Learning signal integrity from scratch (SIPI) -- 3 challenges faced by Si and Si based design methods

Signal integrity challenges

As early as more than ten years ago, signal integrity did not enter vision of the hardware engineers , Engineers deal with interference 、 Noise and other problems “ Three magic weapons ” It's grounding 、 wave filtering 、 shielding , This practice of relying solely on the experience of engineers is obviously very extensive . At that time, most of the working frequencies of digital systems were 100MHz following , The rise time of the signal is also a few nanoseconds , The problem of signal integrity is not serious ; At that time, the level standards of digital systems were mostly TTL、CMOS The level signal line has a large noise margin, so there is no need to worry about the influence of power supply noise ; At that time, the chip power consumption was not as high as it is now CPU、DSP Power consumption of tens of Watts at any time , There is no need to worry about the current capacity and voltage drop of the power supply ; The interface voltage of the chip at that time 、 The nuclear voltage is not as low as it is now, or even to 1V following , Don't worry too much about power supply noise .

However, with the continuous improvement of the performance of electronic communication products 、 The development of semiconductor technology , The maximum transmission rate of the signal has been determined by 2004 Year of 3.125Gbps To the present 28Gbps, At the same time, the power supply voltage keeps decreasing , The demand for current is increasing . These are the challenges of high-speed circuit design , This makes it impossible to solve all kinds of problems caused by signal transmission only through the experience of engineers ：

• Progress in semiconductor technology , Making the chip integration more and more high , The interference between different modules is becoming more and more serious ;
• Increase in signal rate , Make the timing margin smaller and smaller ,DDR5 The rate of has been reached 6.4Gbps.

• The current demand of the system is increasing , send IR Drop And power supply noise is becoming more and more difficult to control ;
• The supply voltage is getting lower and lower , The chip has higher and higher requirements for power supply noise ;
• The signal rises faster and faster , So that its tiny interconnection structure （ For example, chip packaging 、 Connector 、 Through holes, etc ） Will affect the signal quality .
• The signal amplitude keeps decreasing , As a result, the signal noise margin is continuously reduced

So people began to realize the importance of signal integrity and power integrity , At the same time SI and PI Has also begun to show a strong interest in , And make signal integrity analysis become a very important work and indispensable link in the product development process .

Because of that , A professional signal integrity engineer was born , Although for the time being, I am professionally engaged in SI、PI The number of analysis engineers is relatively small （ Although more than a few years ago ）, Most of the SI、PI The analysis is also done by hardware engineers or PCB Engineers are doing part-time jobs . However, we should also see that the development prospect of signal integrity analysis is very broad , With the rapid development of digital systems 、 High integration 、 The development trend of shortening the product development cycle , Signal Integrity Engineer will play a more and more important role in product development .

Design method based on signal integrity analysis

With electron 、 The development of communication technology , Functions of electronic products 、 Performance and other indicators are constantly improving , Electronic products face high density 、 miniaturization 、 Distributed 、 low power consumption 、 High reliability requirements and many other challenges . According to the different ways to deal with these challenges , We can put the product development mode , It can be roughly divided into the following three types ：

• Problem solving

seeing the name of a thing one thinks of its function , Problem solving is the technology that some small companies face 、 Money 、 speed of progress 、 The helpless choice under the pressure of many aspects such as cost . A product is not fully evaluated , But do it first . If problems are found during the commissioning test , Then rack your brains to find a way to solve 、 Remedy , Meiqiming said “ Tackle key problems and overcome difficulties ”. The products designed in this way put aside the elements of luck , It cannot be stable . If the problem cannot be solved , It needs to be revised , Of course, it will certainly affect the progress of product development , It may also cost more .

• Rule Driven

The company has developed to a certain scale , Rich experience accumulated in long-term product development . Translate these experiences into design rules , Apply these rules to drive during product development 、 Constrain the entire development process . Obvious , This development method is compared with “ Problem solving ” There has been a marked improvement . But this method is only in the accumulation of experience into the rules , In the face of more complex 、 When developing products with higher performance requirements , Existing rules may not apply .

Facing new adjustment, new design methods are needed .

• System design type

“ Everything is done in advance , If you don't anticipate, you will lose ”. So we also need a standard design process for hardware product development , yes “ I'll talk about it later ”, Find a solution to the problem , It is still necessary to study the key technologies first 、 The difficulties shall be fully evaluated and analyzed , Find a solution to the problem in the early stage and then carry out , Effective risk aversion ？ Obviously, the latter is more reliable ！

Introduce... Into the hardware development process SI、PI Simulation and analysis can effectively solve the signal problem in the early stage of design 、 Problems caused by power quality , So it will SI、PI It is very necessary to introduce simulation analysis into the hardware development process . At present, some people in China are 、 Some big companies such as Xing , Each has its own design methodology 、 Have a complete SI、PI Simulation analysis process .

Facing the challenge of signal integrity , In the past “ Problem solving ”、“ Rule Driven ” The design method of has been unable to meet the requirements . We have to redefine the hardware development process . In order to predict in time at the early stage of product design 、 Solve problems and risks in signal integrity , At present, the signal integrity analysis has been introduced into the mainstream hardware design process in the industry .

  Flow chart of mainstream hardware design in the industry

At all stages of the design SI The role of simulation analysis is as follows ：

• Schematic design stage ：SI Engineers are involved , After the simulation and analysis of the key signal and the key power supply, the matching scheme of the key signal is given 、 Of key interface devices buffer The selection 、 Filtering scheme of key power supply, etc ; At the same time, the key network PCB Design rules .

• PCB design phase ：SI Engineers and PCB Simulation rules before engineer communication and Implementation , stay PCB After the design is completed, the signal quality and the pre simulation conclusion are verified by post simulation . If you can't meet the requirements, you need to work with the hardware engineer 、PCB Engineers communicate and propose solutions to PCB Design optimization .

On the surface, it seems that... Has been added to the design process SI Simulation analysis , It will lead to the extension of product design cycle , But in fact, it can predict and solve the problem of signal integrity in advance , It can greatly shorten the testing and debugging cycle , Even the probability of success in the first board is greatly improved .