Diff: QEI.h

Revision:

0:e3b759ab4b5c

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+++ b/QEI.h	Mon Aug 02 18:53:02 2010 +0000
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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 */