132 lines
4 KiB
ReStructuredText
132 lines
4 KiB
ReStructuredText
============================================================
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rotary-encoder - a generic driver for GPIO connected devices
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============================================================
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:Author: Daniel Mack <daniel@caiaq.de>, Feb 2009
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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, others also have
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a stable state with both outputs high (half-period mode) and some have
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a stable state in all steps (quarter-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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|<>|
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one step (quarter-period mode)
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For more information, please see
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https://en.wikipedia.org/wiki/Rotary_encoder
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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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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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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. Because the driver uses generic device properties, this can
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be done either via device tree, ACPI, or using static board files, like in
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example below:
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::
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/* board support file example */
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#include <linux/input.h>
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#include <linux/gpio/machine.h>
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#include <linux/property.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 gpiod_lookup_table rotary_encoder_gpios = {
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.dev_id = "rotary-encoder.0",
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.table = {
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GPIO_LOOKUP_IDX("gpio-0",
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GPIO_ROTARY_A, NULL, 0, GPIO_ACTIVE_LOW),
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GPIO_LOOKUP_IDX("gpio-0",
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GPIO_ROTARY_B, NULL, 1, GPIO_ACTIVE_HIGH),
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{ },
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},
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};
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static const struct property_entry rotary_encoder_properties[] __initconst = {
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PROPERTY_ENTRY_U32("rotary-encoder,steps-per-period", 24),
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PROPERTY_ENTRY_U32("linux,axis", ABS_X),
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PROPERTY_ENTRY_U32("rotary-encoder,relative_axis", 0),
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{ },
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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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};
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...
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gpiod_add_lookup_table(&rotary_encoder_gpios);
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device_add_properties(&rotary_encoder_device, rotary_encoder_properties);
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platform_device_register(&rotary_encoder_device);
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...
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Please consult device tree binding documentation to see all properties
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supported by the driver.
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