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copied!<p>In most embedded systems, a processor will be ultimately responsible for sensing the state of various devices which translate external stimuli to digital-level logic voltages (e.g. when a button is pushed, a pin will go low; otherwise it will sit high), and controlling devices which translate logic-level voltages directly into action (e.g. when a pin is high, a light will go on; when low, it will go off). It used to be that processors did not have general-purpose I/O, but would instead have to use a shared bus communicate with devices that could process I/O requests and set or report the state of the external circuits. Although this approach was not entirely without advantages (one processor could monitor or control thousands of circuits on a shared bus) it was inconvenient in many real-world applications.</p> <p>While it is possible for a processor to control any number of inputs and outputs using a four-wire SPI bus or even a two-wire I2C bus, in many cases the number of signals a processor will need to monitor or control is sufficiently small that it's easier to simply include the circuitry to monitor or control some signals directly on the chip itself. Although dedicated interfacing hardware will frequently have output-only or input-only pins (the person choosing the hardware interface chips will know how many signals need to be monitored, and how many need to be controlled), a particular family of processor may be used in some applications that require e.g. 4 inputs and 28 outputs, and other applications that require 28 inputs and 4 outputs. Instead of requiring that different parts be used in applications with different balances between inputs and outputs, it's simpler to just have one part with inputs that can be configured as inputs or outputs, as needed.</p>

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