel_serial_write(cel_serial_port* port, unsigned char const* buf, size_t len) {
+    (void)port;
+    (void)buf;
+    (void)len;
+    /* TODO: platform-specific write */
+    return 0;
+}
+
+void cel_serial_flush(cel_serial_port* port) {
+    (void)port;
+    /* TODO: platform-specific flush */
+}
diff --git a/celrs/serial.h b/celrs/serial.h
new file mode 100644
index 0000000..8972b66
--- /dev/null
+++ b/celrs/serial.h
@@ -0,0 +1,24 @@
+#pragma once
+#include <stddef.h>
+
+/* Opaque serial port handle */
+typedef struct cel_serial_port cel_serial_port;
+
+/* Open a serial port. path is platform-specific:
+ *   Windows: "COM3"
+ *   Linux/macOS: "/dev/ttyUSB0"
+ * baud_rate: 400000 is standard for ELRS CRSF
+ * Returns NULL on failure. */
+cel_serial_port* cel_serial_open(char const* path, int baud_rate);
+
+/* Close and free the serial port */
+void cel_serial_close(cel_serial_port* port);
+
+/* Read up to len bytes. Returns bytes read, 0 on timeout/error. */
+size_t cel_serial_read(cel_serial_port* port, unsigned char* buf, size_t len, int timeout_ms);
+
+/* Write data. Returns bytes written, 0 on error. */
+size_t cel_serial_write(cel_serial_port* port, unsigned char const* buf, size_t len);
+
+/* Flush output buffer */
+void cel_serial_flush(cel_serial_port* port);
diff --git a/deps/Coverage.cmake b/deps/Coverage.cmake
new file mode 100644
index 0000000..01159ef
--- /dev/null
+++ b/deps/Coverage.cmake
@@ -0,0 +1,56 @@
+# ==============================================================================
+# Coverage
+# ==============================================================================
+# gcov-based coverage via --coverage, reported by gcovr.
+# Works with GCC and Clang on Linux and macOS out of the box.
+# Windows requires GCC (e.g. MinGW via scoop install gcc) or a Clang build
+# that includes compiler-rt (clang_rt.profile).
+#
+# Requires: gcovr on PATH (install via: uv tool install gcovr)
+# Usage:    cmake -DENABLE_COVERAGE=ON ...  then  ninja coverage
+# Report:   build/coverage/index.html
+# ==============================================================================
+
+option(ENABLE_COVERAGE "Build with coverage instrumentation" OFF)
+
+if (ENABLE_COVERAGE)
+    if (NOT CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+        message(FATAL_ERROR "ENABLE_COVERAGE requires GCC or Clang")
+    endif()
+
+    find_program(GCOVR_EXE gcovr REQUIRED)
+
+    if (CMAKE_C_COMPILER_ID MATCHES "Clang")
+        # gcovr needs a single-token gcov executable. Wrap llvm-cov gcov in a
+        # script placed in the build dir (guaranteed no spaces in path).
+        # AppleClang ships llvm-cov inside Xcode, reached only via xcrun.
+        if (WIN32)
+            find_program(LLVM_COV_EXE llvm-cov REQUIRED)
+            set(GCOV_EXECUTABLE "${CMAKE_BINARY_DIR}/llvm-gcov.bat")
+            file(WRITE "${GCOV_EXECUTABLE}" "@echo off\r\n\"${LLVM_COV_EXE}\" gcov %*\r\n")
+        elseif (CMAKE_C_COMPILER_ID STREQUAL "AppleClang")
+            find_program(XCRUN_EXE xcrun REQUIRED)
+            set(GCOV_EXECUTABLE "${CMAKE_BINARY_DIR}/llvm-gcov.sh")
+            file(WRITE "${GCOV_EXECUTABLE}" "#!/bin/sh\nexec xcrun llvm-cov gcov \"$@\"\n")
+            file(CHMOD "${GCOV_EXECUTABLE}"
+                PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
+                            GROUP_READ GROUP_EXECUTE
+                            WORLD_READ WORLD_EXECUTE)
+        else()
+            find_program(LLVM_COV_EXE llvm-cov REQUIRED)
+            set(GCOV_EXECUTABLE "${CMAKE_BINARY_DIR}/llvm-gcov.sh")
+            file(WRITE "${GCOV_EXECUTABLE}" "#!/bin/sh\nexec \"${LLVM_COV_EXE}\" gcov \"$@\"\n")
+            file(CHMOD "${GCOV_EXECUTABLE}"
+                PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE
+                            GROUP_READ GROUP_EXECUTE
+                            WORLD_READ WORLD_EXECUTE)
+        endif()
+        message(STATUS "Coverage: enabled (gcovr: ${GCOVR_EXE}, gcov: llvm-cov gcov)")
+    else()
+        set(GCOV_EXECUTABLE "gcov")
+        message(STATUS "Coverage: enabled (gcovr: ${GCOVR_EXE})")
+    endif()
+
+    add_compile_options(--coverage -O0 -g)
+    add_link_options(--coverage)
+endif()
diff --git a/deps/FindCMock.cmake b/deps/FindCMock.cmake
new file mode 100644
index 0000000..6988bb7
--- /dev/null
+++ b/deps/FindCMock.cmake
@@ -0,0 +1,92 @@
+# ==============================================================================
+# Find CMock
+# ==============================================================================
+# This module fetches the CMock mocking framework (depends on Unity).
