Functional timing optimization

Abstract

It is widely believed that any true (or sensitizable) critical path of length /spl ges/2 T must be speeded up in order for a circuit to have a delay <T. I demonstrate that this notion is pessimistic. Many true paths can never affect the delay of the circuit-whenever such a path propagates a signal, some other path that is at least as long also propagates a signal. The theory for a new classification of paths based on the impact on the circuit delay is presented and conditions are given under which a path (or a set of paths) must be speeded up in order to improve the circuit delay. The conditions for the categorization are independent of the delays in the circuit and are valid for all delay assignments. This work indicates that the widely employed notions of true and false paths may be misleading both for timing optimization and delay analysis of logic circuits.