Synchronized Recursive Timed Automata

We present synchronized recursive timed automata (SRTA) that extend timed automata with a stack. Each frame of a stack is composed of rational-valued clocks, and SRTA synchronously increase the values of all the clocks within the stack. Our main contribution is to show that the reachability problem of SRTA is ExpTime-complete. This decidability contrasts with the undecidability for recursive timed automata (RTA) introduced by Trivedi and Wojtczak, and Benerecetti et al. Unlike SRTA, the frames below the top are frozen during the computation at the top frame in RTA. Our construction of the decidability proof is based on the region abstraction for dense timed pushdown automata (TPDA) of Abdulla et al. to accommodate together diagonal constraints and fractional constraints of SRTA. Since SRTA can be seen as an extension of TPDA with diagonal and fractional constraints, our result enlarges the decidable class of pushdown-extensions of timed automata.