Handling Bus Errors

# Handling Bus Errors

When a CAN controller detects an error, it transmits an error frame. This is a special CAN message that causes all other CAN controllers on the bus to notice that an error has occurred.

CANlib will report error frames to the application just like it reports any other CAN message, but the canMSG_ERROR_FRAME flag will be set in the flags parameter when e.g. canRead() returns.

When an error frame is received, its identifier, DLC and data bytes will be undefined. You should test if a message is an error frame before checking its identifier, DLC or data bytes.

In an healthy CAN system, error frames should rarely, if ever, occur. Error frames usually mean there is some type of serious problem in the system, such as a bad connector, a bad cable, bus termination missing or faulty, or another node transmitting at wrong bit rate, and so on.

# SJA1000 Error Codes

Many of our CAN interface boards use the SJA1000 CAN controller. When an error occurs on the bus, the device firmware and driver will pass the error code from the CAN controller to the application by using the CAN identifier field. The identifier will be set to 0x800 (2048) plus the error code from the controller. The flag field will contain the canMSG_ERROR_FRAME flag. The data length will be 0.

You can use the following tables to interpret the error code from the controller. Note that they are applicable only to those CAN interfaces that use SJA1000.

Bit 7 | Bit 6 | Meaning
--- | --- | ---
0 | 0 | bit error
0 | 1 | form error
1 | 0 | stuff error
1 | 1 | other type of error

Bit 5 | Meaning
--- | ---
0 | TX; error occurred during transmission
1 | RX; error occurred during reception

Bit 4..0 | Meaning
--- | ---
0x03 | start of frame
0x02 | ID.28 to ID.21
0x06 | ID.20 to ID.18
0x04 | bit SRTR
0x05 | bit IDE
0x07 | ID.17 to ID.13
0x0F | ID.12 to ID.5
0x0E | ID.4 to ID.0
0x0C | bit RTR
0x0D | reserved bit 1
0x09 | reserved bit 0
0x0B | data length code
0x0A | data field
0x08 | CRC sequence
0x18 | CRC delimiter
0x19 | acknowledge slot
0x1B | acknowledge delimiter
0x1A | end of frame
0x12 | intermission
0x11 | active error flag
0x16 | passive error flag
0x13 | tolerate dominant bits
0x17 | error delimiter
0x1C | overload flag

Example
A message is transmitted on the CAN bus. A stream of error frames are returned. canRead() reads messages with id 0x000008D9, dlc = 0 and flags = 0x20 (canMSG_ERROR_FRAME). What's up?

Using the tables above, we see that 0xD9 means

- Other type of error.
- Error during transmit.
- Acknowledge slot.

so it is likely that the module transmitting the message doesn't get any answer.

Note
  We make no guarantee that this feature will be present in future releases of hardware, drivers and firmware.
  We recommend that you use this feature for troubleshooting a particular installation only. Do not write your software so it relies on this feature.

See also
  Message Flags