Electronic control unit and method having program rewriting function

1. Field of the Invention In an ECU for controlling an engine, a microcomputer executes a program in a mask ROM after releasing the reset, determines a signal of a specific input terminal, and switches in accordance with the foregoing determination result the self operation mode into either one of the normal mode to execute a control program in a flash ROM and the rewrite mode to update and write a data from an external unit into the ROM. A monitor circuit monitors a watchdog pulse from the microcomputer, outputs a reset pulse to the microcomputer when determining an abnormality, and alters the level of the foregoing signal to the microcomputer into the level of the normal mode into the level of the rewrite mode.