Early Output Logic using Anti-Tokens

C.F. Brej and J.D. Garside

Abstract

Delay-insensitive dual-rail and bundled data design methodologies are the two main approaches used for the creation of asynchronous circuits. Bundled data allows the creation of fast, low overhead circuits, whereas dual-rail allows bit-level pipelining and average case performance. This paper describes 'Early output' logic, which combines the positive features of the two methods to create faster asynchronous circuits. This method allows the creation of circuits yielding performance faster than their synchronous counterparts.

Early output implementations allow logic to evaluate results before all inputs are presented. The results move to the next stage, but the current stage stalls while waiting for the late inputs to arrive simply to acknowledge them. This unnecessary wait can be removed by allowing backwards propagating 'Anti-Tokens' to remove the late inputs. The use of anti-tokens and improved semi-decoupled latches allows the removal of many stalls due to unnecessary synchronisations, thus improving the performance of the circuit.

Full paper (123K, PDF) and conference slides (209K PowerPoint).