Kvaser CAN Protocol Course: CAN Error Handling (Part 8)

Summary

This video explains the error handling mechanisms within the CAN protocol, emphasizing the importance of detecting and correcting errors on the bus. It covers various error detection methods, including Bit Monitoring, Bit Stuffing, Frame Check, Acknowledgement Check, and Cyclic Redundancy Check. The video also discusses error confinement, where nodes with faulty transmissions enter different states like Error Passive or Bus Off to prevent further disruption. Additionally, the CAN bus failure modes and fault tolerance in drivers are outlined, showcasing how the system manages failures to ensure communication remains as reliable as possible.

Takeaways

• 😀 Error handling is a critical feature of the CAN protocol, aiming to detect and correct errors in messages on the bus.
• 😀 Every CAN controller detects errors within messages and transmits an Error Flag when an issue is found, which disrupts bus traffic temporarily.
• 😀 There are two key error counters maintained by each node: Transmit Error Counter and Receive Error Counter, each with specific rules for incrementing or decrementing.
• 😀 If the Transmit Error Counter exceeds 127, the node enters 'Error Passive' mode and begins transmitting Passive Error Flags.
• 😀 A node enters the 'Bus Off' state if its Transmit Error Counter exceeds 255, meaning it no longer participates in the bus traffic.
• 😀 The CAN protocol has five error detection mechanisms: Bit Monitoring, Bit Stuffing, Frame Check, Acknowledgement Check, and Cyclic Redundancy Check (CRC).
• 😀 Bit Stuffing helps prevent long sequences of identical bits on the bus, adding an extra bit of opposite polarity to aid in error detection.
• 😀 The Frame Check ensures that parts of the message with fixed formats, such as CRC Delimiters and End of Frame, are correct; otherwise, a Form Error is signaled.
• 😀 Acknowledgement Check verifies that all nodes on the bus that receive a message correctly acknowledge it by sending a dominant level in the Acknowledgement Slot.
• 😀 The Cyclic Redundancy Check (CRC) ensures data integrity by comparing the calculated CRC with the one included in the message, signaling an error if they don't match.

Q & A

What is the purpose of error handling in the CAN protocol?

Error handling in the CAN protocol aims to detect errors in messages on the CAN bus and ensure that erroneous messages are retransmitted correctly, preserving the integrity of the communication system.

How does a CAN node detect errors in messages?

Each CAN node monitors the bus for errors within messages by checking the transmitted signal. If a discrepancy is detected, an Error Flag is transmitted to inform other nodes about the issue.

What happens when an error is detected by a CAN node?

When a CAN node detects an error, it transmits an Error Flag, which causes other nodes to discard the current message. The error counters are then adjusted based on the nature of the error.

What are the two error counters maintained by a CAN node?

The two error counters maintained by a CAN node are the Transmit Error Counter (TEC) and the Receive Error Counter (REC). These counters track errors and help manage the node's behavior in error situations.

How do the Transmit Error Counter and Receive Error Counter differ in their increment rates?

The Transmit Error Counter increments faster when the transmitting node detects an error, whereas the Receive Error Counter increments more slowly in listening nodes. This helps isolate the faulty node, usually the transmitter.

What happens when the error counters exceed certain thresholds in CAN?

When the error counters exceed specified thresholds, the node enters different error states. If the Transmit Error Counter exceeds 127, the node enters Error Passive mode, and if it exceeds 255, the node enters the Bus Off state.

What is the difference between Error Active, Error Passive, and Bus Off states in CAN?

In Error Active mode, the node actively transmits error flags. In Error Passive mode, the node transmits passive error flags, which do not interfere with bus traffic. In Bus Off mode, the node completely ceases communication on the bus.

What are the five error detection mechanisms in the CAN protocol?

The five error detection mechanisms in CAN are: 1) Bit Monitoring, 2) Bit Stuffing, 3) Frame Check, 4) Acknowledgement Check, and 5) Cyclic Redundancy Check (CRC).

How does Bit Stuffing work in CAN?

Bit Stuffing involves inserting a bit of the opposite level after five consecutive bits of the same level in a transmitted message. This helps prevent excessive DC components and provides an additional opportunity for error detection.

What are some common failure modes of the CAN bus and how are they addressed?

Common failure modes of the CAN bus include CAN_H or CAN_L being interrupted, shorted to ground or battery voltage, or loss of connection to the termination network. Some failure modes are survivable with reduced signal-to-noise ratios, while others require fault-tolerant drivers like the TJA1053 for reliable operation.