

"I Sort of Know What CAN Is, but What’s CAN FD?"

Bret Detrick
Lead Application Engineer, dSPACE Inc.

May 6, 2015: This article is not written for those who have a detailed knowledge of CAN clear down to stuff bits, termination, and physical layer topology. I’ve written this blog at a higher level and it is my hope that readers will be able to nod their heads with confidence and go “uh-huh” in a meeting where CAN and CAN FD are mentioned.

For those not familiar with CAN, it is the most common communication bus that links together the ECUs in a vehicle. What is an ECU, you ask? That stands for Electronic Control Unit. An ECU is a computer that controls part of the vehicle. As cars have become more advanced, there has become a need for more and more ECUs and increased communication between these ECUs. For about 20 years, CAN (or Controller Area Network) has been the standard used to network these ECUs.

For those already familiar with this technology, it is common knowledge that the data transfer capacity of a single CAN bus has long been inadequate. Manufacturers are now putting 3, 4, 5 or more CAN buses in their vehicles and it is becoming more difficult to manage. There have been several competing technologies, but nothing that matches the simplicity, flexibility, and most importantly, low cost of CAN. Moving to a new technology would be like a person having to learn a new language. This is where CAN FD comes in and the FD stands for Flexible Data rate.

To explain this quickly, we need to speak about CAN in very simple terms. The CAN bus allows one ECU to broadcast a message to all other ECUs on the bus. It does so at a certain speed called the baud rate. This baud rate is limited by things that only electrical engineers can understand. In the vehicles, the baud rate is currently near its practical limit. However, it has been determined that part of the message can be transmitted at a higher baud rate.

Here’s why: The CAN message has two parts -- the arbitration and data. The arbitration part is when multiple ECUs try to talk at once, but certain rules determine which one may continue. This arbitration part will remain at the current baud rate because it is mostly responsible for the limitation in the baud rate. The data part of the message is when only one ECU is allowed to talk and all others must listen. It has been determined that this part may be safely increased by a factor of 10 or more.

There are other topics to cover -- such as longer data payload (more data per message) and partial networking (where some ECUs ignore the faster rate) -- but I am not going to duplicate Wikipedia here.

Look for CAN FD to start appearing in vehicles in model year 2017 or 2018. Actually, don’t look for it because you won’t see it. But we expect our customers to start talking about it at any time now.

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