1 ============================================================
2 rotary-encoder - a generic driver for GPIO connected devices
3 ============================================================
4
5 :Author: Daniel Mack <daniel@caiaq.de>, Feb 2009
6
7 Function
8 --------
9
10 Rotary encoders are devices which are connected to the CPU or other
11 peripherals with two wires. The outputs are phase-shifted by 90 degrees
12 and by triggering on falling and rising edges, the turn direction can
13 be determined.
14
15 Some encoders have both outputs low in stable states, others also have
16 a stable state with both outputs high (half-period mode) and some have
17 a stable state in all steps (quarter-period mode).
18
19 The phase diagram of these two outputs look like this::
20
21 _____ _____ _____
22 | | | | | |
23 Channel A ____| |_____| |_____| |____
24
25 : : : : : : : : : : : :
26 __ _____ _____ _____
27 | | | | | | |
28 Channel B |_____| |_____| |_____| |__
29
30 : : : : : : : : : : : :
31 Event a b c d a b c d a b c d
32
33 |<-------->|
34 one step
35
36 |<-->|
37 one step (half-period mode)
38
39 |<>|
40 one step (quarter-period mode)
41
42 For more information, please see
43 https://en.wikipedia.org/wiki/Rotary_encoder
44
45
46 Events / state machine
47 ----------------------
48
49 In half-period mode, state a) and c) above are used to determine the
50 rotational direction based on the last stable state. Events are reported in
51 states b) and d) given that the new stable state is different from the last
52 (i.e. the rotation was not reversed half-way).
53
54 Otherwise, the following apply:
55
56 a) Rising edge on channel A, channel B in low state
57 This state is used to recognize a clockwise turn
58
59 b) Rising edge on channel B, channel A in high state
60 When entering this state, the encoder is put into 'armed' state,
61 meaning that there it has seen half the way of a one-step transition.
62
63 c) Falling edge on channel A, channel B in high state
64 This state is used to recognize a counter-clockwise turn
65
66 d) Falling edge on channel B, channel A in low state
67 Parking position. If the encoder enters this state, a full transition
68 should have happened, unless it flipped back on half the way. The
69 'armed' state tells us about that.
70
71 Platform requirements
72 ---------------------
73
74 As there is no hardware dependent call in this driver, the platform it is
75 used with must support gpiolib. Another requirement is that IRQs must be
76 able to fire on both edges.
77
78
79 Board integration
80 -----------------
81
82 To use this driver in your system, register a platform_device with the
83 name 'rotary-encoder' and associate the IRQs and some specific platform
84 data with it. Because the driver uses generic device properties, this can
85 be done either via device tree, ACPI, or using static board files, like in
86 example below:
87
88 ::
89
90 /* board support file example */
91
92 #include <linux/input.h>
93 #include <linux/gpio/machine.h>
94 #include <linux/property.h>
95
96 #define GPIO_ROTARY_A 1
97 #define GPIO_ROTARY_B 2
98
99 static struct gpiod_lookup_table rotary_encoder_gpios = {
100 .dev_id = "rotary-encoder.0",
101 .table = {
102 GPIO_LOOKUP_IDX("gpio-0",
103 GPIO_ROTARY_A, NULL, 0, GPIO_ACTIVE_LOW),
104 GPIO_LOOKUP_IDX("gpio-0",
105 GPIO_ROTARY_B, NULL, 1, GPIO_ACTIVE_HIGH),
106 { },
107 },
108 };
109
110 static const struct property_entry rotary_encoder_properties[] = {
111 PROPERTY_ENTRY_U32("rotary-encoder,steps-per-period", 24),
112 PROPERTY_ENTRY_U32("linux,axis", ABS_X),
113 PROPERTY_ENTRY_U32("rotary-encoder,relative_axis", 0),
114 { },
115 };
116
117 static const struct software_node rotary_encoder_node = {
118 .properties = rotary_encoder_properties,
119 };
120
121 static struct platform_device rotary_encoder_device = {
122 .name = "rotary-encoder",
123 .id = 0,
124 };
125
126 ...
127
128 gpiod_add_lookup_table(&rotary_encoder_gpios);
129 device_add_software_node(&rotary_encoder_device.dev, &rotary_encoder_node);
130 platform_device_register(&rotary_encoder_device);
131
132 ...
133
134 Please consult device tree binding documentation to see all properties
135 supported by the driver.
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