General-purpose input/output

A general-purpose input/output (GPIO) is an uncommitted digital signal pin on an integrated circuit or electronic circuit board whose behavior—including whether it acts as input or output—is controllable by the user at run time. GPIOs have no predefined purpose and are unused by default. If used, the purpose and behavior of a GPIO is defined and implemented by the designer of higher assembly-level circuitry: the circuit board designer in the case of integrated circuit GPIOs, or system integrator in the case of board-level GPIOs. Integrated circuit (IC) GPIOs are implemented in a variety of ways. Some ICs provide GPIOs as a primary function whereas others include GPIOs as a convenient 'accessory' to some other primary function. Examples of the former include the Intel 8255, which interfaces 24 GPIOs to a parallel bus, and various GPIO 'expander' ICs, which interface GPIOs to serial buses such as I²C and SMBus. An example of the latter is the Realtek ALC260 IC, which provides eight GPIOs in addition to its primary function of audio codec. Microcontroller ICs typically include GPIOs. Depending on the application, a microcontroller's GPIOs may comprise its primary interface to external circuitry or they may be just one type of I/O used among several, such as analog I/O, counter/timer, and serial communication. In some ICs, particularly microcontrollers, a GPIO pin may be capable of alternate functions. Often in such cases, it is necessary to configure the pin to operate as a GPIO (vs. its alternate functions) in addition to configuring the GPIO's behavior. Some microcontroller devices (e.g., Microchip dsPIC33 family) incorporate internal signal routing circuitry that allows GPIOs to be programmatically mapped to device pins. FPGAs extend this capability by allowing GPIO pin mapping, instantiation and architecture to be programmatically controlled.