On Synthesis of Speed-Independent Circuits at STG Level

Synthesizing hazard-free asynchronous circuits directly at Signal Transition Graph (STG) level has been shown to need significantly less CPU time than approaches at the stategraph[10, 16, 4]. However, all previous methods at STG level were based on sufficient conditions only. Hence, the synthesized circuit results generally are inferior, due to the incomplete transformation. In this paper, we present a new Characteristic Graph (CG) to encapsulate all feasible solutions of the original STG in reduced size, which compares favorably with the state graph approach. The requirements of speed independent circuits can then be completely transformed into the CG. Furthermore, we derive a necessary and sufficient condition for speed independent implementation based on a predefined general circuit model, which has not yet been reported. With CGs and this condition, we develop a heuristic synthesis algorithm which derives solutions similar to the state-graph approach while requiring significantly less CPU time.