  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.
