Arduino Rotary Encoder with Automatic Debounce

RotaryEncoder.h

/* Rotary encoder handler for arduino. v2.0
 *
 * Original author of version 1.1: Ben Buxton <bb@cactii.net>
 * https://github.com/buxtronix/arduino/tree/master/libraries/Rotary
 *
 * Author of version 2.0 based on 1.1: Frank Martinez <https://twitter.com/mnesarco>
 *
 * Copyright 2011 Ben Buxton. Licensed under the GNU GPL Version 3.
 * Contact: bb@cactii.net
 *
 * Copyright 2022 Frank Martinez.
 * Contact: <https://twitter.com/mnesarco>
 *
 * +------------------------------------------------------+
 * | Changes introduced in version 2.0:                   |
 * |                                                      |
 * | 1. Make it a Header only library.                    |
 * | 2. Removed macros, replaced by c++ templates.        |
 * | 3. Added begin() method because pinMode should not   |
 * |    be called before setup().                         |  
 * +------------------------------------------------------+
 * | Usage:                                               |
 * |                                                      |
 * | Rotary<HalfStepEncoder, INPUT_PULLUP> half(2,3);     |
 * | Rotary<FullStepEncoder, INPUT_PULLUP> full(4,5);     |
 * |                                                      |
 * | void setup()                                         |
 * | {                                                    |
 * |   half.begin();                                      |
 * |   full.begin();                                      |
 * | }                                                    |
 * |                                                      |
 * | loop()                                               |
 * | {                                                    |
 * |   unsigned char v1 = half.process();                 |
 * |   unsigned char v2 = full.process();                 |
 * | }                                                    |
 * |                                                      |
 * | Rest is the same as of 1.1                           |
 * +------------------------------------------------------+
 *
 * A typical mechanical rotary encoder emits a two bit gray code
 * on 3 output pins. Every step in the output (often accompanied
 * by a physical 'click') generates a specific sequence of output
 * codes on the pins.
 *
 * There are 3 pins used for the rotary encoding - one common and
 * two 'bit' pins.
 *
 * The following is the typical sequence of code on the output when
 * moving from one step to the next:
 *
 *   Position   Bit1   Bit2
 *   ----------------------
 *     Step1     0      0
 *      1/4      1      0
 *      1/2      1      1
 *      3/4      0      1
 *     Step2     0      0
 *
 * From this table, we can see that when moving from one 'click' to
 * the next, there are 4 changes in the output code.
 *
 * - From an initial 0 - 0, Bit1 goes high, Bit0 stays low.
 * - Then both bits are high, halfway through the step.
 * - Then Bit1 goes low, but Bit2 stays high.
 * - Finally at the end of the step, both bits return to 0.
 *
 * Detecting the direction is easy - the table simply goes in the other
 * direction (read up instead of down).
 *
 * To decode this, we use a simple state machine. Every time the output
 * code changes, it follows state, until finally a full steps worth of
 * code is received (in the correct order). At the final 0-0, it returns
 * a value indicating a step in one direction or the other.
 *
 * It's also possible to use 'half-step' mode. This just emits an event
 * at both the 0-0 and 1-1 positions. This might be useful for some
 * encoders where you want to detect all positions.
 *
 * If an invalid state happens (for example we go from '0-1' straight
 * to '1-0'), the state machine resets to the start until 0-0 and the
 * next valid codes occur.
 *
 * The biggest advantage of using a state machine over other algorithms
 * is that this has inherent debounce built in. Other algorithms emit spurious
 * output with switch bounce, but this one will simply flip between
 * sub-states until the bounce settles, then continue along the state
 * machine.
 * A side effect of debounce is that fast rotations can cause steps to
 * be skipped. By not requiring debounce, fast rotations can be accurately
 * measured.
 * Another advantage is the ability to properly handle bad state, such
 * as due to EMI, etc.
 * It is also a lot simpler than others - a static state table and less
 * than 10 lines of logic.
 */

#ifndef RotaryEncoder_h
#define RotaryEncoder_h

#include <Arduino.h>

struct HalfStepEncoder {};
struct FullStepEncoder {};

