Given a timed automaton with parametric timings, our objective is to describe a procedure for deriving constraints on the parametric timings in order to ensure that, for each value of parameters satisfying these constraints, the behaviors of the timed automata are time-abstract equivalent. We will exploit a reference valuation of the parameters that is supposed to capture a characteristic proper behavior of the system. The method has been implemented and is illustrated on various examples of asynchronous circuits.