Early evaluation for performance enhancement in phased logic

Data-dependent completion time is a well-known advantage of self-timed circuits, one that allows them to operate at average rather than worst-case execution rates. A technique called early evaluation (EE) that extends this advantage by allowing self-timed modules to produce results before all of their inputs have arrived is described here. The technique can be applied to any combinational function and is integrated into the phased logic (PL) design methodology that accepts synchronous design entry and produces delay-insensitive self-timed circuits. We describe an algorithm that ensures that the resulting delay-insensitive circuits are safe, and develop a generalized method for inserting EE gates into any PL netlist. We give performance results for several benchmark circuits, including a five-stage pipelined CPU and a microprogrammed floating-point unit. Comparisons are made among clocked circuits, PL circuits, and PL circuits with EE. Simulation results show a clear performance benefit for PL circuits that use EE.