A multiplexing circuit uses parallel-configured pairs of resistors and signal sources in a voltage divider network in such a way that a single analog-to-digital input can be used to specify the state of more than one signal source. One circuit includes a microprocessor having an analog-to-digital (ADC) input; a memory communicatively coupled to the microprocessor; and a voltage divider network having an output coupled to the ADC input, wherein the network includes a plurality of resistors paired with a plurality of respective signal sources, and wherein the output is unique for each combination of states of the signal sources in accordance with a known relation that is stored in the memory. The signal sources are selected from two categories of sources: continuous sources and discrete sources, where discrete sources may be binary discrete or random discrete. In one embodiment, the first signal source is either a continuous source or a random discrete source, and the second signal is a binary discrete source.

An input circuit of a one-chip microcomputer is connected to an external switching circuit. When an analog input signal of a significant level generated in the external switching circuit is received at an analog input terminal of the input circuit, an A/D conversion start request signal is generated in an A/D conversion start request generating circuit and is sent to an A/D converter. The operation of the A/D converter is started in response to the A/D conversion start request signal, the analog input signal received at the analog input terminal is converted into digital data, and an A/D conversion finish signal is sent from the A/D converter to a CPU of the one-chip microcomputer. The operation of the CPU is started in response to the A/D conversion finish signal, and the digital data is readout to the CPU. Therefore, the A/D converter, the CPU or a clock is not operated to wait for an analog input signal generated in the external switching circuit, but the A/D converter is operated for the A/D conversion, and the CPU is operated to read out the digital data. Accordingly, an electric power consumed in the A/D converter, the CPU and the clock can be reduced.

A solid-state image pickup apparatus includes a mode setting circuit for allowing the operator to select desired one of the modes matching with the display format of a display which displays a video signal fed thereto. A clock generating circuit generates clocks including a first clock and a second clock higher in frequency than the first clock. A frequency selector selects either one of the first and second clocks in accordance with a mode fed from the mode setting circuit. An image pickup section includes a plurality of photosensitive cells for transforming, by photoelectric transduction, light incident thereto from a scene to be picked up. The image pickup section picks up the scene and produces signal charges representative of the scene in accordance with the output of the frequency selector. A noise reducing circuit reduces noise components included in a signal output from the image pickup section. A digitizing circuit converts the output of the noise reducing circuit to a digital signal in accordance with the first clock. A signal processing circuit processes the output of the digitizing circuit in a manner matching with picture display and/or recording. A controller controls the clock generating circuit, frequency selector, image pickup section, noise reducing circuit, digitizing circuit, and signal processing circuit. The modes include a first mode in which the frequency selector outputs the first clock and a second mode in which it outputs the second clock.

A method and system for powering down an analog-to-digital converter ("ADC") into a sleep mode are disclosed. If the ADC receives a normal set of pulses for a serial clock signal of the ADC, a serial interface controller outputs converted data requested by a user through a serial interface. Also, if the ADC receives a sleep set of pulses for the serial clock signal, a state machine of the ADC powers down the ADC into a sleep mode in which at least parts of the ADC are operated at a reduced power consumption level. Furthermore, if the ADC is in the sleep mode and the ADC receives a wake-up set of pulses for the serial clock signal, the state machine powers back up the ADC from the sleep mode.

A method for placing a device in a selected mode of operation. The method comprises the steps of initializing a device select signal into a first logic state, asserting the device select signal in a second logic state, and returning the device select signal to the first logic state within a first user-controlled time window. A device is also described that includes means for detecting logic state transitions at a device select input and a clock input, and means for changing operating mode of the device in response to a predetermined number of logic state transitions at the clock input, occurring between logic state transitions at the device select input. The selected operating mode may be a reduced power consumption mode, for example, or another operating mode of the device, such as a daisy-chain mode of operation, or a mode that accommodates programming of analog input range.