+#
+# Targets provided:
+#   CMock::CMock - The CMock library target
+#
+# Variables set:
+#   CMock_FOUND       - TRUE if CMock is available
+#   CMock_LIBRARIES   - The CMock library target (CMock::CMock)
+#   CMock_INCLUDE_DIR - Include directories for CMock
+#   CMock_VERSION     - Version of CMock (if available)
+#
+# Generator variables (set when Ruby is found):
+#   CMOCK_SCRIPT      - Path to lib/cmock.rb
+#   RUBY_EXECUTABLE   - Path to ruby interpreter
+# ==============================================================================
+
+if (DEFINED _FINDCMOCK_INCLUDED)
+    return()
+endif()
+set(_FINDCMOCK_INCLUDED TRUE)
+
+find_package(Unity REQUIRED)
+
+if (DEFINED CMock_FIND_VERSION AND NOT CMock_FIND_VERSION STREQUAL "")
+    set(CMOCK_VERSION "${CMock_FIND_VERSION}")
+else()
+    set(CMOCK_VERSION "2.6.0")
+endif()
+
+message(STATUS "Fetching CMock ${CMOCK_VERSION}")
+
+include(FetchContent)
+
+FetchContent_Declare(
+    cmock
+    URL https://github.com/ThrowTheSwitch/CMock/archive/refs/tags/v${CMOCK_VERSION}.zip
+    DOWNLOAD_EXTRACT_TIMESTAMP TRUE
+)
+
+# CMock uses Meson — bypass its build system and compile src/cmock.c directly.
+# FetchContent_MakeAvailable cannot be used here (no CMakeLists.txt in CMock),
+# so we call FetchContent_Populate directly and opt into the old policy.
+cmake_policy(PUSH)
+cmake_policy(SET CMP0169 OLD)
+FetchContent_GetProperties(cmock)
+if (NOT cmock_POPULATED)
+    FetchContent_Populate(cmock)
+endif()
+cmake_policy(POP)
+
+# The Ruby generator expects vendor/unity/auto/type_sanitizer.rb — populate
+# it from the Unity source we already have rather than needing a git submodule
+set(_cmock_vendor_auto "${cmock_SOURCE_DIR}/vendor/unity/auto")
+if (NOT EXISTS "${_cmock_vendor_auto}/type_sanitizer.rb")
+    file(MAKE_DIRECTORY "${_cmock_vendor_auto}")
+    file(COPY "${unity_SOURCE_DIR}/auto/" DESTINATION "${_cmock_vendor_auto}")
+endif()
+
+if (NOT TARGET cmock)
+    add_library(cmock STATIC "${cmock_SOURCE_DIR}/src/cmock.c")
+    target_include_directories(cmock PUBLIC
+        "${cmock_SOURCE_DIR}/src"
+        "${unity_SOURCE_DIR}/src"
+    )
+    target_link_libraries(cmock PUBLIC unity)
+endif()
+
+if (NOT TARGET CMock::CMock)
+    add_library(CMock::CMock ALIAS cmock)
+endif()
+
+set(CMock_FOUND        TRUE)
+set(CMock_LIBRARIES    CMock::CMock)
+set(CMock_VERSION      "${CMOCK_VERSION}")
+set(CMock_INCLUDE_DIR  "${cmock_SOURCE_DIR}/src")
+set(CMOCK_SCRIPT       "${cmock_SOURCE_DIR}/lib/cmock.rb" CACHE FILEPATH "Path to CMock Ruby generator")
+
+if (CMock_INCLUDE_DIR AND TARGET cmock)
+    set_target_properties(cmock PROPERTIES
+        INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "${CMock_INCLUDE_DIR}"
+    )
+endif()
+
+find_program(RUBY_EXECUTABLE ruby)
+if (NOT RUBY_EXECUTABLE)
+    message(WARNING "Ruby not found — CMock code generation unavailable")
+endif()
+
+set(CMOCK_LICENSE_FILE "${cmock_SOURCE_DIR}/LICENSE.txt" CACHE FILEPATH "Path to CMock license file")
diff --git a/deps/FindUnity.cmake b/deps/FindUnity.cmake
new file mode 100644
index 0000000..6d3edb5
--- /dev/null
+++ b/deps/FindUnity.cmake
@@ -0,0 +1,67 @@
+# ==============================================================================
+# Find Unity
+# ==============================================================================
+# This module fetches the Unity unit testing framework.
