CAN FD – in simple terms
CAN FD (CAN with Flexible Data Rate) is the further development of the classic CAN bus. Compared with Classical CAN, CAN FD allows up to 64 bytes of payload per frame and a higher data rate during the data phase. This allows more data to be transferred more efficiently without abandoning the proven basic principle of CAN communication. CAN FD is now used wherever classic CAN networks reach their limits in throughput, diagnostics, or software updates.
What is CAN FD?
CAN FD stands for CAN with Flexible Data Rate. The protocol adds two key protperties to the classic CAN bus:
First, a CAN FD frame can carry significantly more payload than a classic CAN frame. Instead of a maximum of 8 bytes, CAN FD allows up to 64 bytes per message. Second, CAN FD permits a higher transmission speed during the data phase. This significantly increases network efficiency.
CAN FD was developed to better support modern applications with higher data volumes. These include, for example, diagnostic functions, ECU flash programming, measurement data acquisition, and systems with increasing communication density.
CAN FD vs. CAN — the key differences
The most important difference between Classical CAN and CAN FD lies in the combination of greater payload length and higher data throughput. CAN FD was designed to maintain the existing CAN concepts while enabling greater efficiency for modern applications.
How does CAN FD work?
CAN FD is based on the familiar CAN principle, but extends the data frame with additional capabilities. Particularly important are the flexible data rate, the larger payload length, and new indicators in the frame format.
A CAN FD frame can be divided into two speed ranges:
- During the arbitration phase, communication runs at a bit rate suited to the network topology.
- During the data phase, the bit rate can be increased if **Bit Rate Switching (BRS)** is used.
This preserves the bus arbitration mechanism while making the actual data transfer much more efficient.
CAN FD base frame — arbitration phase and data phase
Bit Rate Switching (BRS)
More payload per message
Why CAN FD was developed
As the number of ECUs, diagnostic functions, and software components in machines and vehicles increases, communication requirements also grow. CAN FD was developed to better address these requirements within the familiar CAN world: more data per message, shorter transmission times, and higher efficiency within the existing system concept.
CAN FD frames and important fields
The CAN FD data frame resembles the classic CAN frame, but contains additional information for the extended functions. These include, in particular, indicators for the FD format, bit rate switching, and the status of the transmitting node.
In Classical CAN, only two data frame types were distinguished: Standard Frames (CAN 2.0A) with an 11-bit identifier and Extended Frames (CAN 2.0B) with a 29-bit identifier. These formats remain in place, but within CAN FD they receive new names.
A CAN FD controller is downward-compatible, i.e. the previous formats of the classical CAN are recognized and can be processed. However, this does not work in the reverse direction: the frame formats FBFF and FEFF are unknown to a classical CAN controller and, consequently, error frames will be sent. This is why all devices in the network need to be equipped with a CAN FD controller to be able to use FD frame formats.
The most significant differences in the frame structure of a classical CAN data frame are shown in fig. 1.
CAN FD Base Frame Format
The CAN FD format does not support remote frames, the previous RTR bit after the identifier is replaced by the “Remote Request Substitution” (RRS) bit.
FDF – FD Format
BRS – Bit Rate Switch
ESI – Error State Indicator
Using the ESI (Error State Indicator) bit in the control field, the sender of a message now signals whether it is in the Error Active or Error Passive state, which significantly improves troubleshooting in networks.
To avoid obtaining a worse residual error probability than with Classical CAN due to the potentially higher number of stuff bits in a message, a Stuff-Bit Counter (3 bits + parity) was introduced. The new Stuff Count field is sent at the beginning of the CRC field. The new stuff-count field is sent at the beginning of the CRC field.
DLC in CAN FD
Encoding of data lengths
Even without faster transmission during the data phase, the larger amount of data alone offers considerable advantages. At a nominal bit rate of 1 MBit/s the net transfer rate (i.e. the transfer rate of the data field) of a classical CAN controller is 576 kBit/s. A CAN FD frame with a data size of 64 bytes has a net transfer rate of 903 kBit/s.
Advantages of CAN FD
Higher data throughput
Less overhead
Ideal for diagnostics and updates
The proven CAN principle remains
Typical use cases for CAN FD
ECU programming
Diagnostics and service
Measurement and test engineering
Measurement and test engineering
FAQ about CAN FD
What does CAN FD mean?
CAN FD means CAN with Flexible Data Rate. It is an extension of the classic CAN bus with greater payload length and a higher data rate during the data phase.
What is the difference between Classical CAN and CAN FD?
How much data can CAN FD transfer?
Is CAN FD backward compatible?
Does CAN FD support remote frames?
When should CAN FD be used?
More on the topic of CAN FD
I/O Modules
Acquire analog and digital signals and communicate via CAN FD.
Training
Get more details on CAN FD in the training course.
Protocol stacks
Integrate CAN FD into the product
Available CAN FD Controllers
Development of CAN FD systems is determined by the available CAN controllers. The following list is an overview of currently available CAN FD microcontrollers, but without claiming to be complete.
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