CN117914645A - CAN bus control method and structure with priority - Google Patents

Abstract

The invention discloses a CAN bus control method with priority, which is used for setting the priority of messages, controlling the access of a CAN bus according to the priority of the messages and preferentially sending the messages with high priority; or the priority of the nodes is set, the CAN bus access is controlled according to the priority of the nodes, and the nodes with high priority send messages preferentially. The CAN bus structure with priority comprises a multi-channel LVDS/TTL level conversion circuit and an arbiter, wherein the arbiter is arranged to control CAN bus access, and the arbiter comprises a priority encoder and a multiplexer, and the message transmission is carried out by selecting nodes according to the priority. The invention solves the defects that any node on the traditional CAN bus structure CAN actively send information to other nodes on the network at any time without dividing the information into primary and secondary parts, and has untimely response and poor real-time property.

Description

CAN bus control method and structure with priority

Technical Field

The invention relates to a field bus technology, in particular to a CAN bus control method and structure with priority, and belongs to the technical field of communication.

Background

As the control of various systems of automobiles gradually changes to automation and intelligence, more and more electronic components are in the automobiles, and the electrical systems of the automobiles become increasingly complex. The traditional automobile electronics mostly adopt a point-to-point single communication mode, and are in mutual contact. Each path of signal transmission needs a copper cable to connect and exchange data, so that each ECU needs N interfaces, and the number of interfaces of some ECUs is tens or even thirty, so that a huge wiring system is formed, and the space occupation, the vehicle weight, the cost, the equipment complexity and the instability are greatly increased. In addition, in order to meet the real-time requirements of the electronic systems, the common data (such as the engine speed, the wheel speed, the throttle pedal position and other information) of the automobile need to be shared, and the real-time requirements of each control unit are different. Therefore, the traditional electric network cannot adapt to the development of the modern automobile electronic system, and new automobile bus technology is generated.

To accommodate the need for "reducing the number of wiring harnesses" and "high-speed communication of large amounts of data through multiple LANs", CAN buses have been developed. The CAN bus is an ISO internationally standardized serial communication protocol. The CAN bus has no master-slave division in data communication transmission, any node CAN actively send information to other nodes on the network at any time without dividing the master-slave division, and CAN freely communicate among all communication nodes.

The control system on the modern automobile mainly adopts a CAN bus, which is a communication control system specially designed for automobile electronic product development, and is also applied to the fields of tramcar control, subway control, fire-fighting host networking work, industrial automation production line control, artificial intelligence development and the like besides being applied to automobile control. Because of the inherent nature of the vehicle itself, CAN bus must meet the following requirements: the data transmission speed of the CAN bus must be high, good communication instantaneity must be achieved, the communication command CAN be transmitted to a target area at the first time after being sent, and a typical scene is sudden braking when an automobile encounters accidents in the driving process. Therefore, the highest data transmission speed of the CAN bus is designed to reach 1Mbps, and the information transmission speed of the CAN bus basically has no attenuation in the range of forty meters, thereby meeting the requirements of vehicle communication.

In the conventional CAN bus structure, each node checks the state of the bus before sending the message, and if the bus is occupied, it needs to wait for a period of time and retry sending. This approach, while simple, can result in important messages not being sent in time, thereby reducing the response speed of the system. To solve this problem, a prioritized CAN bus control method and structure CAN be devised.

Disclosure of Invention

The invention aims to provide a CAN bus control method and structure with priority, which are used for solving the defects that any node on the traditional CAN bus structure CAN actively send information to other nodes on a network at any moment without being separated from primary and secondary, and has untimely response and poor real-time performance. The CAN bus structure with priority is provided with an arbiter for controlling CAN bus access, and different nodes are selected for transmission according to the priority of the message, and the CAN bus structure with priority is realized by adopting a circuit with a priority encoder and a multiplexer.

The aim of the invention is realized by the following technical scheme:

The CAN bus control method with priority sets the priority of the messages, controls the CAN bus access according to the priority of the messages, and sends the messages with high priority preferentially; or the priority of the nodes is set, the CAN bus access is controlled according to the priority of the nodes, and the nodes with high priority send messages preferentially.

