The basics of the controller area network (CAN bus) and its use in automotive applications
given network configuration must be given serious consideration upfront. The bus configuration in Figure 1 is very cost-effective and efficient. Rather than wire each sensor and actuator to a controller, the network routes multiple connections via simpler wired connections. The LIN bus uses a single wire for communications, and the CAN bus supports higher speeds using a single twisted pair.
As mentioned, the CAN bus, like all active networks, evolves continuously to meet the needs of the industry. CAN Flexible Data Rate (CAN FD) increases the data rate to 5 Mbps. The latest CAN standard is CAN Extra Long (CAN XL) and supports data rates of up to 10 Mbps or more. CAN FD and CAN XL are backward compatible with classic CAN. The physical layer wiring for the CAN network consists of twisted pair wiring between CAN nodes, as shown in Figure 2.
The bus topology wiring of the CAN network requires 120 ohm (Ω) terminations at both ends of the bus to minimize reflections. Stubs—unterminated open connections—should be minimized. Bus rates depend on the CAN implementation and are affected by the length of the network. The longer the network, the lower the maximum data rate it can sustain. The 1 Mbps rated data rate is for a network length of 40 meters (m) or less.
Figure 3: Shown are the differential signal definitions for the CAN bus CANH and CANL conductors. (Image source: Texas Instruments)
Communication across a CAN network relies on differential signaling using the two wires in the twisted pair designated CANH and CANL (Figure 3). The CAN transceiver driver is required to achieve a 1.5 volt differential signal across the 60 Ω differential load of the terminated twisted pair. The signal levels are referred to as dominant and recessive. The dominant or ‘1’ level has a differential voltage level greater than or equal to 0.9 volts. The recessive or ‘0’ level has a differential voltage of less than 0.5 volts.
The bus driver is capable of actively driving the bus to the dominant state, while the return to the recessive state depends on resistive discharge through the terminations. This accounts for the shorter rise time when transitioning from the recessive to the dominant state. It also allows a dominant bit to overwrite a recessive bit state. This feature is used for acknowledgment and bus arbitration. The twisted pair transmission lines have a propagation delay of 5 nanoseconds per meter (ns/m). CAN
controllers are configured for the propagation delays of the network to which they are coupled, so this propagation delay information is needed to ensure correct bus arbitration and prioritization. CAN transceivers, which physically drive the bus, are available in 8 pin and 14 pin versions. The Texas Instruments TCAN1042GDQ1 is an 8-pin version that supports CAN FD. Conveniently, Texas Instruments uses a common pin mapping for the top eight pins on both the 8 and 14 pin versions, allowing drop-in replacement.
Figure 2: The CAN bus uses terminated, twisted pair wiring, and nodes are drop connected. Each node has a CAN transceiver and a microcontroller unit (MCU) with a CAN controller function. (Image source: Texas Instruments)
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