The need for speed: Turbo-charged CAN

Other Parts Discussed in Post: SN65HVD256

WTHAYTA? What the heck are you talking about? Do you feel that you are always surrounded by people using acronyms and you do not know what they mean? With multiple definitions for the same acronym, you can’t just use HD for half duplex or FD for full duplex. HD nowadays means High Definition, and just recently FD got rechristened to mean Flexible Data-rate. Have you heard of that? If not, stick with me.

Controller Area Network (CAN), which has been a work horse communication standard for many years, generated new buzz over the last year with a new CAN with flexible data-rate (FD) standard. This new CAN FD proposal was created to help free up network bandwidth and more fully utilize the network. It solves two limitations in CAN networks today.

The first shortcoming that limits CAN communication today is the amount of overhead that comes with every message, or frame as the CAN standard refers to it. An easy way to look at this is to compare the number of bits of data that can be sent in one frame versus the total number of bits that need to be sent as overhead. For example, CAN limits the number of bytes that can be sent in the data field to eight bytes of data, which is equivalent to 64 bits. A CAN frame with an 11-bit identifier field will have a total 111 bits in each frame, not including stuff bits which to simplify this blog will not be taken into account. That means that 47 bits, or 42.3 percent of the message, is overhead!

Moving to CAN FD, the data field is extended to allow up to 64 bytes of data, or 512 bits, in one CAN message. Taking the previous example where an 11-bit identifier was used, there are now 512 bits of data in a 568-bit frame. This means that the overhead is only 56 bits, or 9.9 percent of the entire message! This in itself can make a huge difference in freeing up network bandwidth. Imagine being able to say eight times as much information in one breath!

Can full duplex

The second thing that limits CAN communication is the speed, or data rate, at which the information is sent over the bus. This is where the flexible data-rate part of the title comes to play. One of the benefits of CAN is that when multiple nodes try to access the bus at the same time a non-destructive bit-wise arbitration takes place at the beginning of the frame. To ensure that nodes on opposite ends of the bus can properly arbitrate for bus access, the speed of communication is limited by the two-way loop time of the bus. With a propagation delay of roughly five nanoseconds per meter on a twisted pair bus of 24 AWG wire, this delay starts to really reduce the maximum communication speed with longer buses.

CAN FD proposes having two data rates throughout the CAN frame. During the beginning of the frame, when arbitration is occurring and multiple nodes can access the bus, there is a slower data rate. Once arbitration concludes and only one transceiver is accessing the bus, the two-way loop times no longer need to be met and communication can be switched to a faster data rate. By doubling or even quadrupling the data rate that the data field is sent, the length of time it takes to send the total frame decreases by more than 65 percent. Remember back-in-the-day when you had a dial-up modem connection and your webpages used to load line-by-line? Then one day, you switched to broadband and your website loaded almost instantly. Remember how exuberant you were?  That is the CAN FD impact.

Figure 1 shows the benefit of increasing the data rate and/or the size of the payload. Check out TI’s flexible data ready Turbo CAN transceivers, such as the SN65HVD256. Now you know WTHIATA!