Microcontroller Integrated Circuit with Read Only MemoryMicrocontroller integrated circuit comprises a processor core which exchanges data with at least one data processing and storage device. The integrated circuit comprises a mask-programmed read only memory containing a generic program such as a test program which can be executed by the microcontroller. The genetic program includes a basic function for writing data into the data progressing or storage device or devices. The write function is used to load a downloading program. Because a downloading program is not permanently stored in the read only memory, the microcontroller can be tested independently of the application program, and remains standard with regard to the type of memory component with which it can be used in a system.To be more precise, the invention concerns a microcontroller integrated circuit. A microcontroller is usually a VLSI (Very Large Scale Integration) integrated circuit containing all or most of the components of a computer. Its function is not predefined but depends on the program that it executes.A microcontroller necessarily comprises a processor core including a command sequencer (which is a device distributing various control signals to the instructions of a program), an arithmetic and logic unit (for processing the data) and registers (which are specialized memory units). The other components of the computer can be either internal or external to the microcontroller, however. In other words, the other components are integrated into either the microcontroller or auxiliary circuits.These other components of the computer are data processing and storage devices, for example read only or random access memory containing the program to be executed, clocks and interfaces (serial or parallel).As a general rule, a system based on a microcontroller therefore comprises a microchip containing the microcontroller, and a plurality of microchips containing the external data processing and storage devices which are not integrated into the microcontroller. A microcontroller-based system of this kind comprises, for example, one or more printed circuit boards on which the microcontroller and the other components are mounted.It is the application program, I. e. the program which is executed by the microcontroller, which determines the overall operation of the microcontroller system. Each application program is therefore specific to a separate application.In most current applications the application program is too large to be held in the microcontroller and is therefore stored in a memory external to the microcontroller. This program memory, which has only to be read, not written, is generally a reprogrammable read only memory (REPROM).After the application program has been programmed in memory and then started in order to be executed by the microcontroller, the microcontroller system may not function as expected.In the last unfavorable situation this is a minor dysfunction of the system and the microcontroller is still able to dialog with a test station via a serial or parallel interface. This test station is then able to determin the nature of the problem and indicates precisely the type of correction (software and physical) to be applied to the system for it to operate correctly.Unfortunately, most dysfunctions of microcontroller-based system result in a total system lock-up, preventing any dialog with a test station. It is then impossible to determine the type of fault, i.e. whether it is a physical fault (in the microcontroller itself, in an external read only memory, in a peripheral device, on a bus, etc) or a software fault(i.e. an error in the application program).The troubleshooting technique usually employed in these cases of total lock-up is based on the use of sophisticated test devices requiring the application of probes to the pins of the various integrated circuits of the microcontroller-based system under test.There are various problems associated with the use of such test devices for troubleshooting a microcontroller-based system. The probes used in these test devices are very fragile, difficult to apply because of the small size of the circuit and their close packing, and may not make good contact with the circuit.Also, because of their high cost, these test devices are not mass produced. Consequently, faulty microcontroller-based systems can not be repaired immediately, wherever they happen to be located at the time, but must first be returned to a place where a test device is available. Troubleshooting a microcontroller-based system in this way is time-consuming, irksome and costly.To avoid the need for direct action on the microcontroller-based system each time the application program executed by the microcontroller of the system is changed, it is standard practice to use a downloadable read only memory to store the application program, a loading program being written into a mask-progra