The purpose of this paper is to explore the models and forms of reasoning appropriate for analysing the
temporal behaviour of circuits. That leads from various Boolean models appropriate for combinational
and synchronous circuits, to the event model for asynchronous circuits. Circuit behaviour is captured at
each level of abstraction using model-specific operators, whose laws are used to perform algebraic (rather
than semantic) reasoning about transient behaviour. The results have a particularly satisfying form: for
example the hazards of a conditional are calculated algebraically to be a conditional of hazards. A
method is developed for simulating an asynchronous combinational circuit, that starts with an untimed,
logic-level, specification, passes to a timed refinement and then moves to a simple timed automaton for
simulation in UPPAAL. Each stage in the method is supported by a projection relating the models at the
various levels, so that correctness can be confirmed.