DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

Table 3.4 Error Flag Output Timing [19].
Type of Error Output Timing
Bit Error The Error Flag is output beginning with the bit that immediately follows the
Stuffing Error one in which an error was detected
Form Error
ACK Error
CRC Error The Error Flag is output beginning with the bit immediately following the
CRC field (ACK slot bit).
3.5 Fault Confinement and Error States
A major risk with any serial bus system is that a single defective node can shut down the
entire network. To deal with this, the CAN protocol is designed to automatically detect a
faulty node and disconnect it from the network. CAN requires two error counters for every
node, one to keep track of transmit errors and the other to keep track of receive errors. When a
transmission or reception error occurs, the respective counter is increased by a weighted value
depending upon the type of error and which node caused the error. For every successful
transmission or reception, the respective counter is decremented by one. Generally, if a node
recognizes that it is the source of the error then the counter is incremented by nine for receive
errors and eight for transmission errors. Otherwise, each counter is increased by one [18].
Detected errors are made public to all other nodes via Error Frames or Error Flags. The
transmission of an erroneous message is aborted and the frame is repeated as soon as the
message can again win arbitration on the network. Also, each node is in one of three error states,
Error-Active, Error-Passive or Bus-Off. Figure 3.10 shows a graphical representation of these
three error states.
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