When developing your own car head unit, sooner or later you face the question of how it should interact with the vehicle's electronics.
How should power-on and shutdown be handled properly? Where can you get information about reverse gear engagement? How can you detect the lighting state or handle steering wheel buttons?
To solve these problems, you could develop your own controller, connect it to the vehicle's CAN bus, and process the necessary messages yourself.
But as it turns out, there is a much simpler solution — a ready-made CAN Box.
That's exactly what I want to talk about in this article.
A Little Background
The first version of my CaRadio project, built on a Raspberry Pi, is still being used in my car.
During its operation and further development, my ideas about what a DIY car head unit should look like have changed considerably.
One of the important milestones was working on CaRaBack — my own solution for CarPlay.
The CaRaBack development article series is currently being written and is regularly updated.
Working on CaRaBack allowed me to reconsider the architecture of CaRadio. In particular, I realized that building a car head unit does not necessarily require a Raspberry Pi. Another device could serve as the computing platform, provided it meets the requirements for performance, interfaces, and cost.
At the same time, I continued looking for the best way to manage the power supply of the future head unit.
It was during these experiments that I acquired a Chinese Android head unit designed for Volkswagen vehicles.
I'll leave the details of the head unit itself for a separate article series. For now, we're interested in another device that came with it.
It was a CAN Box.
First Encounter with a CAN Box
At first glance, a CAN Box is just a small plastic box with a connector for the wiring harness.
My unit is labeled G-VW-RZ-58. It is designed for Volkswagen, Skoda, and Seat vehicles.
Interestingly, in the kit I received, the CAN Box was not connected to the head unit via UART.
Perhaps the wiring harness was intended for a different head unit model. It's also possible that the CAN Box was used only to generate individual control signals, such as ACC and reverse camera activation.
This still needs to be verified.
When the device ended up on my workbench, I decided to find out what was inside.
I opened the enclosure, examined the circuitry, and started looking for information about the microcontroller, firmware, and communication protocols.
I did find information online, but it was rather fragmented.
You can find discussions about CAN Box compatibility with Chinese head units, individual pinouts, and protocol descriptions. However, there are relatively few detailed resources explaining how these devices are built and how they work.
Gradually, I realized that a CAN Box is much more interesting than it might seem at first.
What Is a CAN Box?
In modern vehicles, electronic control units exchange information over digital communication buses.
One of the most widely used is CAN (Controller Area Network).
It allows different electronic modules in a vehicle to exchange messages without requiring a separate wire for every signal.
For example, information about ignition state, lighting, or reverse gear engagement can be transmitted over the CAN bus.
However, it's important to understand that CAN is primarily a data communication protocol.
By itself, it does not define which particular message means that the ignition has been switched on or a specific button has been pressed.
The meaning of messages depends on the vehicle manufacturer, model, production year, and particular electronic system.
Therefore, simply connecting to the CAN bus is not enough.
You need to know which messages are transmitted in the vehicle and how to interpret them correctly.
This is where a CAN Box comes in.
How a CAN Box Works
A CAN Box is an electronic module that receives messages from the vehicle's CAN bus, analyzes them, and converts them into signals or commands that a connected device can understand.
A typical unit contains:
- A microcontroller with firmware for processing messages.
- A CAN transceiver for the physical connection to the bus.
- Power supply and protection circuits.
- Communication interfaces for connecting to the head unit.
- Output stages for generating control signals.
The operating principle is fairly simple.
The vehicle transmits messages over the CAN bus. The CAN Box receives them, identifies relevant events, and generates corresponding commands.
For example, when reverse gear is engaged, the decoder may send a command to the head unit to switch to the rear-view camera.
When the vehicle lights are switched on, it may indicate that the screen brightness should be adjusted.
And when the ignition state changes, it may generate an ACC signal.
The capabilities of a particular device depend on its hardware design, firmware, and supported vehicle.
CAN Box and Chinese Android Head Units
In Chinese Android head units, a CAN Box often acts as an intermediary between the vehicle's electronics and the head unit itself.
One common communication method is the UART serial interface.
The CAN Box receives vehicle messages, converts them into its own protocol, and sends them to the head unit.
The head unit no longer needs to understand the specifics of the vehicle's CAN bus.
Instead, it receives ready-to-process commands that its software can handle.
For example:
- Ignition state changes.
- Reverse gear engagement.
- Vehicle lighting activation.
- Steering wheel button presses.
- Door status.
- Parking system information.
