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Linux/Documentation/input/rotary-encoder.txt

  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 };
126 

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