Delay Fault Coverage and Performance Tradeoffs

The main disadvantage of the path delay fault model is that to achieve 100% testability every path must be tested. Since the number of paths is usually exponential in circuit size, all known analysis and synthesis techniques for 100% path delay fault testability are infeasible on most circuits. In this paper, we show that 100% delay fault testability is not necessary to guarantee the speed of a combinational circuit. There exist path delay faults which can never impact the circuit delay (computed using any correct timing analysis method) unless some other path delay faults also affect it; hence these delay faults need not be considered in delay fault testing. Next, assuming only the existence of robust delay fault tests for a very small set of paths, we show how the circuit speed can be selected such that 100% robust delay fault coverage is achieved.