Initialization issues in asynchronous circuit synthesis

Abstract

A design specification is said to be functionally uninitializable if an initializable implementation cannot be obtained. Due to the absence of any initialization sequence, a fault simulator or test generator that assumes an unknown starting state will be completely ineffective for uninitializable circuits. We present a novel procedure for synthesizing initializable asynchronous circuits from functionally uninitializable Signal Transition Graphs (STG). After characterizing the necessary conditions for functional uninitializability, we propose a technique that transforms the original STG into an equivalent, functionally initializable STG. We show that the presence of concurrency provides the designer with an extra degree of flexibility when implementing the circuit. It is shown that initializability can be achieved by sacrificing minimal concurrency and without violating the syntactic properties of the STG required for a hazard-free implementation. The synthesis of a trigger module illustrates this procedure.