Design and Verification of Speed-Independent Circuits with Arbitration in Workcraft

The traditional design methods for speed-independent (SI) circuits require their behaviour to be output-persistent. A common source of non-persistence is arbitration that leads to a choice between output signals. It is the designer's responsibility to remove such non-persistent behaviour before proceeding to synthesis, usually by manually factoring the arbitration out into the environment, where the choice is implemented using a mutex element. There are several problems with this approach: Significant manual effort factoring out the mutex and inserting it after synthesis. There is no guarantee that the signals the designer thinks can be implemented by a mutex actually follow the arbitration protocol. Factoring out converts mutex grants into inputs, so verification of output-persistence would miss a situation when a mutex grant is disabled due to premature withdrawal of the corresponding request (this applies to verification of both, the original specification and the circuit implementation). In this paper we demonstrate how these problems were solved by integrating automatic mutex insertion into the SI synthesis flow implemented in Workcraft (https://workcraft.org/).