3 How CAN SIC Reduces Bus Ringing

The CAN bus has two logical states during normal operation: recessive and dominant, as shown in Figure 3-1.

A dominant bus state occurs when driving the bus differentially and corresponds to a logic low on the TXD and RXD pins. A recessive bus state occurs when the bus is biased to V_CC/2 through the high-value internal input resistors (R_IN) of the receiver and corresponds to a logic high on the TXD and RXD pins. A dominant state overwrites the recessive state during arbitration. The recessive-to-dominant signal edge on the CAN bus is usually clean, as it is strongly driven by the transmitter. The differential transmitter output impedance of the CAN transceiver during the dominant phase is approximately 50 Ω and closely matches the network characteristic impedance. For a regular CAN-FD transceiver, the dominant-to-recessive edge is when the driver differential output impedance suddenly goes to approximately 60 kΩ, and the signal reflected back experiences an impedance mismatch, which causes ringing.

Transmitter-based SIC detects the dominant-to-recessive edge on TXD and activates ringing suppression circuitry on the driver output. The CAN driver continues driving the bus recessive strongly until t_{SIC_TX_base} so that reflections diminish and the recessive bit is clean at the sampling point. In this active recessive phase, the transmitter output impedance is low (approximately 100 Ω). Since the reflected signal does not see a huge impedance mismatch, ringing is attenuated considerably. After this phase ends and the device enters a passive recessive phase, the driver output impedance rises to approximately 60 kΩ. Figure 3-2 shows this phenomenon.

An important factor in the active recessive phase strongly driving the bus is that it should only last for a maximum of 530 ns (t_{SIC_TX_base}, as listed in Table 1-1). The data phase of the CAN-FD protocol only lasts for 200 ns max (if operated at 5 Mbps), so this ringing suppression will be active for the entire recessive bit duration, resulting in correct CAN bus and RXD signals. For the arbitration phase, however – where the fastest bit duration is 1 µs for 1-Mbps operation, multiple transmitters can transmit simultaneously, and the dominant bit has to overwrite the recessive bit – the duration of ringing suppression may place some limits on the overall network length and arbitration speed. See the CiA 601-4 specification for more details.