Fiat uses time analysis software Symtavision to achieve 70% CAN bus load

As the number of ECUs in automotive electronic systems increases, the amount of data that needs to be communicated through the CAN bus also increases dramatically. According to previous engineering experience, when the bus load rate exceeds 30%, the probability of message delay increases. How high is the probability of message delay caused by different bus load rates, and how much delay can be caused? These questions are difficult to answer using existing analysis methods and tools. At the same time, two questions are derived (the bus load rate in this article refers to the CAN bus):

Does the FlexRay bus have to be used when the bus load rate exceeds 30%?
Will the message response time exceed the deadline when the bus load rate is below 30%?
The user case of Fiat R&D department using Symtavision can answer the above two questions.

Bus load rate below 30% will cause message delay

The CAN bus load rate is an evaluation parameter of the bus message occupation within a statistical period of time. This is a very macro parameter. For example, in the following example CAN bus: there are 3 ECUs, each sending different CAN messages in different cycles.

Figure 1: Modeling and analyzing CAN bus message delay using SymTA/S

The bus load rate obtained by using SymTA/S analysis is 2.85%, which is very small. However, a detailed analysis of the message Msg_20ms shows that due to bus arbitration, the maximum message response time is 0.564ms (Deadline is 0.5ms); after optimizing the CAN bus, the response time is effectively reduced. This is just a very simple example, which shows that there is message delay even when the bus load rate is lower than 30%. New tools are needed in engineering to evaluate the effectiveness and scalability of the bus. The bus load rate has certain limitations.

Rethinking bus load factor

The CAN bus load rate is only a macro parameter and can no longer fully evaluate the effectiveness and scalability of the bus. The reason for this is that, on the one hand, the electronic systems were relatively simple and the number of bus messages was small in the past, and simply limiting the bus load rate could meet the message response time requirements. However, when the number of ECUs participating in CAN bus communication or the number of bus messages increased, it was no longer possible to ensure that a lower bus load rate would meet the message response time requirements.

On the other hand, from a technical perspective, there was no tool that could help engineers analyze the more detailed factors behind the CAN load rate, and it was impossible to specifically analyze the delay of each message and the impact of the message on the bus. However, in engineering applications, the final definition or evaluation is the deadline of a certain message. When the CAN communication system becomes more complex, the simple bus load rate can no longer be used as a complete technical indicator to evaluate a CAN bus system. A more detailed analysis is needed to determine whether the worst case response time of each message exceeds the deadline.

CAN bus load rate can reach 70% -Fiat R&D engineer

Fiat R&D engineers have long used SymTA/S, a time modeling analysis and verification tool from Symtavision of Germany, to help them develop and verify CAN bus systems, and have used Symtavision's analysis tools in several models. When developing the next generation of models, engineers maximized the communication capabilities of the CAN bus under the premise of increasing the number of system ECUs but not using the FlexRay bus. Of course, the premise of these measures is to ensure that the designed CAN bus meets the requirements for response time (Deadline).

First, Fiat engineers imported the bus model (DBC) file into the SymTA/s tool, and then analyzed it and found that the sending time of some messages exceeded the pre-defined deadline. Then they optimized the bus system and determined that the maximum response time of those messages that exceeded the pre-defined deadline met the requirements. At the same time, analysis found that the load rate of the CAN bus was 70%.

in conclusion

Symtavision's time modeling, analysis and verification tools allow developers to not only analyze the bus load rate when determining whether the CAN bus is feasible, but more importantly, they can analyze and verify the maximum response time of each message, making the analysis of the CAN bus a step deeper.