1. Apparatus for defining an input state vector for application to the circuit inputs of a digital circuit to achieve low power when said digital circuit is in an idle state, the apparatus comprising: an input state set defining module operable to perform independent determinations defining respective sets of the input states of said input state vector; and a conflict resolving module operable to resolve any conflict in the definitions of any one or more of said input states in favor of a definition of said one or more of said input states that achieves the lowest power consumption when said input state vector comprising said one or more of said input states in accordance with said definition is applied to the circuit inputs of said digital circuit in said idle state.