gpio: gpio.txt

File gpio.txt, 6.1 KB (added by Cale Collins, 6 years ago)


11 GPIO Interfaces
22 ===============
44 The documents in this directory give detailed instructions on how to access
55 GPIOs in drivers, and how to write a driver for a device that provides GPIOs
66 itself.
88 Due to the history of GPIO interfaces in the kernel, there are two different
99 ways to obtain and use GPIOs:
1111 - The descriptor-based interface is the preferred way to manipulate GPIOs,
1212 and is described by all the files in this directory excepted gpio-legacy.txt.
1313 - The legacy integer-based interface which is considered deprecated (but still
1414 usable for compatibility reasons) is documented in gpio-legacy.txt.
1616 The remainder of this document applies to the new descriptor-based interface.
1717 gpio-legacy.txt contains the same information applied to the legacy
1818 integer-based interface.
2121 What is a GPIO?
2222 ===============
2424 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
2525 digital signal. They are provided from many kinds of chip, and are familiar
2626 to Linux developers working with embedded and custom hardware. Each GPIO
2727 represents a bit connected to a particular pin, or "ball" on Ball Grid Array
2828 (BGA) packages. Board schematics show which external hardware connects to
2929 which GPIOs. Drivers can be written generically, so that board setup code
3030 passes such pin configuration data to drivers.
3232 System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
3333 non-dedicated pin can be configured as a GPIO; and most chips have at least
3434 several dozen of them. Programmable logic devices (like FPGAs) can easily
3535 provide GPIOs; multifunction chips like power managers, and audio codecs
3636 often have a few such pins to help with pin scarcity on SOCs; and there are
3737 also "GPIO Expander" chips that connect using the I2C or SPI serial buses.
3838 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
3939 firmware knowing how they're used).
4141 The exact capabilities of GPIOs vary between systems. Common options:
4343 - Output values are writable (high=1, low=0). Some chips also have
4444 options about how that value is driven, so that for example only one
4545 value might be driven, supporting "wire-OR" and similar schemes for the
4646 other value (notably, "open drain" signaling).
4848 - Input values are likewise readable (1, 0). Some chips support readback
4949 of pins configured as "output", which is very useful in such "wire-OR"
5050 cases (to support bidirectional signaling). GPIO controllers may have
5151 input de-glitch/debounce logic, sometimes with software controls.
5353 - Inputs can often be used as IRQ signals, often edge triggered but
5454 sometimes level triggered. Such IRQs may be configurable as system
5555 wakeup events, to wake the system from a low power state.
5757 - Usually a GPIO will be configurable as either input or output, as needed
5858 by different product boards; single direction ones exist too.
6060 - Most GPIOs can be accessed while holding spinlocks, but those accessed
6161 through a serial bus normally can't. Some systems support both types.
6363 On a given board each GPIO is used for one specific purpose like monitoring
6464 MMC/SD card insertion/removal, detecting card write-protect status, driving
6565 a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware
6666 watchdog, sensing a switch, and so on.
6969 Common GPIO Properties
7070 ======================
7272 These properties are met through all the other documents of the GPIO interface
7373 and it is useful to understand them, especially if you need to define GPIO
7474 mappings.
7676 Active-High and Active-Low
7777 --------------------------
7878 It is natural to assume that a GPIO is "active" when its output signal is 1
7979 ("high"), and inactive when it is 0 ("low"). However in practice the signal of a
8080 GPIO may be inverted before is reaches its destination, or a device could decide
8181 to have different conventions about what "active" means. Such decisions should
8282 be transparent to device drivers, therefore it is possible to define a GPIO as
8383 being either active-high ("1" means "active", the default) or active-low ("0"
8484 means "active") so that drivers only need to worry about the logical signal and
8585 not about what happens at the line level.
8787 Open Drain and Open Source
8888 --------------------------
8989 Sometimes shared signals need to use "open drain" (where only the low signal
9090 level is actually driven), or "open source" (where only the high signal level is
9191 driven) signaling. That term applies to CMOS transistors; "open collector" is
9292 used for TTL. A pullup or pulldown resistor causes the high or low signal level.
9393 This is sometimes called a "wire-AND"; or more practically, from the negative
9494 logic (low=true) perspective this is a "wire-OR".
9696 One common example of an open drain signal is a shared active-low IRQ line.
9797 Also, bidirectional data bus signals sometimes use open drain signals.
9999 Some GPIO controllers directly support open drain and open source outputs; many
100100 don't. When you need open drain signaling but your hardware doesn't directly
101101 support it, there's a common idiom you can use to emulate it with any GPIO pin
102102 that can be used as either an input or an output:
104104 LOW: gpiod_direction_output(gpio, 0) ... this drives the signal and overrides
105105 the pullup.
107107 HIGH: gpiod_direction_input(gpio) ... this turns off the output, so the pullup
108108 (or some other device) controls the signal.
110110 The same logic can be applied to emulate open source signaling, by driving the
111111 high signal and configuring the GPIO as input for low. This open drain/open
112112 source emulation can be handled transparently by the GPIO framework.
114114 If you are "driving" the signal high but gpiod_get_value(gpio) reports a low
115115 value (after the appropriate rise time passes), you know some other component is
116116 driving the shared signal low. That's not necessarily an error. As one common
117117 example, that's how I2C clocks are stretched: a slave that needs a slower clock
118118 delays the rising edge of SCK, and the I2C master adjusts its signaling rate
119119 accordingly.