Overview of the CAN Bus Protocol

CAN stands for Controller Area Network. This type of communication is inside several types of systems, the most common is automotive. These systems consists of typically wired controllers (microprocessors/microcontrollers) that talk to each other. There are protocols and methods to allow wireless communication, but wired is far more common. Unlike SPI, I2C, USB, and other similar formats, CAN Bus uses a much different format for data communication. CAN Bus is based off of differential voltage levels:

There are two wires used for communication that transmit data at the same time. They are called CAN Hi (High) and CAN Lo (Low) and have different voltage levels that are interpreted by each controller (called CAN Nodes). CAN Hi usually measures from 2.5V to 3.75V while CAN Lo measures from 2.5V to 1.25V. When both lines read 2.5V the signal is called “Recessive” which would be equivalent to a binary value of 1. When CAN Hi goes to 3.75V and CAN Lo goes to 1.25V the signal is called “Dominant” which would be equivalent to a binary value of 0. Conceptually, this may not make sense when talking about digital logic where 0 is Low and 1 is High, but the protocol is looking for the 0 values over the 1 values. So the driver logic is inverse from typical digital logic interpretation.

Basic Wiring Diagram

The wiring has some peculiar aspects that are unique to the protocol. One advantage CAN Bus has over single wire protocols is the ability of nodes to be disconnected without other nodes failing. If CAN Hi or CAN Lo was cut to a particular node, the rest of the system would not care and still send data to the other nodes. Some systems may be capable of transmitting data along one line if the main one went down, but this depends on the company (not all systems can work with a down CAN Hi or CAN Lo line). Performance gets hit on systems that can handle a line being cut though because the 120 ohm resistors have two purposes. They first provide the differential between Hi and Lo along with matching impedance for systems that have higher frequencies. The protocol works best with the differential voltages. The crossed wires also have two purposes (twisted pair). First, they help block EMI emissions from outside sources. Second, they also help with EMI issues when the protocol transmits with faster frequencies.

Notice that there are no other lines, this means that packets (messages) are sent to all nodes at the same time. There is no “address” in a CAN message data frame, but there are methods in the data frame to determine what is accepted or rejected by each node along with priority with certain messages defined by the user. The data has to have bits set in order to avoid collision with other messages, so priority is key for CAN Bus to avoid that problem.

CAN Bus Data Format

As with all protocols, data is organized in specific ways defined by standards established. CAN Bus protocol started with CAN 2.0, updated with ISO 11898, and developments are still being made (there are extended versions of the protocol for longer messages). An entire CAN Frame is typically 14 bytes long. Here is a breakdown of a CAN Message:

  • SOF (Start of Frame): this marks the beginning of a new CAN message, lets the other nodes on the bus synchronize.
  • The arbitration field is the identifier which contains an 11 Bit ID and a bit for RTR (remote transit request) which is set to dominant if requesting info from another node. The ID is what other nodes care about as they are the context for whether or not if each node cares about that message. Think of the ID as a pseudo-address that behaves like a "topic". If other nodes don't care about the "topic" they will ignore the message. Not only that, the ID determines priority too for collision avoidance. If communication happens at the same time, the nodes first wait for the lines to be idle (recessive). If one node outputs a recessive level and the other outputs a dominant level, the dominant voltage is always higher priority. An example is shown below.

Node1 Message: 0011 0110 0000…
Node2 Message: 0010 1000 1100…

From right to left, the fourth bit on the message from Node2 would send first because a dominant ‘0’ on bit 4 occurs. Node 1 senses this and quits sending the message and waits for the line to become idle.

  • R0 is for future development.
  • Data length code is a half byte long and tells how long the actual data being sent in the data field is.
  • The data fields contains the desired data that you want to send to other nodes for programming purposes.
  • CRC (Cyclic Redundancy Check) is for error detection. This bit is overwritten if there are no errors with a dominant level.
  • ACK (Acknowledge) is a recessive bit that confirms that there was no error (dominant if there is an error).
  • EOF (End of Frame) is a 7 bit field that signals the end of the CAN Message frame and disables bit stuffing (synchronization data).
  • The iFS (interframe space) is another 7 bit field that serves as a delay so the receiving nodes can move info to a separate buffer.
There are 5 mechanisms of error correction in a CAN Message frame. 2 are at the bit level and 3 are message level. Bit monitoring and bit stuffing are bit level. Bit monitoring is done by each node and monitors to see if there is an error somewhere (done in the arbitration field), error is flagged in a specific manner. The other bit level error correction is bit stuffing. This adds a sixth bit of an opposite level (R or D) if 5 bits of the same level are sent in a message, this allows synchronizing with the transmitter. This is easily removed by the receiver as each node expects this extra bit.

The two message level mechanisms are frame checking and the CRC/ACK fields. Frame checking verifies that all CAN parts exist and if there is a recessive bit after ACK and CRC. When there is error, there are two thresholds. When the first error occurs, one bit is set to dominant as a flag which increments a counter for a node. If the counter is only at 1, recessive error flags are sent and traffic continues as normal on the bus. However, if the counter reaches 2, this indicates a serious error and that node will shut off. The counters do go down if messages are sent successfully without error.

If you want to experiment with CAN bus on your own, there are a few evaluation boards that can be used:

  1. https://www.digikey.com/short/mjbmpb2b
  2. https://www.digikey.com/short/0m2rd48h