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Documentation / input / rotary-encoder.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.

1	rotary-encoder - a generic driver for GPIO connected devices
2	Daniel Mack <daniel@caiaq.de>, Feb 2009
3	
4	0. Function
5	-----------
6	
7	Rotary encoders are devices which are connected to the CPU or other
8	peripherals with two wires. The outputs are phase-shifted by 90 degrees
9	and by triggering on falling and rising edges, the turn direction can
10	be determined.
11	
12	Some encoders have both outputs low in stable states, whereas others also have
13	a stable state with both outputs high (half-period mode).
14	
15	The phase diagram of these two outputs look like this:
16	
17	                  _____       _____       _____
18	                 |     |     |     |     |     |
19	  Channel A  ____|     |_____|     |_____|     |____
20	
21	                 :  :  :  :  :  :  :  :  :  :  :  :
22	            __       _____       _____       _____
23	              |     |     |     |     |     |     |
24	  Channel B   |_____|     |_____|     |_____|     |__
25	
26	                 :  :  :  :  :  :  :  :  :  :  :  :
27	  Event          a  b  c  d  a  b  c  d  a  b  c  d
28	
29	                |<-------->|
30		          one step
31	
32	                |<-->|
33		          one step (half-period mode)
34	
35	For more information, please see
36		http://en.wikipedia.org/wiki/Rotary_encoder
37	
38	
39	1. Events / state machine
40	-------------------------
41	
42	In half-period mode, state a) and c) above are used to determine the
43	rotational direction based on the last stable state. Events are reported in
44	states b) and d) given that the new stable state is different from the last
45	(i.e. the rotation was not reversed half-way).
46	
47	Otherwise, the following apply:
48	
49	a) Rising edge on channel A, channel B in low state
50		This state is used to recognize a clockwise turn
51	
52	b) Rising edge on channel B, channel A in high state
53		When entering this state, the encoder is put into 'armed' state,
54		meaning that there it has seen half the way of a one-step transition.
55	
56	c) Falling edge on channel A, channel B in high state
57		This state is used to recognize a counter-clockwise turn
58	
59	d) Falling edge on channel B, channel A in low state
60		Parking position. If the encoder enters this state, a full transition
61		should have happened, unless it flipped back on half the way. The
62		'armed' state tells us about that.
63	
64	2. Platform requirements
65	------------------------
66	
67	As there is no hardware dependent call in this driver, the platform it is
68	used with must support gpiolib. Another requirement is that IRQs must be
69	able to fire on both edges.
70	
71	
72	3. Board integration
73	--------------------
74	
75	To use this driver in your system, register a platform_device with the
76	name 'rotary-encoder' and associate the IRQs and some specific platform
77	data with it.
78	
79	struct rotary_encoder_platform_data is declared in
80	include/linux/rotary-encoder.h and needs to be filled with the number of
81	steps the encoder has and can carry information about externally inverted
82	signals (because of an inverting buffer or other reasons). The encoder
83	can be set up to deliver input information as either an absolute or relative
84	axes. For relative axes the input event returns +/-1 for each step. For
85	absolute axes the position of the encoder can either roll over between zero
86	and the number of steps or will clamp at the maximum and zero depending on
87	the configuration.
88	
89	Because GPIO to IRQ mapping is platform specific, this information must
90	be given in separately to the driver. See the example below.
91	
92	---------<snip>---------
93	
94	/* board support file example */
95	
96	#include <linux/input.h>
97	#include <linux/rotary_encoder.h>
98	
99	#define GPIO_ROTARY_A 1
100	#define GPIO_ROTARY_B 2
101	
102	static struct rotary_encoder_platform_data my_rotary_encoder_info = {
103		.steps		= 24,
104		.axis		= ABS_X,
105		.relative_axis	= false,
106		.rollover	= false,
107		.gpio_a		= GPIO_ROTARY_A,
108		.gpio_b		= GPIO_ROTARY_B,
109		.inverted_a	= 0,
110		.inverted_b	= 0,
111		.half_period	= false,
112	};
113	
114	static struct platform_device rotary_encoder_device = {
115		.name		= "rotary-encoder",
116		.id		= 0,
117		.dev		= {
118			.platform_data = &my_rotary_encoder_info,
119		}
120	};
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