Hierarchical Combinational–Sequential System Design

Abstract

Real-world digital circuits consist of combinations of combinational and sequential sub-circuits, connected and configured in a hierarchy. Having examined the behavior of some of these sub-circuits (both combinational and sequential) in detail earlier, we now examine their hierarchical combinations, as in real-world systems. All sequential circuits are clocked, while no combinational circuit is clocked. The integration of these two seemingly contradictory design philosophies is possible via sensitivity lists—i.e., a combinational circuit block is sensitized to each of its input ports, while the sequential circuit block responds to its clock. As before, these design examples are graded with increasing levels of complexity, ending with some widely used digital sub-circuits for integrated circuit testing.