I2C Connection Plugin

The I2C Connection plugin provides access to I2C bus devices on Linux systems, commonly used with Raspberry Pi and embedded devices.

When to Use I2C

  • Temperature and humidity sensors
  • ADCs (Analog-to-Digital Converters)
  • DACs (Digital-to-Analog Converters)
  • OLED/LCD displays
  • Real-time clocks (RTC)
  • EEPROM memory
  • GPIO expanders

Header File 

#include <connectionplugins/connectioni2c.h>

Basic Setup 

class MyDriver : public INDI::DefaultDevice
{
public:
    MyDriver()
    {
        setConnectionMode(CONNECTION_I2C);
        
        i2cConnection = new Connection::I2C(this);
        i2cConnection->setDefaultBusPath("/dev/i2c-1");
        i2cConnection->setDefaultAddress(0x28);  // 7-bit address
        i2cConnection->registerHandshake([&]() { return Handshake(); });
        
        registerConnection(i2cConnection);
    }

private:
    Connection::I2C *i2cConnection = nullptr;
};

Communication Example 

bool MyDriver::readSensor(uint8_t *data, size_t len)
{
    int fd = i2cConnection->getPortFD();
    if (fd < 0)
    {
        LOG_ERROR("I2C device not open");
        return false;
    }

    ssize_t nbytes = read(fd, data, len);
    if (nbytes < 0)
    {
        LOGF_ERROR("I2C read error: %s", strerror(errno));
        return false;
    }

    return nbytes == static_cast<ssize_t>(len);
}

bool MyDriver::writeSensor(const uint8_t *data, size_t len)
{
    int fd = i2cConnection->getPortFD();
    if (fd < 0)
    {
        LOG_ERROR("I2C device not open");
        return false;
    }

    ssize_t nbytes = write(fd, data, len);
    if (nbytes < 0)
    {
        LOGF_ERROR("I2C write error: %s", strerror(errno));
        return false;
    }

    return nbytes == static_cast<ssize_t>(len);
}

Best Practices

  • Address Format: Use 7-bit I2C addresses (0x00-0x7F)
  • Bus Selection: Common buses are /dev/i2c-0, /dev/i2c-1, etc.
  • Permissions: Add user to i2c group for access
  • Device Detection: Scan with i2cdetect before implementing
  • Pull-up Resistors: Ensure proper pull-up resistors (typically 4.7kΩ)
  • Clock Speed: Most devices support 100kHz standard mode

Troubleshooting

Cannot Open I2C Bus 

# Enable I2C on Raspberry Pi
sudo raspi-config
# Interface Options -> I2C -> Enable

# Add user to i2c group
sudo usermod -a -G i2c $USER
# Log out and log back in

Device Not Responding 

# Scan I2C bus
i2cdetect -y 1

# Check connections and pull-up resistors