Using I2C Serial Bus to Connect Raspberry Pi to Arduino

Using I2C Serial Bus to Connect Raspberry Pi to Arduino

At some point, I’ll be looking to extend my Raspberry Pi robot, and one of the ways to do that is to use I2C (Inter-Integrated Circuit) serial devices. So, in this feature, I look at how to access an Arduino board from the Raspberry Pi using this serial bus. I also include both Raspberry Pi and Arduino code to demonstrate the use of the serial interface.

I think it’s true that the Arduino boards are a much more reliable platform than the Raspberry Pi when comes to timing critical functions. And this is due to the Arduino having onboard peripherals such as Timers, ADC and many more built-in modules. Also, the Arduino microcontroller executes program functions in real-time as opposed to an operating system queueing tasks for execution.

INITIAL TESTING

Using the included example code, I was able to test the performance of both the Raspberry Pi B3 and the Raspberry Pi Zero. However, the example code is still a work in progress, and some tuning is still necessary for the final I2C serial application.

On initial testing, the Raspberry Pi B3 outperforms the Pi Zero by more than a hundred times. And this is measured by the number of failed read methods within the stream. So, the more read methods I perform per second; the more I2C bus errors I get.

Also, while mostly affecting the Pi zero, execution of another process can interrupt the I2C stream and cause increased errors. However, you can use exception handling in the code to prevent your program from crashing. You may need to perform retries until the serial interface returns to normal operation.

Setup and I2C Test Code

Arduino and Raspberry Pi I2C Test Circuit
Arduino and Raspberry Pi Test Circuit

The Raspberry Pi is going to need some initial configuration to run the example code. Also, to support the SMBus2 Python library, we need to install some dependencies.

Firstly, we configure the Rasberry Pi to enable the I2C interface. So, open a Terminal session on the Raspberry Pi to execute the following command:  sudo raspi-config. The Raspberry Pi Configuration Tool will now open to allow you to select interface options. Then on the next screen, choose I2C to enable the interface.

Raspberry Pi Software Configuration Tool
Raspberry Pi Software Configuration Tool

Secondly, we now install the dependencies required by the SMBus2 library. It appears, at least on my Raspberry Pi setup, I only need to install one package. Therefore, execute the following in the Pi Terminal:

Lastly, we install SMBus2, which is a drop-in replacement for smbus-cffi/smbus-python in pure Python. Enter the following command in the Terminal:

Test Code For I2C

Arduino and Raspberry Pi I2C Connection
Arduino and Raspberry Pi I2C Connection

The Circuit

To run the example code, you will need to connect the I2C interface between the Raspberry Pi and Arduino. The Raspberry Pi will have the USB power, while the Arduino has power from the computer USB serial connection. The Arduino is a 3.3v Pro Mini compatible running at 8MHz. And due to the separate power supplies, we need to connect the grounds between the Arduino and Raspberry Pi.

However, this circuit will not be compatible with 5v Arduino board versions. While it is possible to disable the Arduino pull-up resistors, a 5v spike on the I2C is still possible during the pull-up transition.

So, to check the correct installation of the software packages, run the following command at the Terminal Prompt. However, you will need the example code uploaded and running on the Arduino. This example code will set the slave address of the Arduino board, and hopefully, this test will detect that address.

You should get an output similar to the following:

i2cdetect Serial Bus Output
i2cdetect Serial Bus Output

The Demo Program

The Raspberry Pi operates as the master while the Arduino is the slave device. The process of the program is as below:

The Raspberry Pi uses the SMBus2 read method to get a counter value from the Arduino device. Additionally, after each successful read, the counter on the Arduino is incremented. When the counter reaches 255, the counter is then reset to Zero.

The Raspberry Pi sends an integer called an offset. This offset then allows the Arduino to run an associated function to generate a return value. So, the return value is the counter value and is returned with the offset value. Then, the Raspberry Pi prints both the offset value and the counter value in the Terminal.

The Raspberry Pi I2C Demo Python Code

Copy this program to the Raspberry Pi and run it in the Terminal:  python3 i2c_master_test.py.

Adjust the sleep time in the Python code in the case of getting too many serial bus errors.

The Arduino I2C Demo Code

Using the Arduino IDE, upload the code to the Arduino device.

And Finally

So, this is a work in progress and I expect to try other methods from the SMBus2 library also.

Related Articles

Pi Wars 2018 Raspberry Pi Headless Install

Raspberry Pi Headless Install Without A Display Guide – Link.

Beginners Guide to Raspberry Pi Configuration

Beginners Guide to Raspberry Pi Configuration – Link.

Buying Featured Items

The purchase price is going to vary greatly depending on how quickly you want the items. Therefore shop around checking out Amazon, Ebay and local electronic stores.

UK Searches:

UK Amazon:

  • Arduino Pro Mini ATMEGA328P – Search
  • Raspberry Pi – Search
  • MicroSD Card – Search
  • Raspberry Pi Compatible Wi-Fi Dongle – Search

US Searches:

US Amazon:

  • Arduino Pro Mini ATMEGA328P – Search
  • Raspberry Pi – Search
  • MicroSD Card – Search
  • Raspberry Pi Compatible Wi-Fi Dongle – Search

On Closing

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