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-rotary-encoder - a generic driver for GPIO connected devices
-Daniel Mack <daniel@caiaq.de>, Feb 2009
-
-0. Function
------------
-
-Rotary encoders are devices which are connected to the CPU or other
-peripherals with two wires. The outputs are phase-shifted by 90 degrees
-and by triggering on falling and rising edges, the turn direction can
-be determined.
-
-Some encoders have both outputs low in stable states, whereas others also have
-a stable state with both outputs high (half-period mode).
-
-The phase diagram of these two outputs look like this:
-
- _____ _____ _____
- | | | | | |
- Channel A ____| |_____| |_____| |____
-
- : : : : : : : : : : : :
- __ _____ _____ _____
- | | | | | | |
- Channel B |_____| |_____| |_____| |__
-
- : : : : : : : : : : : :
- Event a b c d a b c d a b c d
-
- |<-------->|
- one step
-
- |<-->|
- one step (half-period mode)
-
-For more information, please see
- http://en.wikipedia.org/wiki/Rotary_encoder
-
-
-1. Events / state machine
--------------------------
-
-In half-period mode, state a) and c) above are used to determine the
-rotational direction based on the last stable state. Events are reported in
-states b) and d) given that the new stable state is different from the last
-(i.e. the rotation was not reversed half-way).
-
-Otherwise, the following apply:
-
-a) Rising edge on channel A, channel B in low state
- This state is used to recognize a clockwise turn
-
-b) Rising edge on channel B, channel A in high state
- When entering this state, the encoder is put into 'armed' state,
- meaning that there it has seen half the way of a one-step transition.
-
-c) Falling edge on channel A, channel B in high state
- This state is used to recognize a counter-clockwise turn
-
-d) Falling edge on channel B, channel A in low state
- Parking position. If the encoder enters this state, a full transition
- should have happened, unless it flipped back on half the way. The
- 'armed' state tells us about that.
-
-2. Platform requirements
-------------------------
-
-As there is no hardware dependent call in this driver, the platform it is
-used with must support gpiolib. Another requirement is that IRQs must be
-able to fire on both edges.
-
-
-3. Board integration
---------------------
-
-To use this driver in your system, register a platform_device with the
-name 'rotary-encoder' and associate the IRQs and some specific platform
-data with it.
-
-struct rotary_encoder_platform_data is declared in
-include/linux/rotary-encoder.h and needs to be filled with the number of
-steps the encoder has and can carry information about externally inverted
-signals (because of an inverting buffer or other reasons). The encoder
-can be set up to deliver input information as either an absolute or relative
-axes. For relative axes the input event returns +/-1 for each step. For
-absolute axes the position of the encoder can either roll over between zero
-and the number of steps or will clamp at the maximum and zero depending on
-the configuration.
-
-Because GPIO to IRQ mapping is platform specific, this information must
-be given in separately to the driver. See the example below.
-
----------<snip>---------
-
-/* board support file example */
-
-#include <linux/input.h>
-#include <linux/rotary_encoder.h>
-
-#define GPIO_ROTARY_A 1
-#define GPIO_ROTARY_B 2
-
-static struct rotary_encoder_platform_data my_rotary_encoder_info = {
- .steps = 24,
- .axis = ABS_X,
- .relative_axis = false,
- .rollover = false,
- .gpio_a = GPIO_ROTARY_A,
- .gpio_b = GPIO_ROTARY_B,
- .inverted_a = 0,
- .inverted_b = 0,
- .half_period = false,
-};
-
-static struct platform_device rotary_encoder_device = {
- .name = "rotary-encoder",
- .id = 0,
- .dev = {
- .platform_data = &my_rotary_encoder_info,
- }
-};
-