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