Hongke share | bring you a comprehensive understanding of "can bus error" (I) -- can bus error and error frame

In this series of articles , We will give you a detailed introduction to CAN Knowledge about bus errors , Include CAN The basic concept of bus error 、CAN Type of bus error 、CAN Error frame and CAN Node error status , And generate and record through the actual application test CAN error .

1. What is? CAN Bus error ？

Controller area network (Controller Area Network, CAN) It is an important standard for today's automotive and industrial automation systems . Reliability is CAN One of the core advantages of bus protocol , This makes it an ideal choice for safety critical applications . But here's the thing , Error handling right CAN Is critical to the robustness of .

CAN Bus errors can occur for many reasons , Such as cable failure 、 noise 、 Mismatched terminal resistance 、CAN Node failure, etc. . distinguish 、 Classify and solve such CAN The mistake is to ensure that the whole CAN The key to the persistent performance of the system . Error handling can identify and reject wrong information , Enable the sender to retransmit information . Besides , This process helps to identify and disconnect the continuous transmission of error messages CAN node .

Error handling is CAN Standards and each CAN The built-in part of the controller . let me put it another way , Every CAN Nodes deal with fault identification and limitation in the same way . Below we make a simple illustrative example ：

Examples of specific steps ：

• CAN node 1 To transmit a message to CAN Bus , And read every bit it sends In doing so , It finds that the bit that sends dominance is read as recessive
• This is a “ Bit error ”, node 1 An active error flag is raised to notify other nodes actually , This means that nodes 1 take 6 A sequence of dominant bits is sent to the bus
• In turn, ,6 Dominant bits are considered by other nodes “ Bit fill error ” As a response , node 2 and 3 At the same time, an active error flag is raised The sequence of error flags caused by this constitutes “CAN Wrong frame ” Part of
• CAN node 1, transmitter , Put it “ Send error counter ”(TEC) increase 8 CAN node 2 and 3 Put it “ Receive error counter ”(REC) increase 1
• CAN node 1 Automatically retransmit messages result , node 1 Put it TEC Reduce 1, node 2 and 3 Put it REC Reduce 1

2. CAN Wrong frame

In the example above ,CAN node “ Raise the active error flag ”, Thus create “ Wrong frame ” In response to detected CAN error .
To understand how it works , Let's first look at one “ normal ” Of CAN frame （ There is no mistake ）：

Please note that , We highlighted CAN In the frame “ Bitfill ”. Bit padding is CAN A subtle but important part of the standard . Basically it stipulates , whenever CAN The node sends the same logic level （ Explicit or implicit ） Five bit hour , It must send a bit of the opposite level . receive CAN The node will automatically delete this extra bit . This process helps ensure continuous synchronization of the network .

According to the previous example , When CAN node 1 stay CAN When an error is detected during message transmission , It will immediately transmit the same logic level 6 Bit sequence - Also known as the trigger activity error flag .

As mentioned earlier , Such a sequence violates the bit filling rule —— Also known as “ Bit fill error ”. Besides , This error affects all on the network CAN Nodes are visible （ And the... That caused this error flag to appear “ Bit error ” contrary ）. therefore , The increase in error flags can be seen as a “ Overall ” The way of error detection , Be sure to inform everyone CAN node .

Please note that , other CAN The node treats the active error flag as a bit fill error . As a response , They also cause an active error flag .
As we will explain later , It is important to distinguish between error flags . especially , The first error flag （ come from “ Find out ” node ） Usually called “ The main ” Active error flag , And later “ reaction ” The error flag of the node is called “ secondary ” Active error flag （s).

3. CAN Example of error frame

Example 1：6 Bit error flag
In this example , all CAN Nodes are found at the same time CAN There is an error in the message and an error flag is sent at the same time .
The result is that all error markers overlap and the total sequence of dominant bits lasts 6 position . under these circumstances , all CAN Nodes see themselves as “ Find out ”CAN node .

At the same time, it is found that this type of error frame is not very common in practice . however , It may be due to a format error （ for example CRC Delimiters are explicit rather than implicit ） or CAN The transmitter is writing CRC Occurs when a bit error is encountered during the field .

Example 2：12 Bit error flag
In this example ,CAN node 1 Transmit dominant bit , But read it as implicit - This means that it finds a bit error . It sends immediately 6 A sequence of dominant bits . Other nodes are only reading the complete 6 Bit filling error is found after bit , Then they raise the error flag at the same time , So as to produce subsequent 6 Dominant bit sequences - That is total 12 individual .

Example 3：9 Bit error flag
In this example , When CAN node 1 Bit error found and start sending 6 Dominant position , It has been sent 3 Dominant bit sequences . Once the main activity passes half the error flag , node 2 and 3 Identification bit filling error （ because 3 An initial dominant position is followed by another 3 A dominant bit ） And start raising their error flags . The result is that the dominant bit sequence from the error flag becomes 9 Bit length .

The above logic that causes the error flag is reflected in what we call “ Activities ”CAN In error frame . Pay special attention to how the secondary error flags caused by each node overlap each other —— And how the primary and secondary signs may overlap . The result is that the sequence of primary bits from which the error flag is raised may be 6 To 12 Bit length .

The sequence always starts with 8 A recessive bit sequence terminates , Marks the end of the error frame . actually , The active error frame may be in error CAN Different positions in the frame “ Start ”, It depends on when errors are found . However , The result will be the same ： All nodes discard the wrong CAN frame , The sending node can try to resend the failed message .

4. Passive error flag

If CAN The node has changed from its default “ Activities ” State transition to “ passive ” state , Then it will only cause the so-called “ Passive error flag ”. The passive error flag is 6 A sequence of recessive bits . under these circumstances , It is relevant to distinguish between passive error flags caused by the sending node and the receiving node .

Example 4： The transmitter is a passive error
As shown in the figure , If the transmitter （ For example, in our example CAN node 1） Trigger passive error flags （ For example, response bit error ）, This will correspond to 6 A continuous sequence of recessive bits . This is all CAN Node detected as bit fill error . Suppose other CAN The node is still in its error active state , They will trigger 6 Active error flag of dominant bits . In other words , Passive transmitters can still “ convey ”CAN The frame is wrong .

Example 5： The receiver is a passive error
contrary , If the receiver generates a passive error flag , This is actually true for all others on the bus CAN node “ invisible ”（ Because any dominant bit is better than the recessive bit sequence ）. actually , This means that the wrong passive receiver no longer has the ability to destroy others CAN The ability of a node to transmit frames .