System-on-Chip (SOC) processors heavily rely on GPIOs. In many cases, every non-dedicated pin can be configured as a GPIO; and most chips have at least several dozen of them. Programmable logic devices (like FPGAs) can easily provide GPIOs, and I2C-connected chips like power managers, audio codecs, and "GPIO Expanders" often have a few such pins to help with pin scarcity on SOCs. Most PC southbridges have a few dozen GPIO-capable pins.
The exact capabilities of GPIOs vary between systems. Common options:
- Output values are writable (high=1, low=0). Some chips also have options about how that value is driven, so that for example only one value might be driven ... supporting "wire-OR" and similar schemes for the other value.
- Input values are likewise readable (1, 0). Some chips support readback of pins configured as "output", which is very useful in such "wire-OR" cases (to support bidirectional signaling). GPIO controllers may have input de-glitch logic, sometimes with software controls.
- Inputs can often be used as IRQ signals, often edge triggered but sometimes level triggered. Such IRQs may be configurable as system wakeup events, to wake the system from a low power state.
- Usually a GPIO will be configurable as either input or output, as needed by different product boards; single direction ones exist too.
On a given board each GPIO is used for one specific purpose like monitoring, MMC/SD card insertion/removal, detecting card write protect status, driving a LED, configuring a transceiver, and so on.