1 ============
2 Introduction
3 ============
4
5
6 GPIO Interfaces
7 ===============
8
9 The documents in this directory give detailed
10 GPIOs in drivers, and how to write a driver fo
11 itself.
12
13
14 What is a GPIO?
15 ===============
16
17 A "General Purpose Input/Output" (GPIO) is a f
18 digital signal. They are provided from many ki
19 to Linux developers working with embedded and
20 represents a bit connected to a particular pin
21 (BGA) packages. Board schematics show which ex
22 which GPIOs. Drivers can be written genericall
23 passes such pin configuration data to drivers.
24
25 System-on-Chip (SOC) processors heavily rely o
26 non-dedicated pin can be configured as a GPIO;
27 several dozen of them. Programmable logic devi
28 provide GPIOs; multifunction chips like power
29 often have a few such pins to help with pin sc
30 also "GPIO Expander" chips that connect using
31 Most PC southbridges have a few dozen GPIO-cap
32 firmware knowing how they're used).
33
34 The exact capabilities of GPIOs vary between s
35
36 - Output values are writable (high=1, low=0)
37 options about how that value is driven, so
38 value might be driven, supporting "wire-OR
39 other value (notably, "open drain" signali
40
41 - Input values are likewise readable (1, 0).
42 of pins configured as "output", which is v
43 cases (to support bidirectional signaling)
44 input de-glitch/debounce logic, sometimes
45
46 - Inputs can often be used as IRQ signals, o
47 sometimes level triggered. Such IRQs may b
48 wakeup events, to wake the system from a l
49
50 - Usually a GPIO will be configurable as eit
51 by different product boards; single direct
52
53 - Most GPIOs can be accessed while holding s
54 through a serial bus normally can't. Some
55
56 On a given board each GPIO is used for one spe
57 MMC/SD card insertion/removal, detecting card
58 a LED, configuring a transceiver, bit-banging
59 watchdog, sensing a switch, and so on.
60
61
62 Common GPIO Properties
63 ======================
64
65 These properties are met through all the other
66 and it is useful to understand them, especiall
67 mappings.
68
69 Active-High and Active-Low
70 --------------------------
71 It is natural to assume that a GPIO is "active
72 ("high"), and inactive when it is 0 ("low"). H
73 GPIO may be inverted before is reaches its des
74 to have different conventions about what "acti
75 be transparent to device drivers, therefore it
76 being either active-high ("1" means "active",
77 means "active") so that drivers only need to w
78 not about what happens at the line level.
79
80 Open Drain and Open Source
81 --------------------------
82 Sometimes shared signals need to use "open dra
83 level is actually driven), or "open source" (w
84 driven) signaling. That term applies to CMOS t
85 used for TTL. A pullup or pulldown resistor ca
86 This is sometimes called a "wire-AND"; or more
87 logic (low=true) perspective this is a "wire-O
88
89 One common example of an open drain signal is
90 Also, bidirectional data bus signals sometimes
91
92 Some GPIO controllers directly support open dr
93 don't. When you need open drain signaling but
94 support it, there's a common idiom you can use
95 that can be used as either an input or an outp
96
97 **LOW**: ``gpiod_direction_output(gpio, 0)``
98 overrides the pullup.
99
100 **HIGH**: ``gpiod_direction_input(gpio)`` ...
101 the pullup (or some other device) controls th
102
103 The same logic can be applied to emulate open
104 high signal and configuring the GPIO as input
105 source emulation can be handled transparently
106
107 If you are "driving" the signal high but gpiod
108 value (after the appropriate rise time passes)
109 driving the shared signal low. That's not nece
110 example, that's how I2C clocks are stretched:
111 delays the rising edge of SCK, and the I2C mas
112 accordingly.
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