Hazard-free implementation of speed-independent circuits

Abstract

This paper develops a theoretical framework for the hazard-free gate-level implementation of speed-independent circuits specified by event-based models, such as signal transition graphs (for processes with AND causality and input choice) or their extension, called change diagrams (which allow OR-causality). It presents sufficient conditions, called the generalized monotonous cover requirements, for a hazard-free circuit to be built within a standard implementation structure. This structure consists of two-level simple-gate combinational logic and a row of latches, either a C-element or an RS-latch. A set of semantic-preserving transformations is defined that can be applied to an original behavioral description of the circuit so as to produce its specification in the form that satisfies the monotonous cover requirement. The transformations are applied at the event-based representation level (to avoid state explosion) and proved to be effective. The main result of the paper is therefore twofold: 1) the proof that any speed-independent behavior can be implemented at the gate level without hazards and 2) an efficient method for constructing such an implementation. Experimental results show that the proposed method compares very favorably, in area and performance, to the previously known techniques.