Title: FTTI vs FHTI: Safety-Related Timing | ISO 26262 Academy URL: https://iso26262.academy/features/concepts/safety-related-timing Description: ISO 26262 fault-handling time intervals (FTTI, FDTI, FRTI, FHTI): how test rates set detection latency and how to budget detection and reaction time. --- Concept guide · ISO 26262-1 §3 · 10 chapters # Safety-Related Timing Learn to treat time as a safety property: the fault handling intervals, diagnostic test intervals, timing budgets, safe-state transitions, allocation to hardware and software, and verification evidence. 10 Chapters 2 Worked examples Included in [Expert](https://iso26262.academy/pricing#plan-advanced) [Preview free](https://iso26262.academy/register?plan=free&from=%2Fconcepts%2Fsafety-related-timing&utm_source=website&utm_medium=cta&utm_campaign=features_concepts) See the chapters Inside the course 1. 01 **Time Is a Safety Property** 2. 02 **FTTI, FHTI, FDTI** 3. 03 **Diagnostic Test Interval** 4. 04 **The Timing Budget** 5. 05 **Safe-State Transition** The short version ## Quick answers What is the fault tolerant time interval (FTTI) in ISO 26262? The fault tolerant time interval (ISO 26262-1:2018, 3.61) answers one question: once a fault appears in the item, what is the shortest time before a hazardous event could follow if nothing steps in? It is the hard deadline handed down by the hazard analysis, and whatever detection and reaction your system performs must complete inside it. FTTI is a property of the vehicle-level hazard, not of your design: you cannot negotiate it, only budget against it. What is the difference between FTTI and FHTI? FTTI is the deadline; FHTI is your performance against it. The fault handling time interval (FHTI) is the sum of the fault detection time interval (FDTI, from fault occurrence to detection) and the fault reaction time interval (FRTI, from detection to reaching the safe state). Safety requires FDTI plus FRTI to stay below FTTI, and in practice with deliberate margin: setting FHTI equal to FTTI leaves no tolerance for jitter and worst-case stacking. Keep going, free - [Article · 6 min read FTTI Is a Budget, Not a Number](https://iso26262.academy/blog/ftti-is-a-budget-not-a-number) - [Demo exam · no account Test yourself with 7 exam-style questions](https://iso26262.academy/demo-exam) Why this course · ISO 26262, Part 1, Clause 3 ## Why it pays for itself ### Untangle FTTI, FDTI, FRTI, FHTI Learn what each fault handling interval means, which one the hazard fixes and which ones your design sets, and the inequality that ties them together. ### Build a defensible timing budget Work a brake-by-wire budget end to end: reduce the interval to a working budget, allocate detection and reaction, and hold explicit margin for jitter and worst-case stacking. ### Verify timing with evidence Go beyond claiming the budget holds: use fault injection to measure real detection latency, WCET analysis for the reaction path, and assemble a timing evidence package the safety case can rely on. After the course ## What you’ll be able to do ### Use the timing intervals correctly Separate the intervals fixed by the hazard from those set by your design. ### Build a timing budget Allocate detection and reaction time against the fault tolerant time interval with explicit margin. ### Specify diagnostic test intervals Set test rates that fit both the timing budget and latent fault coverage. ### Allocate across hardware and software Split timing at the HSI and account for scheduling and execution time. ### Verify timing with evidence Back budget claims with fault injection, WCET analysis and a timing evidence package. The curriculum · 10 chapters ## Chapter by chapter 1. 01 **Time Is a Safety Property** See why detection takes time, why timing is so often under-budgeted, and where timing appears across ISO 26262. - Detection takes time - The timing chain - Where timing appears 2. 02 **FTTI, FHTI, FDTI** Put the fault handling intervals on one fault-to-safe-state timeline, clear up common confusions, and see why margin is essential. - Interval definitions - Fault timeline - Why margin matters 3. 03 **Diagnostic Test Interval** Define the diagnostic test interval, relate it to the FTTI and to latent fault detection, and specify it in requirements. - DTI defined - DTI and FTTI - Specifying DTI 4. 04 **The Timing Budget** Set up the core timing inequality, work through a brake-by-wire budget, trade detection against reaction time, and document the result. - Core inequality - Brake-by-wire example - Budget trade-offs 5. 05 **Safe-State Transition** Define safe states, break down the reaction path, handle emergency operation and design degraded modes. - Safe state definition - Reaction path - Emergency operation 6. 06 **Multi-Point Detection** Separate single-point from multi-point timing, choose test rates for latent faults and connect them to FMEDA results. - Latent fault metric - Test rates - Link to the FMEDA 7. 07 **Worked Timing Diagram** Read an annotated fault timeline event by event and draw the conclusions a reviewer would draw. - Scenario setup - Annotated timeline - Key takeaways 8. 08 **Allocating Timing to HW/SW** Split detection between hardware and software, account for scheduling and WCET, and document timing at the HSI. - Hardware versus software detection - Scheduling and WCET - Timing at the HSI 9. 