Add expression pedal calibration: cal min/cal max

- Remove fixed 0-4095 mapping, use cal_min/cal_max instead
- Auto-calibrate minimum at boot (assumes pedal at heel)
- 'cal min' sets current position as heel (0)
- 'cal max' sets current position as toe (127)
- Map cal_min->0, cal_max->127 with clamping
- Update 'exp' command to show raw ADC + cal values

include/expression_pedal.h

@@ -11,6 +11,11 @@ public:
     void begin();
     void update(UsbMidiTransport& midi);
     uint8_t get_value() const { return current_value; }
+    uint16_t get_raw() const { return current_raw; }
+    void set_cal_min();
+    void set_cal_max();
+    uint16_t get_cal_min() const { return cal_min; }
+    uint16_t get_cal_max() const { return cal_max; }
 
 private:
     uint8_t adc_pin;
@@ -23,5 +28,9 @@ private:
     static const uint16_t READ_INTERVAL_MS = 20;
     uint32_t last_read_time;
 
+    uint16_t current_raw;
+    uint16_t cal_min;
+    uint16_t cal_max;
+
     uint8_t adc_to_midi(uint16_t adc_value);
 };

src/expression_pedal.cpp

@@ -9,18 +9,24 @@ ExpressionPedal::ExpressionPedal(uint8_t adc_pin)
     , midi_channel(1)
     , midi_cc(4)
     , last_read_time(0)
+    , current_raw(0)
+    , cal_min(0)
+    , cal_max(4095)
 {
 }
 
 void ExpressionPedal::begin() {
-    Serial.printf("[EXP] Expression pedal on ADC1_CH4 (GPIO%d)\n", adc_pin);
+    Serial.printf("[EXP] Expression pedal on ADC1_CH3 (GPIO%d)\n", adc_pin);
     analogReadResolution(12);
     pinMode(adc_pin, INPUT);
     delay(10);
-    uint16_t initial = analogRead(adc_pin);
-    current_value = adc_to_midi(initial);
+    current_raw = analogRead(adc_pin);
+    cal_min = current_raw;
+    current_value = adc_to_midi(current_raw);
     last_sent_value = current_value;
-    Serial.printf("[EXP] Initial ADC: %d -> MIDI: %d\n", initial, current_value);
+    Serial.printf("[EXP] Calibrating... press toe then type 'cal max'\n");
+    Serial.printf("[EXP] Current ADC: %d -> MIDI: %d\n", current_raw, current_value);
+    Serial.printf("[EXP] cal_min=%d cal_max=%d (default)\n", cal_min, cal_max);
 }
 
 void ExpressionPedal::update(UsbMidiTransport& midi) {
@@ -28,8 +34,8 @@ void ExpressionPedal::update(UsbMidiTransport& midi) {
     if (now - last_read_time < READ_INTERVAL_MS) return;
     last_read_time = now;
 
-    uint16_t raw = analogRead(adc_pin);
-    uint8_t new_value = adc_to_midi(raw);
+    current_raw = analogRead(adc_pin);
+    uint8_t new_value = adc_to_midi(current_raw);
 
     if (new_value > current_value + HYSTERESIS ||
         new_value + HYSTERESIS < current_value ||
@@ -44,7 +50,22 @@ void ExpressionPedal::update(UsbMidiTransport& midi) {
     }
 }
 
+void ExpressionPedal::set_cal_min() {
+    cal_min = current_raw;
+    if (cal_min >= cal_max) cal_max = cal_min + 1;
+    Serial.printf("[EXP] cal_min set to %d (ADC raw)\n", cal_min);
+}
+
+void ExpressionPedal::set_cal_max() {
+    cal_max = current_raw;
+    if (cal_max <= cal_min) cal_min = cal_max - 1;
+    Serial.printf("[EXP] cal_max set to %d (ADC raw)\n", cal_max);
+}
+
 uint8_t ExpressionPedal::adc_to_midi(uint16_t adc_value) {
-    uint32_t scaled = (uint32_t)adc_value * 127 / 4095;
-    return (uint8_t)scaled;
+    if (adc_value <= cal_min) return 0;
+    if (adc_value >= cal_max) return 127;
+    uint32_t span = cal_max - cal_min;
+    uint32_t offset = adc_value - cal_min;
+    return (uint8_t)(offset * 127 / span);
 }

src/main.cpp

@@ -87,8 +87,13 @@ void handle_serial_command(const String& cmd) {
         mux.show();
         Serial.println("[CMD] Pixel 1 WHITE (max brightness)");
     } else if (cmd == "exp") {
-        uint16_t raw = analogRead(5);
-        Serial.printf("[CMD] EXP GPIO5 ADC=%d MIDI=%d\n", raw, exp_pedal.get_value());
+        Serial.printf("[CMD] EXP ADC=%d MIDI=%d cal_min=%d cal_max=%d\n",
+                      exp_pedal.get_raw(), exp_pedal.get_value(),
+                      exp_pedal.get_cal_min(), exp_pedal.get_cal_max());
+    } else if (cmd == "cal min") {
+        exp_pedal.set_cal_min();
+    } else if (cmd == "cal max") {
+        exp_pedal.set_cal_max();
     } else if (cmd == "usb") {
         Serial.printf("[CMD] USB mounted: %s\n", TinyUSBDevice.mounted() ? "YES" : "NO");
         Serial.printf("[CMD] USB ready: %s\n", TinyUSBDevice.ready() ? "YES" : "NO");
@@ -132,7 +137,9 @@ void handle_serial_command(const String& cmd) {
         Serial.println("  read       - raw button read");
         Serial.println("  red/green/blue - solid colour");
         Serial.println("  pixel0/pixel1 - single pixel test");
-        Serial.println("  exp        - expression pedal ADC/MIDI value");
+        Serial.println("  exp        - expression pedal ADC/MIDI/cal values");
+        Serial.println("  cal min    - set current pedal position as minimum (heel)");
+        Serial.println("  cal max    - set current pedal position as maximum (toe)");
         Serial.println("  usb        - USB connection status and descriptor info");
         Serial.println("  gpiotest   - raw GPIO pin diagnostic");
         Serial.println("  rawled     - bit-bang WS2812 (no library)");