linux/Documentation/input/rotary-encoder.txt
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   1rotary-encoder - a generic driver for GPIO connected devices
   2Daniel Mack <daniel@caiaq.de>, Feb 2009
   3
   40. Function
   5-----------
   6
   7Rotary encoders are devices which are connected to the CPU or other
   8peripherals with two wires. The outputs are phase-shifted by 90 degrees
   9and by triggering on falling and rising edges, the turn direction can
  10be determined.
  11
  12The phase diagram of these two outputs look like this:
  13
  14                  _____       _____       _____
  15                 |     |     |     |     |     |
  16  Channel A  ____|     |_____|     |_____|     |____
  17
  18                 :  :  :  :  :  :  :  :  :  :  :  :
  19            __       _____       _____       _____
  20              |     |     |     |     |     |     |
  21  Channel B   |_____|     |_____|     |_____|     |__
  22
  23                 :  :  :  :  :  :  :  :  :  :  :  :
  24  Event          a  b  c  d  a  b  c  d  a  b  c  d
  25
  26                |<-------->|
  27                  one step
  28
  29
  30For more information, please see
  31        http://en.wikipedia.org/wiki/Rotary_encoder
  32
  33
  341. Events / state machine
  35-------------------------
  36
  37a) Rising edge on channel A, channel B in low state
  38        This state is used to recognize a clockwise turn
  39
  40b) Rising edge on channel B, channel A in high state
  41        When entering this state, the encoder is put into 'armed' state,
  42        meaning that there it has seen half the way of a one-step transition.
  43
  44c) Falling edge on channel A, channel B in high state
  45        This state is used to recognize a counter-clockwise turn
  46
  47d) Falling edge on channel B, channel A in low state
  48        Parking position. If the encoder enters this state, a full transition
  49        should have happend, unless it flipped back on half the way. The
  50        'armed' state tells us about that.
  51
  522. Platform requirements
  53------------------------
  54
  55As there is no hardware dependent call in this driver, the platform it is
  56used with must support gpiolib. Another requirement is that IRQs must be
  57able to fire on both edges.
  58
  59
  603. Board integration
  61--------------------
  62
  63To use this driver in your system, register a platform_device with the
  64name 'rotary-encoder' and associate the IRQs and some specific platform
  65data with it.
  66
  67struct rotary_encoder_platform_data is declared in
  68include/linux/rotary-encoder.h and needs to be filled with the number of
  69steps the encoder has and can carry information about externally inverted
  70signals (because of used invertig buffer or other reasons).
  71
  72Because GPIO to IRQ mapping is platform specific, this information must
  73be given in seperately to the driver. See the example below.
  74
  75---------<snip>---------
  76
  77/* board support file example */
  78
  79#include <linux/input.h>
  80#include <linux/rotary_encoder.h>
  81
  82#define GPIO_ROTARY_A 1
  83#define GPIO_ROTARY_B 2
  84
  85static struct rotary_encoder_platform_data my_rotary_encoder_info = {
  86        .steps          = 24,
  87        .axis           = ABS_X,
  88        .gpio_a         = GPIO_ROTARY_A,
  89        .gpio_b         = GPIO_ROTARY_B,
  90        .inverted_a     = 0,
  91        .inverted_b     = 0,
  92};
  93
  94static struct platform_device rotary_encoder_device = {
  95        .name           = "rotary-encoder",
  96        .id             = 0,
  97        .dev            = {
  98                .platform_data = &my_rotary_encoder_info,
  99        }
 100};
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