Toggle: A Coverage-Guided Random Stimulus Generator

Despite the increasing research effort in formal verification, constraintbased random simulation remains an integral part of design validation, especially for large design components where formal techniques do not scale. However, effective simulation often requires the construction of complex constraints to stimulate important aspects of the design. In this paper we present Toggle, a novel solution which automatically identifies those regions of the design which are not sufficiently exercised in random simulation. Toggle then generates legal random stimulus of the primary inputs of the design to improve coverage over those regions, by augmenting the toggling activity of the signals internal to the region. In addition, Toggle can also be used to toggle a set of user-specified signals anywhere in the design. Experimental results indicate that Toggle can stimulate regions of a design in much fewer simulation cycles than random simulation, leading to simulation runs which can potentially expose bugs sooner. 1