Abstract: A power voltage of a power source circuit is produced by amplifying a threshold voltage difference of an operational amplifier by an amplification factor corresponding to a dividing ratio of a resistance dividing circuit. The power source circuit is integrated on a CMOS substrate together with a computer block. A plurality of analog switches are associated with the resistance dividing circuit to stabilize the power voltage of the power source circuit. One of these switches is selectively closed to change the dividing ratio of the resistance dividing circuit in accordance with actuation data stored in a control register.

Abstract: A signal processing circuit includes a signal selecting device operative for selecting and outputting one of plural input signals. A filter connected to the signal selecting device filters the signal selected by the signal selecting device with a changeable center frequency of a pass band of the filtering. A frequency changing device is operative for temporarily changing the center frequency when the signal selected by the signal selecting device is changed from one to another. The filter includes, for example, a switched-capacitor filter.

Abstract: A knock sensing apparatus for an internal combustion engine which can detect a failure in a knock sensor signal system correctly even when a changeover operation due to hardware is performed at a place between a knock sensor and a band pass filter. In this knock sensing apparatus, the failure detection by a failure detecting circuit for detecting a failure in a signal system from knock sensors is invalidated for a predetermined period after one of the knock sensors has been changed over to the other knock sensor, or one of two kinds of pass bands of a band pass filter has been changed over to the other pass band, to prevent a decrease in failure detection accuracy due to the signal delaying action of the switched capacitor filter.