Sequential optimization in the absence of global reset

Abstract

We study the problem of optimizing synchronous sequential circuits. There have been previous efforts to optimize such circuits. However, all previous attempts make implicit or explicit assumptions about the design or the environment of the design. For example, it is widespread practice to assume the existence of a hardware reset line and consequently a fixed power-up state; in the absence of the same, a common premise is that the design's environment will apply an initializing sequence. We review the concept of safe replaceability which does away with these assumptions and the delay-safe replaceability notion, which is applicable when the design's output is not used for a certain number of cycles after power-up. We then develop procedures for optimizing the combinational next-state and output logic, as well as routines for reencoding the state space and removing state bits under these replaceability criteria. Experimental results demonstrate the effectiveness of our algorithms.