The method for setting the priority of the message by using the CAN bus control method with the priority comprises the following steps: the messages are assigned different priorities with identifiers contained in the message frames.

Further, the low priority message is represented by a low value identifier and the high priority message is represented by a high value identifier.

In the above-mentioned CAN bus control method with priority, the messages are sent according to the priority order of the messages, if a higher priority message is generated, the transmission of the current message is interrupted, and the higher priority message is sent immediately.

According to the CAN bus control method with the priority, node messages are sent according to the priority sequence of the nodes, if a higher priority node generates a message, message transmission of the current node is interrupted, and the message of the higher priority node is sent immediately.

In the above-mentioned CAN bus control method with priority, a priority shielding bit is set in the CAN controller, so as to shield transmission of low-priority messages, and the messages are transmitted only when the priority of the messages is higher than the shielded level.

The CAN bus structure with priority comprises a CAN bus CAN_ H, CAN _L and each CAN node, wherein each CAN node is connected to the CAN bus, the CAN bus structure further comprises a multi-channel LVDS/TTL level conversion circuit and an arbiter, the LVDS/TTL level conversion circuit converts differential signals into TTL level signals, the dominant differential signals are logic 1, the recessive differential signals are logic 0, the number of channels of the LVDS/TTL level conversion circuit is the same as that of CAN nodes, the arbiter is arranged to control CAN bus access, message transmission is carried out according to the priority selection nodes, the arbiter comprises a priority encoder and a multiplexer, the input end of each channel of the LVDS/TTL level conversion circuit is respectively connected with the CAN_ H, CAN _L of each node of the CAN bus, the multiplexing output end of the LVDS/TTL level conversion circuit is connected with the priority encoder and the input end of the multiplexer, the output end of the priority encoder is connected with the control end of the multiplexer, and each output end of the multiplexer is respectively connected with the interrupt ports EXTI of MCU of each node; the priority encoder control logic is to: at a certain moment, a node sends a dominant differential signal, if the priority of a node A in the nodes sending the signal is highest according to a preset priority, a priority encoder outputs a priority coding value signal representing the node A to a control end of a multi-path selector, and if no node sends the dominant differential signal, a CAN bus is idle; the multiplexer control logic is to: according to the priority code value signal of the node A received by the control end of the multiplexer, a logic 1 TTL level signal of the node A is selected to be conducted to the MCU interrupt EXTI of the CAN node A, and the activated node MCU controls the CAN transceiver to send data to the bus.

Further, the LVDS/TTL level conversion circuit in the CAN bus structure with priority comprises a MAX485 chip, a capacitor C1 and a resistor R1, wherein two ends of the capacitor C1 are respectively connected with a 3.3V power supply and a power supply ground, a VCC pin and a GND pin of the MAX485 chip are respectively connected with a 3.3V power supply and a power supply ground, one end of the resistor R1 is grounded, the other end of the resistor R1 is connected with RE non-DE pins of the MAX485 chip, signals of the CAN bus are accessed from CAN_H and CAN_L pins of the MAX485 chip, and converted TTL levels are output from RO ends of the MAX485 chip.

Compared with the prior art, the invention has the beneficial effects that:

1. controllability of message transmission: by defining and distributing priorities of different messages or nodes, the high-priority messages or nodes can be ensured to acquire faster transmission rights on the bus, so that the real-time performance and reliability of critical messages are ensured. This is critical for applications requiring real-time data transmission, such as automotive control systems and industrial automation.

2. Collision and impact reduction: in a conventional CAN bus, multiple nodes may collide and collide when attempting to send a message at the same time, resulting in a message being lost or retransmitted. The probability of collision can be reduced by introducing the message priority, and the bus utilization rate and stability are improved.

3. Better system performance: by reasonably distributing the priorities of the messages or nodes, the messages of the key tasks can have higher transmission priorities, so that the performance and response time of the whole system are improved.

