High-speed CAN bus physical layer requirements for wire harnesses

2022-08-05
Introduction:
The current vehicle has more and more functions, including driving assistance functions: such as 360-degree panoramic surround view, automatic parking, lane departure warning, lane keeping, ACC (adaptive cruise control), forward collision warning, reverse side warning, blind spot assist, etc.; comfort functions: such as air suspension, seat mirror adjustment memory, etc., the realization of these functions are dependent on the increase of the corresponding controller to achieve, in addition to the current The controller design basically has to consider functional safety, its control strategy is more perfect, but also more complex, the number of signal interaction between the corresponding controllers is also extremely increased, these huge numbers of signals must be used with the help of data transmission buses such as CAN, CANFD, MOST, FlexRay and the current gradually started to use Ethernet, this article on the most commonly used high-speed CAN bus, briefly introduced the following In this article, we will briefly introduce the CAN2.0 protocol and the requirements of the wiring harness with the most commonly used high-speed CAN bus.
 

What is CAN?

CAN (Controller Area Network) controller local area network, is BOSCH in order to solve the vehicle increased signal transmission first proposed, but also the ISO international standardization of serial communication protocol.
CAN's characteristics.
①Multi-master control: and broadcast type, when the bus is idle, all units can send messages to the bus, and the priority of message ID (identifier) is identified by bit-by-bit arbitration, and the one with higher priority gets the right to send.
② Flexibility of the system: The units connected to the bus do not have information similar to "address", so when adding units to the bus, the hardware and software of the other units connected to the bus and the application layer do not need to be changed.
③Remote data request: You can request other units to send data by sending "remote frames".
④ Error detection, error notification and error recovery: All units can detect errors, and the detected errors will immediately notify all other units, and the unit that is sending messages will be forced to end the current transmission once an error is detected.
⑤ Fault Closure: CAN can determine whether the type of error is a temporary data error on the bus or a continuous data error (e.g., internal unit failure, driver failure, disconnection, etc.). By this function, when a continuous data error occurs on the bus, the unit causing this error can be isolated from the bus.
 Connection: The number of units connected on the bus is limited by the time delay on the bus and the electrical load. Decrease the communication speed and the number of connectable units increases, increase the communication speed and the number of connectable units decreases.

OSI basic reference model

Communication protocols usually group related communication tasks by layers, which enables greater flexibility in the application of bus systems.
The hardware and software of CAN are divided into multiple layers.
① Application layer: The information is displayed in the data structure of the application. This data is transferred to the object layer.
② Object layer: The role is to manage the messages. The object layer decides when to send those messages and also performs a receive checksum on the received messages.
 Transport layer: The transport layer transmits messages to the object layer and turns the messages it gets from the object layer that need to be sent into a form that the physical layer can send. In addition, the transport layer is responsible for arbitration or error identification and error marking.
④Physical layer: The lowest layer, consisting of the physical components of the network, such as wires and voltages.
 

Message Format

CAN supports two different message formats, defined in CAN2.0A and CAN2.0B, respectively. CAN2.0A has an 11-bit identifier, while CAN2.0B has a 29-bit identifier, and both have the same data frame format, consisting of frame start, arbitration field, control field, data field, CRC, ACK field, and frame end.
 

Bus Transmission

①Bus logic state and coding
CAN bus has two logical states, i.e. explicit and implicit, explicit represents binary bit "0", invisible represents binary bit "1", after receiving the bus message, the CAN transceiver converts the signal level into logical state, i.e. CAN_H level and After receiving the bus message, the CAN transceiver converts the signal level into a logical state, i.e. CAN_H level and CAN_L level are subtracted to obtain a difference level.
High-speed CAN transmits recessive state bits at a level of 2.5V on both CAN_H /CAN_L, and at a level of 3.5V on CAN_H and 1.5V on CAN_L when transmitting dominant bits.
The low-speed CAN has a level of 0V on CAN_H and 5V on CAN_L when transmitting implicit status bits, and a level of 3.6V on CAN_H and 1.4V on CAN_L when transmitting explicit status bits.

②Terminal reflection cancellation
When an open bus transmits electronic signals, terminal reflections are generated and interfere with communication. To eliminate terminal reflections, a 120 ohm terminal resistor is added to each end of the bus.

CAN requirements for wiring harness

①Concept of characteristic impedance

The baud rate of high-speed CAN bus is 500Kbit/S. At high frequencies, the signal propagates through the cable in the form of electromagnetic waves, while describing the resistance system and characteristic impedance of the wire for electromagnetic waves in the case of high frequencies.
The characteristic impedance of the cable is the ratio of the electric field strength and magnetic field strength transmitted in the cable (V/m)/(A/m) = ohms, and the formula for calculating the characteristic impedance is as follows:


R = resistivity per unit length of this conductor material (in the case of DC), ohms.
G = coefficient of conductivity of the insulation, ohms
J = imaginary number, +90 degrees phase angle
Л=3.1416
L=inductance per unit length of cable
C = electrical capacity per unit length of cable
At high frequencies, the two terms of f in the formula become large, when R and G can be neglected and the formula is simplified to



So in the case of high enough f, the characteristic impedance and frequency are not related, the characteristic impedance is equal to the square root of the inductance/capacity, and the square root of the product of inductance and capacitance.
 

②The measurement method of characteristic impedance

The characteristic impedance of the cable describes the working characteristics of the cable at high frequencies, universal is to measure the resistance with DC current, so you can not use a multimeter to measure the impedance of the cable, can be measured by professional equipment to measure the impedance of a section of the cable in the case of an open circuit at the far end Zoc, and then measure the impedance of the short circuit at the far end Zsc, and then use the product of the two open that is the characteristic impedance of the wire.

③Impedance matching

If there is a mismatch between the source output impedance, the characteristic impedance of the cable and the load input impedance, there are reflections. When these reflected waves collide with the signal generator (source), they are re-emitted and mixed with the normal signals being emitted, and it is difficult to distinguish which are the original signals and which are re-reflected waves.
 

④ Capacitance between twisted pair wires

In order to ensure the integrity of signal transmission, the CAN twisted pair must meet the requirements of capacitance between the wire and the conductor, and the test method is as follows.

⑤ Propagation delay time

To ensure real-time signal transmission, the transmission time of the signal on the cable must meet the requirements, and the measurement method of signal delay is based on EIA-364-103.

The following are the requirements of a company for high-speed CAN bus parameters on the cable.


 

Summary

In addition to the above requirements, high-speed CAN requirements for the wire harness, the insulation layer of the wire, twisted pair distance, pin in the plug-in location, routing requirements (to avoid the antenna coaxial cable and easily affected signal lines, while avoiding high-current lines), CAN lines in the design, more from the perspective of signal transmission theory to consider, as opposed to other power supply lines and ground lines.