DFT and FIT Contribution
Introduction
DFT mechanisms directly influence the FIT rate by detecting structural faults before they become dangerous. Their contribution is essential for quantifying residual risk and supporting the safety case.
Why FIT Contribution Matters
- Required for PMHF (Probabilistic Metric for Hardware Failure) calculation
- Provides quantitative evidence for the safety case
- Influences ASIL decomposition strategies
- Determines diagnostic coverage and residual fault rates
Key Concepts
Mapping Fault Coverage to FIT Reduction
- Fault coverage reduces the population of dangerous faults
- Higher coverage → lower residual FIT
- DFT results feed directly into FMEDA calculations
Startup vs Runtime Detection
- Startup tests (LBIST, MBIST) detect latent faults before operation
- Runtime tests (monitors, periodic LBIST) detect faults during mission time
- Combined detection reduces the overall dangerous failure rate
Residual Fault Analysis
- Residual faults = faults not detected by any safety mechanism
- These faults contribute directly to the dangerous FIT
- DFT determines how many faults remain after applying all tests
Diagnostic Coverage Metrics
- DC = Detected / (Detected + Undetected)
- Higher DC reduces the residual FIT
- DC must be justified with DFT evidence (ATPG, LBIST, MBIST, monitors)
Formulas, examples
Figure 1: DFT → FIT Contribution
DFT mechanisms reduce the dangerous FIT rate by detecting structural faults before they can affect safety‑related functions. Raw FIT represents the intrinsic defect rate of the block, while ATPG, LBIST, MBIST, and runtime monitors remove a portion of these faults through their diagnostic coverage. The remaining undetected faults form the residual FIT, which is used in PMHF calculations and contributes directly to the safety case and ASIL justification.
Figure 2: FIT Reduction Example
DFT mechanisms reduce the dangerous FIT rate by removing a portion of the intrinsic faults through their diagnostic coverage. Each mechanism contributes to the overall fault coverage, and the remaining undetected faults form the residual FIT. This residual FIT is used in the PMHF calculation and directly influences the safety case and ASIL justification.
Figure 3:
Residual faults are the faults that remain undetected after all DFT mechanisms have been applied. The fault space is reduced by ATPG, LBIST, MBIST, and runtime monitors, but any undetected faults contribute directly to the residual FIT. These residual faults are used in the PMHF calculation and form the basis for the safety case and ASIL justification.
