Quadrature encoder interface by Aaron Berk
Fork of QEI by
QEI.cpp
- Committer:
- aminomar
- Date:
- 2016-05-04
- Revision:
- 1:5f28ceec8280
- Parent:
- 0:5c2ad81551aa
File content as of revision 1:5f28ceec8280:
#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 it follows the sequence, then pulses ++ 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_++; }