// Values returned by 'process'
// No complete step yet.
static constexpr unsigned char DIR_NONE     = 0x0;
// Clockwise step.
static constexpr unsigned char DIR_CW       = 0x10;
// Anti-clockwise step.
static constexpr unsigned char DIR_CCW      = 0x20;

/*
 * The below state table has, for each state (row), the new state
 * to set based on the next encoder output. From left to right in,
 * the table, the encoder outputs are 00, 01, 10, 11, and the value
 * in that position is the new state to set.
 */

static constexpr unsigned char R_START        = 0x0;

// Full Step States
static constexpr unsigned char RF_CW_FINAL     = 0x1;
static constexpr unsigned char RF_CW_BEGIN     = 0x2;
static constexpr unsigned char RF_CW_NEXT      = 0x3;
static constexpr unsigned char RF_CCW_BEGIN    = 0x4;
static constexpr unsigned char RF_CCW_FINAL    = 0x5;
static constexpr unsigned char RF_CCW_NEXT     = 0x6;

template<typename EncoderType = FullStepEncoder>
constexpr unsigned char ttable[7][4] = {
  // R_START
  {R_START,     RF_CW_BEGIN,  RF_CCW_BEGIN, R_START},
  // RF_CW_FINAL
  {RF_CW_NEXT,  R_START,      RF_CW_FINAL,  R_START | DIR_CW},
  // RF_CW_BEGIN
  {RF_CW_NEXT,  RF_CW_BEGIN,  R_START,      R_START},
  // RF_CW_NEXT
  {RF_CW_NEXT,  RF_CW_BEGIN,  RF_CW_FINAL,  R_START},
  // RF_CCW_BEGIN
  {RF_CCW_NEXT, R_START,      RF_CCW_BEGIN, R_START},
  // RF_CCW_FINAL
  {RF_CCW_NEXT, RF_CCW_FINAL, R_START,      R_START | DIR_CCW},
  // RF_CCW_NEXT
  {RF_CCW_NEXT, RF_CCW_FINAL, RF_CCW_BEGIN, R_START},
};

// Half Step States
static constexpr unsigned char RH_CCW_BEGIN    = 0x1;
static constexpr unsigned char RH_CW_BEGIN     = 0x2;
static constexpr unsigned char RH_START_M      = 0x3;
static constexpr unsigned char RH_CW_BEGIN_M   = 0x4;
static constexpr unsigned char RH_CCW_BEGIN_M  = 0x5;

template<>
constexpr unsigned char ttable<HalfStepEncoder>[6][4] = {
  // R_START (00)
  {RH_START_M,            RH_CW_BEGIN,     RH_CCW_BEGIN,  R_START},
  // RH_CCW_BEGIN
  {RH_START_M | DIR_CCW,  R_START,         RH_CCW_BEGIN,  R_START},
  // RH_CW_BEGIN
  {RH_START_M | DIR_CW,   RH_CW_BEGIN,     R_START,       R_START},
  // RH_START_M (11)
  {RH_START_M,            RH_CCW_BEGIN_M,  RH_CW_BEGIN_M, R_START},
  // RH_CW_BEGIN_M
  {RH_START_M,            RH_START_M,      RH_CW_BEGIN_M, R_START | DIR_CW},
  // RH_CCW_BEGIN_M
  {RH_START_M,            RH_CCW_BEGIN_M,  RH_START_M,    R_START | DIR_CCW},
};

template<typename EncoderType = FullStepEncoder, uint8_t PinMode = INPUT_PULLUP>
class Rotary
{
  public:
    Rotary(char _pin1, char _pin2) {
      pin1 = _pin1;
      pin2 = _pin2;
      state = R_START;
    }

    void begin()
    {
      pinMode(pin1, PinMode);
      pinMode(pin2, PinMode);
    }

    unsigned char process() {
      // Grab state of input pins.
      unsigned char pinstate = (digitalRead(pin2) << 1) | digitalRead(pin1);
      // Determine new state from the pins and state table.
      state = ttable<EncoderType>[state & 0xf][pinstate];
      // Return emit bits, ie the generated event.
      return state & 0x30;
    }

  private:
    unsigned char state;
    unsigned char pin1;
    unsigned char pin2;
};

#endif