+#
+# Targets provided:
+#   Unity::Unity - The Unity library target
+#
+# Variables set:
+#   Unity_FOUND       - TRUE if Unity is available
+#   Unity_LIBRARIES   - The Unity library target (Unity::Unity)
+#   Unity_INCLUDE_DIR - Include directories for Unity
+#   Unity_VERSION     - Version of Unity (if available)
+# ==============================================================================
+
+if (DEFINED _FINDUNITY_INCLUDED)
+    return()
+endif()
+set(_FINDUNITY_INCLUDED TRUE)
+
+if (DEFINED Unity_FIND_VERSION AND NOT Unity_FIND_VERSION STREQUAL "")
+    set(UNITY_VERSION "${Unity_FIND_VERSION}")
+else()
+    set(UNITY_VERSION "2.6.1")
+endif()
+
+message(STATUS "Fetching Unity ${UNITY_VERSION}")
+
+include(FetchContent)
+
+FetchContent_Declare(
+    unity
+    URL https://github.com/ThrowTheSwitch/Unity/archive/refs/tags/v${UNITY_VERSION}.zip
+    DOWNLOAD_EXTRACT_TIMESTAMP TRUE
+)
+
+FetchContent_MakeAvailable(unity)
+
+# Unity sets INTERFACE_SYSTEM_INCLUDE_DIRECTORIES to a path inside the build
+# tree, which CMake rejects on newer versions. The path stays in
+# INTERFACE_INCLUDE_DIRECTORIES so headers are still found.
+if (TARGET unity)
+    set_target_properties(unity PROPERTIES INTERFACE_SYSTEM_INCLUDE_DIRECTORIES "")
+endif()
+
+if (NOT TARGET Unity::Unity)
+    if (TARGET unity)
+        add_library(Unity::Unity ALIAS unity)
+    else()
+        message(FATAL_ERROR "Could not fetch Unity; no target unity or Unity::Unity available")
+    endif()
+endif()
+
+set(Unity_FOUND        TRUE)
+set(Unity_LIBRARIES    Unity::Unity)
+set(Unity_VERSION      "${UNITY_VERSION}")
+set(Unity_INCLUDE_DIR  "${unity_SOURCE_DIR}/src")
+
+
+if (TARGET unity)
+    target_compile_definitions(unity PUBLIC
+        UNITY_OUTPUT_COLOR
+        UNITY_INCLUDE_PRINT_FORMATTED
+    )
+endif()
+
+set(UNITY_LICENSE_FILE "${unity_SOURCE_DIR}/LICENSE.txt" CACHE FILEPATH "Path to Unity license file")
diff --git a/deps/Flags.cmake b/deps/Flags.cmake
new file mode 100644
index 0000000..28a12cd
--- /dev/null
+++ b/deps/Flags.cmake
@@ -0,0 +1,27 @@
+# ==============================================================================
+# Compiler Flags
+# ==============================================================================
+# Sets BASE_OPTIONS (warning flags) and BASE_DEFINITIONS.
+# Apply per-target via target_compile_options / target_compile_definitions
+# to avoid polluting fetched dependencies.
+#
+# Requires: Platform.cmake (for IS_CLANG_OR_GCC / IS_MSVC)
+# ==============================================================================
+
+set(BASE_DEFINITIONS "")
+set(BASE_LIBRARIES   "")
+
+set(BASE_OPTIONS "")
+if (IS_CLANG_OR_GCC)
+    set(BASE_OPTIONS
+        "-Wall"
+        "-Wextra"
+        "-Werror"
+    )
+elseif (IS_MSVC)
+    set(BASE_OPTIONS
+        "/W4"
+        "/WX"
+        "/utf-8"
+    )
+endif()
diff --git a/deps/IDE.cmake b/deps/IDE.cmake
new file mode 100644
index 0000000..1577f6c
--- /dev/null
+++ b/deps/IDE.cmake
@@ -0,0 +1,17 @@
+# ==============================================================================
+# IDE Integration
+# ==============================================================================
+# Groups dependency targets into folders for Visual Studio / Xcode.
+# Has no effect on the build.
+# ==============================================================================
+
+function(set_target_folder target folder)
+    if (TARGET ${target})
+        set_target_properties(${target} PROPERTIES FOLDER ${folder})
+    endif()
+endfunction()
+
+if (CMAKE_GENERATOR MATCHES "Visual Studio" OR CMAKE_GENERATOR MATCHES "Xcode")
+    set_target_folder(unity deps)
+    set_target_folder(cmock deps)
+endif()
diff --git a/deps/Platform.cmake b/deps/Platform.cmake
new file mode 100644
index 0000000..5a5fdcb
--- /dev/null
+++ b/deps/Platform.cmake
@@ -0,0 +1,64 @@
+# ==============================================================================
+# Platform Detection
+# ==============================================================================
+# This module detects the current platform and compiler, setting IS_* variables
+# that can be used throughout the build system for conditional logic.
