1.CAN bus classification
CAN stands for: Controller Area Network
1.1 classification purposes:
Different automobile different system requirements on the bus, such as: engine control system, the transmission rate is fast; amenity system, since the dispersion controlled object, the transmission cable is easily damaged, but the transmission rate is not very high requirements.
CAN bus is divided into high speed and low speed CAN CAN, two main differences: different transmission rates, High Speed CAN rate: 125kbit / s 1Mbit / s, low speed CAN ~: 5 ~ 125kbit / s
1.2 CAN applications different rates
High-speed CAN (CAN-C): an engine control, automatic transmission control, driving stability systems, instrument cluster
Low-speed CAN (CAN-B): air-conditioning control, seat adjustment, window lifter, roof control, mirror control, light control, navigation system controls
2 Introduction stratified
2.1 Physical Layer
2.1.1 Topology
CAN bus uses two topologies: bus topology and star topology
2.1.1.1 bus topology
Does not require the main controller, all CAN nodes connected directly to the CAN bus, each node can obtain information on the bus.
High Speed CAN bus structure
Low-speed CAN bus structure
Advantages: good electrical properties, reflected in:
- A node fails, the remaining bus system, the communication performance is not affected.
- When you add a new node, we do not do a lot of changes, can be coupled with the existing bus system.
High Speed CAN bus topology using low speed CAN bus topology may be employed, low Terminating resistors on CAN, nodes located at both ends of the CAN transceiver.
2.1.1.2 Star topology
Requires the use of a central coupling node, the node only as a relay, the message to either a single node branch. Since the signal transmission time to keep unchanged, and the length of the wire-type bus topology, the star topology to be the same.
Low-speed CAN star structure
Star topology, the main low-speed CAN uses.
2.1.2 Node
2.1.2.1 nodes
By the microcontroller to run the application, CAN controller and a CAN transceiver components.
Microcontroller: CAN controller controls provided CAN data to be transmitted, received data read CAN.
CAN controller: manages message transmission and reception, the binary data to be transmitted into a bit data transfer current required for the current transmission bit transmitted by the controller to the CAN transceiver pin Txd.
CAN transceiver: enhancement signal, generating the desired level of differential data transmission, enhancing the current bits sent on the bus (CAN_H and CAN_L) in a serial manner.
For receiving direction, the received CAN message to the CAN controller through the CAN transceiver is receiving pin RxD.
Section 2.1.2.2 point limit
Up to 30 nodes can be connected.
2.1.3 logic state of the bus and the voltage level
Two logic levels state, dominant and invisible. On behalf of the dominant level "0" stealth level represents a "1."
High-speed and low-speed CAN CAN bus using different dominant and recessive transmission level logic state.
CAN High Speed CAN and low voltage levels are as follows:
2.1.4 type transmission line
2.1.4.1 wire transmission
Most wire transmission using the CAN bus, using a twisted-pair coupling and decoupling non-twisted pair, and two wires are CAN_H CAN_L.
Two transmission lines at different levels, the level difference, the same interference filter out the two conductors, bus reduce the sensitivity to interference.
2.1.4.2 single-wire transmission
Single-wire transmission prerequisite is that all bus nodes common ground. This common "to" replace the function of the second wire.
Only for low-speed single-wire CAN body electronics and comfort electronic system.
2.1.4.3 extending knowledge
A. Coupling: an electronic circuit, a signal sent by another circuit element by one circuit.
Decoupling: electronic circuits, portions of the power supply circuit causes a voltage fluctuation generated removed, these fluctuations to avoid mutual interference the respective circuits.
Signal coupling method: a capacitor, transformer, optocoupler and the like.
Decoupling method: binding portions of the power circuit, the parallel capacitor.
B. UTP: unshielded twisted pair i.e., the metal interlayer using a wire, electromagnetic reflection, absorption to achieve the shielding effect, installation requirements and all accessories connector must have shielding functions and grounded. In military applications, transportation, government.
Non UTP: Unshielded Twisted Pair i.e., according to their equilibrium (e.g., twisted to each other cancel each other out) to resist interference is generally used in ordinary commercial buildings, residential, no special requirements.
2.1.5 terminating resistor
2.1.5.1 terminating resistor
The reflected wave can be absorbed on the network, enhanced signal strength, the two values should equal the parallel termination resistors characteristic impedance of the transmission line on the communication frequency (120 ohms) impedance match does not cause signal reflections.
Terminating resistor can be installed in the CAN controller.
2.1.5.2 terminating resistor Objective:
Bus when transmitting electronic signals, will produce reflection interference communication terminal. To eliminate reflection termination at both ends of the bus are a terminating resistor of 120Ω.
2.1.5.3 extending knowledge:
Reflection termination:
When a high-frequency signal transmission, the signal transmission line wavelength is relatively short, a signal transmission line termination in a reflected wave formed, influence on the original signal, so terminating resistor at the end of the transmission line, signals arriving
After the end of the transmission line is not reflected.
And the low-frequency signals, without terminating resistor. However, when long-term transmission, typically in order to avoid signal reflections and echoes, but also at the receiving end of the access terminal resistor.
2.2 Data Link Layer
Key data frame