Software configurable CAN bus termination circuit with memory
Nina Rinne · Tampere University Institutional Repository (Tampere University) · 2025
The CAN bus utilizes a differential signaling topology, where one conductor of a twisted pair is referred to as CAN H (high), and the other as CAN L (low). For proper data transmission on the bus, its farthest ends must be terminated with a resistor matching the characteristic impedance of the wires. The termination resistor can either be installed directly in the wiring harness between the conductors or integrated into the circuit board of an electronic control unit (ECU). Custom cable assemblies with termination resistors increase costs and complexity for customers, whereas ECUs with pre-installed termination resistors can only be used at the physical ends of the bus. Consequently, manufacturers often create multiple ECU variants, some with termination installed and others without, to meet varying customer needs. A controllable termination switch, integrated into the circuit board of an ECU, enables switching the termination on or off via a control signal. Such a solution allows ECUs to be used at any position on the CAN bus, as the termination can be activated only for nodes located at the bus ends. This flexibility reduces design complexity and allows adding or removing ECUs from the network without altering the hardware. The main goal of this thesis was to develop a configurable CAN termination circuit that meets the requirements imposed by the harsh operating environment of vehicles and mobile machinery. In the theory portion of this thesis, literature review was used to obtain necessary background information about transmission line theory, Controller Area Network, and to define the requirements for the design. Furthermore, existing implementations of switchable CAN termination circuits were studied and their shortcomings determined. In the empirical research portion, various measurements were conducted on the designed prototype. The requirements included high temperature tolerance, resistance to vibration and mechanical shocks, low power consumption, compatibility with the common-mode voltage range specified for the CAN bus and preservation of the termination state without continuous control signal. Moreover, components used in the design had to be reliable and durable over time. During testing, the termination state in the designed prototype was preserved without continuous control signal and was configurable by a set of control pulses. Therefore, the prototype can be used in applications where low-power mode is implemented and no continuous control signal from MCU is available. Furthermore, the total current consumption was under 1 mA across the whole supply voltage range of 4.5 V to 27 V and ambient temperature range from -25°C to 105°C. The measurements also showed that the designed prototype does not effectively increase the loop delay of the CAN node. The common mode voltage tolerance of -2 V to 7 V was not achieved with the original design. However, with minor changes to the circuit, the higher tolerance of -12 V to 12 V is possible to achieve. Lastly, the components used in the design were simple, low-cost and widely available. To fully implement the prototype circuit into an ECU, further electromagnetic compatibility (EMC) testing is required. Additionally, the design can be simplified to further reduce current consumption and cost, and possibly increase its common mode voltage tolerance.