4. Configurability: the inventive method allows the system designer to configure to determine the priority of a message according to the needs of a particular application. This flexibility allows customization to be made according to the needs of the application, improving the applicability of the system.

5. The invention has the CAN bus structure design of priority, does not change the original CAN bus circuit structure, controls CAN bus access by arranging an arbiter, and selects different nodes for transmission according to the priority. The expansion is flexible and convenient, and the deployment is easy.

The CAN bus structure with priority CAN improve the controllability, performance and reliability of the CAN bus communication system, and is particularly suitable for application scenes requiring high real-time performance and reliability.

Drawings

FIG. 1 is a diagram of a CAN bus structure with priority in accordance with the invention;

FIG. 2 is a block diagram of embodiment 2 of the CAN bus architecture design of the invention;

FIG. 3 is a flow chart showing the MCU priority arbitration scheme according to embodiment 2 of the present invention;

FIG. 4 is a schematic diagram of an LVDS cell design embodying the present invention;

FIG. 5 is a schematic diagram of a CAN transceiver circuit embodying the invention;

FIG. 6 is a CAN node design circuit design embodying the invention;

Fig. 7 is a flowchart of the MCU program of the CAN node embodying the present invention.

Detailed Description

The invention will be further described with reference to the drawings and the specific examples.

Based on the design thought of setting priority, the following 5 kinds of technical schemes can be adopted, and the following analysis is respectively illustrated:

1. setting an identifier for each message frame, wherein the identifier is provided with a priority, and the priority has a plurality of levels; for example, a low value identifier is used to represent a low priority message and a high value identifier is used to represent a high priority message.

2. Messaging uses a polling and preemption mechanism: defining a fixed polling sequence, sending the messages according to the priority sequence, interrupting the current transmission if a high-priority message is generated, and immediately sending the high-priority message; this mechanism is called preemption. In the preemption mechanism, high priority messages interrupt the low priority messages being transmitted, ensuring that the high priority messages can be transmitted as soon as possible.

3. Setting a priority shielding bit in the CAN controller for shielding the transmission of the low-priority message; when a high priority message is received, which is higher than the masked level, the message is transmitted.

4. Using a bus access control protocol scheme: a bus access control protocol may be designed in which different message priorities are defined and transmission according to the priorities is implemented. For example, an arbiter may be provided to control bus access, with different nodes being selected for transmission based on the priority of the message.

5. Using a network management tool: some CAN bus systems provide a network management tool that CAN set the priority of messages by configuration software. This allows flexibility in dynamically changing the priority of messages at run-time.

The adoption of method 1 may be achieved by introducing a priority field of the frame identifier. Each CAN frame has a frame Identifier (ID) that includes a priority field for sequential scheduling of frame transmissions on the bus. The priority of the frame as it is sent over the bus is determined by this priority field.

In a prioritized CAN bus architecture, when multiple nodes need to transmit CAN frames, each node will transmit according to the priority of the frame, and frames with higher priorities will be transmitted preferentially. Thus, important frames can be ensured to be transmitted in time, and the real-time requirement is met. For example, one node needs to send an urgent control command, while another node simply sends a piece of general status information. According to the priority field, the frame of the emergency control instruction will have a higher priority and will be sent preferentially before the normal state information. It should be noted that the prioritized CAN bus structure requires corresponding configuration and programming in the software of the CAN controller and nodes to ensure that the priority fields of the frames are correctly identified and processed. In addition, there are some CAN controllers and nodes that may need to support a hierarchy of priorities in order to better manage multiple nodes and schedule between different priorities.

Before sending the message, each node allocates a priority to the message according to a certain rule, and then performs AND operation with the priority mask bit of the node. A message may be sent only if the result is 1, otherwise waiting is required.

The setting of the priority mask bits may be determined according to system requirements, and one common method of setting is to set the mask bits to a binary number, where each bit represents the mask state of the corresponding priority. For example, if the mask bit is "1100", then messages indicating priorities 3 and 4 will be masked and cannot be sent. Whereas messages with priority 1 and 2 may be sent.

