On testing the path delay faults of a microprocessor using its instruction set

This paper addresses the problem of testing path delay faults in a microprocessor using instructions. It is observed that a structurally testable path (i.e., a path testable through at-speed scan) in a microprocessor might not be testable by its instructions simply because no instruction sequence can produce the desired test sequence which can sensitize the paths and capture the fault effect into the destination output/flip-flop at-speed. These paths are called functionally untestable paths. We discuss the impact of delay defects on the functionally untestable paths on the overall circuit performance and illustrate that they do not need to be tested if the delay defect does not cause the path delay to exceed twice the clock period. Identification of such paths helps determine the achievable path delay fault coverage and reduce the subsequent test generation effort. The experimental results for two microprocessors (Parwan and DLX) indicate that a significant percentage of structurally testable paths are functionally untestable and thus need not be tested