This paper presents a Computer Aided Testability tool named STA (System Testability Assistant), aiming at proposing a testability strategy for system as early as possible in the design process. Given a system description, STA first compiles it in order to create system, boards, and circuits objects, and then proceeds bottom-up, from the circuit to the system, in order evaluate the actual testability of the whole system, recommend a testability strategy and testability features inclusion in some modules, and provides support for including these testability features in the system by calling external generators or synthesis for testability tools.