TDD for Embedded C: Project Layout + Unity & CMock (practical)

Keep tests separate from production, wire up a tiny runner, and mock hardware at the header boundary. Here’s the minimal, repeatable setup that works with CubeIDE/MDK/IAR or Make.

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📁 Project anatomy (add, don’t tangle)

/Core/Src            -> app code (e.g., main.c, modules)
/Drivers             -> HAL/CMSIS or your drivers
/Build, /Debug       -> IDE artifacts (auto-generated)
/TDD                 -> ✅ tests live here (excluded from production build)
  /tests             -> test_*.c (Unity tests)
  /unity             -> Unity sources (vendor drop-in)
  /cmock             -> CMock sources + generated mocks
  /mocks             -> <auto-generated> mock_*.c/.h

👉 In CubeIDE: right-click TDD → Resource Configurations → Exclude from Build… (all configs).

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✅ Unity: tiny test + runner

// TDD/tests/test_my_module.c
#include "unity.h"
#include "my_module.h"

void setUp(void) {}
void tearDown(void) {}

void test_get_value_returns_5(void) {
    TEST_ASSERT_EQUAL_UINT8(5, get_value());
}

// TDD/tests/test_runner.c
#include "unity.h"
extern void test_get_value_returns_5(void);
int main(void){
    UNITY_BEGIN();
    RUN_TEST(test_get_value_returns_5);
    return UNITY_END();
}

Build a test target that compiles: unity/*.c, tests/*.c, and module(s) under test.
(Optionally -DUNIT_TEST for test-only code paths.)

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🔌 CMock: mock the hardware boundary

// Drivers/driver.h  (production header to be mocked)
uint8_t driver_get_byte(void);

// TDD/tests/test_sensor.c
#include "unity.h"
#include "mock_driver.h"     // generated by CMock from driver.h
#include "sensor.h"          // module under test

void test_sensor_reads_driver_value(void){
    driver_get_byte_ExpectAndReturn(0x2A);
    TEST_ASSERT_EQUAL_UINT8(0x2A, sensor_read());
}

# TDD/cmock.yml  (example CMock config)
:mock_path: "TDD/mocks"
:plugins: [expect, ignore, return_thru_ptr]
:enforce_strict_ordering: true

Run CMock over headers → emits mock_driver.* into /TDD/mocks. Compile tests against mocks, not real drivers.

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🧪 Build wiring (Make-ish sketch)

TEST_SRCS := TDD/tests/*.c TDD/unity/*.c TDD/mocks/*.c Core/Src/sensor.c
PROD_SRCS := $(filter-out TDD/%,$(wildcard Core/Src/*.c Drivers/**/*.c))

# test target links TEST_SRCS; production target links PROD_SRCS only

In IDEs, make a Test build config that includes TDD/… and excludes hardware files you’re mocking.

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⚡ Pro Tips

• Keep tests fast & pure: no HAL clocks/ISR—mock them.

• One behavior per test; readable names: test_<unit>_<behavior>_<expectation>.

• Generate mocks from headers you own (stable API).

• Gate test-only code with #ifdef UNIT_TEST.

• Failures first: write the test, see it fail, make it pass, refactor.

🎯 Conclusion

Separate tests, add Unity runner, mock hardware with CMock—now your embedded code is testable without boards, fast, and refactor-friendly.

Written By: Musaab Taha

This article was improved with the assistance of AI.