• MCU: ATmega328PB-AU
• Framework: Arduino-compatible AVR C++
• Build system: PlatformIO
• Key libraries: Wire built in, TM1637Display

Ini
[env:atmega328pb]
platform = atmelavr
board = ATmega328PB
framework = arduino
board_build.f_cpu = 8000000L
monitor_speed = 115200
lib_deps =
    avishorp/TM1637@^1.2.0

Cpp
#include <Arduino.h>
#include <Wire.h>
#include <TM1637Display.h>
#include <math.h>

// Schematic pin mapping
static constexpr uint8_t PIN_TM_CLK = 5;   // U1 PD5 -> U3 CLK
static constexpr uint8_t PIN_TM_DIO = 6;   // U1 PD6 -> U3 DIO
static constexpr uint8_t LIS3MDL_ADDR = 0x1C; // SA1 tied low; use 0x1E if board/library variant reports alternate address

// LIS3MDL registers
static constexpr uint8_t REG_WHO_AM_I = 0x0F;
static constexpr uint8_t REG_CTRL_REG1 = 0x20;
static constexpr uint8_t REG_CTRL_REG2 = 0x21;
static constexpr uint8_t REG_CTRL_REG3 = 0x22;
static constexpr uint8_t REG_CTRL_REG4 = 0x23;
static constexpr uint8_t REG_OUT_X_L = 0x28;

TM1637Display display(PIN_TM_CLK, PIN_TM_DIO);

struct MagRaw {
  int16_t x;
  int16_t y;
  int16_t z;
};

// Calibration values: update after vehicle/dashboard calibration sweep.
float hardIronX = 0.0f;
float hardIronY = 0.0f;
float softScaleX = 1.0f;
float softScaleY = 1.0f;

bool i2cWrite8(uint8_t reg, uint8_t value) {
  Wire.beginTransmission(LIS3MDL_ADDR);
  Wire.write(reg);
  Wire.write(value);
  return Wire.endTransmission() == 0;
}

bool i2cRead(uint8_t reg, uint8_t *buf, size_t len) {
  Wire.beginTransmission(LIS3MDL_ADDR);
  Wire.write(reg);
  if (Wire.endTransmission(false) != 0) return false;
  size_t got = Wire.requestFrom(LIS3MDL_ADDR, (uint8_t)len);
  if (got != len) return false;
  for (size_t i = 0; i < len; ++i) buf[i] = Wire.read();
  return true;
}

bool initLIS3MDL() {
  uint8_t who = 0;
  if (!i2cRead(REG_WHO_AM_I, &who, 1)) return false;

  // Ultra-high-performance XY, 10 Hz ODR: CTRL_REG1 = 0b01110000
  // +/-4 gauss full scale: CTRL_REG2 = 0b00000000
  // Continuous-conversion mode: CTRL_REG3 = 0b00000000
  // Ultra-high-performance Z: CTRL_REG4 = 0b00001100
  bool ok = true;
  ok &= i2cWrite8(REG_CTRL_REG1, 0x70);
  ok &= i2cWrite8(REG_CTRL_REG2, 0x00);
  ok &= i2cWrite8(REG_CTRL_REG3, 0x00);
  ok &= i2cWrite8(REG_CTRL_REG4, 0x0C);
  return ok;
}

bool readMag(MagRaw &m) {
  uint8_t b[6];
  if (!i2cRead(REG_OUT_X_L | 0x80, b, sizeof(b))) return false;
  m.x = (int16_t)((uint16_t)b[1] << 8 | b[0]);
  m.y = (int16_t)((uint16_t)b[3] << 8 | b[2]);
  m.z = (int16_t)((uint16_t)b[5] << 8 | b[4]);
  return true;
}

int headingDegrees(const MagRaw &m) {
  float x = ((float)m.x - hardIronX) * softScaleX;
  float y = ((float)m.y - hardIronY) * softScaleY;
  float heading = atan2f(y, x) * 180.0f / PI;
  if (heading < 0.0f) heading += 360.0f;
  int deg = (int)lroundf(heading);
  if (deg >= 360) deg -= 360;
  return deg;
}

void showHeading(int deg) {
  // Shows 000-359 on the three wired digits. Leading zeroes are useful for compass readout.
  display.showNumberDec(deg, true, 3, 0);
}

void setup() {
  Serial.begin(115200);
  Wire.begin();
  Wire.setClock(100000);

  display.setBrightness(2); // Keep low for solar power budget; raise if needed.
  display.clear();

  if (!initLIS3MDL()) {
    Serial.println(F("LIS3MDL init failed"));
    display.showNumberDec(888, true, 3, 0);
  } else {
    Serial.println(F("Solar compass firmware started"));
  }
}

void loop() {
  static uint32_t lastRead = 0;
  if (millis() - lastRead < 250) return;
  lastRead = millis();

  MagRaw mag;
  if (readMag(mag)) {
    int deg = headingDegrees(mag);
    showHeading(deg);
    Serial.print(F("Heading: "));
    Serial.println(deg);
  } else {
    Serial.println(F("Mag read failed"));
    display.showNumberDec(999, true, 3, 0);
  }
}

• Build: pio run -e atmega328pb
• Flash: use an AVR ISP programmer because the schematic does not include a USB interface; command depends on programmer, e.g. pio run -e atmega328pb -t upload --upload-port .
• Calibrate after installation by rotating the assembled unit in the vehicle and updating the hard-iron/soft-iron constants.