A Separation Principle for Embedded System Interfacing

In designing systems, engineers decompose the problem into smaller, more manageable tasks. A classic example of this is the

separation principle

from control systems which allows one to decompose the design of an optimal feedback control system into two independent tasks by designing (a) an observer, and (b) a controller. We investigate an analogous result for embedded system interfacing that will allow separation of the design of the input and output hardware interfaces while still guaranteeing the ability of the software to meet the system requirements. We define the notions of observability (controllability) of the system requirements with respect to the input (output) interface. We show that for a system that can be modeled by a functional four-variable model, observability and controllability allow for the separation of the design of the input and output interfaces. We also show that this separation is not always possible for systems that need the general, relational four-variable model. By strengthening either observability or controllability, we restrict the choice of input or output interfaces, but ensure separability of their designs.