Tilt compass on an Arduino Mega

// Tilt compass: an MPU-9250 reads which way is down and which way is north,
// with asukiaaa's MPU9250 library, on a Mega: the library's floating point
// and atan2() take more flash than an Uno example is allowed.
//
//   MPU-9250  SDA -> SDA, SCL -> SCL, VCC -> 5 V, GND -> GND
//
// Inside the chip are two devices. The accelerometer and gyroscope answer
// at 0x68, like an MPU6050. The AK8963 magnetometer sits behind them, and
// beginMag() sets the bypass bit so it answers at 0x0C on the same two
// wires. The accelerometer gives the tilt; the magnetometer gives the
// Earth's field, whose horizontal part points north.
//
// magHorizDirection() is atan2 of the magnetometer's X and Y: the angle of
// north from the chip's own axes, which only means a compass bearing while
// the board is held level.
//
// Things to try. Drag the heading slider and watch the direction follow.
// Then tip the board: the tilt changes and so does the direction, because
// the field also points down into the ground. A tilt-compensated compass
// uses the accelerometer to undo that.

#include <Wire.h>
#include <MPU9250_asukiaaa.h>

MPU9250_asukiaaa imu;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  imu.setWire(&Wire);
  uint8_t id = 0;
  imu.readId(&id);
  Serial.print("WHO_AM_I 0x");
  Serial.println(id, HEX);
  imu.beginAccel(ACC_FULL_SCALE_2_G);
  imu.beginMag();
}

void loop() {
  if (imu.accelUpdate() == 0) {
    float pitch = atan2(-imu.accelX(), imu.accelZ()) * 180.0 / PI;
    float roll = atan2(imu.accelY(), imu.accelZ()) * 180.0 / PI;
    Serial.print("pitch ");
    Serial.print((int)lround(pitch));
    Serial.print("  roll ");
    Serial.print((int)lround(roll));
  }
  if (imu.magUpdate() == 0) {
    Serial.print("  direction ");
    Serial.print((int)lround(imu.magHorizDirection()));
  }
  Serial.println();
  delay(500);
}