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+/* Copyright 2020 Christopher Courtney, aka Drashna Jael're (@drashna) <drashna@live.com>
+ * Copyright 2020 Ploopy Corporation
+ * Copyright 2022 Leorize <leorize+oss@disroot.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#include "opt_encoder.h"
+#include "util.h"
+#include <stdbool.h>
+#include <stdint.h>
+
+/* An alternative implementation for interpreting the encoder status:
+ *
+ * From graphing the phototransistor voltages, the peak and baseline appears to
+ * be rather stable. Therefore there is no need to average them out, and instead
+ * just simply store the min and max voltages of each phototransistor.
+ *
+ * This algorithm then distinguish between high and low states by employing an
+ * approach similar to a Schmitt trigger: a low and high threshold is defined
+ * for each phototransistor based on their min and max voltages.
+ *
+ * Currently, the thresholds are:
+ *
+ * * High threshold: The upper quarter of the voltage range.
+ * * Low threshold: The lower quarter of the voltage range.
+ *
+ * these thresholds are defined for each phototransistor.
+ *
+ * For a state to cross from high -> low, it must fall below the low threshold.
+ * Similarly, to cross from low -> high, the voltage must be higher than the
+ * high threshold.
+ *
+ * Having two distinct thresholds filters out the bulk of noise from the
+ * phototransistors.
+ *
+ * For converting the resulting high and low signals into rotation, a simple
+ * quadrature decoder is used.
+ */
+
+/* The minimum value returned by the ADC */
+#define ENCODER_MIN 0
+/* The maximum value returned by the ADC */
+#define ENCODER_MAX 1023
+
+/* Utilities for composing the encoder state */
+#define MAKE_STATE(HI_A, HI_B) (((uint8_t)((HI_A) & 0x1) << 1) | ((uint8_t)((HI_B) & 0x1)))
+#define STATE_A(st) ((st & 0x2) >> 1)
+#define STATE_B(st) (st & 0x1)
+#define LOLO MAKE_STATE(0, 0)
+#define HILO MAKE_STATE(1, 0)
+#define LOHI MAKE_STATE(0, 1)
+
+typedef enum {
+ CALIBRATION, /* Recalibrate encoder state by waiting for a 01 -> 00 or 10 -> 00 transistion */
+ DECODE /* Translate changes in the encoder state into movement */
+} encoder_state_t;
+
+static encoder_state_t mode;
+
+static uint8_t lastState;
+
+static uint16_t lowA;
+static uint16_t highA;
+static uint16_t lowB;
+static uint16_t highB;
+
+#define MOVE_UP 1
+#define MOVE_DOWN -1
+#define MOVE_NONE 0
+#define MOVE_ERR 0x7F
+static const uint8_t movement[] = {
+ // 00 -> 00, 01, 10, 11
+ MOVE_NONE, MOVE_DOWN, MOVE_UP, MOVE_ERR,
+ // 01 -> 00, 01, 10, 11
+ MOVE_UP, MOVE_NONE, MOVE_ERR, MOVE_DOWN,
+ // 10 -> 00, 01, 10, 11
+ MOVE_DOWN, MOVE_ERR, MOVE_NONE, MOVE_UP,
+ // 11 -> 00, 01, 10, 11
+ MOVE_ERR, MOVE_UP, MOVE_DOWN, MOVE_NONE
+};
+
+void opt_encoder_init(void) {
+ mode = CALIBRATION;
+ lastState = 0;
+
+ lowA = ENCODER_MAX;
+ lowB = ENCODER_MAX;
+ highA = ENCODER_MIN;
+ highB = ENCODER_MIN;
+}
+
+int8_t opt_encoder_handler(uint16_t encA, uint16_t encB) {
+ int8_t result = 0;
+
+ highA = MAX(encA, highA);
+ lowA = MIN(encA, lowA);
+ highB = MAX(encB, highB);
+ lowB = MIN(encB, lowB);
+
+ /* Only compute the thresholds after a large enough range is established */
+ if (highA - lowA > SCROLL_THRESH_RANGE_LIM && highB - lowB > SCROLL_THRESH_RANGE_LIM) {
+ const int16_t lowThresholdA = (highA + lowA) / 4;
+ const int16_t highThresholdA = (highA + lowA) - lowThresholdA;
+ const int16_t lowThresholdB = (highB + lowB) / 4;
+ const int16_t highThresholdB = (highB + lowB) - lowThresholdB;
+
+ uint8_t state = MAKE_STATE(
+ STATE_A(lastState) ? encA > lowThresholdA : encA > highThresholdA,
+ STATE_B(lastState) ? encB > lowThresholdB : encB > highThresholdB
+ );
+
+ switch (mode) {
+ case CALIBRATION:
+ if ((lastState == HILO && state == LOLO)
+ || (lastState == LOHI && state == LOLO))
+ mode = DECODE;
+ else
+ mode = CALIBRATION;
+ break;
+
+ case DECODE:
+ result = movement[lastState * 4 + state];
+ /* If we detect a state change that should not be possible,
+ * then the wheel might have moved too fast and we need to
+ * recalibrate the encoder position. */
+ mode = result == MOVE_ERR ? CALIBRATION : mode;
+ result = result == MOVE_ERR ? MOVE_NONE : result;
+
+ break;
+ }
+
+ lastState = state;
+ }
+
+ return result;
+}