+#
+# Compiler flags set:
+#   IS_CLANG_OR_GCC  - TRUE if using Clang or GCC compiler
+#   IS_MSVC          - TRUE if using Microsoft Visual C++ compiler
+#
+# Platform flags set:
+#   IS_WINDOWS       - TRUE if building for Windows
+#   IS_LINUX         - TRUE if building for Linux
+#   IS_MACOS         - TRUE if building for macOS
+#   IS_IOS           - TRUE if building for iOS
+#   IS_ANDROID       - TRUE if building for Android
+#   IS_EMSCRIPTEN    - TRUE if building for WebAssembly via Emscripten
+# ==============================================================================
+
+# ------------------------------------------------------------------------------
+# Compiler Detection
+# ------------------------------------------------------------------------------
+set(IS_CLANG_OR_GCC FALSE)
+set(IS_MSVC         FALSE)
+
+if(MSVC)
+    set(IS_MSVC TRUE)
+elseif(CMAKE_C_COMPILER_ID MATCHES "Clang|GNU")
+    set(IS_CLANG_OR_GCC TRUE)
+endif()
+
+# ------------------------------------------------------------------------------
+# Platform Detection
+# ------------------------------------------------------------------------------
+set(IS_WINDOWS    FALSE)
+set(IS_LINUX      FALSE)
+set(IS_MACOS      FALSE)
+set(IS_IOS        FALSE)
+set(IS_ANDROID    FALSE)
+set(IS_EMSCRIPTEN FALSE)
+
+if(EMSCRIPTEN)
+    message(STATUS "Platform: Emscripten")
+    set(IS_EMSCRIPTEN TRUE)
+elseif(ANDROID)
+    message(STATUS "Platform: Android")
+    set(IS_ANDROID TRUE)
+elseif(APPLE)
+    if(IOS)
+        message(STATUS "Platform: iOS")
+        set(IS_IOS TRUE)
+    else()
+        message(STATUS "Platform: macOS")
+        set(IS_MACOS TRUE)
+    endif()
+elseif(WIN32)
+    message(STATUS "Platform: Windows")
+    set(IS_WINDOWS TRUE)
+elseif(UNIX)
+    message(STATUS "Platform: Linux")
+    set(IS_LINUX TRUE)
+else()
+    message(FATAL_ERROR "Unknown platform!")
+endif()
diff --git a/deps/Sanitizers.cmake b/deps/Sanitizers.cmake
new file mode 100644
index 0000000..d793dcd
--- /dev/null
+++ b/deps/Sanitizers.cmake
@@ -0,0 +1,28 @@
+# ==============================================================================
+# Sanitizers
+# ==============================================================================
+# AddressSanitizer (ASan) support.
+# Works with GCC/Clang on Linux and macOS, Clang on Windows (requires
+# compiler-rt with sanitizers), and MSVC on Windows (/fsanitize=address).
+#
+# Usage: cmake -DENABLE_ASAN=ON ...
+# ==============================================================================
+
+option(ENABLE_ASAN "Build with AddressSanitizer" OFF)
+
+if (ENABLE_ASAN)
+    if (ENABLE_COVERAGE)
+        message(FATAL_ERROR "ENABLE_ASAN and ENABLE_COVERAGE cannot be used together")
+    endif()
+
+    if (MSVC)
+        add_compile_options(/fsanitize=address)
+        message(STATUS "ASan: enabled (MSVC)")
+    elseif (CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+        add_compile_options(-fsanitize=address -fno-omit-frame-pointer)
+        add_link_options(-fsanitize=address)
+        message(STATUS "ASan: enabled (${CMAKE_C_COMPILER_ID})")
+    else()
+        message(FATAL_ERROR "ENABLE_ASAN: unsupported compiler ${CMAKE_C_COMPILER_ID}")
+    endif()
+endif()
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
new file mode 100644
index 0000000..818aa23
--- /dev/null
+++ b/tests/CMakeLists.txt
@@ -0,0 +1,93 @@
+find_package(Unity REQUIRED)
+find_package(CMock REQUIRED)
+
+set(MOCK_GEN_DIR "${CMAKE_CURRENT_BINARY_DIR}/mocks")
+file(MAKE_DIRECTORY "${MOCK_GEN_DIR}")
+
+# Generate a CMock mock from a header and attach it to a target.
+# Usage: cmock_generate_mock(<target> <absolute-path-to-header>)
+# CMock names generated files Mock<Name>.h/.c (capital M, no separator)
+function(cmock_generate_mock target header)
+    if (NOT RUBY_EXECUTABLE)
+        message(FATAL_ERROR "Ruby is required for CMock generation")
+    endif()
+    get_filename_component(name "${header}" NAME_WE)
+    get_filename_component(header_dir "${header}" DIRECTORY)
+    set(mock_src "${MOCK_GEN_DIR}/Mock${name}.c")
+    set(mock_hdr "${MOCK_GEN_DIR}/Mock${name}.h")
+    add_custom_command(
+        OUTPUT  "${mock_src}" "${mock_hdr}"
+        COMMAND "${RUBY_EXECUTABLE}" "${CMOCK_SCRIPT}"
+                "--mock_path=${MOCK_GEN_DIR}"
+                "${header}"
+        DEPENDS "${header}"
+        COMMENT "CMock: generating Mock${name}"
+        VERBATIM
+    )
+    target_sources("${target}" PRIVATE "${mock_src}")
+    target_include_directories("${target}" PRIVATE "${MOCK_GEN_DIR}" "${header_dir}")
+endfunction()
+
+set(TEST_TARGETS "")
+
+# CRSF tests — pure functions (CRC, parse, build), no mock needed
+add_executable(test_crsf test_crsf.c)
+target_include_directories(test_crsf PRIVATE "${CMAKE_SOURCE_DIR}")
+target_link_libraries(test_crsf PRIVATE celrs_crsf Unity::Unity)
+target_compile_features(test_crsf PRIVATE c_std_23)
+add_test(NAME test_crsf COMMAND test_crsf)
+list(APPEND TEST_TARGETS test_crsf)
+
+# Serial tests — mocks log_write.h for any logging calls
+add_executable(test_serial test_serial.c)
+target_include_directories(test_serial PRIVATE "${CMAKE_SOURCE_DIR}")
+target_link_libraries(test_serial PRIVATE celrs_serial Unity::Unity CMock::CMock)
+target_compile_features(test_serial PRIVATE c_std_23)
+cmock_generate_mock(test_serial "${CMAKE_SOURCE_DIR}/celrs/log_write.h")
+add_test(NAME test_serial COMMAND test_serial)
+list(APPEND TEST_TARGETS test_serial)
+
+# Logger tests — mocks log_write.h so output calls are intercepted
+add_executable(test_logger test_logger.c)
+target_include_directories(test_logger PRIVATE "${CMAKE_SOURCE_DIR}")
+target_link_libraries(test_logger PRIVATE celrs_logger Unity::Unity CMock::CMock)
+target_compile_features(test_logger PRIVATE c_std_23)
+cmock_generate_mock(test_logger "${CMAKE_SOURCE_DIR}/celrs/log_write.h")
+add_test(NAME test_logger COMMAND test_logger)
+list(APPEND TEST_TARGETS test_logger)
+
+# 'check' builds all suites and runs CTest with full Unity output.
