How to solve the key problems facing the current CAN network application layer protocol design?
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Just like the birth of integrated circuits and microprocessors, the advent of data bus technology has become an important milestone in the development of automotive electronics technology. Nowadays, with the increasing application of CAN bus technology in the field of automotive electronics, the importance of the consistent specification of the protocol has gradually emerged. According to the OSI model defined by ISO (International Organization for Standardization), the CAN protocol defines the physical layer and data link layer specifications, which facilitates the development of application layer protocols that meet the needs of different auto manufacturers. If you need to build a more complete system, you need to choose the appropriate application layer protocol based on CAN.
In order to help engineers in the industry to further understand the importance of automotive CAN network application layer protocol development, as well as master the automotive CAN network application layer protocol design technology and solve its key problems, how to solve the current problem in the electronic engineering design website of the electronic engineering album Key issues facing CAN network application layer protocol design? In the panel discussion, Dr. Dong Yinping, senior product specialist of Mentor Graphics, was invited to interact with the engineers to further discuss the topic of automotive CAN network application layer protocol development. Based on this panel discussion, we summarize some of the problems that often arise in the design of automotive CAN network application layer protocols for readers' reference.
Analyze the CAN protocol application layer and application
Dr. Dong Yinping said that the application layer and application are different. The application layer refers to the application layer of the communication function. It does not define and describe application parameters, it provides only the communication interface between the communication function and the application. Includes: defining communication services, transmitting process data, diagnostic information, and calibration information. Device monitoring and network management are also generally defined as part of the application layer, and some also incorporate part of the transport layer into the application layer, such as more than 8 bytes of data transmission.
The application is completely referred to as application code such as control algorithms. It defines the data and parameters associated with the control algorithm.
In current ECU development, the application code contains application layer code. The disadvantages are the following three aspects:
1. The application has changed and you must check to see if the application layer still meets the requirements.
2. The communication protocol changes and the entire application and application layer code must be recompiled and tested. This problem is one of the main reasons why OEMs cannot play a leading role in the development of the agreement. Therefore, many domestic automakers have new agreements, and it is hoped that some large foreign suppliers will encounter great obstacles when implementing new agreements. One is unwilling to do it, and the other is that the cost of redevelopment is high.
3. Seriously hindered the reuse of nodes and designs. Because the application and application layers are combined, it is difficult to achieve plug and play.
The solution is to standardize the interface, which is to split the application layer from the application and standardize the interface. One of the features of AUTOSAR is the standardized interface for plug and play. Mentor Graphics' VTP is also a typical example.
Is the importance of CAN testing important in protocol development?
Dr. Dong Yinping said that the importance of testing and design is reflected in different design ideas. At present, the methods of application layer protocol can be divided into two categories, one is the method of testing as the center of gravity, and the other is the method of designing the center of gravity.
The first method is also known as voting or testing. This is an engineering design method. Each supplier requests the agreement, the integration requirements of the OEM, the feasibility of the test through the test, and the subsequent release of the agreement. In addition to verifying the implementation of the protocol, an important task of testing is to test the protocol design, trying to solve the problem of unreasonable ID allocation, message conflicts, and so on. The focus of this approach is testing, so testing is important.
The second method is the system level design method. This is a theoretical design method, which proposes completely different requirements. The supplier only needs to provide corresponding parameters and calculate the network communication characteristics according to a certain theoretical model, such as signal delay and bus load. Based on this, considering the design requirements, the ID of each message is allocated through a certain scheduling algorithm. The core of the method is to optimize these characteristic parameters to ensure the real-time performance of the vehicle network communication. Therefore, in this design method, design is the focus.
Dr. Dong Yinping also pointed out that testing is not the same as verification. He said: The verification indicates that we have a standard to test whether the tested object is in conformity. However, the current automotive electronics test can not be called verification, because it also tests whether the protocol design is correct, and whether there is any flaw in the protocol design. The input of the verification is the characteristic of the object to be tested, the standard is referred to, and the output is whether or not the two are in conformity.
In the current test, the input has both the characteristics of the measured object and the so-called standard. The output is whether the protocol needs to be modified, whether the system is feasible, and whether the design meets the requirements. Note that this leads to a difficult place to understand. We want to test whether the ECU meets the standards, but we are still skeptical about this standard based on the test results. So unless you have a very strong skill and experience, you are always embarrassed between the two, wasting time and money. China is currently in this state, but we have to rely on testing to promote the development of technology, which is hard to imagine. What we need is a new development idea.
Without design, there is no way to test at all. On the contrary, the design determines the system of testing. For example, the electrical characteristics of the vehicle network, such as the terminal resistance and capacitance of the ECU, are related to the specific network environment. It may be different because the wiring harness is different. Designers know the causes and possible problems that affect these parameters, and these can be the inputs to building test methods.
Dr. Dong Yinping added that testing is more important, but if you have to compare it, it is more important to choose design. In fact, the development of the electronics industry can be a reference. Why is EDA tools so important in electronic design? And now the international trend is based on system-level design, modeling and so on. The focus is on the system level! Dr. Dong Yinping also stressed that if you only pay attention to testing, then it is the old way of developing abroad. It is a departure from the system-level design ideas, and it does not consider the trend of international development.
In which ways should the agreement be constructed?
The basic elements of the CAN protocol are the mapping of IDs, periods, and signals to messages. Therefore, the main tasks of constructing the protocol are ID allocation, defining message periods, and determining the mapping relationship between signals and messages. The design of these three aspects is equally important. The main factors to be considered in the design are the real-time requirements of data transmission (so-called timing), the relative importance of data, and the time requirements of data-related application control algorithms.
Dr. Dong Yinping pointed out that the design of the agreement is actually a very complicated work. For the domestic, because we lack the corresponding experience, the foreign technology is also used for us. ? So far, this is still the main obstacle to the development of China's technology.
There are also some existing standards in the world, such as CANopen, SAE J1939.SAE J1939 This is an agreement between the application and the truck electronic control network led by the agreement of the automotive engineers. However, it is mainly applied to the electronic control system of the truck and cannot be directly transferred to the car control system. However, with the development of automotive electronics, the division of labor in automotive electronics design is becoming more and more detailed. This part of the work also has computer-aided design that manufacturers provide tools to implement protocols. For example, Mentor Graphics' VNA is an automated protocol design software.
What is the structure of the program code that implements the communication protocol?
Dr. Dong Yinping said: First, the CAN communication function includes the physical layer, the data link layer and the application layer. The physical layer and data link layer have been implemented in hardware and are now standardized with ready-made components (CAN controllers and transceivers). Therefore, the CAN controller and transceiver are added to the MCU, and the corresponding driver is implemented by the software to realize the CAN communication function. However, this is not enough for automotive electronics applications because the data link layer has many functions that are not defined such as functional implementation between messages with logical relationships, network management, and so on.
Therefore, the structure of the program code of the communication protocol should be the underlying driver + application code (application code of the communication function). If you consider the current technological development of automotive electronics embedded software, the future structure should be the underlying driver + application code + abstraction layer. The automotive system software open system standard AUTOSAR is basically the same idea. There are also many software vendors that offer off-the-shelf solutions. ECU software development only requires control algorithms on top of the data-based read-write interface provided by the solution. The benefit of this is that software designers can use their expertise to centrally design control algorithms and ensure their reliability. Such products such as Mentor Graphics' embedded software (VTP + Network Management + Diagnostics...) are examples of such applications.
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