An Observability-based Code Coverage Metric for Functional Simulation

Abstract — Functional simulation is the most widely used method for design verification. At various levels of abstrac-tion, e.g., behavioral, register-transfer level and gate level, the designer simulates the design using a large number of vectors attempting to debug and verify the design. A major problem with functional simulation is the lack of good metrics and tools to evaluate the quality of a set of functional vectors. Metrics used currently are based on instruction counts and are quite simplis-tic. Designers are forced to use ad-hoc methods to terminate functional simulation, e.g., CPU time limitations. We propose a new metric for measuring the extent of design verification provided by a set of functional simulation vectors. This metric is universal, and can be used uniformly for all de-signs. Our metric computes observability information to deter-mine whether effects of errors that are activated by the program stimuli can be observed at the circuit outputs. We provide preliminary experimental evidence that supports the validity of the proposed metric. We believe that using this metric in design verification will result in higher-quality func-tional tests and improved correctness checking. I