CAN-bus
Info
It was first developed by Bosch in 1983, and since its official introduction in 1986,
it has been used in most cars currently produced and has been applied in various other fields.
Protocols
CAN2.0A / CAN2.0B / CANFD
CAN2.0A: The normal format.
CAN2.0B: The extended format of CAN2.0A.
CANFD: Flexible bitrate & max 64 byte data.
This page is based on CAN 2.0A.
Signal interpretation is easy if you only know the frame and the stuff bit.
CAN bus
The key to CAN communication is to send and receive messages with the volt difference of CAN bus H/L.
If there is a voltage difference, it is dominent , and if there is no voltage difference, it is receiving .
The signal value that the MCU receives through the transceiver is
picture [1] CANBUS states
| state | value |
|---|---|
| dominent | 0 |
| recessive | 1 |
Frame
It is can2.0A frame (44 + 8N bit).
| SOF(1) | ID(11) | RTR(1) | IDE(1) | RB(1) | DLC(4) | DATA(8N) | CRC(15) | DEL(1) | ACK(2) | EOF(7) |
RB: reserved
message competition
In CAN interface, multiple nodes transmit messages at the same time.
But, CAN has one bus, It is half-duplex system.
So, CAN messages compete with each other.
The highest priority message is delivered by comparing the ID value between the SOF and RTR of the CAN frame.
The remaining nodes are ready to be sent again.
The dominant is LOW. The recessive is HIGH.
Suppose 3 nodes are connected by a CAN interface.
The message being sent is determined by the priority of the ID.
picture [2] CAN messages competition
All bits from 10 to 6 of the message id progress to the same dominant and recessive bit.
Only NODE 2 of bit 5 is a recessive bit, and is eliminated from message competition.
at bit 2, NODE 1 is eliminated from message competition for the same reason.
Comparing the CANBUS state value with the ID value of NODE3, it is the same.
Stuff bit
If the same bit comes out five times in a row, the next bit must use one opposite bit.
00000'1'01 ... 11111'0'01 ...
Errors
CAN bus errors can occur for several reasons
faulty cables, noise, incorrect termination, malfunctioning CAN nodes, etc …
Error handling identifies and rejects erroneous messages, enabling a sender to re-transmit the message.
Further, the process helps identify and disconnect CAN nodes that consistently transmit erroneous messages.
step-by-step
picture [3] CAN errors handling sequence
1. CAN node 1 transmits a message onto the CAN bus - and reads every bit it sends
2. In doing so, it discovers that one bit that was sent dominant was read recessive
3. This is a 'Bit Error' and node 1 raises an Active Error Flag to inform other nodes
4. In practice, this means that node 1 sends a sequence of 6 dominant bits onto the bus
5. In turn, the 6 dominant bits are seen as a 'Bit Stuffing Error' by other nodes
6. In response, nodes 2 and 3 simultaneously raise an Active Error Flag
7. This sequence of raised error flags comprise part of a 'CAN error frame'
8. CAN node 1, the transmitter, increases its 'Transmit Error Counter' (TEC) by 8
9. CAN nodes 2 and 3 increase their 'Receive Error Counter' (REC) by 1
10. CAN node 1 automatically re-transmits the message - and now succeeds
11. As a result, node 1 reduces its TEC by 1 and nodes 2 and 3 reduce their REC by 1
CAN error types
| Error types | Node | Info |
|---|---|---|
| Bit Error | Transmitter | Occurs when opposite bit is detected during self-examination of the message. |
| Bit Stuffing Error | Receiver | Node detects a sequence of 6 bits of the same logical level between SOF and CRC. |
| Form Error | Receiver | Node detects a bit of an invalid logical level in the SOF/EOF fields or ACK/CRC delimiters. |
| ACK Error | Transmitter | Node transmits a CAN message, but the ACK slot is not made dominant by receiver(s). |
| CRC Error | Receiver | Node calculates a CAN messages CRC that differs from the transmitted CRC field value. |
CAN node states
The canbus status depends on the error counter value.
TEC : Transmit Error Counter
REC : Receive Error Counter
Canbus is default to error-active .
If the value of TEC REC is greater than 127,
Canbus state changed the error-passive state .
If the TEC value exceeds 255, the node is disconnected from the bus.
It is BUS-OFF .
picture [4] CAN node states
Error counter changed by:
| EC | value | Info |
|---|---|---|
| TEC | +8 | Transmitter raises primary error flag. |
| TEC | -1 | Transmitter successfully sends message. |
| REC | +8 | Receiver raises primary error flag. |
| REC | +1 | Receiver raises secondary error flag. |
| REC | -1 | Receiver successfully receives message. |
Bus states:
A CAN node enters the Error Passive state if the REC or TEC exceed 127
A CAN node enters the Bus Off state if the TEC exceeds 255
Example
An example of can oscilloscope Waveform (between CAN-rx and GND).
picture [5] CAN example
- Raw data
010100000100000100011000100010010001000110011101111101110000110
- Analyze the raw data according to the frame
RTR CRC.D | | | | | | 0 101 0000 0'1'0000 0'1'00011 0001 0001 0010 0010 0011 0011 1011 1110 1110 0001 10 | | | | | | | | | <---------------------------> <----------------> || | | | | | | | | | Data: 11 22 33 CRC || | | | | | | | | | || s '5' '0' s '0' I s r D AA o t D t 0 L CC f u E u C KK f f . f f D
This message is ID: 500, DATA: 11 22 33
Reference
TEXAS INSTRUMENTS Introduction to theController Area Network (CAN)
CSS ELECTRONICS
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