Finding reset nondeterminism in RTL designs - scalable X-analysis methodology and case study

Due to increases in design complexity, routing a reset signal to all registers is becoming more difficult. One way to solve this problem is to reset only certain registers and rely on a software initialization sequence to reset other registers. This approach, however, may allow unknown values (also called X-values) in uninitialized registers to leak to other registers, leaving the design in a nondeterministic state. Although logic simulation can find some X-problems, it is not accurate and may miss bugs. A recent approach based on symbolic simulation can handle Xs accurately; however, it is not scalable. In this work we analyze the characteristics of X-problems and propose a methodology that leverages the accuracy of formal X-analysis and can scale to large designs. This is achieved by our novel partitioning techniques and the intelligent use of waveforms as stimulus. We applied our methodology to an industrial design and successfully identified several Xs unknown to the designers, including three real bugs, demonstrating the effectiveness of our approach.