+# USES_TERMINAL keeps ANSI colors alive through Ninja's output buffering.
+add_custom_target(check
+    COMMAND ${CMAKE_CTEST_COMMAND}
+            --test-dir "${CMAKE_BINARY_DIR}"
+            --output-on-failure
+            --progress
+    WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
+    USES_TERMINAL
+    DEPENDS ${TEST_TARGETS}
+)
+
+if (ENABLE_COVERAGE)
+    set(COVERAGE_DIR "${CMAKE_BINARY_DIR}/coverage")
+    add_custom_target(coverage
+        COMMAND ${CMAKE_CTEST_COMMAND} --output-on-failure
+        COMMAND ${CMAKE_COMMAND} -E make_directory "${COVERAGE_DIR}"
+        COMMAND ${GCOVR_EXE}
+                --gcov-executable "${GCOV_EXECUTABLE}"
+                --root "${CMAKE_SOURCE_DIR}"
+                --filter "${CMAKE_SOURCE_DIR}/celrs/"
+                --exclude "${CMAKE_SOURCE_DIR}/tests/"
+                --exclude ".*Mock.*"
+                --exclude ".*unity.*"
+                --exclude ".*cmock.*"
+                --html-details "${COVERAGE_DIR}/index.html"
+                --txt
+                --print-summary
+                "${CMAKE_BINARY_DIR}"
+        WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
+        USES_TERMINAL
+        DEPENDS ${TEST_TARGETS}
+        COMMENT "Coverage report: ${COVERAGE_DIR}/index.html"
+    )
+endif()
diff --git a/tests/test_crsf.c b/tests/test_crsf.c
new file mode 100644
index 0000000..e138754
--- /dev/null
+++ b/tests/test_crsf.c
@@ -0,0 +1,115 @@
+#include "unity.h"
+#include "celrs/crsf.h"
+#include <string.h>
+
+void setUp(void) {}
+void tearDown(void) {}
+
+/* CRC tests */
+void test_crc_empty(void) {
+    uint8_t data[1] = {0};
+    TEST_ASSERT_EQUAL_UINT8(0x00, cel_crsf_crc(data, 0));
+}
+
+void test_crc_single_byte(void) {
+    uint8_t data[1] = {0x01};
+    uint8_t crc = cel_crsf_crc(data, 1);
+    TEST_ASSERT_TRUE(crc != 0); /* non-trivial */
+}
+
+void test_crc_known_value(void) {
+    /* CRSF heartbeat frame data (dest+src+type+size+payload):
+     * {0x10, 0x80, 0x03, 0x02, 0x80, 0x01}
+     * Known CRC for this sequence */
+    uint8_t data[6] = {0x10, 0x80, 0x03, 0x02, 0x80, 0x01};
+    uint8_t crc = cel_crsf_crc(data, 6);
+    TEST_ASSERT_TRUE(crc != 0);
+    /* Verify idempotency */
+    uint8_t crc2 = cel_crsf_crc(data, 6);
+    TEST_ASSERT_EQUAL_UINT8(crc, crc2);
+}
+
+/* Frame parse tests */
+void test_parse_invalid_header(void) {
+    cel_crsf_frame frame;
+    uint8_t buf[8] = {0x00, 0x10, 0x80, 0x03, 0x02, 0x80, 0x01, 0x00};
+    TEST_ASSERT_EQUAL_INT(-1, cel_crsf_frame_parse(&frame, buf, 8));
+}
+
+void test_parse_too_short(void) {
+    cel_crsf_frame frame;
+    uint8_t buf[2] = {0xC8, 0x10};
+    TEST_ASSERT_EQUAL_INT(-1, cel_crsf_frame_parse(&frame, buf, 2));
+}
+
+void test_parse_null_frame(void) {
+    uint8_t buf[8] = {0xC8, 0x10, 0x80, 0x03, 0x02, 0x80, 0x01, 0x00};
+    TEST_ASSERT_EQUAL_INT(-1, cel_crsf_frame_parse(NULL, buf, 8));
+}
+
+void test_parse_null_buf(void) {
+    cel_crsf_frame frame;
+    TEST_ASSERT_EQUAL_INT(-1, cel_crsf_frame_parse(&frame, NULL, 8));
+}
+
+/* Frame build tests */
+void test_build_heartbeat(void) {
+    uint8_t dst[256];
+    uint8_t payload[2] = {0x80, 0x01};
+    size_t len = cel_crsf_frame_build(dst, 0x00, 0x80, 0x03, payload, 2);
+
+    TEST_ASSERT_GREATER_THAN(0, len);
+    TEST_ASSERT_EQUAL_UINT8(CEL_CRSF_FRAME_HEADER, dst[0]);
+    TEST_ASSERT_EQUAL_UINT8(0x00, dst[1]);  /* destination */
+    TEST_ASSERT_EQUAL_UINT8(0x80, dst[2]);  /* source */
+    TEST_ASSERT_EQUAL_UINT8(0x03, dst[3]);  /* type: heartbeat */
+    TEST_ASSERT_EQUAL_UINT8(0x02, dst[4]);  /* size */
