Patent ID: 7664973

Claim:
An integrated circuit that operates with application of an external power source supplying a first source voltage, the circuit comprising: a source voltage monitoring unit that monitors the level of the first source voltage supplied from the external power source, and that determines whether the first source voltage is supplied from the external power source; a clock selecting unit that is supplied with a first clock of a first frequency and a second clock of a second frequency lower than the first frequency, and that selects and outputs the first clock when it is determined by the source voltage monitoring unit that the first source voltage is supplied and the second clock when it is determined by the source voltage monitoring unit that the first source voltage is not supplied; a processor that operates with supply of either one of the first clock and the second clock output by the clock selecting unit and controls the operation of the integrated circuit wherein the integrated circuit operates with application of either one of the external power source and an internal power source supplying a second source voltage, whose level is lower than that of the first source voltage, and wherein the source voltage monitoring unit controls so as to apply the first source voltage when determining that the first source voltage is supplied, and so as to apply the second source voltage when determining that the first source voltage is not supplied.