9.1.1 Start of Frame (SOF)
The start of frame field indicates the beginning of a data frame and is common to both standard and extended formats.
9.1.2 Arbitration Field
Arbitration is used to resolve bus conflicts that occur when several devices at once start sending messages on the bus. The arbitration field indicates the priority of a frame, and it is different in the standard and extended formats. In the standard format there are 11 bits, and up to 2032 IDs can be set. The extended format ID consists of 11 base IDs plus 18 extended IDs. Up to 2032×218 discrete IDs can be set.
During the arbitration phase, each transmitting device transmits its identifier and compares it with the level on the bus. If the levels are equal, the device continues to transmit. If the device detects a dominant level on the bus while it is trying to transmit a recessive level, it quits transmitting and becomes a receiving device. After arbitration only one transmitter is left on the bus, and this transmitter continues to send its control field, data field, and other data.
The process of arbitration is illustrated in Figure 9.9 by an example consisting of three nodes having identifiers:
Node 1: 11100110011 Node 2: 11100111111 Node 3: 11100110001
Figure 9.9: Example CAN bus arbitration
Assuming the recessive level corresponds to 1 and the dominant level to 0, the arbitration is performed as follows:
• All the nodes start transmitting simultaneously, first sending SOF bits.
• Then they send their identifier bits. The 8th bit of Node 2 is in the recessive state, while the corresponding bits of Nodes 1 and 3 are in the dominant state. Therefore Node 2 stops transmitting and returns to receive mode. The receiving phase is indicated by a gray field.
• The 10th bit of Node 1 is in the recessive state, while the same bit of Node 3 is in dominant state. Thus Node 1 stops transmitting and returns to receive mode.
• The bus is now left to Node 3, which can send its control and data fields freely.
Notably, the devices on the bus have no addresses. Instead, all the devices pick up all the data on the bus, and every node must filter out the messages it does not want.
9.1.3 Control Field
The control field is 6 bits wide, consisting of 2 reserved bits and 4 data length code (DLC) bits, and indicates the number of data bytes in the message being transmitted. This field is coded as shown in Table 9.1, where up to 8 transmit bytes can be coded with 6 bits.
Table 9.1: Coding the control field
| No. of data bytes | DLC3 | DLC2 | DLC1 | DLC0 |
|---|---|---|---|---|
| 0 | D | D | D | D |
| 1 | D | D | D | R |
| 2 | D | D | R | D |
| 3 | D | D | R | R |
| 4 | D | R | D | D |
| 5 | D | R | D | R |
| 6 | D | R | R | D |
| 7 | D | R | R | R |
| 8 | R | Dor R | D or R | D or R |
D: Dominant level, R: Recessive level.
9.1.4 Data Field
The data field carries the actual content of the message. The data size can vary from 0 to 8 bytes. The data is transmitted with the MSB first.
9.1.5 CRC Field
The CRC field, consisting of a 15-bit CRC sequence and a 1-bit CRC delimiter, is used to check the frame for a transmission error. The CRC calculation includes the start of frame, arbitration field, control field, and data field. The calculated CRC and the received CRC sequence are compared, and if they do not match, an error is assumed.
9.1.6 ACK Field
The ACK field indicates that the frame has been received normally. This field consists of 2 bits, one for ACK slot and one for ACK delimiter.
9.2 Remote Frame
The remote frame is used by the receiving unit to request transmission of a message from the transmitting unit. It consists of six fields (see Figure 9.10): start of frame, arbitration field, control field, CRC field, ACK field, and end of frame field. A remote frame is the same as a data frame except that it lacks a data field.
Figure 9.10: Remote frame
9.3 Error Frame
Error frames are generated and transmitted by the CAN hardware and are used to indicate when an error has occurred during transmission. An error frame consists of an error flag and an error delimiter. There are two types of error flags: active, which consists of 6 dominant bits, and passive, which consists of 6 recessive bits. The error delimiter consists of 8 recessive bits.
9.4 Overload Frame
The overload frame is used by the receiving unit to indicate that it is not yet ready to receive frames. This frame consists of an overload flag and an overload delimiter. The overload flag consists of 6 dominant bits and has the same structure as the active error flag of the error frame. The overload delimiter consists of 8 recessive bits and has the same structure as the error delimiter of the error frame.
9.5 Bit Stuffing
The CAN bus makes use of bit stuffing, a technique to periodically synchronize transmit-receive operations to prevent timing errors between receive nodes. After 5 consecutive bits with the same level, one bit of inverted data is added to the sequence. If, during sending of a data frame or remote frame, the same level occurs in 5 consecutive bits anywhere from the start of frame to the CRC sequence, an inverted bit is inserted in the next (i.e., the sixth) bit. If, during receiving of a data frame or remote frame, the same level occurs in 5 consecutive bits anywhere from the start of frame to CRC sequence, the next (sixth) bit is deleted from the received frame. If the deleted sixth bit is at the same level as the fifth bit, an error (stuffing error) is detected.
9.6 Types of Errors
The CAN bus identifies five types of errors:
• Bit error
• CRC error
• Form error
• ACK error
• Stuffing error
Bit errors are detected when the output level and the data level on the bus do not match. Both transmit and receive units can detect bit errors. CRC errors are detected only by receiving units. CRC errors are detected if the calculated CRC from the received message and the received CRC do not match. Form errors are detected by the transmitting or receiving units when an illegal frame format is detected. ACK errors are detected only by the transmitting units if the ACK field is found recessive. Stuffing errors are detected when the same level of data is detected for 6 consecutive bits in any field that should have been bit-stuffed. This error can be detected by both the transmitting and receiving units.