+    TEST_ASSERT_EQUAL_UINT8(0x80, dst[5]);  /* payload[0] */
+    TEST_ASSERT_EQUAL_UINT8(0x01, dst[6]);  /* payload[1] */
+}
+
+void test_build_roundtrip(void) {
+    uint8_t dst[256];
+    uint8_t payload[4] = {0xAA, 0xBB, 0xCC, 0xDD};
+    size_t len = cel_crsf_frame_build(dst, 0x10, 0x80, 0x01, payload, 4);
+
+    /* Parse the built frame back */
+    cel_crsf_frame frame;
+    TEST_ASSERT_EQUAL_INT(0, cel_crsf_frame_parse(&frame, dst, len));
+    TEST_ASSERT_EQUAL_UINT8(0x10, frame.destination);
+    TEST_ASSERT_EQUAL_UINT8(0x80, frame.source);
+    TEST_ASSERT_EQUAL_UINT8(0x01, frame.type);
+    TEST_ASSERT_EQUAL_UINT8(4, frame.size);
+    TEST_ASSERT_EQUAL_UINT8(0xAA, frame.payload[0]);
+    TEST_ASSERT_EQUAL_UINT8(0xDD, frame.payload[3]);
+}
+
+void test_build_null_dst(void) {
+    uint8_t payload[2] = {0x01, 0x02};
+    TEST_ASSERT_EQUAL_UINT(0, cel_crsf_frame_build(NULL, 0x00, 0x80, 0x03, payload, 2));
+}
+
+void test_build_null_payload(void) {
+    uint8_t dst[256];
+    size_t len = cel_crsf_frame_build(dst, 0x10, 0x80, 0x03, NULL, 0);
+    TEST_ASSERT_GREATER_THAN(0, len);
+    /* Should still have valid CRC for empty payload */
+    cel_crsf_frame frame;
+    TEST_ASSERT_EQUAL_INT(0, cel_crsf_frame_parse(&frame, dst, len));
+}
+
+int main(void) {
+    UNITY_BEGIN();
+    RUN_TEST(test_crc_empty);
+    RUN_TEST(test_crc_single_byte);
+    RUN_TEST(test_crc_known_value);
+    RUN_TEST(test_parse_invalid_header);
+    RUN_TEST(test_parse_too_short);
+    RUN_TEST(test_parse_null_frame);
+    RUN_TEST(test_parse_null_buf);
+    RUN_TEST(test_build_heartbeat);
+    RUN_TEST(test_build_roundtrip);
+    RUN_TEST(test_build_null_dst);
+    RUN_TEST(test_build_null_payload);
+    return UNITY_END();
+}
diff --git a/tests/test_logger.c b/tests/test_logger.c
new file mode 100644
index 0000000..919731a
--- /dev/null
+++ b/tests/test_logger.c
@@ -0,0 +1,73 @@
+#include "unity.h"
+#include "celrs/logger.h"
+#include "Mocklog_write.h"
+
+void setUp(void)    { Mocklog_write_Init(); cel_logger_set_level(CEL_LOG_DEBUG); }
+void tearDown(void) { Mocklog_write_Verify(); Mocklog_write_Destroy(); }
+
+void test_log_debug_emits_at_debug_level(void) {
+    cel_log_write_Expect("[DEBUG] hello");
+    cel_log_debug("hello");
+}
+
+void test_log_info_emits_at_debug_level(void) {
+    cel_log_write_Expect("[INFO] world");
+    cel_log_info("world");
+}
+
+void test_log_warn_emits_at_warn_level(void) {
+    cel_logger_set_level(CEL_LOG_WARN);
+    cel_log_write_Expect("[WARN] alert");
+    cel_log_warn("alert");
+}
+
+void test_log_err_emits_at_warn_level(void) {
+    cel_logger_set_level(CEL_LOG_WARN);
+    cel_log_write_Expect("[ERROR] fatal");
+    cel_log_err("fatal");
+}
+
+void test_log_debug_suppressed_at_info_level(void) {
+    cel_logger_set_level(CEL_LOG_INFO);
+    cel_log_debug("silent");
+}
+
+void test_log_info_suppressed_at_warn_level(void) {
+    cel_logger_set_level(CEL_LOG_WARN);
+    cel_log_info("silent");
+}
+
+void test_log_warn_suppressed_at_error_level(void) {
+    cel_logger_set_level(CEL_LOG_ERROR);
+    cel_log_warn("silent");
+}
+
+void test_log_none_suppresses_all(void) {
+    cel_logger_set_level(CEL_LOG_NONE);
+    cel_log_debug("silent");
+    cel_log_info("silent");
+    cel_log_warn("silent");
+    cel_log_err("silent");
+}
+
+void test_level_can_be_raised_then_lowered(void) {
+    cel_logger_set_level(CEL_LOG_ERROR);
+    cel_log_info("silent");
+    cel_logger_set_level(CEL_LOG_DEBUG);
+    cel_log_write_Expect("[INFO] now visible");
+    cel_log_info("now visible");
+}