The CAN bus structure design adopting the priority shielding bits CAN effectively improve the response speed of the system and optimize the use efficiency of the bus. In the design and implementation process, the setting method of the priority and the length of the shielding bit, the priority allocation rule of the node and the like can be determined according to the actual requirements of the system.

The method 1, the method 2, the method 3 and the method 5 are realized by software, and are difficult to meet particularly high real-time requirements, for example, the reaction time of an automobile in emergency to take emergency braking is less than 0.5s, and the requirements of some industrial control fields are even smaller. Therefore, the preferable technical scheme is as follows: the CAN bus structure with priority provided by the invention adopts hardware to realize priority judgment and selection, defines an arbiter to control CAN bus access, selects different nodes to transmit according to the priority of the message, and CAN be realized by using a circuit with a priority encoder and a multiplexer. For example, a CAN bus network has N nodes, n=4, and is formed by CAN1, CAN2, CAN3, and CAN4 nodes, and an exemplary circuit is shown in fig. 1, where CAN1, CAN2, CAN3, and CAN4 nodes are connected to a standard CAN bus network (can_ H, CAN _l), and two ends are connected to 120Ω termination resistors. CAN nodes are typically divided into three parts: CAN transceiver, CAN controller, MCU. Usually, some single-chip microcomputer is internally integrated with corresponding CAN controller peripheral equipment, such as a common single-chip microcomputer STM 32. The U1 circuit module is a LVDS (Low Voltage DIFFERENTIAL SIGNALING) to TTL (transmitter-transmitter Logic) circuit, which converts the differential signal into a TTL level signal, because the U2 priority encoder and the U3 multiplexer can only process the TTL level signal.

The U2 priority encoder is a 4-2 priority encoder with 4-bit input and 2-bit output. A. B, C, D is the input signal of the U2 priority encoder, and the output signals S0, S1. Its function is shown in truth table 1:

table 1U2 priority encoder truth table

The priority A is highest, and the priority order is A > B > C > D.

The U3 multiplexer is a 4-4 combinational logic circuit with 4-bit input and 4-bit output. A. B, C, D is the input signal of the U3 multiplexer, selects the control signals S0, S1, and outputs the signals Q1, Q2, Q3, Q4. The functional truth value is shown in table 2:

table 2U3 multiplexer truth table

The CAN bus structure with priority works on the principle that:

Assume that there are CAN1, CAN2, CAN3 and CAN4 nodes that send CAN messages of different priorities onto the bus and are controlled by the arbiter. CAN1 has the highest priority and the priority order is CAN1> CAN2> CAN3> CAN4 in turn.

The CAN controller obtains the bus level by judging the potential difference of two wires of CAN_H and CAN_L, and the CAN bus level is divided into a dominant level and a recessive level. The dominant level represents a logic "0", where the can_h level is higher than can_l, 3.5V and 1.5V, respectively, and the potential difference is 2V. The invisible level represents logic "1", and at this time, the voltages of can_h and can_l are both about 2.5V, and the potential difference is 0V. The bus idle state is always implicit.

1. Node CAN1 sends a high priority CAN message. At this time, the CAN_H level is higher than CAN_L, and the potential difference is 2V. The signal is input to a U1 circuit module, a logic '1' TTL level signal is generated, and the U2 priority encoder encodes the signal to obtain an 11 encoded value with high priority. The U3 multiplexer selects the encoded value and activates the corresponding control signal a (Q1) to output 1. At this time, even if there is a node with a lower priority than it, a message is sent, such as a CAN2 node, and the corresponding "S0S1" code value is not generated because the logic "1" ttl level signal generated by the CAN2 node is masked by the U2 priority encoder.

2. The control signal A (Q1) outputs 1 to activate, sends a signal to the MCU of the node CAN1 indicating that the node CAN1 CAN send its high priority CAN message onto the bus.

