I2C Bus (Development Overview)

I2C is a packet-switched serial communication protocol that allows multiple master devices to connect to multiple slave devices using only 2 wires per connection. It is intended for attaching lower-speed peripheral ICs to processors and microcontrollers in short-distance, intra-board communication.

Pixhawk/PX4 support it for:

• Connecting off board components that require higher data rates than provided by a strict serial UART, such as rangefinders.
• Compatibility with peripheral devices that only support I2C.
• Allowing multiple devices to attach to a single bus (useful for conserving ports). For example, LEDs, Compass, rangefinders etc.

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The page Hardware > I2C Peripherals contains information about how to use (rather than integrate) I2C peripherals and solve common setup problems.

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IMUs (accelerometers/gyroscopes) should not be attached via I2C (typically the SPI bus is used). The bus is not fast enough even with a single device attached to allow vibration filtering (for instance), and the performance degrades further with every additional device on the bus.

Integrating I2C Devices

Drivers should #include <drivers/device/i2c.h> and then provide an implementation of the abstract base class I2C defined in I2C.hpp for the target hardware (i.e. for NuttX here).

A small number of drivers will also need to include headers for their type of device (drv_*.h) in /src/drivers/ - e.g. drv_led.h.

To include a driver in firmware you must add the driver to the board-specific cmake file that corresponds to the target you want to build for. You can do this for a single driver:

CONFIG_DRIVERS_DISTANCE_SENSOR_LIGHTWARE_LASER_I2C=y

You can also include all drivers of a particular type.

CONFIG_COMMON_DISTANCE_SENSOR=y

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For example, you can see/search for CONFIG_DRIVERS_DISTANCE_SENSOR_LIGHTWARE_LASER_I2C in the px4_fmu-v4_default configuration.

I2C Driver Examples

To find I2C driver examples, search for i2c.h in /src/drivers/.

Just a few examples are:

• drivers/distance_sensor/lightware_laser_i2c - I2C driver for Lightware SF1XX LIDAR.
• drivers/distance_sensor/lightware_laser_serial - Serial driver for Lightware SF1XX LIDAR.
• drivers/ms5611 - I2C Driver for the MS5611 and MS6507 barometric pressure sensor connected via I2C (or SPI).

Further Information

• I2C (Wikipedia)
• I2C Comparative Overview (learn.sparkfun.com)
• Driver Framework