CAN Bit Timing
Controller Area Network (CAN) is a broadcast, differential serial bus standard, originally developed in the 1980s by Robert Bosch GmbH, for connecting electronic control units (ECUs). CAN was specifically designed to be robust in electromagnetically noisy environments and can utilize a differential balanced line like RS-485. It can be even more robust against noise if twisted pair wire is used. Although initially created for automotive purposes (as a vehicle bus), nowadays it is used in many embedded control applications (e.g., industrial) that may be subject to noise. The messages it sends are small (8 data bytes max) but are protected by a CRC-15 (polynomial 0x62CC) that guarantees a Hamming bit length of 6 (so up to 5 bits in a row corrupted will be detected by any node on the bus).
Bit rates up to 1 Mbit/s are possible at network lengths below 40 m. Decreasing the bit rate allows longer network distances (e.g. 125 kbit/s at 500 m).
The CAN data link layer protocol is standardized in ISO 11898-1 (2003). This standard describes mainly the data link layer — composed of the Logical Link Control (LLC) sublayer and the Media Access Control (MAC) sublayer — and some aspects of the physical layer of the OSI Reference Model. All the other protocol layers are left to the network designer's choice.
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CAN features an automatic 'arbitration free' transmission. A CAN message that is transmitted with highest priority will 'win' the arbitration, and the node transmitting the lower priority message will sense this and back off and wait.
This is achieved by CAN transmitting data through a binary model of "dominant" bits and "recessive" bits where dominant is a logical 0 and recessive is a logical 1. This means open collector, or 'wired or' physical implementation of the bus (but since dominant is 0 this is sometimes referred to as wired-AND). If one node transmits a dominant bit and another node transmits a recessive bit then the dominant bit "wins" (a logical AND between the two).
| Truth tables for dominant/recessive and logical AND | |||||||||||||||||||
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So, if you are transmitting a recessive bit, and someone sends a dominant bit, you see a dominant bit, and you know there was a collision. (All other collisions are invisible.) The way this works is that a dominant bit is asserted by creating a voltage across the wires while a recessive bit is simply not asserted on the bus. If anyone sets a voltage difference, everyone sees it, hence, dominant. Thus there is no delay to the higher priority messages, and the node transmitting the lower priority message automatically attempts to re-transmit 6 bit clocks after the end of the dominant message.
When used with a differential bus, a Carrier Sense Multiple Access/Bitwise Arbitration (CSMA/BA) scheme is often implemented: if two or more devices start transmitting at the same time, there is a priority based arbitration scheme to decide which one will be granted permission to continue transmitting. The CAN solution to this is prioritised arbitration (and for the dominant message delay free), making CAN very suitable for real time prioritised communications systems.
During arbitration, each transmitting node monitors the bus state and compares the received bit with the transmitted bit. If a dominant bit is received when a recessive bit is transmitted then the node stops transmitting (i.e., it lost arbitration). Arbitration is performed during the transmission of the identifier field. Each node starting to transmit at the same time sends an ID with dominant as binary 0, starting from the high bit. As soon as their ID is a larger number (lower priority) they'll be sending 1 (recessive) and see 0 (dominant), so they back off. At the end of ID transmission, all nodes but one have backed off, and the highest priority message gets through unimpeded.
Each node in a CAN network has its own clock, and no clock is sent during data transmission. Synchronization is done by dividing each bit of the frame into a number of segments: Synchronisation, Propagation, Phase 1 and Phase 2. The Length of each phase segment can be adjusted based on network and node conditions. The sample point falls between Phase Buffer Segment 1 and Phase Buffer Segment 2, which helps facilitate continuous synchronization. Continuous synchronization in turn enables the receiver to be able to properly read the messages.
Based on levels of abstraction, the structure of the CAN protocol can be described in terms of the following layers:
A CAN network can be configured to work with two different message (or "frame") formats: the standard or base frame format (or CAN 2.0 A), and the extended frame format (or CAN 2.0 B). The only difference between the two formats is that the “CAN base frame” supports a length of 11 bits for the identifier, and the “CAN extended frame” supports a length of 29 bits for the identifier, made up of the 11-bit identifier (“base identifier”) and an 18-bit extension (“identifier extension”). The distinction between CAN base frame format and CAN extended frame format is made by using the IDE bit, which is transmitted as dominant in case of an 11-bit frame, and transmitted as recessive in case of a 29-bit frame. CAN controllers that support extended frame format messages are also able to send and receive messages in CAN base frame format. All frames begin with a start-of-frame (SOF) bit that, obviously, denotes the start of the frame transmission.
CAN has four frame types:
The data frame is the only frame for actual data transmission. There are two message formats:
The CAN standard requires the implementation must accept the base frame format and may accept the extended frame format, but must tolerate the extended frame format.
