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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 disc 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.
//****************************************************************************/

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

QEI::QEI(PinName channelA, PinName channelB, PinName index, int pulsesPerRev) {

    channelA_ = new InterruptIn(channelA);
    channelB_ = new InterruptIn(channelB);
    index_    = new InterruptIn(index);

    pulses_       = 0;
    revolutions_  = 0;
    pulsesPerRev_ = pulsesPerRev;

    //Workout what the current state is.
    int chanA = channelA_->read();
    int chanB = channelB_->read();

    //2-bit state.
    currState_ = (chanA << 1) | (chanB);
    prevState_ = currState_;

    channelA_->rise(this, &QEI::encode);
    channelA_->fall(this, &QEI::encode);
    channelB_->rise(this, &QEI::encode);
    channelB_->fall(this, &QEI::encode);
    //Index is optional.
    if (index !=  NC) {
        index_->rise(this, &QEI::index);
    }

}

void QEI::reset(void) {

    pulses_      = 0;
    revolutions_ = 0;

}

int QEI::getCurrentState(void) {

    return currState_;

}

int QEI::getPulses(void) {

    return pulses_;

}

// There are four possible states for a quadrature encoder which correspond to
// 2-bit gray code.
//
// A state change is only valid if of only one bit has changed.
// A state change is invalid if both bits have changed.
//
// Clockwise Rotation ->
//
//    00 01 11 10 00
//
// <- Counter Clockwise Rotation
//
// If we observe any valid state changes going from left to right, we have
// moved one pulse clockwise [we will consider this "backward" or "negative"].
//
// If we observe any valid state changes going from right to left we have
// moved one pulse counter clockwise [we will consider this "forward" or
// "positive"].
//
// We might enter an invalid state for a number of reasons which are hard to
// predict - if this is the case, it is generally safe to ignore it, update
// the state and carry on, with the error correcting itself shortly after.
void QEI::encode(void) {

    int change = 0;
    int chanA  = channelA_->read();
    int chanB  = channelB_->read();

    //2-bit state.
    currState_ = (chanA << 1) | (chanB);

    //Entered an invalid state, or no change.
    if ((currState_ ^ prevState_) == INVALID || currState_ == prevState_) {
        //Even if the state was invalid, it will eventually
        //correct itself if we simply update the state.
        prevState_ = currState_;
    }
    //Entered a valid state.
    else {
        //2 bit state. Right hand bit of prev XOR left hand bit of current
        //gives 0 if clockwise rotation and 1 if counter clockwise rotation.
        change = (prevState_ & PREV_MASK) ^ ((currState_ & CURR_MASK) >> 1);

        if (change == 0) {
            change = -1;
        }

        pulses_ -= change;
        prevState_ = currState_;
    }

}

void QEI::index(void) {

    revolutions_++;

}