Validation of RTL descriptions remains one of the principal bottlenecks in the circuit design process. Random simulation based methods for functional validation suffer from fundamental limitations and may be inappropriate or too expensive. In fact, for some circuits, a large number of vectors is required in order to make the circuit reach hard to test constructs and obtain accurate values for their testability. In this work, we present a static, non-simulation based, method for the determination of the controllability of RTL constructs that is efficient and gives accurate feedback to the designers in what regards the presence of hard to control constructs in their RTL code. The method takes as input a Verilog RTL description, solves the Chapman-Kolmogorov equations that describe the steady-state of the circuit and outputs the computed values for the controllability of the RTL constructs. To avoid the exponential blow-up that results from writing one equation for each circuit state and solving the resulting system of equations, an approximation method is used. We present results showing that the approximation is effective and describe how the method can be used to bias a random test generator in order to achieve higher coverage using a smaller number of vectors. 1
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