A microcontroller device is fabricated in a semiconductor integrated circuit chip for controlling an external system with which the microcontroller device is to be installed in circuit. The device includes a CPU, a program memory, a data memory, and peripherals for use in controlling the external system. An on-chip peripheral timing module establishes the timing of and degree of control exercised over the external system by a generating a repetitive event configured as a pulse waveform. The timing module is adapted for selectively setting the period of the pulse waveform to designate the frequency of occurrence of the repetitive event for the timing of control and to provoke a corresponding periodic interrupt of the CPU. The width of the pulse in each period of the pulse waveform is selectively modulated to set the degree of control. The timing module selectively varies the frequency of periodic interrupts of the CPU without affecting the period of the pulse waveform. In one embodiment, the selective variation of frequency of the periodic interrupts is performed by a divider circuit.