Finite state machine synthesis with fault tolerant test function

Abstract

According to a recent synthesis for testability proposal, a test function specified as a finite state machine with the same number of state variables as the given object machine, is incorporated into the state diagram prior to synthesis. Since a complete verification of the test machine is not practical, an often used heuristic sets and observes each state variable. The two machines share logic and a fault can result in partial or total loss of the test function. We show that the tests generated under the assumption that the entire test function is intact can become invalid. We propose a new method of synthesizing PLA-based finite state machines with fault tolerant test machines. Our approach eliminates testing of the test function. A constrained logic minimization phase insures that faults have predictable effect on the state diagram of the composite machine (object machine embedded with the test function). This allows effective use of the test function during test generation even in the presence of faults that effect both object and test machines. Only a combinational test generator is required for test generation. Each combinational vector is augmented by appropriate initialization and propagation sequences. Unlike prior approaches, ourO(log₂ n) length test sequence isguaranteed to detect any targeted crosspoint fault. Experimental results on the MCNC Logic Synthesis Workshop finite state machine benchmark set are given as evidence of practicality of the proposed approach.