Classical CAN vs. CAN FD Protocols: the Differences that Matter for DesignersNovember 24, 2020 by Antonio Anzaldua Jr.
While CAN FD has been around since 2011, this protocol is still, in many cases, the preferable option to Classical CAN for many designers. Here's why.
Controller Area Network with Flexible Data Rate (CAN FD) is a data communication protocol that has evolved alongside modern electronics, allowing design engineers to improve communication reliability in industrial and automation control systems.
In the late 1980s, the Classical CAN protocol was established to increase vehicle functionality. Essentially, anything that involves movement in machinery is utilizing a CAN system. However as time went on, the demand for sophisticated electronic control units (ECU) and safety features such as advanced driver-assistance systems (ADAS) increased exponentially.
CAN FD rollout in vehicles has begun in 2020. Image used courtesy of CSS Electronics
To meet the growing demands, Classical CAN was required to jump ahead into a new protocol that could withstand the heavy data traffic that was inevitably approaching.
The Design Benefits of CAN FD Over Classical CAN
With the growing rate of data being transferred and received simultaneously, CAN FD emerged as the industry-standard protocol. So what changes were made to help manufacturers and designers maintain data-communication reliability?
Engineers know the importance of cable length and how it impacts systems across the board, as seen in a recently-failed satellite launch in Spain—the result of erroneously-assembled cables. It can cause voltage drops, delay signals, and lessen the amount of current delivered to connections since resistance is proportional to the length of the cable or conductor.
For Classical CAN, having longer cables would create less bandwidth throughout the system, a terrible tradeoff that designers had to work around. CAN FD is actually not affected by the cable length. Regardless of the amount of cable used, the bandwidth will remain constant, and in some cases may increase slightly.
Classical CAN networks would only allow 8 data bytes to be transferred per node, which would be sufficient back in the 1980s. However, modern electronic control units (ECUs) add an inrush of data that would not be able to quickly decipher logic in an 8-bit connection. Utilizing CAN FD protocol, the system is able to hold 64 data bytes per frame, which is eight times more than Classical CAN can endure.
Data Transfer Speed
The speed of data transfer has significantly increased by implementing the CAN FD protocol. In a Classical CAN system, if one node connection were to receive two signals of data, the protocol would choose the most dominant bits to pass through and ignore the others.
Because CAN FD supports a faster bit-rate, a single message can hold more data. Image used courtesy of NI
This would cause a delay in the system and would increase the differential voltage on the shared bus. Due to its dual bit-rate connections, the design is able to multitask and transfer bits regardless of its dominance.
Reliability is a key factor for automation and industrial control systems, especially prior to and during the testing and measurement design phases. One way to ensure reliability is to employ a cyclic redundancy check (CRC), which is another differentiator between CAN FD and Classical CAN. The CRC was upgraded in the new protocol.
This 21-bit diagnostic report is able to fault check for data, searching for undetected errors within the system. The original report only allowed for 15 bits with nearly zero fixed bits for communication reliability, providing no redundancy toward the system. With CAN FD, the network operates with four fixed bits designated for strengthening communication lines.
Frame of Classic CAN vs. CAN FD. Image used courtesy of CSS Electronics
Microchip’s 8-bit MCU Family for CAN FD Networks
CAN FD is becoming more prominent in the industry—even beyond control and automation systems. Recently, Microchip introduced its first 8-bit microcontroller family for CAN FD networks, the PIC18Q84. Microchip’s MCU focuses on securely increasing data transfer in complex designs.
The MCU is equipped with core independent peripherals (CIPs) that allow systems to transmit and receive data through the CAN FD bus. The combo package is able to handle tasks without the system’s CPU. Microchip says the new situation features near-zero latency to decrease delays in the automotive interface or sensors.
Microchip’s PIC18Q84 family offers a 32-bit CRC frame and 129kB of flash memory on all 48 pins. Image used courtesy of Microchip
As the associate VP of marketing for Microchip’s 8-bit microcontroller business unit, Greg Robinson believes the solution will go beyond automation and industrial controls: “CAN FD will continue to play a critical role in delivering faster data transfer rates for applications, ranging from the connected car to industrial automation and smart homes,” he explains.
CAN FD Engenders More Design Flexibility
In the past, engineers needed to avoid critical message delays while maintaining appropriate wire lengths when creating a Classical CAN network. Now, CAN FD allows for more design flexibility in electric vehicles (EVs), ECU flashing, robotics, and safe driving systems.
While CAN FD should meet the demand curve in heavily-trafficked data networks for the time being, the ever-increasing amount of data transfer may necessitate another upgrade in CAN protocols in the near future.