This paper investigates the way state diagrams can be best represented in the polychronous model of computation. In this relational model, the basic objects are signals, which are related through data-flow equations. Signals are associated with logical clocks, which provide the capability to describe systems in which components obey to multiple clock rates. We propose a model of finite-state automata, called polychronous automata, which is based on clock relations. A specificity of this model is that an automaton is submitted to clock constraints. This allows one to specify a wide range of control-related configurations, either reactive, or restrictive with respect to their control environment. A semantic model is defined for these polychronous automata, that relies on a Boolean algebra of clocks.