driver-api/gpio/intro.rst

17 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
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
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
47     sometimes level triggered. Such IRQs may be configurable as system
48     wakeup events, to wake the system from a low power state.
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 --------------------------
72 ("high"), and inactive when it is 0 ("low"). However in practice the signal of a
76 being either active-high ("1" means "active", the default) or active-low ("0"
78 not about what happens at the line level.
81 --------------------------
82 Sometimes shared signals need to use "open drain" (where only the low signal
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.
97  **LOW**: ``gpiod_direction_output(gpio, 0)`` ... this drives the signal and
104 high signal and configuring the GPIO as input for low. This open drain/open
107 If you are "driving" the signal high but gpiod_get_value(gpio) reports a low
109 driving the shared signal low. That's not necessarily an error. As one common