The use of SDL (Specification and Description Language)has arisen as a promising way of dealing with the increasing complexity of embedded real-time systems. However, SDL does not take into account non-functional aspects, such as the time requirements that are especially important in the context of real-time systems. Furthermore, the associated code generators do not integrate the management of temporal constraints. A real-time semantic extension to SDL for real-time systems modelling is proposed. The real-time profile is added to SDL in order to prepare the real-time code generation. To illustrate the proposal, an example of a speed regulator is presented.