+
+int main(void) {
+    UNITY_BEGIN();
+    RUN_TEST(test_log_debug_emits_at_debug_level);
+    RUN_TEST(test_log_info_emits_at_debug_level);
+    RUN_TEST(test_log_warn_emits_at_warn_level);
+    RUN_TEST(test_log_err_emits_at_warn_level);
+    RUN_TEST(test_log_debug_suppressed_at_info_level);
+    RUN_TEST(test_log_info_suppressed_at_warn_level);
+    RUN_TEST(test_log_warn_suppressed_at_error_level);
+    RUN_TEST(test_log_none_suppresses_all);
+    RUN_TEST(test_level_can_be_raised_then_lowered);
+    return UNITY_END();
+}
diff --git a/tests/test_serial.c b/tests/test_serial.c
new file mode 100644
index 0000000..dc95205
--- /dev/null
+++ b/tests/test_serial.c
@@ -0,0 +1,72 @@
+#include "unity.h"
+#include "celrs/serial.h"
+#include "Mocklog_write.h"
+
+void setUp(void)    { Mocklog_write_Init(); }
+void tearDown(void) { Mocklog_write_Verify(); Mocklog_write_Destroy(); }
+
+void test_open_valid_path(void) {
+    cel_serial_port* port = cel_serial_open("COM3", 400000);
+    TEST_ASSERT_NOT_NULL(port);
+    cel_serial_close(port);
+}
+
+void test_open_null_path(void) {
+    TEST_ASSERT_NULL(cel_serial_open(NULL, 400000));
+}
+
+void test_open_preserves_path(void) {
+    cel_serial_port* port = cel_serial_open("/dev/ttyUSB0", 400000);
+    TEST_ASSERT_NOT_NULL(port);
+    /* path is stored internally; verify by roundtrip behavior */
+    cel_serial_close(port);
+}
+
+void test_close_null(void) {
+    /* Should not crash */
+    cel_serial_close(NULL);
+}
+
+void test_read_returns_zero_stub(void) {
+    cel_serial_port* port = cel_serial_open("COM3", 400000);
+    TEST_ASSERT_NOT_NULL(port);
+    uint8_t buf[16];
+    /* Stub implementation returns 0 */
+    size_t n = cel_serial_read(port, buf, sizeof(buf), 100);
+    TEST_ASSERT_EQUAL_UINT(0, n);
+    cel_serial_close(port);
+}
+
+void test_write_returns_zero_stub(void) {
+    cel_serial_port* port = cel_serial_open("COM3", 400000);
+    TEST_ASSERT_NOT_NULL(port);
+    uint8_t buf[4] = {0xC8, 0x10, 0x80, 0x03};
+    /* Stub implementation returns 0 */
+    size_t n = cel_serial_write(port, buf, sizeof(buf));
+    TEST_ASSERT_EQUAL_UINT(0, n);
+    cel_serial_close(port);
+}
+
+void test_flush_no_crash(void) {
+    cel_serial_port* port = cel_serial_open("COM3", 400000);
+    TEST_ASSERT_NOT_NULL(port);
+    cel_serial_flush(port); /* should not crash */
+    cel_serial_close(port);
+}
+
+void test_flush_null(void) {
+    cel_serial_flush(NULL); /* should not crash */
+}
+
+int main(void) {
+    UNITY_BEGIN();
+    RUN_TEST(test_open_valid_path);
+    RUN_TEST(test_open_null_path);
+    RUN_TEST(test_open_preserves_path);
+    RUN_TEST(test_close_null);
+    RUN_TEST(test_read_returns_zero_stub);
+    RUN_TEST(test_write_returns_zero_stub);
+    RUN_TEST(test_flush_no_crash);
+    RUN_TEST(test_flush_null);
+    return UNITY_END();
+}
diff --git a/tools/CMakeLists.txt b/tools/CMakeLists.txt
new file mode 100644
index 0000000..f705304
--- /dev/null
+++ b/tools/CMakeLists.txt
@@ -0,0 +1,4 @@
+add_executable(tool_telemetry telemetry.c)
+target_include_directories(tool_telemetry PRIVATE "${CMAKE_SOURCE_DIR}")
+target_compile_features(tool_telemetry PRIVATE c_std_23)
+target_link_libraries(tool_telemetry PRIVATE celrs_crsf celrs_serial celrs_logger celrs_log_write)
diff --git a/tools/telemetry.c b/tools/telemetry.c
new file mode 100644
index 0000000..5a22636
--- /dev/null
+++ b/tools/telemetry.c
@@ -0,0 +1,112 @@
+#include "celrs/crsf.h"
+#include "celrs/serial.h"
+#include "celrs/logger.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* TX power index to dBm mapping (ELRS standard) */
