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In embedded systems, many external inputs are provided by sensors. There are many kinds of sensors. A sensor can be as simple as an on/off switch or as complex as an accelerometer or range finder. The manner in which a sensor is connected to an input port is determined by the character of the sensor's output signal. Some sensors provide digital signals that can be connected directly to an I/O port. Other sensors provide analog signals. Since a computer can handle only digital signals, analog to digital conversion is needed.
One good place to read about recent advances in sensor technology is Sensor Magazine (current web address is http://www.sensorsmag.com/articles/article_index/main.shtml). In general, the sensory-computer interface is not a simple matter. Special circuits are usually needed for connecting sensors to the computer. An in depth description on this topic can be found in Introduction to Instrumentation and Measurements, CRC Press, authored by Robert B. Northrop.