• MCU: ATSAMD21G18A-AU
• Framework: Arduino SAMD core
• Build system: PlatformIO
• Purpose: Read LIS3MDL heading data, read dimmer ADC, monitor charger status, and drive IS31FL3731 display matrix.

Ini
[env:zeroUSB]
platform = atmelsam
board = zeroUSB
framework = arduino
monitor_speed = 115200
lib_deps =

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

static constexpr uint8_t LIS3MDL_ADDR = 0x1C;
static constexpr uint8_t IS31_ADDR = 0x74;

static constexpr uint8_t PIN_DIMMER_ADC = A0;
static constexpr uint8_t PIN_MAG_INT = 4;
static constexpr uint8_t PIN_MAG_DRDY = 5;
static constexpr uint8_t PIN_CHG_STAT1 = 6;
static constexpr uint8_t PIN_CHG_STAT2 = 7;

static uint8_t displayFrame[3] = {0, 0, 0};

static void i2cWrite8(uint8_t addr, uint8_t reg, uint8_t value) {
  Wire.beginTransmission(addr);
  Wire.write(reg);
  Wire.write(value);
  Wire.endTransmission();
}

static uint8_t i2cRead8(uint8_t addr, uint8_t reg) {
  Wire.beginTransmission(addr);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(addr, (uint8_t)1);
  if (Wire.available()) return Wire.read();
  return 0;
}

static int16_t i2cRead16LE(uint8_t addr, uint8_t reg) {
  Wire.beginTransmission(addr);
  Wire.write(reg | 0x80);
  Wire.endTransmission(false);
  Wire.requestFrom(addr, (uint8_t)2);
  uint8_t lo = Wire.available() ? Wire.read() : 0;
  uint8_t hi = Wire.available() ? Wire.read() : 0;
  return (int16_t)((hi << 8) | lo);
}

static bool initLIS3MDL() {
  uint8_t who = i2cRead8(LIS3MDL_ADDR, 0x0F);
  if (who != 0x3D) return false;
  i2cWrite8(LIS3MDL_ADDR, 0x20, 0x70); // CTRL_REG1: 80 Hz, high-performance XY
  i2cWrite8(LIS3MDL_ADDR, 0x21, 0x00); // CTRL_REG2: +/-4 gauss
  i2cWrite8(LIS3MDL_ADDR, 0x22, 0x00); // CTRL_REG3: continuous conversion
  i2cWrite8(LIS3MDL_ADDR, 0x23, 0x0C); // CTRL_REG4: high-performance Z
  i2cWrite8(LIS3MDL_ADDR, 0x24, 0x40); // CTRL_REG5: block data update
  return true;
}

static void initIS31FL3731() {
  i2cWrite8(IS31_ADDR, 0xFD, 0x0B); // Function register page
  i2cWrite8(IS31_ADDR, 0x0A, 0x00); // Audio sync off
  i2cWrite8(IS31_ADDR, 0x00, 0x00); // Picture mode
  i2cWrite8(IS31_ADDR, 0x01, 0x00); // Display frame 0
  i2cWrite8(IS31_ADDR, 0x0B, 0x00); // Shutdown normal operation

  i2cWrite8(IS31_ADDR, 0xFD, 0x00); // Frame 0 page
  for (uint8_t r = 0x00; r <= 0x11; r++) i2cWrite8(IS31_ADDR, r, 0x00);
  for (uint8_t r = 0x24; r <= 0xB3; r++) i2cWrite8(IS31_ADDR, r, 0x20);
}

static uint8_t sevenSegForChar(char c) {
  switch (c) {
    case 'N': return 0b00110111;
    case 'E': return 0b01111001;
    case 'S': return 0b01101101;
    case 'W': return 0b00111110;
    case ' ': return 0;
    default: return 0;
  }
}

static void setDisplayText(const char text[3], uint8_t brightness) {
  i2cWrite8(IS31_ADDR, 0xFD, 0x00);
  for (uint8_t digit = 0; digit < 3; digit++) {
    displayFrame[digit] = sevenSegForChar(text[digit]);
    for (uint8_t seg = 0; seg < 8; seg++) {
      bool on = displayFrame[digit] & (1 << seg);
      uint8_t pwmReg = 0x24 + digit * 16 + seg;
      i2cWrite8(IS31_ADDR, pwmReg, on ? brightness : 0);
    }
  }
}

static float readHeadingDegrees() {
  int16_t x = i2cRead16LE(LIS3MDL_ADDR, 0x28);
  int16_t y = i2cRead16LE(LIS3MDL_ADDR, 0x2A);
  float heading = atan2f((float)y, (float)x) * 180.0f / PI;
  if (heading < 0.0f) heading += 360.0f;
  return heading;
}

static void headingToText(float deg, char out[3]) {
  const char *dir = "N  ";
  if (deg >= 22.5f && deg < 67.5f) dir = "NE ";
  else if (deg >= 67.5f && deg < 112.5f) dir = "E  ";
  else if (deg >= 112.5f && deg < 157.5f) dir = "SE ";
  else if (deg >= 157.5f && deg < 202.5f) dir = "S  ";
  else if (deg >= 202.5f && deg < 247.5f) dir = "SW ";
  else if (deg >= 247.5f && deg < 292.5f) dir = "W  ";
  else if (deg >= 292.5f && deg < 337.5f) dir = "NW ";
  out[0] = dir[0];
  out[1] = dir[1];
  out[2] = dir[2];
}

static uint8_t readBrightness() {
  uint16_t raw = analogRead(PIN_DIMMER_ADC);
  return (uint8_t)map(raw, 0, 1023, 2, 255);
}

static void printChargerStatus() {
  bool stat1High = digitalRead(PIN_CHG_STAT1);
  bool stat2High = digitalRead(PIN_CHG_STAT2);
  Serial.print("STAT1="); Serial.print(stat1High);
  Serial.print(" STAT2="); Serial.println(stat2High);
}

void setup() {
  pinMode(PIN_MAG_INT, INPUT);
  pinMode(PIN_MAG_DRDY, INPUT);
  pinMode(PIN_CHG_STAT1, INPUT);
  pinMode(PIN_CHG_STAT2, INPUT);
  analogReadResolution(10);

  Serial.begin(115200);
  Wire.begin();
  Wire.setClock(100000);

  delay(100);
  bool magOk = initLIS3MDL();
  initIS31FL3731();
  Serial.println(magOk ? "LIS3MDL OK" : "LIS3MDL missing");
}

void loop() {
  char text[3];
  float heading = readHeadingDegrees();
  headingToText(heading, text);
  uint8_t brightness = readBrightness();
  setDisplayText(text, brightness);

  Serial.print("Heading="); Serial.print(heading, 1);
  Serial.print(" Display="); Serial.write(text, 3);
  Serial.print(" Brightness="); Serial.println(brightness);
  printChargerStatus();

  delay(250);
}

• Build: pio run
• Flash using SWD-capable workflow supported by the chosen SAMD programmer/debug probe.
• Serial monitor, if a UART/USB serial path is later added: pio device monitor

• The schematic currently reserves SWD pins for programming. No USB or UART debug connector is wired yet.
• Magnetometer calibration is required in final firmware for accurate in-vehicle heading.
• The 7-segment character map is a starter approximation; tune glyphs once display orientation is confirmed.