technical reportWe study synchronous digital circuits in an abstract setting. A circuit is viewed as a collection of modules connected through their boundary ports, where each port assumes a fixed direction (input or output) over one cycle of operation, and can change directions across cycles. No distinction is made between clock inputs and non-clock inputs. A cycle of operation consists of the application of a set of inputs followed by the stabilization of the module state before the next inputs are applied (i.e. fundamental mode operation is assumed). The states and inputs of a module are modeled symbolically, in a functional notation. This enables us to study not only finite-state controllers, but also large data paths, possibly with unbounded amounts of state. We present the abstract syntax for modules, well-formedness checks on the syntax, the formal semantics in terms of the denotation of a module, and the rule for composing two modules interconnected and operating in parallel, embodied in the operator par. It is shown that par preserves well-formedness, and denotes conjunction. These results are applicable to virtually every kind of synchronous circuit (e.g. VLSI circuits that employ single or multiphase clocks, circuits that employ switch or gate logic structures, circuits that employ uni- or bi-directional ports, etc.), thanks to the small number of assumptions upon which the HOP model is set up
