TOPIC : Fault Redundancy

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TOPIC Fault Redundancy
UNIT 2 Fault modeling
Module 2.3 Fault redundancy and Fault collapsing
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Redundancy

• A combinational circuit with an undetectable
  fault can be simplified by removing at least a
  gate or that gate input.
• For example, suppose if a stuck-at-1 fault at an
  input of n-input AND gate is undetectable, then
  that input can be removed. The circuit now
  reduces to (n-1) input AND gate.
• Similarly if a stuck-at-0 fault at an input of
  AND gate is undetectable, the gate can be
  replaced with a constant signal 0.

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Contd

• A combinational circuit with an undetectable
  fault can be redundant.
• Redundancy is also possible, when a set of K
  lines are cut and MltK lines connect such that the
  output response function does not change.
• A combinational circuit in which all the faults
  are detectable is said to be irredundant circuit.

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Triple Modular Redundancy(TMR)

• It is a fault tolerant design.
• Input is given to all the three systems and their
  outputs are given to the majority voter circuit M
  whose output response is the primary output.
• If any one of the three systems is faulty, the
  other two systems can mask it by using majority
  voter circuit M.

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Need of un-wanted gates

• Output ab bc ac, which can be simplified
  to abac, which implies gate Y can be removed.
• If gate Y is removed, there will be a hazard when
  the input changes from 111 to 011, for an
  instance 0-pulse will appear.
• Note The fault n stuck-at-0 is undetectable.