Digital Spirit Level

// Digital Spirit Level
// ADXL362 (SPI, 3.3V, via level converter, CS on D10) + 16x2 LCD
//
// Gravity is a constant 1 g downward. How that 1 g divides between the
// axes gives the tilt, so the angle comes from atan2 of one axis against
// the other two -- no fluid, no bubble, and it can be re-zeroed anywhere.

#include <SPI.h>
#include <LiquidCrystal.h>

const int CS_PIN = 10;

// ADXL362 SPI instruction bytes
const byte CMD_WRITE_REG = 0x0A;
const byte CMD_READ_REG  = 0x0B;

// ADXL362 registers
const byte REG_XDATA_L   = 0x0E;   // X low; Y and Z follow consecutively
const byte REG_FILTER_CTL= 0x2C;
const byte REG_POWER_CTL = 0x2D;

LiquidCrystal lcd(7, 6, 5, 4, 3, 2);   // RS, E, D4, D5, D6, D7

float zeroPitch = 0.0, zeroRoll = 0.0;

void writeReg(byte reg, byte value) {
  digitalWrite(CS_PIN, LOW);
  SPI.transfer(CMD_WRITE_REG);
  SPI.transfer(reg);
  SPI.transfer(value);
  digitalWrite(CS_PIN, HIGH);
}

// Burst-reads X, Y, Z as three consecutive 12-bit signed values.
void readXYZ(int &x, int &y, int &z) {
  digitalWrite(CS_PIN, LOW);
  SPI.transfer(CMD_READ_REG);
  SPI.transfer(REG_XDATA_L);
  byte xl = SPI.transfer(0x00), xh = SPI.transfer(0x00);
  byte yl = SPI.transfer(0x00), yh = SPI.transfer(0x00);
  byte zl = SPI.transfer(0x00), zh = SPI.transfer(0x00);
  digitalWrite(CS_PIN, HIGH);

  x = (int)((xh << 8) | xl);
  y = (int)((yh << 8) | yl);
  z = (int)((zh << 8) | zl);

  // 12-bit two's complement sign-extended into 16 bits
  if (x > 2047) x -= 4096;
  if (y > 2047) y -= 4096;
  if (z > 2047) z -= 4096;
}

void readAngles(float &pitch, float &roll) {
  int x, y, z;
  readXYZ(x, y, z);
  pitch = atan2((float)x, sqrt((float)y * y + (float)z * z)) * 180.0 / PI;
  roll  = atan2((float)y, sqrt((float)x * x + (float)z * z)) * 180.0 / PI;
}

void setup() {
  Serial.begin(9600);
  pinMode(CS_PIN, OUTPUT);
  digitalWrite(CS_PIN, HIGH);

  SPI.begin();
  SPI.setDataMode(SPI_MODE0);
  SPI.setBitOrder(MSBFIRST);

  lcd.begin(16, 2);
  lcd.print(F("Digital Level"));

  delay(10);
  writeReg(REG_FILTER_CTL, 0x13);  // +/-2 g range, 100 Hz output data rate
  writeReg(REG_POWER_CTL,  0x02);  // measurement mode (0x00 would be standby)
  delay(100);

  // Whatever it is sitting on at power-up becomes 'flat'.
  readAngles(zeroPitch, zeroRoll);
  lcd.clear();
}

void loop() {
  float pitch, roll;
  readAngles(pitch, roll);
  pitch -= zeroPitch;
  roll  -= zeroRoll;

  lcd.setCursor(0, 0);
  lcd.print(F("Pitch "));
  lcd.print(pitch, 1);
  lcd.print(F("\xDF   "));   // degree symbol in the HD44780 character set

  lcd.setCursor(0, 1);
  lcd.print(F("Roll  "));
  lcd.print(roll, 1);
  lcd.print(F("\xDF   "));

  Serial.print(pitch, 2); Serial.print(F(",")); Serial.println(roll, 2);
  delay(200);
}