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[] __initconst = { 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 struct platform_device rotary_encoder_device = { 118 .name = "rotary-encoder", 119 .id = 0, 120 }; 121 122 ... 123 124 gpiod_add_lookup_table(&rotary_encoder_gpios); 125 device_add_properties(&rotary_encoder_device, rotary_encoder_properties); 126 platform_device_register(&rotary_encoder_device); 127 128 ... 129 130Please consult device tree binding documentation to see all properties 131supported by the driver. 132