Untestable faults in DFT

Faults list in design are categorized into sub categories. Faults class are mainly divided into

  • Testable(TE)–> Faults can be tested by some patterns.
  • Untestable(UT)–> Faults foe which no pattern exits to detect the faults

Untestable Faults: Are the faults for which no pattern exit to either detect or possible detect them. These faults cannot cause any functional failures. And so the tools excludes them while calculating the test coverage. Types of Untestable faults are

  • Unused (UU)
  • Tied (TI)
  • Blocked(BL)
  • Redundant Faults (RE)
  • Unused (UU)
    • Any floating pins not used in the design come under UU faults
    • The unused faults class includes all the faults on circuit unconnected to any observation point
  • Tied (TI)
    • This faults includes faults on gates where the point of the faults is tied to a value identical to the fault stuck value
  • Blocked (BL)
    • Due to tied logic in the design few faults are blocked and these are categories into Blocked faults. By adding the observable test point we can increase the coverage report.
  • Redundant (RE)
    • The faults which are undetectable by the tool by any pattern , are classified as redundant faults