Real time reactive systems interact with their environment, using inputs and outputs, within specified time constraints. For such systems, a functional misbehavior or a deviation from the specified time constraints may have catastrophic consequences. Therefore, ensuring the correctness of real time systems becomes very important. We introduce the potential faults which can be encountered in a timed system implementation. We adapt an existing test cases generation technique, based on state characterization set, to generate timed test cases from a timed system specification. We model a timed system with a Timed Input Output Automaton (TIOA), which is a variant of the Alur and Dill model (R. Alur and D. Dill, 1994). In order to generate the timed test suite, the TIOA is first transformed into a Nondeterministic Timed Finite State Machine (NTFSM) with a given granularity. We illustrate our method with an example.