The CAN-bus specification specifies the format, usage, and timing of each type of frame. And the frame processing and verification are automatically completed in the CAN controller. Once the error is detected, the data frame being transmitted will stop immediately and will be resent again when the bus is idle until the transmission is successful. The process does not require the CPU. Interference is eliminated by the transmitter unless the error is accumulated.
The reliability of CAN-bus is very high, but in some cases errors occur. In order to enable reliable transmission of data on the bus, the CAN-bus specification specifies the format, use and timing of various types of frames. . Below we summarize what errors will occur in CAN communication.
Figure 1
Bit error: The transmitting node will also sample and collect the sent bit stream while transmitting data. If the monitored bus level does not match the level being sent, a bit error (Bit Check Error) will occur;
Bit stuffing error: To solve the common shortcomings in asynchronous serial communication - the communication clocks of each node are not synchronized, CAN-bus adopts a synchronous method, which stipulates that every 5 bits of time length is synchronized, when transmitting signals The consecutive 5 bits are the same, it is necessary to insert a reverse evaluation bit (called "padding bit"), if the signal transmitted on the cable does not satisfy this rule, a Bit Stuff Error will occur;
CRC error: The CRC value sent by the sender is calculated by the transmitter. The receiver uses the same CRC algorithm to calculate the CRC value of the receiver. If the CRC value calculated by the receiver does not match the received CRC value, the receiver will generate the CRC value. Class error (CRC Error);
Format error: This type of format error (Frame Check Error) occurs when the transmitted data frame format does not match any of the legal frame formats.
Acknowledgement error: If the ACK segment sending node does not receive a response (dominant bit) from the receiving node, an Acknowledgment Error will occur, so this type of error occurs when a single node sends a data frame.
The error frame acts as a signal light during bus signal transmission. If a communication error is detected during reception and transmission, an error frame is sent. The error frame consists of an error identifier and an error delimiter. There are two types of error identifiers: "active error" and "passive error".
In the active error state, as soon as an error is detected, it immediately "actively" issues an error flag. The so-called "error flag", which itself is a "wrong bit sequence" (six consecutive dominant bits, does not meet the "up to 5 consecutive same-sex bits" requirement of the CAN protocol), the purpose is to "actively" tell everyone : Even if you didn't find the error "I just found out", now I am "leading by example" wrong! You should see this mistake!
If it is in a passive state and an error is detected, it can only blink and "passively" wait for someone else (actively reporting the wrong site) to report an error. When waiting, it can not move the bus until it recognizes the "wrong bit" issued by the active error reporting site. Sequence, it was a sigh of relief: someone officially reported it wrong! Then he can go to the competition bus, and it will dry up.
In order to prevent a device from continuously transmitting and receiving data due to its own reasons (such as hardware damage) and continuously destroying data frames, thereby affecting other normal node communication, the CAN-bus specification stipulates that each CAN controller has a transmission error counter and A receive counter. According to the count value, the CAN nodes will be in different device states. The transition relationship between states is shown in the following figure:
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