3. The node CAN1 sends its CAN message to the bus according to the control signal.

Assuming no other message is transmitted on the bus at this time, node CAN3 sends a low priority CAN message. The signal is input to the U1 circuit block output C. The U2 priority encoder encodes the encoded data to obtain a code value with lower priority. The multiplexer selects the encoded value and activates the corresponding control signal output C. Control signal C output 1 is active and signals node C that node C CAN send its low priority CAN message onto the bus. The node CAN3 sends its CAN message to the bus according to the control signal.

In this way, the arbiter selects different nodes for transmission according to the priority of the message, thereby realizing the CAN bus access control based on the priority. The function of the arbiter in the above invention can also be realized by programming by using a singlechip, and the circuit structure is shown in fig. 2:

Module description in circuit diagram: the U2 module is an MCU (single chip microcomputer) system, A, B, C, D signals are connected to an MCU input pin, and signals Q1, Q2, Q3 and Q4 are connected to an MCU output pin. And realizing CAN bus priority arbitration through programming. The specific workflow is illustrated in fig. 3: the program first checks condition 1. If the condition 1 signal a=1 is true, the signal q1=1 is output. If condition 1 is false, then condition 2 is checked continuously. If the condition 2 signal b=1 is true, the signal q2=1 is output. If condition 2 is false, then condition 3 is checked continuously. If the condition 3 signal c=1 is true, the signal q3=1 is output. If condition 3 is false, a default operation output signal q4=1 is performed. Finally, the procedure ends. And from the beginning again, the cycle is repeated.

Embodiment one:

This embodiment is an embodiment of a CAN bus structure design with priority. As shown in fig. 1, the U1 circuit module is composed of four LVDS unit circuits, and to convert the differential signal into a TTL level signal, a differential signal receiver chip, such as SN75176 or MAX485, may be used. These chips can convert the differential signals to TTL level signals and provide level conversion and driving capabilities. In this embodiment, MAX485 chip design is selected, as shown in fig. 4, pins RE not and DE jointly control signal receiving and transmitting of MAX485, when DR is low level, RE not valid DE is invalid, and signals from CAN bus enter MAX485 from can_h and can_l, and then are output from RO terminal. When the CAN controller sends data, the bus is dominant, and the CAN_H level is higher than the CAN_L, and is respectively 3.5V and 1.5V, and the potential difference is 2V. Y outputs TTL level signals. The transmit pin DI is not used.

As shown in fig. 5, the transceiver circuit of the CAN node is implemented by selecting NXP semiconductor SN65HVD230 from the CAN transceiver, and RXD and TXD are respectively data receiving and transmitting pins for connecting with the transceiver end of the CAN controller. The two ends of the CAN_ H, CAN _L are used for connecting other devices on the CAN bus, and all the devices are connected to the CAN bus in a parallel mode.

The embodiment of the CAN node circuit is shown in fig. 6, the MCU selects a singlechip STM32F103R6, a CAN transceiver is integrated inside, and pins TXD and RXD of the CAN transceiver are connected with pins PA12 (CAN_TX) and PA11 (CAN_RX) of the STM32F103R 6. EXTI interrupt the signal connection pin PC0. When the CAN bus priority arbiter outputs a signal Qx (X=1, 2, 3, 4), wherein the signal line of Qx=1 is provided with EXTI interrupt signals which are valid, an interrupt service routine is executed, and the MCU controls the CAN transceiver to transmit data; on the signal line with qx=0, EXTI interrupt signal is inactive, and MCU controls CAN transceiver to prepare to receive data. The MCU program flow chart is shown in FIG. 7. CAN node work flow: firstly, monitoring the state of a bus, and when the bus is idle, preparing data transmission and reception. Firstly judging whether to send data, if yes, judging whether EXTI signals are valid, otherwise, waiting for receiving the data; and transmitting data when EXTI signals are valid, otherwise waiting for data reception. The procedure ends.

The U2 priority encoder and the U3 multiplexer select corresponding types of functional logic devices according to a truth table, or an FPGA chip is applied to design according to functional logic.