+static int const s_tx_power_dbm[] = {
+    0, 20, 26, 30, 32, 34, 36, 38,
+    0, 0, 0, 0, 0, 0, 0, 0
+};
+
+/* Parse link telemetry payload (5 bytes) from CRSF frame type 0x02 */
+static int telemetry_parse_link(int16_t* rssi, uint8_t* link_quality,
+                                int8_t* snr, int* tx_power_dbm,
+                                uint8_t* rssi_rc,
+                                uint8_t const* payload, size_t len) {
+    if (rssi == NULL || payload == NULL) return -1;
+    if (len < 5) return -1;
+
+    *rssi         = (int16_t)payload[0];       /* 0-100% */
+    *link_quality = payload[1];                /* 0-100% */
+    *snr          = (int8_t)payload[2];        /* signed dB */
+    uint8_t power_idx = payload[3];
+    *tx_power_dbm = (power_idx < sizeof(s_tx_power_dbm) / sizeof(s_tx_power_dbm[0]))
+                        ? s_tx_power_dbm[power_idx]
+                        : 0;
+    *rssi_rc      = payload[4];               /* 0-100% */
+
+    return 0;
+}
+
+static void print_usage(char const* prog) {
+    printf("Usage: %s <serial_port> [interval_ms]\n", prog);
+    printf("  serial_port  : COM3 (Windows) or /dev/ttyUSB0 (Linux)\n");
+    printf("  interval_ms  : poll interval in ms (default 200)\n");
+}
+
+int main(int argc, char const* argv[]) {
+    if (argc < 2) {
+        print_usage(argv[0]);
+        return 1;
+    }
+
+    char const* port_path = argv[1];
+    int interval_ms = 200;
+    if (argc >= 3) {
+        interval_ms = atoi(argv[2]);
+        if (interval_ms <= 0) interval_ms = 200;
+    }
+
+    /* Open serial port */
+    cel_serial_port* port = cel_serial_open(port_path, 400000);
+    if (port == NULL) {
+        cel_log_err("Failed to open serial port");
+        return 1;
+    }
+
+    char msg[256];
+    snprintf(msg, sizeof(msg), "Connected to %s (400000 baud)", port_path);
+    cel_log_info(msg);
+
+    /* Send heartbeat to establish CRSF link */
+    uint8_t hb_payload[2] = {CEL_CRSF_ADDRESS_TBS_GROUND_STATION, 0x01};
+    uint8_t hb_frame[256];
+    size_t hb_len = cel_crsf_frame_build(hb_frame, CEL_CRSF_ADDRESS_FC_BROADCAST,
+                                          CEL_CRSF_ADDRESS_TBS_GROUND_STATION,
+                                          CEL_CRSF_FRAMETYPE_HEARTBEAT, hb_payload, 2);
+    cel_serial_write(port, hb_frame, hb_len);
+
+    printf("RX\tLINK\tSNR\tTXP\tRSSI_RC\n");
+
+    /* Read loop */
+    uint8_t buf[256];
+    int frames = 0, errors = 0;
+
+    for (int i = 0; i < 20; i++) { /* read up to 20 telemetry frames */
+        size_t n = cel_serial_read(port, buf, sizeof(buf), interval_ms);
+        if (n == 0) continue;
+
+        cel_crsf_frame frame;
+        if (cel_crsf_frame_parse(&frame, buf, n) != 0) {
+            errors++;
+            continue;
+        }
+
+        if (frame.type != CEL_CRSF_FRAMETYPE_PACKET_LINK_TELEMETRY) {
+            continue; /* skip non-telemetry frames */
+        }
+
+        int16_t rssi;
+        uint8_t link_quality;
+        int8_t snr;
+        int tx_power;
+        uint8_t rssi_rc;
+
+        if (telemetry_parse_link(&rssi, &link_quality, &snr,
+                                 &tx_power, &rssi_rc,
+                                 frame.payload, frame.size) == 0) {
+            frames++;
+            printf("%d\t%d\t%d\t%d\t%d\n",
+                   rssi, link_quality, snr, tx_power, rssi_rc);
+        }
+    }
+
+    snprintf(msg, sizeof(msg), "Frames: %d, Errors: %d", frames, errors);
+    cel_log_info(msg);
+
+    cel_serial_close(port);
+    return 0;
+}