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//****************************************************************************/
//@section LICENSE
//
//Copyright (c) 2010 ARM Limited
//
//Permission is hereby granted, free of charge, to any person obtaining a copy
//of this software and associated documentation files (the "Software"), to deal
//in the Software without restriction, including without limitation the rights
//to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//copies of the Software, and to permit persons to whom the Software is
//furnished to do so, subject to the following conditions:
//
//The above copyright notice and this permission notice shall be included in
//all copies or substantial portions of the Software.
//
//THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
//THE SOFTWARE.
//****************************************************************************/
//@section DESCRIPTION
//
// Quadrature Encoder Interface.
//
// A quadrature encoder consists of two code tracks on a disk which are 90
// degrees out of phase. It can be used to determine how far a wheel has
// rotated, relative to a known starting position.
//
// Only one code track changes at a time leading to a more robust system than
// a single track, because any jitter around any edge won't cause a state
// change as the other track will remain constant.
//
// Encoders can be a homebrew affair, consisting of infrared emitters/receivers
// and paper code tracks consisting of alternating black and white sections;
// alternatively, complete disk and PCB emitter/receiver encoder systems can
// be bought, but the interface, regardless of implementation is the same.
//
//               +-----+     +-----+     +-----+
// Channel A     |  ^  |     |     |     |     |
//            ---+  ^  +-----+     +-----+     +-----
//               ^  ^
//               ^  +-----+     +-----+     +-----+
// Channel B     ^  |     |     |     |     |     |
//            ------+     +-----+     +-----+     +-----
//               ^  ^
//               ^  ^
//               90deg
//
// This interface uses X4 encoding which calculates the pulse count based on
// reading the current state after each rising and falling edge of either
// channel.
//
//               +-----+     +-----+     +-----+
// Channel A     |     |     |     |     |     |
//            ---+     +-----+     +-----+     +-----
//               ^     ^     ^     ^     ^
//               ^  +-----+  ^  +-----+  ^  +-----+
// Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
//            ------+  ^  +-----+  ^  +-----+     +--
//               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
//               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
// Pulse count 0 1  2  3  4  5  6  7  8  9  ...
//
// An optional index channel can be used which determines when a full
// revolution has occured.
//
// If a 4 pules per revolution encoder was used, the following would be
// observed.
//
//               +-----+     +-----+     +-----+
// Channel A     |     |     |     |     |     |
//            ---+     +-----+     +-----+     +-----
//               ^     ^     ^     ^     ^
//               ^  +-----+  ^  +-----+  ^  +-----+
// Channel B     ^  |  ^  |  ^  |  ^  |  ^  |     |
//            ------+  ^  +-----+  ^  +-----+     +--
//               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
//               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
//               ^  ^  ^  +--+  ^  ^  +--+  ^
//               ^  ^  ^  |  |  ^  ^  |  |  ^
// Index      ------------+  +--------+  +-----------
//               ^  ^  ^  ^  ^  ^  ^  ^  ^  ^
// Pulse count 0 1  2  3  4  5  6  7  8  9  ...
// Rev.  count 0          1           2
//
// Rotational position in degrees can be calculated by:
//
// (pulse count / X * N) * 360
//
// Where X is the encoding type [in our case X=4], and N is the number of
// pulses per revolution.
//
// Linear position can be calculated by:
//
// (pulse count / X * N) * (1 / PPI)
//
// Where X is encoding type [in our case X=4], N is the number of pulses per
// revolution, and PPI is pulses per inch, or the equivalent for any other
// unit of displacement. PPI can be calculated by taking the circumference
// of the wheel or encoder disk and dividing it by the number of pulses per
// revolution.
//****************************************************************************/

#ifndef QEI_H
#define QEI_H

//****************************************************************************/
// Includes
//****************************************************************************/
#include "mbed.h"

//****************************************************************************/
// Defines
//****************************************************************************/
#define PREV_MASK 0x1 //Mask for the previous state in determining direction
//of rotation.
#define CURR_MASK 0x2 //Mask for the current state in determining direction
//of rotation.
#define INVALID   0x3 //XORing two states where both bits have changed.

/**
 * Quadrature Encoder Interface.
 */
class QEI {

public:

    /**
     * Constructor.
     *
     * Reads the current values on channel A and channel B to determine the
     * initial state.
     *
     * Attaches the encode function to the rise/fall interrupt edges of
     * channels A and B to perform X4 encoding.
     *
     * Attaches the index function to the rise interrupt edge of channel index
     * (if it is used) to count revolutions.
     *
     * @param channelA mbed pin for channel A input.
     * @param channelB mbed pin for channel B input.
     * @param index    mbed pin for optional index channel input,
     *                 (pass NC if not needed).
     * @param pulsesPerRev Number of pulses in one revolution.
     */
    QEI(PinName channelA, PinName channelB, PinName index, int pulsesPerRev);

    /**
     * Reset the encoder.
     *
     * Sets the pulses and revolutions count to zero.
     */
    void reset(void);

    /**
     * Read the state of the encoder.
     *
     * @return The current state of the encoder as a 2-bit number, where:
     *         bit 1 = The reading from channel B
     *         bit 2 = The reading from channel A
     */
    int getCurrentState(void);

    /**
     * Read the number of pulses recorded by the encoder.
     *
     * @return Number of pulses which have occured.
     */
    int getPulses(void);

private:

    /**
     * Update the pulse count.
     *
     * Called on every rising/falling edge of channels A/B.
     *
     * Reads the state of the channels and determines whether a pulse forward
     * or backward has occured, updating the count appropriately.
     */
    void encode(void);

    /**
     * Called on every rising edge of channel index to update revolution
     * count by one.
     */
    void index(void);

    InterruptIn* channelA_;
    InterruptIn* channelB_;
    InterruptIn* index_;

    int          pulsesPerRev_;
    int          revolutions_;
    int          prevState_;
    int          currState_;

    volatile int pulses_;

};

#endif /* QEI_H */