GY-85 IMU 9-Aixs ITG3200/ITG3205 ADXL345 HMC5883L Sensor Module

Pinout & Connection Guide

The GY-85 module features a 6-pin configuration for easy connection.

Pin Definitions

I2C Pin Mapping for Common Development Boards

Wiring Diagram (Arduino Uno)

GY-85 Module          →    Arduino Uno
─────────────────────────────────────────
VCC                   →    5V
GND                   →    GND
SCL                   →    A5 (SCL)
SDA                   →    A4 (SDA)
XDA (optional)        →    Leave unconnected
XCL (optional)        →    Leave unconnected

Important Notes:

• The module includes built‑in logic level conversion, making it safe for direct connection to both 3.3V and 5V systems.

• Most GY-85 modules include onboard pull-up resistors, but if you experience communication issues, you may need to add external 4.7kΩ pull-ups on SDA and SCL lines.

• The XDA and XCL pins are auxiliary I2C connections used to daisy‑chain additional I2C sensors (e.g., a second accelerometer). For basic operation, leave them unconnected.

Sensor Details

ITG3205/ITG3200 – 3‑Axis Digital Gyroscope

The ITG3205 is a single‑chip MEMS gyroscope that measures angular velocity (rate of rotation) around the X, Y, and Z axes. Key features include:

• 16‑bit ADC for high‑resolution output

• Fixed ±2000°/s measurement range

• Programmable low‑pass filter for noise reduction

• Built‑in temperature sensor for drift compensation

• Low power: 6.5 mA active current, 5 µA standby

• Fast I2C communication up to 400 kHz

Output scaling: The raw output of the ITG3205 is converted to degrees per second using the scale factor of 14.375 LSB per °/s.

ADXL345 – 3‑Axis Digital Accelerometer

The ADXL345 measures both static acceleration (gravity) for tilt sensing and dynamic acceleration from motion or shock. Key features include:

• User‑selectable ranges: ±2g, ±4g, ±8g, or ±16g

• 10‑bit fixed resolution or 13‑bit full‑resolution mode (4 mg/LSB at ±16g)

• Ultra‑low power: 23 µA in measurement mode, 0.1 µA in standby

• Activity/inactivity monitoring and free‑fall detection

• 32‑level FIFO buffer to reduce host processor load

The ADXL345 is ideal for tilt measurement and orientation detection.

HMC5883L – 3‑Axis Digital Magnetometer

The HMC5883L measures the Earth’s magnetic field to determine absolute heading (compass direction). Key features include:

• 12‑bit ADC with 2 mG per LSB resolution

• ±8 Gauss measurement range

• Heading accuracy of 1° to 2° (with proper calibration)

• Sampling rate up to 160 Hz (6 ms measurement time)

• Integrated degaussing straps for resetting the sensor

• Built‑in self‑test capability

For accurate heading, the magnetometer must be calibrated to compensate for magnetic interference from nearby components (motors, wires, batteries). Additionally, tilt compensation using accelerometer data is required for heading accuracy when the sensor is not perfectly level.

Usage Guide

Software Setup (Arduino IDE)

The GY-85 module is supported by several Arduino libraries. The most comprehensive is Jeff Rowberg’s I2Cdevlib, which provides individual libraries for each sensor.

Step 1: Install Required Libraries

The I2Cdevlib collection includes:

• ADXL345 (accelerometer)

• ITG3200 (gyroscope – compatible with ITG3205)

• HMC5883L (magnetometer)

Download the libraries from https://github.com/jrowberg/i2cdevlib or install via Library Manager if available.

Step 2: Initialize All Three Sensors

/*
  GY-85 9-Axis IMU – Complete Sensor Initialization
  Reads all three sensors simultaneously
*/

#include <Wire.h>
#include "I2Cdev.h"
#include "ADXL345.h"
#include "ITG3200.h"
#include "HMC5883L.h"

// Create sensor objects
ADXL345 accel;
ITG3200 gyro;
HMC5883L mag;

// Variables for sensor data
int16_t ax, ay, az;     // Accelerometer raw values
int16_t gx, gy, gz;     // Gyroscope raw values
int16_t mx, my, mz;     // Magnetometer raw values

void setup() {
  Serial.begin(115200);
  Wire.begin();
  
  // Initialize Accelerometer
  accel.initialize();
  Serial.print("Accelerometer: ");
  Serial.println(accel.testConnection() ? "Connected" : "Failed");
  
  // Initialize Gyroscope
  gyro.initialize();
  Serial.print("Gyroscope: ");
  Serial.println(gyro.testConnection() ? "Connected" : "Failed");
  
  // Initialize Magnetometer
  mag.initialize();
  Serial.print("Magnetometer: ");
  Serial.println(mag.testConnection() ? "Connected" : "Failed");
  
  Serial.println("GY-85 Ready");
}

void loop() {
  // Read all three sensors
  accel.getAcceleration(&ax, &ay, &az);
  gyro.getRotation(&gx, &gy, &gz);
  mag.getHeading(&mx, &my, &mz);
  
  // Display readings
  Serial.print("Accel: ");
  Serial.print(ax); Serial.print(", ");
  Serial.print(ay); Serial.print(", ");
  Serial.println(az);
  
  Serial.print("Gyro:  ");
  Serial.print(gx); Serial.print(", ");
  Serial.print(gy); Serial.print(", ");
  Serial.println(gz);
  
  Serial.print("Mag:   ");
  Serial.print(mx); Serial.print(", ");
  Serial.print(my); Serial.print(", ");
  Serial.println(mz);
  
  Serial.println("-------------------");
  
  delay(100);
}

Step 3: Convert Raw Gyroscope Data to Degrees per Second

The ITG3205 outputs raw values that must be scaled using the sensitivity factor:

float convertToDegreesPerSecond(int16_t rawValue) {
  // Sensitivity = 14.375 LSB per °/s
  return rawValue / 14.375;
}

Step 4: Calculate Tilt‑Compensated Heading

For accurate heading, you must compensate for tilt using accelerometer data:

float calculateHeading(float magX, float magY, float magZ, 
                       float accX, float accY, float accZ) {
  // Calculate pitch and roll from accelerometer
  float pitch = atan2(-accX, sqrt(accY * accY + accZ * accZ));
  float roll  = atan2(accY, accZ);
  
  // Tilt compensation for magnetometer
  float magX_comp = magX * cos(pitch) + magZ * sin(pitch);
  float magY_comp = magX * sin(roll) * sin(pitch) 
                  + magY * cos(roll) 
                  - magZ * sin(roll) * cos(pitch);
  
  // Calculate heading (in radians)
  float heading = atan2(magY_comp, magX_comp);
  
  // Convert to degrees (0-360)
  if (heading < 0) heading += 2 * PI;
  return heading * 180 / PI;
}