The frame format is as follows:
| Field name | Length (bits) | Purpose |
|---|---|---|
| Start-of-frame | 1 | Denotes the start of frame transmission |
| Identifier | 11 | A (unique) identifier for the data |
| Remote transmission request (RTR) | 1 | Must be dominant (0)Optional |
| Identifier extension bit (IDE) | 1 | Must be dominant (0)Optional |
| Reserved bit (r0) | 1 | Reserved bit (it must be set to dominant (0), but accepted as either dominant or recessive) |
| Data length code (DLC) | 4 | Number of bytes of data (0-8 bytes) |
| Data field | 0-8 bytes | Data to be transmitted (length dictated by DLC field) |
| CRC | 15 | Cyclic redundancy check |
| CRC delimiter | 1 | Must be recessive (1) |
| ACK slot | 1 | Transmitter sends recessive (1) and any receiver can assert a dominant (0) |
| ACK delimiter | 1 | Must be recessive (1) |
| End-of-frame (EOF) | 7 | Must be recessive (1) |
One restriction placed on the identifier is that the first 7 bits cannot be all recessive bits. (I.e., the 16 identifiers 1111111xxxx are invalid.)
The frame format is as follows:
| Field name | Length (bits) | Purpose |
|---|---|---|
| Start-of-frame | 1 | Denotes the start of frame transmission |
| Identifier A | 11 | First part of the (unique) identifier for the data |
| Substitute remote request (SRR) | 1 | Must be recessive (1)Optional |
| Identifier extension bit (IDE) | 1 | Must be recessive (1)Optional |
| Identifier B | 18 | Second part of the (unique) identifier for the data |
| Remote transmission request (RTR) | 1 | Must be dominant (0) |
| Reserved bits (r0, r1) | 2 | Reserved bits (it must be set dominant (0), but accepted as either dominant or recessive) |
| Data length code (DLC) | 4 | Number of bytes of data (0-8 bytes) |
| Data field | 0-8 bytes | Data to be transmitted (length dictated by DLC field) |
| CRC | 15 | Cyclic redundancy check |
| CRC delimiter | 1 | Must be recessive (1) |
| ACK slot | 1 | Transmitter sends recessive (1) and any receiver can assert a dominant (0) |
| ACK delimiter | 1 | Must be recessive (1) |
| End-of-frame (EOF) | 7 | Must be recessive (1) |
The two identifier fields (A & B) combined form a 29-bit identifier.
•Generally data transmission is performed on an autonomous basis with the data source node (e.g. a sensor) sending out a Data Frame. It is also possible, however, for a destination node to request the data from the source by sending a Remote Frame. •There are 2 differences between a Data Frame and a Remote Frame. Firstly the RTR-bit is transmitted as a dominant bit in the Data Frame and secondly in the Remote Frame there is no Data Field.
i.e.
RTR = 0 ; DOMINANT in data frame
RTR = 1 ; RECESSIVE in remote frame
In the very unlikely event of a Data Frame and a Remote Frame with the same identifier being transmitted at the same time, the Data Frame wins arbitration due to the dominant RTR bit following the identifier. In this way, the node that transmitted the Remote Frame receives the desired data immediately.
The first field is given by the superposition of ERROR FLAGS contributed from different stations. The following second field is the ERROR DELIMITER.
The overload frame contains the two bit fields Overload Flag and Overload Delimiter. There are two kinds of overload conditions that can lead to the transmission of an overload flag:
The start of an overload frame due to case 1 is only allowed to be started at the first bit time of an expected intermission, whereas overload frames due to case 2 start one bit after detecting the dominant bit. Overload Flag consists of six dominant bits. The overall form corresponds to that of the active error flag. The overload flag’s form destroys the fixed form of the intermission field. As a consequence, all other stations also detect an overload condition and on their part start transmission of an overload flag. Overload Delimiter consists of eight recessive bits. The overload delimiter is of the same form as the error delimiter.
Data frames and remote frames are separated from preceding frames by a bit field called interframe space. Overload frames and error frames are not preceded by an interframe space and multiple overload frames are not separated by an interframe space. Interframe space contains the bit fields intermission and bus idle and, for error passive stations, which have been transmitter of the previous message, suspend transmission.
In CAN frames, a bit of opposite polarity is inserted after five consecutive bits of the same polarity. This practice is called bit stuffing, and is due to the "Non Return to Zero" (NRZ) coding adopted. The "stuffed" data frames are destuffed by the receiver. Since bit stuffing is used, six consecutive bits of the same type (111111 or 000000) are considered an error. Bit stuffing implies that sent data frames could be larger than one would expect by simply enumerating the bits shown in the tables above.
There are several CAN physical layer standards:
ISO 11898-2 uses a two-wire balanced signaling scheme. It is the most used physical layer in car powertrain applications and
industrial control networks.
ISO 11898-4 standard defines the time-triggered communication on CAN (TTCAN). It is based on the CAN data link layer protocol providing a system clock for the scheduling of messages.
SAE J1939 standard uses a two-wire twisted pair, -11 has a shield around the pair while -15 does not. SAE 1939 is widely used in agricultural & construction equipment.
ISO 11783-2 uses four unshielded twisted wires; two for CAN and two for terminating bias circuit (TBC) power and ground. This bus is used on agricultural tractors. This bus is intended to provide interconnectivity with any implementation adhering to the standard.
As the CAN standard does not include tasks of application layer protocols, such as flow control, device addressing, and transportation of data blocks larger than one message, many implementations of higher layer protocols were created. Among these are DeviceNet, CANopen, SDS, CANaerospace, J1939, NMEA 2000, CAN Kingdom, SafetyBUS p, and MilCAN.