Based on kernel version 4.10.8. Page generated on 2017-04-01 14:43 EST.
1 rotary-encoder - a generic driver for GPIO connected devices 2 Daniel Mack <daniel@caiaq.de>, Feb 2009 3 4 0. Function 5 ----------- 6 7 Rotary encoders are devices which are connected to the CPU or other 8 peripherals with two wires. The outputs are phase-shifted by 90 degrees 9 and by triggering on falling and rising edges, the turn direction can 10 be determined. 11 12 Some encoders have both outputs low in stable states, others also have 13 a stable state with both outputs high (half-period mode) and some have 14 a stable state in all steps (quarter-period mode). 15 16 The phase diagram of these two outputs look like this: 17 18 _____ _____ _____ 19 | | | | | | 20 Channel A ____| |_____| |_____| |____ 21 22 : : : : : : : : : : : : 23 __ _____ _____ _____ 24 | | | | | | | 25 Channel B |_____| |_____| |_____| |__ 26 27 : : : : : : : : : : : : 28 Event a b c d a b c d a b c d 29 30 |<-------->| 31 one step 32 33 |<-->| 34 one step (half-period mode) 35 36 |<>| 37 one step (quarter-period mode) 38 39 For more information, please see 40 https://en.wikipedia.org/wiki/Rotary_encoder 41 42 43 1. Events / state machine 44 ------------------------- 45 46 In half-period mode, state a) and c) above are used to determine the 47 rotational direction based on the last stable state. Events are reported in 48 states b) and d) given that the new stable state is different from the last 49 (i.e. the rotation was not reversed half-way). 50 51 Otherwise, the following apply: 52 53 a) Rising edge on channel A, channel B in low state 54 This state is used to recognize a clockwise turn 55 56 b) Rising edge on channel B, channel A in high state 57 When entering this state, the encoder is put into 'armed' state, 58 meaning that there it has seen half the way of a one-step transition. 59 60 c) Falling edge on channel A, channel B in high state 61 This state is used to recognize a counter-clockwise turn 62 63 d) Falling edge on channel B, channel A in low state 64 Parking position. If the encoder enters this state, a full transition 65 should have happened, unless it flipped back on half the way. The 66 'armed' state tells us about that. 67 68 2. Platform requirements 69 ------------------------ 70 71 As there is no hardware dependent call in this driver, the platform it is 72 used with must support gpiolib. Another requirement is that IRQs must be 73 able to fire on both edges. 74 75 76 3. Board integration 77 -------------------- 78 79 To use this driver in your system, register a platform_device with the 80 name 'rotary-encoder' and associate the IRQs and some specific platform 81 data with it. 82 83 struct rotary_encoder_platform_data is declared in 84 include/linux/rotary-encoder.h and needs to be filled with the number of 85 steps the encoder has and can carry information about externally inverted 86 signals (because of an inverting buffer or other reasons). The encoder 87 can be set up to deliver input information as either an absolute or relative 88 axes. For relative axes the input event returns +/-1 for each step. For 89 absolute axes the position of the encoder can either roll over between zero 90 and the number of steps or will clamp at the maximum and zero depending on 91 the configuration. 92 93 Because GPIO to IRQ mapping is platform specific, this information must 94 be given in separately to the driver. See the example below. 95 96 ---------<snip>--------- 97 98 /* board support file example */ 99 100 #include <linux/input.h> 101 #include <linux/rotary_encoder.h> 102 103 #define GPIO_ROTARY_A 1 104 #define GPIO_ROTARY_B 2 105 106 static struct rotary_encoder_platform_data my_rotary_encoder_info = { 107 .steps = 24, 108 .axis = ABS_X, 109 .relative_axis = false, 110 .rollover = false, 111 .gpio_a = GPIO_ROTARY_A, 112 .gpio_b = GPIO_ROTARY_B, 113 .inverted_a = 0, 114 .inverted_b = 0, 115 .half_period = false, 116 .wakeup_source = false, 117 }; 118 119 static struct platform_device rotary_encoder_device = { 120 .name = "rotary-encoder", 121 .id = 0, 122 .dev = { 123 .platform_data = &my_rotary_encoder_info, 124 } 125 };