DELAY TESTING OF DIGITAL CIRCUITS BY OUTPUT WAVEFORM ANALYSIS

A new method for delay fault testing of digital circuits is presented. Unlike catastrophic failures that simply have incorrect steady-state logic values at the circuit outputs, delay faults change the shape of the output waveforms by moving the signal transitions in time. Therefore, since the output waveforms contain information about the circuit delays, instead of only latching the outputs at the sampling time, the output waveforms between samples are analyzed as well. Two classes of output waveform analysis are discussed. In the first technique, the output waveform is observed for any changes after the sampling time, since in a fault-free circuit, the outputs are expected to have stabilized at the desired logic values. In the second technique, information is extracted from the faulty and fault-free waveforms before the sampling time, and compared for any differences. Circuits for the waveform analyzers are presented to show that the method is feasible, and experimental results are given.