CAN sniffing explained

Monitoring the CAN bus is complicated, because so much data flows in modern cars. So identifying individual messages from the stream of data is difficult.

A common way of identifying a specific message is called “divide and conquer”. The idea came from a classic computer algorithm, of same name, used in many aspect (like sorting). Easily explained, you know that the message you are looking for can be compared to finding a needle in a haystack. With the divide and conquer algorithm, you divide the haystack in two equal parts and look through half the haystack for the needle. If you don’t find the needle, you break up the remaining half in two new stacks, and looking in one of these. You continue this process until you find the needle.

You can apply this to finding CAN messages as well. Let's say you want to discover the command for controlling the power windows, then the steps involved are:

• Record messages flowing while pressing the window switch in the car.
• Divide the recorded messages into two halfs.
• Replay/send all data from one part to the car’s CAN bus. If the window rolls down you have the right part.
• Keep doing this procedure until you have identified the exact message you are looking for.

The following CAN data dump is 34 lines out of 1797 lines, for a 5 second recording:

Once you identified the specific command, you need to replay/send it to make sure you have the right one.

All of this sounds very complicated (which it can be), but with AutoPi all of this has been automated in a simple tool you can access from the AutoPi Cloud.

Using AutoPi to discover new commands in my car

With the AutoPi Cloud, the tedious process of finding CAN commands for your vehicle has been automated in a simple user interface.

Above image shows the user interface used to discover new CAN commands. The steps are very simple. You need to physically be in your car to do the discovery:

• Open the CAN explorer. It will tell you if there is connection to the car
• Press the record button
• Now perform the function you want to record 5 times. Like pressing the “left front window” switch
• Press the stop record button

The system will now analyze the data received and output the possible commands found. Because of the large data amount, the system will sometimes output a few number of possible commands. You now have to possibility to narrow it down by replaying the found commands one-by-one. When you found the command you were looking for, you simply press the save button. You newly found command is stored for future use by you and for all other users with a similar car.

Using discovered CAN commands to build something cool

So what can you do with your new CAN command? Why not use it to tie up a cool speech command to your car.

The AutoPi Cloud comes built in with an If-This-Then-That trigger system. Using this system you can add your own triggers and trigger the new found CAN command.

The image above shows the interface for adding new triggers to the system. Adding a new trigger is very simple.

• Select your input source. This is the beacon. As an example select the “googleAssistantSpeech” source
• Add a critearia, this is the text spoken. We will add “Roll down left front window”
• Now you add your output source. this is the reactor. We select the “carIntegrator” and from the dropdown menu, we find our new command “left front window down” and select this
• Press save

The system will automatically upload the new functionality to your AutoPi Dongle. Now all there is left for you is to test it by actually speaking to you car: “Hey AutoPi, roll down front left window”