How to solve the key problems facing the current CAN network application layer protocol design?
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 increasingly widespread application of CAN bus technology in the field of automotive electronics, the importance of the consistent specification of its protocol has gradually emerged. According to the OSI model defined by ISO (International Organization for Standardization), the CAN protocol defines physical layer and data link layer specifications, which provides convenience for different car manufacturers to develop application layer protocols that meet their own needs. If you need to build a more complete system, you also need to select the appropriate application layer protocol on the basis of CAN.
In order to help engineers in the industry to further understand the importance of the development of the CAN CAN application layer protocol, as well as to master the automotive CAN network application layer protocol design technology and solve its key problems, "How to solve the current What are the key issues facing CAN network application layer protocol design? 》 Denmin Dong, Senior Product Specialist of Mentor Graphics, was invited to interact with engineers and further discussed the topic of the development of automotive CAN network application layer protocol. Based on this special discussion, we summarize some of the problems that often arise in the design of the application layer protocol of the automotive CAN network for readers' reference.
Analysis of CAN protocol application layer and application program
Dr. Dong Yinping said that the application layer and the application are different. The application layer refers to the application layer of the communication function. It does not define and describe application parameters, it only provides communication interfaces for communication functions and application programs. Including: 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 for implementation, such as data transmission of more than 8 bytes.
The application program completely refers to the application code such as the control algorithm. It defines the data and parameters related to the control algorithm.
In the current ECU development, the application code contains the application layer code. The disadvantage lies in the following three aspects:
1. The application program changes, you must check whether the application layer can meet the requirements.
2. The communication protocol changes, and the entire application program 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 OEMs have new agreements, hoping that some large foreign suppliers will encounter great obstacles when implementing the new agreements. One is reluctance to do it, and the other is the high cost of redevelopment.
3. Seriously hinders the reuse of nodes and designs. Because the application and application layer are fused together, it is difficult to achieve a plug-and-play effect.
The solution is to standardize the interface, that is, to split the application layer from the application and standardize the interface. One of the features of AUTOSAR is the standardized interface, which enables plug and play. Mentor Graphics' VTP is also a typical example.
Is CAN testing important or protocol formulation important?
Dr. Dong Yinping said that the importance of testing and design is reflected in different design ideas. At present, the methods developed by the 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 as the center of gravity.
The first method is also called the voting method or the test method. This is an engineering design method. Each supplier puts forward requirements for the agreement, integrates the requirements of the OEM, verifies the feasibility of the agreement through testing, and then releases the agreement. In addition to verifying the implementation of the protocol, the test function also has an important task to test the protocol design, trying to solve the problem of unreasonable ID allocation and message conflicts. The focus of this method is testing, so testing is more important.
The second method is the system-level design method. This is a theoretical design method. It proposes completely different requirements. The supplier only needs to provide the corresponding parameters and calculate the network communication characteristics, such as signal delay and bus load, according to a certain theoretical model. Based on this, considering the design requirements, through a certain scheduling algorithm, the ID of each message is assigned. 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 key.
Dr. Dong Yinping also pointed out that testing is not equivalent to verification. He said: Verification means that we have a standard to test whether the tested object meets. However, the current test of automotive electronics cannot be called verification, because it also tests whether the protocol design is correct and whether there are defects in the protocol design. The input to the verification is the characteristics of the measured object, the reference is to the standard, and the output is whether the two are in compliance.
In the current test, both the characteristics of the measured object and the so-called standards are input. The output is whether the protocol needs to be modified, whether the system is feasible, and whether the design meets the needs. Note that this leads to an incomprehensible place. We want to test whether the ECU meets the standard, but we are doubting this standard based on the test results. So unless there is a very strong accumulation of technology and experience, otherwise you will always hover 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, it is difficult to imagine. What we need is a new development idea.
Without design, there is no testing at all. On the contrary, design determines the testing system. For example, the electrical characteristics of the vehicle network, such as the terminal resistance and capacitance of the ECU, are all related to the specific network environment. It may be different because of different wiring harness. The designer knows the reasons and possible problems that affect these parameters, and these can all be input to establish the test method.
Dr. Dong Yinping added that the test is more important, but if it must be compared, it is more inclined to choose the design is more important. In fact, the development of the electronics industry can be used as a reference. Why are EDA tools so important in electronic design? And the current international trend is based on system-level design, modeling, etc. The focus is on the system level! Dr. Dong Yinping also emphasized that if you only focus on testing, then you are taking the old road of foreign development, deviating from the system-level design ideas, and not considering the trend of international development.
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