Category Archives: 02. Sw requirement

Guide to Functional Decomposition – ppt video online download


Contents What is functional decomposition? Why is functional decomposition used? When is functional decomposition used? How to perform functional decomposition A sample functional decomposition – step 1 A sample functional decomposition – step 2 A sample functional decomposition – step 3 What level to decompose functions to? Further assistance

소스: Guide to Functional Decomposition – ppt video online download

It is posed for functional FTA.

 

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Similarity between Academic paper and Requirements


When researcher writes academic paper, one of important thing is to make structure. Usually research is one’s own study so it is hard to understand for people who are not interested in the topic. So s/he has to consider how to reader can understand easily.

S/he also consider internal consistency for better understanding.

I realized that writing requirement is quite similar to write an academic paper. If requirements are written without consideration of structure, it is hard to understand. Readers cannot understand what are system’s sub systems, what are functions allocated to sub-systems.

It is not technical point of view. But if they are not clear it is hard to defense against audit, because auditor will confuse it and will not understand what you are saying.

Then s/he will not give a good grade.

 

 

“Doing Nothing” is better than “Doing Wrong”


It is related to identify safety requirement. For safety related systems, to avoid unintended function is essential. But I sometimes find that some engineers have a rule that watchdog function shall be regarded as a safety function.

Is it really true?

It will be critical for company’s financial profit because of quality problem. It surely might be a problem but in safety aspect watchdog is not always related to safety.

Let’s assume that there is a system(sys_A) that report some information to other system(sys_B) which controls vehicle’s movement. A requirement allocated to sys_B is determined as ASIL C, while sys_A is determined as ASIL A or B.

Let assume that sys_B can perceive Sys_A’s liveness by monitoring message that Sys_A sent.

In safety aspect, Sys_A’s safe state might be dead state. It means that when Sys_A detect a fault that leads to violate safety function that is allocated to Sys_A, it is better to do nothing than do wrong.

In this case, what do you think that watchdog fault is safety related or non-safety related ?

In some cases, I found that watchdog function is not a safety function. It is just a general function.

Identifying safety requirement is so important for functional safety manager. Functional safety engineer or functional safety manager have to consider it carefully.

It surely leads to engineers confusing when they regard non-safety functions as safety function. All safety activities after requirement phase will be messed up because there will not be consistent in their work products. Their safety arguments will be no longer convincing.