The basics of the controller area network (CAN bus) and its use in automotive applications
The Control Field contains the length of the following data field. The data field can have up to eight data bytes in classic CAN. If data is being transmitted (data frame), then the data field is followed by the cyclic redundancy check (CRC) which is used to check for transmission errors. At this point, the driver puts the bus into a recessive state so the receiver can acknowledge (ACK) successful receipt by asserting a dominant state. The ACK bit is surrounded by delimiter bits on each side. After the ACK bit, the controller sends a seven-bit end-of-frame (EOF) message followed by a seven-bit interframe spacing, marking the minimum time before another frame can be transmitted.
The transceiver is compatible with a Texas Instruments TMS5703137CGWTQEP microcontroller (MCU), which has three CAN controllers available (Figure 4). 14 pin CAN transceivers have additional functions, including auxiliary power buses and extended low-power and sleep modes. The Texas Instruments TCAN1043GDRQ1 is a 14 pin CAN transceiver that supports full 5 Mbps operation under CAN FD. It is also compatible with the TMS5703137CGWTQEP MCU (Figure 5).
CAN protocol The CAN bus follows a standardized protocol that allows the network to address data and commands where needed (Figure 6). Between transmissions, the bus is in an idle state at the recessive voltage level. The start of the transmission frame (SOF) is marked by the transmission of a dominant bit. A message identifier in the next field is followed by a remote transmission request (RTR)— which indicates whether the frame contains data or is requesting data. These two fields are used to establish message priority and arbitrate access to the bus.
Figure 4: A typical 8-pin transceiver application with the transceiver connected to the transmit (TXD) and receive (RXD) ports of an MCU which includes multiple CAN bus controllers. (Image source: Texas Instruments)
Figure 6: The CAN protocol provides a structure for transferring data across the bus and verifying that it is received. It also provides error checking. (Image source: Texas Instruments)
Figure 5: A typical application of a 14 pin CAN transceiver incorporating extended low-power and sleep modes controlled via the extra six pins. (Image source: Texas Instruments)
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