Automating the Generation of Programs Maximizing the Repeatable Constant Switching Activity in Microprocessor Units via MaxSAT

Throughout device testing, one key parameter to be considered is the switching activity (SWA) of the circuit under test (CUT). To avoid unwanted scenarios due to excessive power consumption during test, in most cases the SWA of the CUTs must be retained to a minimal value when the test stimulus is applied. However, there are specific cases where the opposite, namely, the SWA maximization within the CUT, or a certain submodule of it, can be proven beneficial. For example, during dynamic burn-in testing we aim at maximizing the internal stress by applying suitable stimuli. This can be done in a functional manner by following the software-based self-test paradigm. However, generating such suitable programs represents a costly and arduous task for the test engineers. We consider the case where the CUT is a pipelined processor core and we aim to maximize the SWA of certain core submodules. We present a comprehensive methodology based on formal methods, able to automatically generate the best two-instruction stress-inducing sequence for the targeted processor module. The generated stimulus is composed of a short, arbitrarily long repeatable sequence of a pair of assembly instructions, thus, guaranteeing the maximum possible constant SWA. The proposed method was applied to the OpenRISC 1200 and the RI5CY (PULP) processor cores demonstrating its effectiveness when compared to other methods. We show that the time for generating the best repeatable instruction sequence is limited in most cases, while the generated sequence can always achieve a significantly higher repeatable and constant SWA than other solutions.