rotary-encoder.rst

  1============================================================
  2rotary-encoder - a generic driver for GPIO connected devices
  3============================================================
  4
  5:Author: Daniel Mack <daniel@caiaq.de>, Feb 2009
  6
  7Function
  8--------
  9
 10Rotary encoders are devices which are connected to the CPU or other
 11peripherals with two wires. The outputs are phase-shifted by 90 degrees
 12and by triggering on falling and rising edges, the turn direction can
 13be determined.
 14
 15Some encoders have both outputs low in stable states, others also have
 16a stable state with both outputs high (half-period mode) and some have
 17a stable state in all steps (quarter-period mode).
 18
 19The phase diagram of these two outputs look like this::
 20
 21                  _____       _____       _____
 22                 |     |     |     |     |     |
 23  Channel A  ____|     |_____|     |_____|     |____
 24
 25                 :  :  :  :  :  :  :  :  :  :  :  :
 26            __       _____       _____       _____
 27              |     |     |     |     |     |     |
 28  Channel B   |_____|     |_____|     |_____|     |__
 29
 30                 :  :  :  :  :  :  :  :  :  :  :  :
 31  Event          a  b  c  d  a  b  c  d  a  b  c  d
 32
 33                |<-------->|
 34	          one step
 35
 36                |<-->|
 37	          one step (half-period mode)
 38
 39                |<>|
 40	          one step (quarter-period mode)
 41
 42For more information, please see
 43	https://en.wikipedia.org/wiki/Rotary_encoder
 44
 45
 46Events / state machine
 47----------------------
 48
 49In half-period mode, state a) and c) above are used to determine the
 50rotational direction based on the last stable state. Events are reported in
 51states b) and d) given that the new stable state is different from the last
 52(i.e. the rotation was not reversed half-way).
 53
 54Otherwise, the following apply:
 55
 56a) Rising edge on channel A, channel B in low state
 57	This state is used to recognize a clockwise turn
 58
 59b) Rising edge on channel B, channel A in high state
 60	When entering this state, the encoder is put into 'armed' state,
 61	meaning that there it has seen half the way of a one-step transition.
 62
 63c) Falling edge on channel A, channel B in high state
 64	This state is used to recognize a counter-clockwise turn
 65
 66d) Falling edge on channel B, channel A in low state
 67	Parking position. If the encoder enters this state, a full transition
 68	should have happened, unless it flipped back on half the way. The
 69	'armed' state tells us about that.
 70
 71Platform requirements
 72---------------------
 73
 74As there is no hardware dependent call in this driver, the platform it is
 75used with must support gpiolib. Another requirement is that IRQs must be
 76able to fire on both edges.
 77
 78
 79Board integration
 80-----------------
 81
 82To use this driver in your system, register a platform_device with the
 83name 'rotary-encoder' and associate the IRQs and some specific platform
 84data with it. Because the driver uses generic device properties, this can
 85be done either via device tree, ACPI, or using static board files, like in
 86example below:
 87
 88::
 89
 90	/* board support file example */
 91
 92	#include <linux/input.h>
 93	#include <linux/gpio/machine.h>
 94	#include <linux/property.h>
 95
 96	#define GPIO_ROTARY_A 1
 97	#define GPIO_ROTARY_B 2
 98
 99	static struct gpiod_lookup_table rotary_encoder_gpios = {
100		.dev_id = "rotary-encoder.0",
101		.table = {
102			GPIO_LOOKUP_IDX("gpio-0",
103					GPIO_ROTARY_A, NULL, 0, GPIO_ACTIVE_LOW),
104			GPIO_LOOKUP_IDX("gpio-0",
105					GPIO_ROTARY_B, NULL, 1, GPIO_ACTIVE_HIGH),
106			{ },
107		},
108	};
109
110	static const struct property_entry rotary_encoder_properties[] = {
111		PROPERTY_ENTRY_U32("rotary-encoder,steps-per-period", 24),
112		PROPERTY_ENTRY_U32("linux,axis",		      ABS_X),
113		PROPERTY_ENTRY_U32("rotary-encoder,relative_axis",    0),
114		{ },
115	};
116
117	static const struct software_node rotary_encoder_node = {
118		.properties = rotary_encoder_properties,
119	};
120
121	static struct platform_device rotary_encoder_device = {
122		.name		= "rotary-encoder",
123		.id		= 0,
124	};
125
126	...
127
128	gpiod_add_lookup_table(&rotary_encoder_gpios);
129	device_add_software_node(&rotary_encoder_device.dev, &rotary_encoder_node);
130	platform_device_register(&rotary_encoder_device);
131
132	...
133
134Please consult device tree binding documentation to see all properties
135supported by the driver.