Abstract: A method for controlling a microcomputer is provided. The microcomputer is operable without completely expending power of backup power supply when main power is shutdown. During the power shutdown, the microcomputer is connected to a capacitor as a backup power device which supplies small amount of current for a short period. Thus, the microcomputer can execute software for a given period. By changing an operation mode of the microcomputer from high speed mode to low speed mode, power consumption of the microcomputer can be significantly reduced. The method uses software to suppress power consumption of the microcomputer to the minimum and to measure a duration time of power shutdown.