SW Critical Path & Dependent Failure Analysis
Find the software paths that can violate a safety goal, rate component criticality, expose the couplings that defeat independence, and spend verification effort where it changes the outcome.
- Chapters
- 7
- Chapters
- Figures and interactive visuals
- 6
- Figures and interactive visuals
- Worked examples
- 2
- Worked examples
Quick answers
- What is software critical path analysis in functional safety?
- Software critical path analysis identifies the paths through which a software fault can lead to a safety goal violation. It traces safety-related inputs through processing to safety-critical outputs using control flow, data flow and call relationships, then classifies components and signals by criticality. The result shows where safety mechanisms, reviews and tests have the most effect.
- What is dependent failure analysis (DFA) in ISO 26262?
- DFA checks whether elements that are assumed to fail independently actually do. Redundancy, monitoring and ASIL decomposition all rest on independence, and common cause or cascading failures can quietly void it. ISO 26262-9, Clause 7 covers the analysis of dependent failures: the architecture is examined for initiators and coupling factors, and each one is shown to be absent or controlled by a safety measure.
Why it pays for itself
Verification effort where it matters
Rating components and signals by criticality lets you concentrate reviews, tests and safety mechanisms on the paths that can violate a safety goal, instead of spreading effort evenly across code that cannot.
Common causes made visible
Coupling factor classes systematically expose the shared power, buses, memory, teams and tools that quietly defeat redundancy, before one common cause takes out both channels.
Templates, not blank pages
Templates, tool recommendations and checklists for both analyses mean your first analysis starts from structure instead of an empty spreadsheet.
What you’ll be able to do
Trace critical paths
Follow safety-related signals from input to output and name the paths that can violate a safety goal.
Rate component criticality
Classify software components and use the rating to prioritize reviews and tests.
Find couplings early
Expose shared resources, inputs and development factors that defeat independence.
Support decomposition
Use DFA as the evidence that decomposed elements are sufficiently independent.
Place mechanisms deliberately
Put safety mechanisms on rated critical paths with a documented rationale.
Chapter by chapter
- 01
Overview
See what critical path analysis and dependent failure analysis each answer, and when in a project to perform them.
- Critical path analysis
- Dependent failure analysis
- When to perform them
- 02
Critical Path Analysis
Trace safety-related signals through the software step by step and classify components and signals by criticality.
- Step-by-step process
- Signal path tracer
- Criticality classes
- 03
Dependent Failure Analysis
Apply coupling factor classes to find dependent failures and see how memory partitioning and E2E protection contain them.
- Coupling factor classes
- Common cause versus cascading
- MPU and E2E mitigation
- 04
Methodology
Understand why DFA is the precondition for ASIL decomposition and follow the methodology for both analyses.
- Decomposition independence gate
- CPA methodology
- Safety goal tracing
- 05
Templates & Tools
Start from structured templates, recommended tools and checklists for both analyses.
- CPA templates
- DFA templates
- Analysis checklists
- 06
Examples
Apply both analyses to a steer-by-wire system and a brake-by-wire system, including a fault-handling timing budget.
- Steer-by-wire CPA
- Brake-by-wire DFA
- Timing budget
- 07
Best Practices
Adopt practices for both analyses, connect them to ISO 26262 expectations and measure analysis quality.
- CPA best practices
- DFA best practices
- Example KPIs
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Safety-signal path tracer
Follow a signal through the software and classify its criticality.
Common cause versus cascading
How one coupling defeats redundancy in different ways.
Memory partition with MPU
A wild write trapped before it corrupts another component.
E2E frame protection
Protection of messages on a shared bus.
Steer-by-wire timing budget
Fault handling against the fault tolerant time interval.
Steer-by-wire paths and brake-by-wire couplings
Two systems show the analyses in practice: critical paths in a steer-by-wire architecture and dependent failure couplings in a brake-by-wire design.
- Critical paths for normal operation, failure detection and fallback
- Coupling factors in a brake-by-wire design
- A steer-by-wire fault-handling timing budget
- Mitigations for the couplings found
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Who this guide is for
- Software architects who must show independence between components of different ASILs
- Verification leads deciding where review and test budget actually reduces risk
- Safety engineers performing dependent failure analysis on redundant architectures
- Developers of fail-operational systems such as brake-by-wire and steer-by-wire
Frequently Asked Questions
Common questions about SW Critical Path & Dependent Failure Analysis
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Where this topic leads next.
- SoftwareAgile Development Methods in ISO 26262Learn how to integrate Scrum and SAFe with ISO 26262 without compromising safety evidence or audit readiness. Covers safety backlogs, ASIL-aware definition of done, sprint-level traceability, and automated verification gates.
- SoftwareISO 26262 and Automotive SPICEOEMs demand an Automotive SPICE capability level and ISO 26262 compliance in the same statement of work, and new teams treat them as two mountains to climb. This concept shows they are one mountain seen from two sides: a shared V-model, one traceability backbone and a single set of work products.
- Safety analysesSafety AnalysesLearn how HARA, FMEA, FTA, STPA, DFA, and HAZOP complement each other across the ISO 26262 lifecycle. Covers method selection criteria, analysis sequencing, and a guide to choosing the right technique for each risk.