“CAN FD is ideal for applications requiring high safety and reliability, such as robotics, elevators and transport systems, as well as medical and healthcare systems. The reliability requirements required by automotive applications are also very beneficial in these use cases.
Due to the higher bandwidth requirements in the automotive sector, the CAN (Controller Area Network) specification was extended to flexible data rates and the new iteration is called CAN FD.
CAN has the advantages of cost, flexibility and robustness, all of which are very beneficial for non-automotive applications in many fields. The market opportunity for CAN FD expansion is even wider. This article introduces the basics of CAN and CAN FD, as well as different application implementations using different physical layers or higher layer protocols, with CAN as the data link layer.
First, let’s talk about the advantages of CAN over standard serial communications like RS232 or RS485. Because CAN has higher communication speed and error detection, it has excellent robustness and lower cost.
cost and flexibility
The most important driver for the automotive industry is to reduce the amount of wiring in the car. Because of the twisted pair wiring, wiring is relatively easy, and it is lightweight and inexpensive. Terminating resistors are necessary for high-speed operation of CAN and CAN FD. Flexibility is a great advantage as it is very easy to scale the system with more nodes.
Error detection and robustness
CAN and CAN FD contain very robust error checking mechanisms. Bit stuffing and monitoring work at the first layer, while frame checking, acknowledgement and cyclic redundancy check work at the second layer of the OSI model.
Bit stuffing adds an alternating bit after five consecutive high or low bits. Six consecutive bits with the same level indicate an error. Bit monitoring reads back every message sent. If there is a discrepancy (except for the quorum or confirmation fields), an error is detected. A big advantage is that errors can be detected very promptly.
Cyclic redundancy checks are implemented differently on CAN and CAN FD due to different data lengths. Framing errors (sometimes also called format or format errors) use a predefined value that must be the same on the receiver side. Every message needs to be acknowledged. These three error checking mechanisms work well at the message level.
In conclusion, CAN and CAN FD are very robust and reliable with many different error checks. No data is lost during message transmission and message collisions are prevented. Each node waits for a period of inactivity before sending. In case two, messages are sent at the same time, the sender detects which message has a higher priority and disables the lower priority message. Compared to Ethernet where both messages are stopped and sent later, the message with the highest priority on CAN goes through.
High speed and low latency
CAN supports data rates up to 1Mbps. With CAN FD, the data rate of the control and data areas can be increased according to the maximum clock of the CAN FD controller. The rate during the arbitration phase remains at a maximum of 1Mbps.
The delay of CAN is less than 145us, while CAN FD with 8Msps and 8Byte data is less than 58us.
Short data frames have an advantage in terms of latency. The transmission and decoding of the entire packet is faster, so the response time is also faster. This effect is even greater with higher transfer rates on CAN FD. Compared to TCP/IP communication, which is designed for large data volumes, the packets are larger and therefore the latency increases. This means that CAN FD (depending on the amount of data) may have lower response times and show better overall real-time performance than 10 or 100Mbit TCP/IP communication.
Regarding the number of nodes, there is theoretically no limit as each message can be sent to a different node. In practice, each node causes signal reflections on the bus, and the transmission quality depends on the CAN transceiver and the physical layer implementation.
This is also the reason for the speed limit over long distances. Typical values are up to 25 nodes on CAN and 8 nodes on CAN FD.
Examples of applications other than automotive
Why use CAN FD outside of automotive applications? Because of the huge advantages mentioned above. CAN and CAN FD are used in a wide variety of industries, including:
• Elevators and lifts
• Access control, light control and security door openers
• air conditioner
• Fleet tracking, vehicle tracking
• Document predictive maintenance, telematics, insurance and black boxes
• Healthcare equipment
• Industrial drives
• Game console
• between host and chain actuator
A good use case for an MCU with two CAN FD controller units and TrustZone and safety is a control unit in building automation, separating the safety part from the non-safety part. A CAN FD controller can be used on the safety side to control critical components such as door openers, sliding doors and ID card readers. The second CAN FD can be used for non-critical control components in building automation such as light switch buttons, bulbs and doors in buildings.
Another use case for dual CAN FD units is gateway functionality, eg in large building automation systems, large cabinets and communication expansion modules. There are many different use cases for MCUs with integrated CAN FD controllers such as actuators, sensors and controls.
CAN FD is ideal for applications requiring high safety and reliability, such as robotics, elevators and transport systems, as well as medical and healthcare systems. The reliability requirements required by automotive applications are also very beneficial in these use cases.