Fault detection by output response comparison of identical circuits using half-frequency compatible sequences

When multiple copies of the same functional unit are available in a design, fault detection can be achieved by comparing the output responses of two copies (or two identical circuits). This obviates the need for storing test responses or for computing signatures of test responses. We propose the testing of identical circuits using a deterministic test sequence by running the circuits at different speeds. This allows the detection of delay faults even if the two copies are affected by similar faults. The proposed method is particularly suitable in applications where functional sequences are used to bin products or to detect delay faults. We consider the case where one of the copies is run at the full circuit speed while the other copy is run at half the full frequency. To allow output response comparison under these conditions, we describe a procedure for designing an input sequence that will produce identical output vectors from the two copies every second time unit, assuming that both copies are fault free. We show that the existence of such input sequences depends on the initial state. Moreover, it is advantageous to start the two copies from different initial states. Experimental results show that very high fault coverages can be achieved by using such sequences.