hysteresis characteristics as shown in fig5 an output voltage vs from the schmitt circuit 30 is kept at a low level after the main power source 22 is turned on and until an output voltage vr from the reset circuit 28 gradually increases to exceed an upper level shift voltage vp and then it goes to a high level the schmitt circuit 30 generates a signal of high level when the output voltage vr from the reset circuit 28 gradually decreases from the main power source voltage vm to reach a lower level shift voltage vn the schmitt circuit 30 generates a signal of low level when the output voltage from the reset circuit 28 becomes still lower fig6 shows a circuit diagram of the microcomputer system shown in fig4 referring to fig6 the main power source 22 and the auxiliary power source 24 are connected through switches sw1 and sw2 respectively to diodes 20 1 and 20 2 constituting the voltage selector 20 a series circuit of a resistor 28 1 and a capacitor 28 2 constituting the reset circuit 28 is coupled between the output terminal of the voltage selector 20 and ground a diode 28 3 is connected in parallel with the resistor 28 1 the junction of the resistor 28 1 and the capacitor 28 2 are coupled to the reset terminal of the microcomputer 26 through a resistor 30 1 and inverters 30 2 and 30.