Software Safety Requirements Specification - Functional Safety (ISO 26262)

Deriving Software Safety Requirements

Requirements Derivation Chain

  Safety Goal [ASIL-D]
  'Avoid unintended AEB activation'
         │
         ▼
  Functional Safety Requirement (FSR) [ASIL-D]
  'FCAB shall only issue AEB request when radar AND camera
   independently confirm object with TTC < 1.5s'
         │
         ▼
  Technical Safety Requirement (TSR) [ASIL-D]
  'AEB decision software module shall cross-validate
   radar object list and camera object list before
   generating AEB_Request signal; if either sensor
   data is missing or invalid, AEB_Request = 0'
         │
         ▼
  Software Safety Requirement (SSR) [ASIL-D]
  'SSR-AEB-001: Function AEB_Decision_Compute() shall
   set output aeb_request = TRUE only when ALL of:
   (a) radar_object_confidence ≥ 0.95,
   (b) camera_object_confidence ≥ 0.95,
   (c) fused_TTC ≤ 1.5s,
   (d) no active sensor fault flags.
   Any other combination: aeb_request = FALSE.'
         │
         ▼
  Software Unit Design
  Unit: AEB_Decision.c → function AEB_Decision_Compute()

SSR Properties for ASIL-D Software

Property	Requirement	Example of Violation	Best Practice
Unambiguous	Only one interpretation possible	'Check sensor data is valid' (what is valid?)	'Sensor data shall be accepted only when: (a) value in [0.3V, 4.7V] AND (b) timeout < 50ms'
Verifiable	Can be tested/inspected objectively	'System shall be reliable'	'System shall detect 95% of sensor faults within FHTI = 10ms'
Traceable	References parent TSR and child test case	Requirement with no ID; no test case	Every SSR has unique ID; linked to TSR above and test below
Complete	All safety-relevant behaviour specified	Missing 'what happens on fault' state	Every SSR specifies nominal AND fault behaviour
Consistent	No contradictions with other SSRs	SSR-001: 'aeb = TRUE when TTC < 1.5s'; SSR-002: 'aeb = TRUE when TTC < 2s'	Use requirements tool (DOORS, Polarion) with conflict detection

Requirements Traceability Matrix

Markdownreq_traceability.md

# Requirements Traceability: AEB Decision Function

| SSR ID      | SSR Text (abbreviated)              | Parent TSR   | Test Case(s)          | Status  |
|-------------|-------------------------------------|--------------|-----------------------|---------|
| SSR-AEB-001 | aeb_request = TRUE only when all conditions met | TSR-AEB-003 | TC-AEB-001, TC-AEB-002, TC-AEB-003 | TESTED ✓ |
| SSR-AEB-002 | aeb_request = FALSE when radar timeout > 50ms   | TSR-AEB-004 | TC-AEB-010            | TESTED ✓ |
| SSR-AEB-003 | aeb_request = FALSE when camera fault active    | TSR-AEB-004 | TC-AEB-011            | TESTED ✓ |
| SSR-AEB-004 | AEB_Decision_Compute() shall complete in ≤ 1ms  | TSR-AEB-005 | TC-AEB-020 (timing)   | TESTED ✓ |
| SSR-AEB-005 | Function shall not use global variables         | TSR-AEB-006 | Code review CR-AEB-001| REVIEWED ✓ |
| SSR-AEB-006 | MISRA C:2012 Rule 14.3 compliance               | TSR-AEB-006 | Static analysis SA-001| ANALYSED ✓ |

## Coverage Metrics
- Total SSRs: 47
- SSRs with test case(s): 45 (96%)
- SSRs covered by review: 2 (non-testable: coding guidelines)
- Open gaps: 0
- Test pass rate: 45/45 (100%)

Summary

Software safety requirements are the software team's primary contract with the safety case. Every SSR must be verifiable (testable or reviewable), and the traceability matrix must show bidirectional links: from safety goal down to test case (forward), and from test case back up to safety goal (backward). An SSR that has no test case and no review record is an open safety gap — the assessor will identify it immediately. Requirements management tools (DOORS, Polarion, Jama) enforce these traces automatically and generate coverage reports; using a spreadsheet for ASIL-D requirements management is technically possible but creates significant manual overhead and is prone to gaps.

🔬 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.