Ini
[env:fj_compass_samd21]
platform = atmelsam
board = mkrzero
framework = arduino
monitor_speed = 115200
lib_deps =
  avishorp/TM1637@^1.2.0

Cpp
#include 
#include 
#include 
#include 

// Schematic net mapping
#define PIN_I2C_SDA PIN_WIRE_SDA
#define PIN_I2C_SCL PIN_WIRE_SCL
#define PIN_DISP_CLK 16
#define PIN_DISP_DIO 17
#define PIN_CAL_BTN 27
#define PIN_MAG_INT 15

// MMC5883MA I2C address is commonly 0x30 for the MEMSIC MMC58xx family.
// Confirm against the exact populated device before production release.
#define MMC5883_ADDR 0x30

TM1637Display display(PIN_DISP_CLK, PIN_DISP_DIO);

struct MagSample {
  float x;
  float y;
  float z;
};

MagSample hardIronOffset = {0.0f, 0.0f, 0.0f};
MagSample softIronScale = {1.0f, 1.0f, 1.0f};
unsigned long lastUpdateMs = 0;
bool calibrationRequested = false;

static bool i2cWrite8(uint8_t reg, uint8_t value) {
  Wire.beginTransmission(MMC5883_ADDR);
  Wire.write(reg);
  Wire.write(value);
  return Wire.endTransmission() == 0;
}

static bool i2cRead(uint8_t reg, uint8_t *buf, size_t len) {
  Wire.beginTransmission(MMC5883_ADDR);
  Wire.write(reg);
  if (Wire.endTransmission(false) != 0) return false;
  size_t n = Wire.requestFrom(MMC5883_ADDR, len);
  for (size_t i = 0; i = 360) bearing = 0;
  display.showNumberDec(bearing, false, 3, 1);
}

static void runCalibrationRoutine() {
  display.showNumberDec(8888, false);
  Serial.println("Calibration: rotate vehicle slowly through full circle, away from large metal objects if possible.");

  MagSample minV = {999999.0f, 999999.0f, 999999.0f};
  MagSample maxV = {-999999.0f, -999999.0f, -999999.0f};
  unsigned long start = millis();

  while (millis() - start  1.0f) softIronScale.x = avgSpan / spanX;
  if (spanY > 1.0f) softIronScale.y = avgSpan / spanY;
  if (spanZ > 1.0f) softIronScale.z = avgSpan / spanZ;

  Serial.println("Calibration complete. Store these constants in NVM for production firmware.");
  calibrationRequested = false;
}

void setup() {
  Serial.begin(115200);
  delay(1000);

  pinMode(PIN_CAL_BTN, INPUT_PULLUP);
  pinMode(PIN_MAG_INT, INPUT);

  Wire.begin();
  Wire.setClock(400000);

  display.setBrightness(3, true);
  display.clear();

  initMagnetometer();
  Serial.println("FJ Cruiser digital compass firmware started.");
}

void loop() {
  if (digitalRead(PIN_CAL_BTN) == LOW) {
    delay(30);
    if (digitalRead(PIN_CAL_BTN) == LOW) calibrationRequested = true;
  }

  if (calibrationRequested) {
    runCalibrationRoutine();
  }

  if (millis() - lastUpdateMs >= 200UL) {
    lastUpdateMs = millis();
    MagSample raw;
    if (readMagnetometerRaw(raw)) {
      MagSample calibrated = applyCalibration(raw);
      float heading = headingDegrees(calibrated);
      showHeading(heading);
      Serial.print("Heading: ");
      Serial.print(heading, 1);
      Serial.print(" deg ");
      Serial.println(cardinalFromHeading(heading));
    } else {
      display.showNumberDec(0, false);
      Serial.println("Magnetometer read failed");
    }
  }
}

• Run calibration after mounting in the FJ Cruiser dashboard because nearby steel, wiring harnesses, speakers, and brackets affect heading.
• Keep the magnetometer physically separated from the buck inductor and display current paths.
• Store final hard-iron offsets and soft-iron scale factors in nonvolatile memory after production firmware is finalized.
• Add tilt compensation later if the dashboard mounting angle or off-road pitch/roll error is significant.