09 **Verifying Timing** Verify timing claims with fault injection, WCET analysis and system-level tools, then build the timing evidence package. - Fault injection - WCET analysis - Evidence package 10. 10 **Pitfalls** Review the timing pitfalls that recur in projects and check a timing budget before it goes to review. - Timing pitfalls - Budget review - Common mistakes Read all 10 chapters with Expert. Start with a free account: the complete Functional Safety Concept guide and 5 practice exams a month, then upgrade when you need this guide. [Expert](https://iso26262.academy/pricing#plan-advanced) [Preview free](https://iso26262.academy/register?plan=free&from=%2Fconcepts%2Fsafety-related-timing&utm_source=website&utm_medium=cta&utm_campaign=features_concepts) [Try the 7-question demo exam](https://iso26262.academy/demo-exam) Worked Example ## Brake-by-Wire Timing Budget Chapter 4 builds a timing budget for a fictional brake-by-wire pressure control, and Chapter 7 reads a full fault timeline event by event. - The core inequality the budget has to satisfy - Allocation of detection and reaction time - Trade-offs between faster detection and faster reaction - How the budget is documented for review Brake-by-wire budget Function: pressure control of a fictional brake-by-wire system Unlock in course Built for ## Who this guide is for - System engineers deriving timing requirements from safety goals and safe states - Software and hardware leads splitting a fault-handling budget across the HSI - Diagnostics engineers specifying diagnostic test intervals for latent fault coverage - Anyone whose safety concept says "within FTTI" without a number behind it ## Frequently Asked Questions Common questions about Safety-Related Timing The diagnostic test interval is the period between successive executions of a periodic diagnostic. It sets a floor on detection latency: a fault occurring just after a test runs waits nearly a full DTI before the next chance of detection, so worst-case FDTI is roughly the DTI plus the test's own execution time. That makes DTI a requirement, not an implementation detail: it must be specified so the resulting FDTI fits the timing budget, and for latent faults the test rate ties directly into the latent fault coverage the FMEDA claims. Start from the FTTI given by the hazard analysis and reserve margin before allocating anything. Derive the detection side from the diagnostic test interval and the execution time of the check, and the reaction side from interrupt latency, task switching, command output and actuator settling. Their sum must fit inside the working budget with the margin intact. Then verify the numbers with fault injection and WCET analysis instead of trusting the spreadsheet. The course works this through for a fictional brake-by-wire pressure control. The guide covers time as a safety property in 10 chapters: the fault handling intervals and the diagnostic test interval, the timing budget, safe-state transitions and emergency operation, timing for multi-point faults, a worked timing diagram, allocation to hardware and software, verification and pitfalls. A brake-by-wire budget serves as the running example. A free account starts you off, and the Pro and Expert plans unlock more of the library. ## Stuck on this in your project? Send one question to a functional safety engineer. You get a written answer with the clause references. From **$290** with a written quote before any charge [See how we can help](https://iso26262.academy/features/consulting?service=question#sample) ## Start the course today A free account unlocks one full concept guide, 3 work product templates, 1 guided process, the Markov and RBD simulators, and 5 practice exams per month. The Pro and Expert plans unlock more of the 93-guide library. No credit card required. [Preview free](https://iso26262.academy/register?plan=free&from=%2Fconcepts%2Fsafety-related-timing&utm_source=website&utm_medium=cta&utm_campaign=features_concepts) [View Pricing Plans](https://iso26262.academy/pricing) Continue with ## Where this topic leads next. [Browse all 93 guides](https://iso26262.academy/features/concepts#library) - Concept phase [Safety Design Decisions Across Vehicle Domains](https://iso26262.academy/features/concepts/domain-safety-design-decisions) Nine automotive domains walk the exact same ISO 26262 Part 3-6 chain (item definition, HARA, FSC, TSC, HSI, hardware and software), yet arrive at radically different technical answers. See why a traction inverter, a brake ECU and a cockpit display controller end up nothing alike. [Expert](https://iso26262.academy/pricing#plan-advanced) 11 chapters - System level [Technical Safety Concept](https://iso26262.academy/features/concepts/technical-safety-concept) Learn how to translate safety goals and FSC into technical safety requirements (TSRs), system architecture, and HW/SW allocation. Covers safety mechanisms, diagnostic coverage, FMEA integration, and a complete worked example with templates. [Expert](https://iso26262.academy/pricing#plan-advanced) 17 chapters - Concept phase [Item Definition & Concept-Phase Initiation](https://iso26262.academy/features/concepts/item-definition-concept-phase) Get the first ISO 26262-3 step right: define the item boundary, interfaces, operating modes, and assumptions of use that HARA, safety goals, and the whole safety case inherit from the concept phase. [Expert](https://iso26262.academy/pricing#plan-advanced) 10 chapters