Embodiment two:

The present embodiment is an embodiment of the CAN bus structure design 2 with priority. As shown in FIG. 2, a single chip microcomputer circuit is changed from the design of a combinational logic circuit of a priority encoder and a multiplexer, and the design flow is shown in FIG. 3.

In addition to the above embodiments, other embodiments of the present invention are possible, and all technical solutions formed by equivalent substitution or equivalent transformation are within the scope of the present invention.

Claims (8)

1. The CAN bus control method with the priority is characterized in that the priority of the messages is set, the CAN bus access is controlled according to the priority of the messages, and the messages with high priority are sent preferentially; or the priority of the nodes is set, the CAN bus access is controlled according to the priority of the nodes, and the nodes with high priority send messages preferentially.

2. The method for controlling the CAN bus with the priority according to claim 1, wherein the method for prioritizing the messages is as follows: the messages are assigned different priorities with identifiers contained in the message frames.

3. The CAN bus control method with priority of claim 2 wherein the low priority message is represented by a low value identifier and the high priority message is represented by a high value identifier.

4. A CAN bus control method with priority as claimed in claim 2, characterized in that the messages are sent in the order of their priority, if higher priority messages are generated, the transmission of the current message is interrupted and the higher priority messages are sent immediately.

5. The CAN bus control method of claim 1 wherein node messages are transmitted in a priority order of nodes, if a higher priority node generates a message, message transmission of a current node is interrupted, and a message of a higher priority node is immediately transmitted.

6. The CAN bus control method of claim 1 wherein a priority mask bit is set in the CAN controller for masking transmission of low priority messages, the messages being transmitted only when the priority of the messages is higher than the masked level.

7. The CAN bus structure with priority comprises a CAN bus CAN_ H, CAN _L and each CAN node, wherein each CAN node is connected to the CAN bus, and the CAN bus structure is characterized by further comprising a multi-channel LVDS/TTL level conversion circuit and an arbiter, wherein the LVDS/TTL level conversion circuit converts differential signals into TTL level signals, the dominant differential signals are logic 1, the recessive differential signals are logic 0, the number of channels of the LVDS/TTL level conversion circuit is the same as that of the CAN nodes, the arbiter is arranged to control CAN bus access, the nodes are selected according to the priority to carry out message transmission, the arbiter comprises a priority encoder and a multiplexer, the input end of each channel of the LVDS/TTL level conversion circuit is respectively connected with the CAN_ H, CAN _L of each node of the CAN bus, the multiplexing output end of the LVDS/TTL level conversion circuit is connected with the priority encoder and the input end of the multiplexer, the output end of the priority encoder is connected with the control end of the multiplexer, and each output end of the multiplexer is respectively connected with the interrupt EXTI of each node MCU; the priority encoder control logic is to: at a certain moment, a node sends a dominant differential signal, if the priority of a node A in the nodes sending the signal is highest according to a preset priority, a priority encoder outputs a priority coding value signal representing the node A to a control end of a multi-path selector, and if no node sends the dominant differential signal, a CAN bus is idle; the multiplexer control logic is to: according to the priority code value signal of the node A received by the control end of the multiplexer, a logic 1 TTL level signal of the node A is selected to be conducted to the MCU interrupt EXTI of the CAN node A, and the activated node MCU controls the CAN transceiver to send data to the bus.

8. The CAN bus structure with priority as set forth in claim 7, wherein the LVDS/TTL level conversion circuit comprises a MAX485 chip, a capacitor C1 and a resistor R1, wherein the two ends of the capacitor C1 are respectively connected with a 3.3V power supply and a power supply ground, the VCC pin and the GND pin of the MAX485 chip are respectively connected with the 3.3V power supply and the power supply ground, one end of the resistor R1 is grounded, the other end of the resistor R1 is connected with the RE non-DE pin of the MAX485 chip, the signals of the CAN bus are accessed from the CAN_H pin and the CAN_L pin of the MAX485 chip, and the converted TTL level is output from the RO end of the MAX485 chip.