The sufficiency and necessity of circuit design

Circuit design includes two specific work areas: schematic diagram and circuit board diagram. These two areas are highly unified and have obvious differences. It can be done by one professional engineer or divided into two professional engineers. The problem solved by the schematic diagram is to grasp the demand analysis and verify the design plan. Its input is the task book (or derived hardware design requirements) and the hardware design plan. Its output is a schematic diagram that can guide the work of the circuit board. As the name suggests, this work focuses on the principle content. The problems solved by circuit board engineers are closer to reality and can be seen and modeled. It is closely related to device characteristics, layout, circuit board manufacturing specifications, etc.

　　When doing structural design, most structures are closely related to the built-in circuit board, such as: appearance size, installation and fixation, external interface, device placement, circuit layout, external radiation, and external radiation resistance. Many of these factors are not understood by structural design professionals. The most important thing is that in many cases, users are not professional, so our company is forced to have certain hardware R&D capabilities. The extent to which we can help users in hardware R&D is determined based on user needs.

　　In the process of product design in this kind of engineering practice, the author summarizes some of his personal experience into two aspects: sufficiency and necessity. These two aspects are closely related to engineering practice, and their main purpose is to first ensure that the design meets the requirements and secondly ensure the optimal solution.

　　The main meaning of sufficiency is to fully guarantee the design requirements and meet various indicators and performance requirements. Necessity considers whether the solution is optimal, involving aspects such as cost, processability, and maintenance. Only when the two are organically combined can the designed product be qualified.

　　In addition to meeting customer needs, sufficiency also needs to meet some potential needs. Specifically, it is a reasonable margin, considering the possibility of future upgrades. This is also very important. Be sure to avoid the situation where a redesign is required in the future to add an IO port. The reasonable margin is difficult to evaluate. For example, if the input bandwidth is required to be 5M, is it reasonable to design it to be 8M or 10M? This is considered by the director and is generally written in the plan. If a chip has basically the same price of 8M and 20M, then of course the 20M one will be used, because the margin is large.

　　Necessity is to examine the design plan in reverse, whether it is necessary to design it this way, whether there is a disproportionate cost to pay for a certain demand, etc. It also directly examines whether the overall hardware solution is relatively optimized, whether there are better solutions, etc. If a chip is facing discontinuation, it is necessary to replace it with another chip with better prospects in the design. In short, you must consider it carefully and aim for the long term.

　　Let's talk about the cost issue. To put it in a more serious way, cost is also a core issue. The larger the batch of products, the more cost should be considered. I took a job around 2004 and was paid 20,000 yuan. I was very happy. The job was to optimize a pic program from 4.3K to less than 4K. There was no source code, only burning code. I didn't understand why I did this at the time. It took me a week to optimize it section by section. Finally, it was optimized to 3.7K. The functions were completely maintained. Anyone who knows assembly can do it. Pure statement optimization is enough.

　　Later I found out that they were an OEM integrator who didn't understand technology at all. The technology they bought used an 8K PIC. By switching to a 4K PIC, they could save more than 300,000 yuan a year. I was completely stunned.

　　Just talking about empty words will make you sleepy, so here's something a little more powerful:

　　1: Resistance

　　Note that the power margin should be more than 1 times. Don't use 0805 just because it is 1/8W. You should use 1206 at least. Otherwise, the board may be hotter in that place. There is a small price difference between 1% and 5% when buying in bulk. When buying 10 plates, the difference can be tens of yuan, but it can be basically ignored. If there are no other surface mount components on the board, don't choose surface mount resistors, which will bring additional welding costs. If there are surface mount components, try to use surface mount resistors to save costs, welding fees, space, and good electrical characteristics.

　　2: Capacitor

　　The cost varies greatly, and is closely related to the capacitor material and packaging. First, clarify the requirements in order to determine the materials, and then choose the appropriate package. Note that it is sufficient to fully meet the requirements, because the price difference is very large. The voltage margin of the electrolytic capacitor should be more than doubled. For example, a 5V circuit should use at least a 9V capacitor, because the electrolytic capacitor has poor thermal stability and the withstand voltage drops sharply at a slightly higher temperature. The withstand voltage should not be too large. For a 5V circuit, a 35V capacitor will be much larger in size and the price will also be much different.

　　3: Peripheral chips

　　Be sure to check whether it will be discontinued in the future, and listen to the opinions and recommendations of agents or technical support. They are familiar with the product series, but you must distinguish whether they may be biased in order to promote a certain chip. Sometimes using mature chips does not necessarily mean a good price. For example, the market price of the classic 6264 is now more than tens of yuan, and it is often out of stock, while the commonly used 62256 is only 4 yuan.

　　When designing a circuit, you must be patient and carefully examine each component one by one. It is okay to say that someone was not scared when he was young, but it is absolutely wrong to say that hardware engineers are not prone to making mistakes.