// minimal MPU-6050, MPU-9250, GY-521 pitch (x) and roll (y).

#include<Wire.h>
const int MPU_addr1 = 0x68;
float xa, roll, ya, pitch, za;

void setup() {
  Wire.begin();                       // begin wire communication
  Wire.beginTransmission(MPU_addr1);
  Wire.write(0x6B);                   // send adress (0x68)
  Wire.write(0);                      // reset (place a 0 into the 6B register)
  Wire.endTransmission(true);         //end the transmission
  Serial.begin(9600);
}

void loop() {
  Wire.beginTransmission(MPU_addr1);
  Wire.write(0x3B);  //send starting register address, accelerometer high byte
  Wire.endTransmission(false); //restart for read
  Wire.requestFrom(MPU_addr1, 6); //get six bytes accelerometer data
  
  int t = Wire.read();
  xa = (t << 8) | Wire.read();
  t = Wire.read();
  ya = (t << 8) | Wire.read();
  t = Wire.read();
  za = (t << 8) | Wire.read();

  // formula from https://wiki.dfrobot.com/How_to_Use_a_Three-Axis_Accelerometer_for_Tilt_Sensing
  
  roll = atan2(ya, za) * 180.0 / PI;
  pitch = atan2(-xa, sqrt(ya * ya + za * za)) * 180.0 / PI;  //account for roll already applied

  Serial.print("roll = ");
  Serial.print(roll, 1);
  Serial.print(", pitch = ");
  Serial.println(pitch, 1);
  delay(250);
}