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Documentation / input / rotary-encoder.txt




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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 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, others also have
13	a stable state with both outputs high (half-period mode) and some have
14	a stable state in all steps (quarter-period mode).
15	
16	The phase diagram of these two outputs look like this:
17	
18	                  _____       _____       _____
19	                 |     |     |     |     |     |
20	  Channel A  ____|     |_____|     |_____|     |____
21	
22	                 :  :  :  :  :  :  :  :  :  :  :  :
23	            __       _____       _____       _____
24	              |     |     |     |     |     |     |
25	  Channel B   |_____|     |_____|     |_____|     |__
26	
27	                 :  :  :  :  :  :  :  :  :  :  :  :
28	  Event          a  b  c  d  a  b  c  d  a  b  c  d
29	
30	                |<-------->|
31		          one step
32	
33	                |<-->|
34		          one step (half-period mode)
35	
36	                |<>|
37		          one step (quarter-period mode)
38	
39	For more information, please see
40		https://en.wikipedia.org/wiki/Rotary_encoder
41	
42	
43	1. Events / state machine
44	-------------------------
45	
46	In half-period mode, state a) and c) above are used to determine the
47	rotational direction based on the last stable state. Events are reported in
48	states b) and d) given that the new stable state is different from the last
49	(i.e. the rotation was not reversed half-way).
50	
51	Otherwise, the following apply:
52	
53	a) Rising edge on channel A, channel B in low state
54		This state is used to recognize a clockwise turn
55	
56	b) Rising edge on channel B, channel A in high state
57		When entering this state, the encoder is put into 'armed' state,
58		meaning that there it has seen half the way of a one-step transition.
59	
60	c) Falling edge on channel A, channel B in high state
61		This state is used to recognize a counter-clockwise turn
62	
63	d) Falling edge on channel B, channel A in low state
64		Parking position. If the encoder enters this state, a full transition
65		should have happened, unless it flipped back on half the way. The
66		'armed' state tells us about that.
67	
68	2. Platform requirements
69	------------------------
70	
71	As there is no hardware dependent call in this driver, the platform it is
72	used with must support gpiolib. Another requirement is that IRQs must be
73	able to fire on both edges.
74	
75	
76	3. Board integration
77	--------------------
78	
79	To use this driver in your system, register a platform_device with the
80	name 'rotary-encoder' and associate the IRQs and some specific platform
81	data with it.
82	
83	struct rotary_encoder_platform_data is declared in
84	include/linux/rotary-encoder.h and needs to be filled with the number of
85	steps the encoder has and can carry information about externally inverted
86	signals (because of an inverting buffer or other reasons). The encoder
87	can be set up to deliver input information as either an absolute or relative
88	axes. For relative axes the input event returns +/-1 for each step. For
89	absolute axes the position of the encoder can either roll over between zero
90	and the number of steps or will clamp at the maximum and zero depending on
91	the configuration.
92	
93	Because GPIO to IRQ mapping is platform specific, this information must
94	be given in separately to the driver. See the example below.
95	
96	---------<snip>---------
97	
98	/* board support file example */
99	
100	#include <linux/input.h>
101	#include <linux/rotary_encoder.h>
102	
103	#define GPIO_ROTARY_A 1
104	#define GPIO_ROTARY_B 2
105	
106	static struct rotary_encoder_platform_data my_rotary_encoder_info = {
107		.steps		= 24,
108		.axis		= ABS_X,
109		.relative_axis	= false,
110		.rollover	= false,
111		.gpio_a		= GPIO_ROTARY_A,
112		.gpio_b		= GPIO_ROTARY_B,
113		.inverted_a	= 0,
114		.inverted_b	= 0,
115		.half_period	= false,
116		.wakeup_source	= false,
117	};
118	
119	static struct platform_device rotary_encoder_device = {
120		.name		= "rotary-encoder",
121		.id		= 0,
122		.dev		= {
123			.platform_data = &my_rotary_encoder_info,
124		}
125	};
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