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General-purpose input/output (GPIO) is a generic pin on an integrated circuit whose behavior, including whether it is an input or output pin, can be controlled by the user at run time.
GPIO pins have no special purpose defined, and go unused by default. The idea is that sometimes the system integrator building a full system that uses the chip might find it useful to have a handful of additional digital control lines, and having these available from the chip can avoid the effort of having to arrange additional circuitry to provide them. For example, the Realtek ALC260 chips (audio codec) have 8 GPIO pins, which go unused by default. Some system integrators (Acer Inc. laptops) employing the ALC260 use the first GPIO (GPIO0) to turn on the amplifier used for the laptop's internal speakers and external headphone jack.
GPIOs are used in:
- Devices with pin scarcity: integrated circuits such as system-on-a-chip, embedded and custom hardware, and programmable logic devices (for example, FPGAs)
- Multifunction chips: power managers, audio codecs, and video cards
- Embedded applications (for example, Arduino, BeagleBone, PSoC kits and Raspberry Pi) make heavy use of GPIO for reading from various environmental sensors (IR, video, temperature, 3-axis orientation, and acceleration), and for writing output to DC motors (via PWM), audio, LCD displays, or LEDs for status.
GPIO capabilities may include:
- GPIO pins can be configured to be input or output
- GPIO pins can be enabled/disabled
- Input values are readable (typically high=1, low=0)
- Output values are writable/readable
- Input values can often be used as IRQs (typically for wakeup events)
GPIO peripherals vary quite widely. In some cases, they are very simple, a group of pins that can be switched as a group to either input or output. In others, each pin can be set up flexibly to accept or source different logic voltages, with configurable drive strengths and pull ups/downs. The input and output voltages are typically, though not universally, limited to the supply voltage of the device with the GPIOs on and may be damaged by greater voltages.
A GPIO pin's state may be exposed to the software developer through one of a number of different interfaces, such as a memory mapped peripheral, or through dedicated IO port instructions.
Some GPIOs have 5 V tolerant inputs: even when the device has a low supply voltage (such as 2 V), the device can accept 5 V without damage.
A GPIO port is a group of GPIO pins (typically 8 GPIO pins) arranged in a group and controlled as a group.