Patent ID: 7404097

Claim:
A vehicle-installed microcomputer system comprising: an A/D converter converting an analog signal sent from an external sensor circuit into a digital signal; a microcomputer which operates in one of a regular mode where a predetermined arithmetic computation on said digital signal is performed, and a sleep mode where said predetermined arithmetic computation is prohibited from being performed; a first power supply circuit supplied with electric power from outside to produce a first power supply voltage to be applied to said microcomputer; and a second power supply circuit supplied with electric power from outside through a power interruption switch to produce a second power supply voltage to be applied to said A/D converter and said external sensor circuit, said second power supply circuit having an output voltage accuracy higher than that of said first power supply circuit; wherein said microcomputer is configured to close said power interruption switch to allow said second power supply circuit to be supplied with electric power in said regular mode, and to open said power interruption switch to prohibit said second power supply circuit from being supplied with electric power in said sleep mode.