A microcontroller device is fabricated in a semiconductor integrated circuit chip to control an external system with which the device is to be installed in circuit. The device has a CPU, a program memory for storing program instructions to be implemented by the CPU, a data memory for storing data including data pertaining to parameters of the external system to be controlled by operation of the CPU according to the instructions, and various peripherals. A brown-out protection circuit monitors the level of the supply voltage for the IC chip relative to a ground reference level, to reset the device as protection against its malfunction in response to reduction of an arithmetic difference between the supply voltage level and the ground reference level to a value less than a predetermined threshold operating voltage level. Reset defines a cessation of operation of the device while maintaining status quo of implementation of program instructions by the CPU and data stored in the data memory. A discriminator distinguishes between a reduction representative of a brown-out event warranting invoking a reset of the device and a reduction representative of mere transitory voltage swings commonly occurring in the device operation not warranting invoking a reset of the device. This serves to avoid both malfunctions and unnecessary resetting of the device in control of the external system.