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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, 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 };
121 

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