A microcontroller comprises a microprocessor (1), a test interface (4) and an internal non-erasable memory (2). First control means (6) are provided which are able to activate and deactivate the test interface (4), and second control means (7) are provided which are able to activate and deactivate the internal non-erasable memory (2). The microprocessor (1) of the microcontroller comprises control outputs (101) which are connected with the first and second control means (6, 7). With appropriate timing of activation and deactivation of the test interface (4) and the internal non-erasable memory (2), the microcontroller offers the possibility of preventing an unauthorized access to contents of the internal non-erasable memory (2) without limiting the usability of the test interface (4) for the development of application programs. The microcontroller further offers the possibility to entirely block access to data which are stored in the microcontroller or have been processed within the microcontroller via the test interface (4).