1 Theory of Operation

The ISO 11898 specification details the physical layer requirements for CAN bus communications. CAN is a low-level communication protocol over a twisted pair cable, similar to RS-485

An important feature of CAN is that the bus is not actively driven during logic ‘High’ transmission, referred to as ‘recessive.’ During this time, both bus lines are typically at the same voltage, approximately VCC /2. The bus is only driven during ‘dominant’ transmission, or during logic ‘Low.’ In Dominant, the bus lines are driven such that (CANH – CANL) ≥ 1.5V. This allows a node transmitting a ‘High’ to detect if another node is trying to send a ‘Low’ at the same time. This is used for non-destructive arbitration, where nodes start each message with an address (priority code) to determine which node gets to use the bus. The node with the lowest binary address wins arbitration and continues with its message. There is no need to back-off and retransmit like other protocols.

CAN receivers measure differential voltage on the bus to determine the bus level. Since 3.3V transceivers generate the same differential voltage (≥1.5V) as 5V transceivers, all transceivers on the bus (regardless of supply voltage) can decipher the message. In fact, the other transceivers cannot tell there if anything is different about the differential voltage levels.

Figure 1-2 shows bus voltages for 5V transceivers as well as 3.3V transceivers. For 5V CAN, CANH and CANL are weakly biased at about 2.5V (V CC /2) during recessive. The recessive common-mode voltage for 3.3V CAN is biased higher than V CC /2, typically about 2.3V. This is done to better match the common mode point of the 5V CAN transceivers and minimize the common mode changes on the bus between 3.3V and 5V transceivers. Since CAN was defined as a differential bus with wide common mode allowing for ground shifts (DC offsets between nodes) this isn’t needed for operation, but minimizes emissions in a mixed network. In addition, by using split termination to filter the common mode of the network a significant reduction in emissions is possible. The ISO 11898-2 standard states that transceivers must operate with a common-mode range of -2V to 7V, so the typical 0.2V common-mode shift between 3.3V and 5V transceivers doesn’t pose a problem.

Previously, 3.3V CAN transceivers were not used for automotive applications because they cannot meet strict EMC requirements required by major automotive manufacturers. This application note references legacy 3.3V CAN family such as SN65HVD23x, which were not approved for use in heterogeneous automotive networks. TI's new generation of automotive 3.3V CAN transceivers, TCAN3403-Q1 and TCAN3404-Q1, overcome this challenge and pass IEC 62228-3:2019 under both homogeneous and heterogeneous network conditions. For more detailed information, see How Automotive-Qualified Electromagnetic-Compliant 3.3V CAN FD Transceivers Improve ECU Performance.