A system for generating an ideal rise or fall time includes: a first current source, for providing a first current; an adjustable capacitive component, coupled to the first current source, for generating an output signal according to a total capacitance controlled by a comparison signal; a signal conversion circuit, coupled to the adjustable capacitive component, for restoring charges stored in the adjustable capacitive component to a predetermined value when a voltage level of the output signal reaches a reference value to generate a clock-like signal; and a comparison circuit, coupled to the signal conversion circuit and the adjustable capacitive component, for comparing a period of the clock-like signal with a reference period of a reference clock signal and generating the comparison signal to adjust the total capacitance of the adjustable capacitive component when periods are not the same.