Add resistance capability for BLE Sense

Adds the functionality to measure resistance with the Nano 33 BLE Sense.

Author: manuelzomer

examples/Nano33BLESenseFirmware/Nano33BLESenseFirmware.ino

@@ -12,6 +12,9 @@ const int VERSION = 0x00000001;
 const float TEMPERATURE_CALIBRATION = -5.0;
 
 #define SCIENCE_KIT_UUID(val) ("555a0002-" val "-467a-9538-01f0652c74e8")
+#define RESISTANCE_PIN A0
+#define INPUT_VOLTAGE 3.3
+#define VOLTAGE_BUFFER_SIZE 16
 
 //#define DEBUG 0
 
@@ -26,8 +29,12 @@ BLEFloatCharacteristic         humidityCharacteristic     (SCIENCE_KIT_UUID("001
 BLEUnsignedIntCharacteristic   proximityCharacteristic    (SCIENCE_KIT_UUID("0017"), BLENotify);
 BLECharacteristic              colorCharacteristic        (SCIENCE_KIT_UUID("0018"), BLENotify, 4 * sizeof(int));
 BLEUnsignedShortCharacteristic soundPressureCharacteristic(SCIENCE_KIT_UUID("0019"), BLENotify);
+BLEFloatCharacteristic         resistanceCharacteristic   (SCIENCE_KIT_UUID("0020"), BLENotify);
 
+short voltageBufferIndex = 0;
+bool voltageBufferFilled = false;
 short soundSampleBuffer[256];
+short voltageSampleBuffer[VOLTAGE_BUFFER_SIZE];
 
 void onPDMdata() {
   // query the number of bytes available
@@ -45,6 +52,26 @@ uint16_t getSoundAverage() {
   return sqrt(avg);
 }
 
+void readVoltage() {
+  voltageSampleBuffer[voltageBufferIndex] = analogRead(RESISTANCE_PIN);
+  voltageBufferIndex++;
+  if (!voltageBufferFilled && voltageBufferIndex == VOLTAGE_BUFFER_SIZE) {
+    voltageBufferFilled = true;
+  }
+  voltageBufferIndex=voltageBufferIndex%VOLTAGE_BUFFER_SIZE;
+}
+
+uint16_t getVoltageAverage() {
+  uint32_t avg = 0;
+  for (int i = 0; i < VOLTAGE_BUFFER_SIZE; i++) {
+    avg += voltageSampleBuffer[i];
+  }
+  if (voltageBufferFilled) {
+    return avg/VOLTAGE_BUFFER_SIZE;
+  }
+  return avg/voltageBufferIndex;
+}
+
 // String to calculate the local and device name
 String name;
 unsigned long lastNotify = 0;
@@ -75,6 +102,8 @@ void setup() {
 
   delay(2000);
 
+  pinMode(RESISTANCE_PIN, INPUT); // Used for reading resistance
+
   if (!APDS.begin()) {
     printSerialMsg("Failed to initialized APDS!");
     blinkLoop();
@@ -142,6 +171,7 @@ void setup() {
   service.addCharacteristic(proximityCharacteristic);
   service.addCharacteristic(colorCharacteristic);
   service.addCharacteristic(soundPressureCharacteristic);
+  service.addCharacteristic(resistanceCharacteristic);
 
   versionCharacteristic.setValue(VERSION);
 
@@ -213,4 +243,11 @@ void updateSubscribedCharacteristics() {
     float pressure = BARO.readPressure();
     pressureCharacteristic.writeValue(pressure);
   }
+
+  if(resistanceCharacteristic.subscribed()){
+    readVoltage();
+    uint16_t measuredValue = getVoltageAverage();
+    float measuredVoltage = measuredValue / 1024.0f * INPUT_VOLTAGE;
+    resistanceCharacteristic.writeValue(measuredVoltage);
+  }
 }