File content as of revision 0:fc0482177a6d:
/**
* @author Aaron Berk
*
* @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
*
* The interface uses X2 encoding by default which calculates the pulse count
* based on reading the current state after each rising and falling edge of
* channel A.
*
* +-----+ +-----+ +-----+
* Channel A | | | | | |
* ---+ +-----+ +-----+ +-----
* ^ ^ ^ ^ ^
* ^ +-----+ ^ +-----+ ^ +-----+
* Channel B ^ | ^ | ^ | ^ | ^ | |
* ------+ ^ +-----+ ^ +-----+ +--
* ^ ^ ^ ^ ^
* ^ ^ ^ ^ ^
* Pulse count 0 1 2 3 4 5 ...
*
* This interface can also use 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 ...
*
* It defaults
*
* An optional index channel can be used which determines when a full
* revolution has occured.
*
* If a 4 pules per revolution encoder was used, with X4 encoding,
* 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 [e.g. X4 encoding => 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 [e.g. X4 encoding => X=44], 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,
Encoding encoding) : channelA_(channelA), channelB_(channelB),
index_(index) {
pulses_ = 0;
revolutions_ = 0;
pulsesPerRev_ = pulsesPerRev;
encoding_ = encoding;
//Workout what the current state is.
int chanA = channelA_.read();
int chanB = channelB_.read();
//2-bit state.
currState_ = (chanA << 1) | (chanB);
prevState_ = currState_;
//X2 encoding uses interrupts on only channel A.
//X4 encoding uses interrupts on channel A,
//and on channel B.
channelA_.rise(this, &QEI::encode);
channelA_.fall(this, &QEI::encode);
//If we're using X4 encoding, then attach interrupts to channel B too.
if (encoding == X4_ENCODING) {
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_;
}
int QEI::getRevolutions(void) {
return revolutions_;
}
// +-------------+
// | X2 Encoding |
// +-------------+
//
// When observing states two patterns will appear:
//
// Counter clockwise rotation:
//
// 10 -> 01 -> 10 -> 01 -> ...
//
// Clockwise rotation:
//
// 11 -> 00 -> 11 -> 00 -> ...
//
// We consider counter clockwise rotation to be "forward" and
// counter clockwise to be "backward". Therefore pulse count will increase
// during counter clockwise rotation and decrease during clockwise rotation.
//
// +-------------+
// | X4 Encoding |
// +-------------+
//
// 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);
if (encoding_ == X2_ENCODING) {
//11->00->11->00 is counter clockwise rotation or "forward".
if ((prevState_ == 0x3 && currState_ == 0x0) ||
(prevState_ == 0x0 && currState_ == 0x3)) {
pulses_++;
}
//10->01->10->01 is clockwise rotation or "backward".
else if ((prevState_ == 0x2 && currState_ == 0x1) ||
(prevState_ == 0x1 && currState_ == 0x2)) {
pulses_--;
}
} else if (encoding_ == X4_ENCODING) {
//Entered a new valid state.
if (((currState_ ^ prevState_) != INVALID) && (currState_ != prevState_)) {
//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_++;
}