driver-api/gpio/intro.rst

17 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
20 represents a bit connected to a particular pin, or "ball" on Ball Grid Array
25 System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
26 non-dedicated pin can be configured as a GPIO; and most chips have at least
30 also "GPIO Expander" chips that connect using the I2C or SPI serial buses.
31 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
36   - Output values are writable (high=1, low=0). Some chips also have
38     value might be driven, supporting "wire-OR" and similar schemes for the
41   - Input values are likewise readable (1, 0). Some chips support readback
42     of pins configured as "output", which is very useful in such "wire-OR"
44     input de-glitch/debounce logic, sometimes with software controls.
46   - Inputs can often be used as IRQ signals, often edge triggered but
50   - Usually a GPIO will be configurable as either input or output, as needed
53   - Most GPIOs can be accessed while holding spinlocks, but those accessed
57 MMC/SD card insertion/removal, detecting card write-protect status, driving
58 a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware
69 Active-High and Active-Low
70 --------------------------
73 GPIO may be inverted before is reaches its destination, or a device could decide
76 being either active-high ("1" means "active", the default) or active-low ("0"
81 --------------------------
83 level is actually driven), or "open source" (where only the high signal level is
85 used for TTL. A pullup or pulldown resistor causes the high or low signal level.
86 This is sometimes called a "wire-AND"; or more practically, from the negative
87 logic (low=true) perspective this is a "wire-OR".
89 One common example of an open drain signal is a shared active-low IRQ line.
95 that can be used as either an input or an output:
101  the pullup (or some other device) controls the signal.