  The languages of infinite timed words accepted by timed automata are
traditionally defined using Buchi-like conditions. These acceptance conditions
focus on the set of locations visited infinitely often along a run, but
completely ignore quantitative timing aspects. In this paper we propose a
natural quantitative semantics for timed automata based on the so-called
frequency, which measures the proportion of time spent in the accepting
locations. We study various properties of timed languages accepted with
positive frequency, and in particular the emptiness and universality problems.
