Software Unit Testing & Integration Testing - Functional Safety (ISO 26262)

Structural Coverage Requirements by ASIL

Coverage Criterion	Definition	ASIL-A	ASIL-B	ASIL-C	ASIL-D
Statement coverage	Every statement executed at least once	Rec	Rec	—	—
Branch coverage	Every branch (true/false) executed	—	Rec	Rec	—
MC/DC coverage	Modified Condition/Decision Coverage	—	—	Rec	Required

MC/DC: Modified Condition/Decision Coverage

MC/DC Explained with Example

  Function: should_brake(bool radar_ok, bool camera_ok, float ttc)
  Decision:  if (radar_ok && camera_ok && ttc < 1.5f)

  MC/DC requires: for each CONDITION in the decision,
  demonstrate that it INDEPENDENTLY affects the DECISION outcome
  (while keeping all other conditions fixed)

  For radar_ok (C1), camera_ok (C2), ttc<1.5 (C3):

  Test  C1     C2     C3      Decision   Purpose
  ──────────────────────────────────────────────────────────────────
  T1    TRUE   TRUE   TRUE    TRUE   ←─┐ C1 independence pair with T2
  T2    FALSE  TRUE   TRUE    FALSE  ←─┘ (C1 changed; others fixed; decision changes)
  T3    TRUE   FALSE  TRUE    FALSE     C2 independence: T1(TRUE) vs T3(FALSE)
  T4    TRUE   TRUE   FALSE   FALSE     C3 independence: T1(TRUE) vs T4(FALSE)

  MC/DC = 4 test cases (not 2^3 = 8 for full branch coverage)
  Much more efficient than full combinatorial coverage for complex conditions

  ASIL-D requirement: MC/DC for all safety-relevant decisions
  Tool support: VectorCAST, LDRA, Cantata, Tessy (measure MC/DC automatically)

  MC/DC coverage gaps indicate untested safety-relevant paths:
  a gap in C3 independence means 'we never tested that ttc≥1.5 keeps AEB inactive'
  → this is a safety coverage gap for the AEB activation function

Unit Test Example: AEB Decision Function

Ctest_aeb_decision.c

/* Unit test: AEB_Decision_Compute() with MC/DC coverage */
#include "unity.h"   /* Unity test framework */
#include "AEB_Decision.h"

/* Test fixtures */
static AEB_Inputs_t g_inputs;
static AEB_Outputs_t g_outputs;

void setUp(void) {
    memset(&g_inputs, 0, sizeof(g_inputs));
    memset(&g_outputs, 0, sizeof(g_outputs));
}

/* MC/DC test set for the primary decision condition */

/* T1: All conditions TRUE → AEB request TRUE */
void test_aeb_all_conditions_met(void) {
    g_inputs.radar_conf  = 0.97f;   /* > 0.95 */
    g_inputs.camera_conf = 0.96f;   /* > 0.95 */
    g_inputs.fused_ttc   = 1.2f;    /* < 1.5s */
    g_inputs.fault_active= FALSE;
    AEB_Decision_Compute(&g_inputs, &g_outputs);
    TEST_ASSERT_TRUE(g_outputs.aeb_request);
}

/* T2: radar_conf below threshold → AEB FALSE (C1 independence) */
void test_aeb_radar_conf_low(void) {
    g_inputs.radar_conf  = 0.80f;   /* ≤ 0.95 */
    g_inputs.camera_conf = 0.96f;
    g_inputs.fused_ttc   = 1.2f;
    g_inputs.fault_active= FALSE;
    AEB_Decision_Compute(&g_inputs, &g_outputs);
    TEST_ASSERT_FALSE(g_outputs.aeb_request);
}

/* T3: camera_conf below threshold → AEB FALSE (C2 independence) */
void test_aeb_camera_conf_low(void) {
    g_inputs.radar_conf  = 0.97f;
    g_inputs.camera_conf = 0.80f;   /* ≤ 0.95 */
    g_inputs.fused_ttc   = 1.2f;
    g_inputs.fault_active= FALSE;
    AEB_Decision_Compute(&g_inputs, &g_outputs);
    TEST_ASSERT_FALSE(g_outputs.aeb_request);
}

/* T4: TTC above threshold → AEB FALSE (C3 independence) */
void test_aeb_ttc_safe(void) {
    g_inputs.radar_conf  = 0.97f;
    g_inputs.camera_conf = 0.96f;
    g_inputs.fused_ttc   = 2.0f;    /* ≥ 1.5s */
    g_inputs.fault_active= FALSE;
    AEB_Decision_Compute(&g_inputs, &g_outputs);
    TEST_ASSERT_FALSE(g_outputs.aeb_request);
}

/* T5: Fault active → AEB FALSE regardless of conditions */
void test_aeb_fault_active(void) {
    g_inputs.radar_conf  = 0.97f;
    g_inputs.camera_conf = 0.96f;
    g_inputs.fused_ttc   = 1.2f;
    g_inputs.fault_active= TRUE;    /* fault flag set */
    AEB_Decision_Compute(&g_inputs, &g_outputs);
    TEST_ASSERT_FALSE(g_outputs.aeb_request);  /* safety: no AEB when fault active */
}

int main(void) {
    UNITY_BEGIN();
    RUN_TEST(test_aeb_all_conditions_met);
    RUN_TEST(test_aeb_radar_conf_low);
    RUN_TEST(test_aeb_camera_conf_low);
    RUN_TEST(test_aeb_ttc_safe);
    RUN_TEST(test_aeb_fault_active);
    return UNITY_END();
}

Summary

MC/DC is the most rigorous structural coverage criterion required by ISO 26262 for ASIL-D software. It ensures that every condition in a compound decision expression has been demonstrated to independently affect the decision outcome — a weaker criterion like branch coverage can achieve 100% without ever testing some conditions independently. The MC/DC test set for the AEB decision function (T1–T4) shows that only 4 test cases are needed to achieve MC/DC for a three-condition compound decision, versus 8 test cases for exhaustive combinatorial coverage. Coverage measurement tools (VectorCAST, LDRA) instrument the code and report MC/DC coverage automatically during test execution.

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.