I2C Communications between Raspberry Pi and Arduino – Part Three

Today’s goal is to send a string from the Arduino to the Raspberry Pi. The setup is the same as from day two.

After several attempts and stupid mistakes, I was finally able to get a “Hello World” message from the Arduino to the Raspberry Pi.

Here is the code for the Arduino

Here is the code for the Raspberry Pi

Compile the code
gcc testi2c03.c -o testi2c03

Run the code
./testi2c03 254

We can see that once the string ends, the data on the I2C buss is 255. Let’s tweak the code on the Raspberry Pi to stop once we receive 255.

Compile the code
gcc testi2c03b.c -o testi2c03b

Then run the application
./testi2c03b 254

We can see that the output is now cleaner.

Let’s do even better and print the string as a string instead of a list of characters.

Compile the code
gcc testi2c03c.c -o testi2c03c

Then run the application
./testi2c03c 1 2 3 254 250 251

Yes, I included additional arguments this time. The code was setup to handle this which is really nice. This allows us to teak the code if we like to print out what the values actually are and get some additional information. So let’s create a new application which will do exactly that but will not take in any arguments. I am also going to add a few other things such as detecting if we are using a Raspberry Pi Rev 1 or Rev 2 as well as scanning the I2C Bus.

I was doing some searching on valid I2C addresses and found a great reference article from Total  Phase at 7-bit, 8-bit, and 10-bit I2C Slave Addressing. The article provides a diagram showing the valid range of 7-bit I2C addresses.


From this diagram, we can see that the address used in the examples is a reserved address. I will change the address in the Arduino code so that it is in the valid address range.

Here is a modified version of the code which finds all connected I2C devices. Determines which ones are the sensors that we are interested in, and reads values from each one. This will be a great program for making certain that the design works and all of the sensors are working.

Arduino Code

Raspberry Pi Code


Compiling the Raspberry Pi code is a bit different as we need to link the math library. In order to do this, we need to add -lm to the command line.

gcc testi2c03d.c -o testi2c03d -lm


Here is the results of running the application.


The passing of a string was successful however there are several standards which may be better suited to the goal that I have in mind. One worth further consideration is the System Management Bus (SMBus). For the moment, I am leaving the code as is since the information that I need to send may be sent as simple integer responses. A future enhancement will be to get a better messaging system in place.

The next step is to replace the Arduino with a ATTiny85 and get it all working.

Leave a Reply