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135 lines
4.1 KiB
135 lines
4.1 KiB
============================================================ |
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rotary-encoder - a generic driver for GPIO connected devices |
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============================================================ |
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:Author: Daniel Mack <[email protected]>, 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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Channel A ____| |_____| |_____| |____ |
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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[] = { |
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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 const struct software_node rotary_encoder_node = { |
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.properties = rotary_encoder_properties, |
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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_software_node(&rotary_encoder_device.dev, &rotary_encoder_node); |
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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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