Of course, not every CAN Box supports all these functions.
Furthermore, UART protocols may differ between manufacturers.
But there is an interesting point here.
If a Chinese Android head unit can receive and process these commands, why couldn't a custom-built device do the same?
There is absolutely no need to use Android for this.
You only need to connect to the appropriate interface and implement the protocol handler.
It could be a Raspberry Pi, another single-board computer, or a microcontroller.
CAN Box as an Alternative to a Custom Power Controller
This is where I discovered the possibility that interested me the most.
When developing a Raspberry Pi-based car head unit, one of the challenges is proper power management.
Unlike a conventional car radio, a single-board computer runs an operating system.
To shut down Linux properly, it's advisable to provide enough time between receiving the shutdown command and actually cutting off the power supply.
In the first version of CaRadio, this problem has already been solved: the Raspberry Pi automatically shuts down when the key is removed from the ignition.
However, while developing the new version, I considered various power management approaches, including using a separate microcontroller.
After discovering the CAN Box, I realized that in many cases a custom controller might not be necessary at all.
A ready-made CAN Box can detect the ignition state from vehicle messages and generate an ACC signal.
This signal can be used to control the head unit's power supply.
If a proper Linux shutdown is required, it is enough to notify the computer when the ACC state changes and provide a short delay before switching off the power MOSFET.
For example, 20–30 seconds.
Such a delay can be implemented with a relatively simple electronic circuit, without developing a separate programmable controller.
It's also worth noting that, in practice, Raspberry Pi-based devices can operate for years even when their power is cut off directly.
However, a hard power cut during a write operation still carries the risk of file system corruption.
Therefore, it's better to provide a proper shutdown mechanism.
Of course, before using a CAN Box, you need to verify exactly how the particular model generates ACC and how it behaves when the ignition is switched off.
But the overall concept turned out to be much simpler than developing a custom controller from scratch.
Is It Worth Developing Your Own CAN Controller?
From a technical perspective, developing your own device is entirely possible.
You would need a microcontroller, a CAN transceiver, a power supply circuit, protection components, and appropriate software.
Then you would have to investigate the vehicle's CAN messages, identify the required message IDs, and understand the transmitted data formats.
This is an interesting challenge, especially if your goal is to study automotive communication protocols.
But if the task is simply to obtain a few control signals, the question of cost-effectiveness arises.
A ready-made CAN Box already contains the necessary electronics and firmware for supported vehicles.
Instead of developing the hardware and decoding CAN messages yourself, you can use its existing outputs or investigate the much simpler communication protocol used with the head unit.
Considering component costs, PCB manufacturing, and development time, buying a ready-made CAN Box may be significantly more economical.
Of course, this applies only if the purchased decoder is compatible with the vehicle and provides the required functions.
What's Inside?
Let's return to my CAN Box.
As mentioned earlier, the device came with a Chinese Android head unit.
The enclosure is labeled G-VW-RZ-58 and includes information about the hardware and software versions.
After opening the enclosure, I was able to examine the circuit board and its main components.
Despite its small size, the device contains everything needed to communicate with the vehicle's CAN bus and transmit processed information to external equipment.
The microcontroller firmware is particularly interesting.
It determines which CAN messages the device recognizes and how they are converted into control signals.
That's why two CAN Box modules that look almost identical may differ significantly in functionality.
An Unexpected Conclusion
After examining the device, I came to a rather unexpected conclusion.
A CAN Box like this would have been a great fit even for the first version of CaRadio.
It could have provided an ACC signal directly from the vehicle's CAN bus and used it for automatic power management.
And with UART connected, it could also provide additional vehicle events normally processed by Chinese Android head units.
All of this without having to develop a custom CAN controller.
Perhaps if I had known earlier how these decoders work and what capabilities they offer, I would have simply purchased a CAN Box separately.
And it's quite possible that the need to switch to a Chinese Android head unit would never have arisen.
On the other hand, it was precisely these experiments that introduced me to the CAN Box.
And now I want to investigate its operation in greater detail.
What's Next?
In the next part, we'll try to move from theory to practice.
We'll connect the CAN Box to the vehicle's CAN bus and see how it reacts to different events.
We'll test ACC generation and its response to reverse gear engagement and lighting activation.
We'll also try to investigate UART communication and determine which commands the CAN Box can send to the head unit.
The main goal is to understand how easily a ready-made automotive decoder can be used in custom electronics projects without being tied to a particular Android head unit model or computing platform.