SparkFun Triple Axis Accelerometer Breakout - ADXL335

/*
 * ADXL335 3-Axis Analog Accelerometer with Arduino
 * ------------------------------------------------------------
 * The ADXL335 gives a separate analog voltage for each axis (X, Y, Z).
 * 0 g sits near half the supply voltage; acceleration/tilt shifts each
 * output by ~330 mV per g (at a 3.3 V supply).
 *
 *   >>> The ADXL335 is a 3.3 V part (1.8-3.6 V). Do NOT power it from 5 V. <<<
 *
 * For accurate readings, power the sensor from 3.3 V and wire that SAME
 * 3.3 V rail to the Arduino's AREF pin, then use analogReference(EXTERNAL)
 * so the ADC and the sensor share one reference voltage.
 *
 * Wiring (Arduino Uno / Nano, 10-bit ADC):
 *   ADXL335 VCC  -> 3.3 V
 *   ADXL335 GND  -> GND
 *   ADXL335 X    -> A0
 *   ADXL335 Y    -> A1
 *   ADXL335 Z    -> A2
 *   Arduino 3.3V -> AREF        (the same rail that feeds VCC)
 *   ADXL335 ST   -> leave unconnected (self-test pin)
 *
 * Calibration:
 *   1. Set CALIBRATE_MODE = true, upload, open Serial Monitor (9600 baud).
 *   2. Lay the board flat: Z reads ~1 g, X and Y read ~0 g. Note the
 *      0 g raw count for each axis -> ZERO_X / ZERO_Y / ZERO_Z.
 *   3. Point one axis straight up, then straight down; the two raw counts
 *      are 2 g apart, so (up - down) / 2 = COUNTS_PER_G.
 *   4. Set CALIBRATE_MODE = false and re-upload.
 */

const bool CALIBRATE_MODE = false;  // true = print raw ADC counts for calibration

// ----- Pins -----
const uint8_t PIN_X = A0;
const uint8_t PIN_Y = A1;
const uint8_t PIN_Z = A2;

// ----- Calibration constants (update from CALIBRATE_MODE output) -----
const int   ZERO_X       = 512;     // raw count at 0 g
const int   ZERO_Y       = 512;     // raw count at 0 g
const int   ZERO_Z       = 512;     // raw count at 0 g
const float COUNTS_PER_G = 102.0f;  // ~330 mV/g / (3.3 V / 1023) with 3.3 V AREF

const uint8_t SAMPLES = 16;         // averaged reads per axis (noise reduction)

int readAxis(uint8_t pin) {
  long sum = 0;
  for (uint8_t i = 0; i < SAMPLES; i++) sum += analogRead(pin);
  return (int)(sum / SAMPLES);
}

float toG(int raw, int zero) {
  return (raw - zero) / COUNTS_PER_G;
}

void setup() {
  Serial.begin(9600);
  // MUST set EXTERNAL before any analogRead() when AREF is driven externally,
  // otherwise the internal reference is shorted to AREF (can damage the MCU).
  analogReference(EXTERNAL);
  for (uint8_t i = 0; i < 8; i++) { analogRead(PIN_X); delay(2); } // let ADC settle
}

void loop() {
  int rawX = readAxis(PIN_X);
  int rawY = readAxis(PIN_Y);
  int rawZ = readAxis(PIN_Z);

  if (CALIBRATE_MODE) {
    Serial.print("RAW  X="); Serial.print(rawX);
    Serial.print("  Y=");    Serial.print(rawY);
    Serial.print("  Z=");    Serial.println(rawZ);
  } else {
    Serial.print("X="); Serial.print(toG(rawX, ZERO_X), 2); Serial.print(" g  ");
    Serial.print("Y="); Serial.print(toG(rawY, ZERO_Y), 2); Serial.print(" g  ");
    Serial.print("Z="); Serial.print(toG(rawZ, ZERO_Z), 2); Serial.println(" g");
  }

  delay(200);
}