Summary of some knowledge of circuit design

Nowadays, electronic products are inseparable from the support of various circuits, so how to design a good circuit? Many engineers often complain that when they come into contact with circuit design, many circuit designs are too complicated, and their experience and knowledge about hardware circuits It’s dizzying. Things like signal integrity, EMI, and PCB design will definitely confuse you. Don't be anxious, take everything slowly.

Today I would like to discuss some circuit design tips, dedicated to those who have just started or are about to start designing hardware circuits, so that everyone can avoid "detours" on the road of "hardware circuit design".

1) General idea

To design hardware circuits, the big framework and architecture must be understood, but it is not easy to do this. For some large frameworks, your boss or teacher may have already thought about it, and you just implement the ideas concretely; but for some, you need to design the framework yourself, so you need to figure out what functions you want to achieve, and then find out if there is another one that can achieve the same or similar functions. Reference circuit board (you must know how to use other people's results as much as possible. The more experienced engineers are, the more they will know how to learn from other people's results).

2) Understand the circuit

If you find the reference design, congratulations, you can save a lot of time (including pre-design and post-debugging). Copy it immediately? NO, it is better to understand it first. On the one hand, it can improve our circuit understanding ability and avoid errors in design.

3) No reference design found?

First determine the large IC chip, look for the datasheet, and see if its key parameters meet your requirements. Which are the key parameters you need, and whether you can understand these key parameters are all manifestations of the hardware engineer's ability. This also requires a long period of time. Accumulate slowly. During this period, you must be good at asking questions, because others can often enlighten you with just one sentence about things you don’t understand, especially hardware design.

4) Three main design parts of hardware circuit

Schematic, PCB, bill of materials (BOM) sheet.

Schematic design is to transform the previous ideas into circuit schematics. It is very similar to the circuit diagram in our textbook. PCB involves the actual circuit board, which is a netlist converted from the schematic diagram (the netlist is the bridge between the schematic diagram and the PCB), and the packages of specific components are placed (layout) on the circuit board. Then connect its electrical signals (wiring) according to the flying wires (also called pre-pulled wires). After completing the PCB layout and routing, it is necessary to summarize which components will be used, so we will use the BOM table.

5) What tools are used?

Prote, that is, altimuml, is easy to use and is relatively popular in China. It is sufficient for general work and is suitable for beginner designers. In fact, no matter whether you use a simple protel or a complex cadence tool, the major aspects of hardware design are the same (the operations on protel are similar to windwos and are post-command type; while cadence's products concept & allegro are pre-command type, so you are used to them. After switching to protel, I suddenly turned to cadence tools, which is why I am not used to them.)

All major aspects of design must include:

1) Schematic design.

2)PCB design.

3) Make a BOM list.

Let’s briefly talk about the design process (steps)

(1) Establishment of principle library. To place a new component on the schematic, we must create a library of the changed component. The library mainly defines the pin definition and properties of the new component, and represents it in a specific graphic form (what we often see is a rectangle (representing its IC BODY) with many short lines around it (representing the IC pins)) .

Protel is extremely simple to create libraries, and because so many people use it, ready-made libraries can be found for many components, which is extremely convenient for users. You should understand the differences between icbody, icpins, inputpin, output pin, analog pin, digital pin, power pin, etc.

(2) After you have sufficient libraries, you can draw pictures on the schematic diagram and connect related components through wires according to the requirements of the datasheet and system design. Add line and text comments where relevant.

The difference between wire and line is that the former has electrical properties and the latter does not. Wire is suitable for connecting the same network, and line is suitable for annotating graphics. At this time, some basic concepts should be understood, such as: wire, line, bus, part, footprint, etc.

(3) After completing this step, we can generate a netlist. This netlist is the bridge between the schematic diagram and the pcb. The schematic diagram is a form that we can recognize. If the computer wants to convert it into a pcb, it must convert the schematic diagram into the form it recognizes, netlist, and then process and convert it into a pcb.

(4) After getting the netlist, draw the pcb immediately? Don’t worry, do the ERC first. ERC is the abbreviation for Electrical Rules Examination. It can troubleshoot some basic design errors in schematic diagrams, such as multiple outputs being connected together.

(But be sure to check your schematic diagram carefully and not rely too much on the tool. After all, the tool cannot understand your system. It just checks purely based on some basic rules.)

(5) Did you get a PCB from netlist, a bunch of densely packed components, and countless flying wires that shocked you? Haha, don’t rush, you have to take your time.

(6) Determine the size of the board and frame. Draw a board frame in the keepout area (or mechanical area), which will limit the area for your wiring. It is necessary to consider the board length and board width (sometimes, the board thickness) according to the needs. Of course, layering also has to be considered.

(Stacking means how many layers a board has and how to apply it. For example, there are 4 layers in total. The top layer is for signals, the first layer in the middle is for power, the second layer in the middle is for ground, and the bottom layer is for signals).

Let’s first explain the terminology in (2). Post-command, for example, if we want to copy an object (component), we must first select the object, then press ctrl+C, and then press ctrl+V (the copy command occurs after the object is selected). This method is used by both windows and protel.

But concept is another way, which we call pre-command. Similarly, if we want to copy something, first press ctrl+C, then select the object, and then click outside (the copy command occurs before the object is selected).

1) After determining the board frame, it is time to layout (place) the components. This step of layout is extremely critical. It often determines the difficulty of later wiring. Which components should be placed on the front and which components should be placed on the back must be considered. But these are all matters of benevolence and wisdom; the placement can be different from different angles. In fact, if you draw the schematic diagram yourself and understand the functions of all components, you will naturally have a clear understanding of component placement (if you let someone who does not draw the schematic diagram place the components, the results will often surprise you^_^).

For beginners, just pay attention to the isolation of analog components and digital components, as well as the placement of mechanical locations, and also pay attention to the topology of the power supply.

2)The next step is wiring. This is often interactive with layout. Experienced people can often see where successful wiring can be done from the beginning. If some places are difficult to wire, the layout will need to be changed. For FPGA design, it is often necessary to modify the schematic diagram to make the wiring smoother. There are many factors involved in wiring and layout issues. For high-speed digital parts, they become complicated because they involve signal integrity issues, but often these issues are difficult to quantify or even difficult to calculate. Therefore, when the signal frequency is not very high, routing should be the first principle.

3) OK? Don’t worry, check with DRC first. This must be checked. DRC will mark the completion coverage of wiring and the places where rules are violated. According to this, we can check and correct them one by one.

4) Some PCBs also need to add copper coating (which may increase the cost) and make the outlet part into a teardrop (the factory may add it for you). The final PCB file is converted into a gerber file and can be delivered to PCB production. (Some of them can be supplied directly to the PCB, and the factory will transfer them to gerber for you).

5) To assemble the PCB, prepare the BOM, which can generally be exported directly from the schematic. However, it should be noted that you must be mentally aware of which components in the schematic diagram should be installed and which components should not be installed.

For small batches or research boards, it is convenient to use Excel to manage them yourself (large companies often require professional software to manage). For novices, it is not recommended to hand the first version directly to the assembly factory or welding factory to weld all the BOM materials, as this will not be convenient for troubleshooting problems. The best way is to prepare the components yourself according to the BOM list. After the board arrives, install the components and debug step by step. The above is a summary of some knowledge points in circuit design. I hope you will gain something from reading it.