Hiking compass on an Arduino Uno

// Hiking compass: an LSM303AGR reads which way is down and which way is
// north, and an LTR-390 reads the sun's UV index, two things worth knowing
// on a hill.
//
//   LSM303AGR   SDA -> SDA   SCL -> SCL
//   LTR-390     SDA -> A4    SCL -> A5
//
// The compass heading comes from the magnetometer's two level axes, so it is
// only right while the board is flat; the accelerometer says how far it is
// tipped. A UV index of 3 or more wants sun cream.
//
// Things to try. Drag the heading slider round. Tip the board with the
// roll slider. Drag the UV slider past 3.

#include <Wire.h>
#include <Adafruit_LSM303_Accel.h>
#include "Adafruit_LTR390.h"

const uint8_t MAG = 0x1E;

Adafruit_LSM303_Accel_Unified accel(1);
Adafruit_LTR390 uv;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  if (!accel.begin()) {
    Serial.println("No LSM303");
  }
  // The magnetometer half: continuous at 10 Hz.
  Wire.beginTransmission(MAG);
  Wire.write(0x60);
  Wire.write(0x00);
  Wire.endTransmission();
  uv.begin();
  uv.setMode(LTR390_MODE_UVS);
  uv.setGain(LTR390_GAIN_18);
  uv.setResolution(LTR390_RESOLUTION_18BIT);
  Serial.println("Hiking compass");
}

void loop() {
  delay(200);
  int16_t m[3];
  Wire.beginTransmission(MAG);
  Wire.write(0x68);
  Wire.endTransmission(false);
  Wire.requestFrom(MAG, (uint8_t)6);
  for (uint8_t i = 0; i < 3; i++) {
    uint8_t lo = Wire.read();
    m[i] = (int16_t)(Wire.read() << 8 | lo);
  }
  float heading = atan2((float)m[1], (float)m[0]) * 180 / PI;
  if (heading < 0) heading += 360;
  sensors_event_t event;
  accel.getEvent(&event);
  float uvi = uv.getUVI();
  Serial.print("heading ");
  Serial.print(heading, 0);
  Serial.print("  tilt ");
  Serial.print(atan2(event.acceleration.y, event.acceleration.z) * 180 / PI, 0);
  Serial.print("  UV ");
  Serial.println(uvi, 1);
  if (uvi >= 3) {
    Serial.println("Sun cream");
  }
  delay(300);
}