Diff: QEI.h

Revision:

0:5c2ad81551aa

Child:

1:aea205976bf8

diff -r 000000000000 -r 5c2ad81551aa QEI.h
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+++ b/QEI.h	Thu Sep 02 16:48:55 2010 +0000
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+/**
+ * @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.
+ */
+
+#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:
+
+    typedef enum Encoding {
+
+        X2_ENCODING,
+        X4_ENCODING
+
+    } Encoding;
+
+    /**
+     * 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.
+     * @param encoding The encoding to use. Uses X2 encoding by default. X2
+     *                 encoding uses interrupts on the rising and falling edges
+     *                 of only channel A where as X4 uses them on both
+     *                 channels.
+     */
+    QEI(PinName channelA, PinName channelB, PinName index, int pulsesPerRev, Encoding encoding = X2_ENCODING);
+
+    /**
+     * 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);
+
+    /**
+     * Read the number of revolutions recorded by the encoder on the index channel.
+     *
+     * @return Number of revolutions which have occured on the index channel.
+     */
+    int getRevolutions(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);
+
+    Encoding encoding_;
+
+    InterruptIn channelA_;
+    InterruptIn channelB_;
+    InterruptIn index_;
+
+    int          pulsesPerRev_;
+    int          prevState_;
+    int          currState_;
+
+    volatile int pulses_;
+    volatile int revolutions_;
+
+};
+
+#endif /* QEI_H */