Classification of Sequential Circuits as Causal Functions

In sequential circuits, the current output may depend on both past and current inputs. However, certain kinds of sequential circuits do not refer to all of the past inputs to generate the current output; they only refer to a subset of past inputs. This paper investigates which subset of past inputs a sequential circuit refers to, and proposes a new classification of sequential circuits based on this criterion. The conventional classification of sequential circuits distinguishes between synchronous and asynchronous circuits. In contrast, the new classification consolidates synchronous circuits and multiple clock domain circuits into the same category.