1. General idea
Designing the hardware circuit, the big framework and architecture need to be figured out, but it is really not easy to do this. For some big frameworks, your boss and teacher may have already thought about them, and you just implement the ideas; but there are also some big frameworks that need to be designed by yourself, so you need to figure out what functions to implement, and then find out whether the same or similar functions can be implemented. the reference circuit board.
2. No reference design found?
It's ok. First determine the big IC chip and find the datasheet to see if its key parameters meet your requirements, which are the key parameters you need, and whether you can understand these key parameters are the embodiment of the hardware engineer's ability, which also requires a long-term Accumulate slowly. During this period, you should be good at asking questions, because you don't understand something, others can often wake you up with a single sentence, especially hardware design.
3. Design the hardware circuit
The hardware circuit design is mainly composed of three parts: schematic diagram, pcb and bill of materials BOM.
(1). Schematic design is to convert the previous ideas into circuit schematics, which are very similar to the circuit diagrams in our textbooks.
(2). The pcb involves the actual circuit board. It is based on the netlist converted from the schematic diagram (the netlist is the bridge between the communication schematic diagram and the pcb), and the specific component packages are placed (layout) in the circuit board and then connect its electrical signals (wiring) according to the flying leads.
(3). After the pcb layout is completed, which components should be used should be summarized, so we will use the BOM table.
4. What tools do you use?
In fact, no matter whether you use simple protel or complex cadence tools, the major aspects of hardware design are the same.
Design Process
1. Schematic library establishment
To place a new component on the schematic, we must create a library of components. The library mainly defines the pin definition and its properties of the new component, and represents it in a specific graphic form (what we often see is a rectangle (representing its IC BODY), surrounded by many short lines (representing IC pins)) . The difference between ic body, ic pins, input pin, output pin, analog pin, digital pin, power pin, etc. should be made clear.
2. Draw the schematic
After you have enough libraries, you can draw on the schematic diagram. According to the requirements of the datasheet and system design, connect the relevant components through wires, and add line and text comments in the relevant places. The difference between wire and line is that the former has electrical properties while the latter does not. wire is suitable for connecting the same network, and line is suitable for annotating graphics. Should figure out some basic concepts, such as: wire, line, bus, part, footprint and so on.
3. Generate netlist
After completing the previous step, we can generate the netlist, which is the bridge between the schematic and the pcb. The schematic diagram is a form we can recognize. To convert it into a pcb, the computer must convert the schematic diagram into the form netlist it recognizes, and then process and convert it into a pcb.
4. Electrical rule check
Get netlist and draw pcb right away? Don't worry, do ERC first, ERC is an abbreviation for Electrical Rules Check. It can troubleshoot some basic design errors of schematic diagrams, such as connecting multiple outputs together. (But be sure to check your schematics carefully, don't rely too much on the tool, after all, the tool doesn't understand your system, it just checks purely based on some basic rules.)
5. Get pcb
Got a pcb from netlist, a bunch of dense components, and countless flying leads, does it surprise you? Don't worry, take your time.
6. Determine the size of the board frame
Draw a board frame in the keepout area (or mechanical area), which will limit the area you can route. It is necessary to consider the length and width of the board according to the needs (sometimes, the thickness of the board must also be considered). Of course, the stacking also has to be considered. (Lamination means that there are several layers of the board and how to apply it. For example, the board has a total of 4 layers, the top layer is for signal, the first layer in the middle is for power supply, the second layer in the middle is for ground, and the bottom layer is for signal).
7. Layout
After the board frame is determined, the components are placed, and the layout step 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 should be considered. For beginners, pay attention to the isolation of analog components, digital components, and the placement of mechanical positions, and pay attention to the topology of the power supply.
8. Wiring
The next step is wiring, which is often interactive with layout. Experienced people can often see where the wiring can be successful at the beginning. If some places are difficult to route, the layout needs to be changed. For fpga design, the schematic diagram is often changed to make the wiring smoother. There are many factors involved in routing and layout issues. For high-speed digital parts, they are complicated by signal integrity issues, but these issues are often difficult to quantify or even quantify. Therefore, when the signal frequency is not very high, the routing should be the first principle.
9. Matters needing attention after layout and routing
Check with DRC check first. The DRC will mark the wiring completion coverage and the violations of the rules, and check and correct them one by one according to this.
Some pcbs also need to add copper, make the outlet part into teardrops, and convert the final pcb file into a gerber file, which can be delivered to pcb production.
To assemble the pcb to prepare the bom table, it can generally be exported directly from the schematic diagram. However, it should be noted that which parts of the schematic diagram should be installed and which parts should not be installed. For small batches or research boards, it is also convenient to use excel to manage it yourself (large companies often require professional software to manage). For the novice, the first version is not recommended to be directly handed over to the assembly factory or welding factory to weld all the bom materials, which is inconvenient for troubleshooting. The best way is to prepare the components yourself according to the bom table. After the board arrives, add components and debug step by step.
Debug issues
What do you do first when you get the board, don't rush to power supply to see the function, hardware debugging cannot be completed in one step. First take a multimeter to see if the key network is abnormal, mainly to see if there is a short circuit between the power supply and the ground (although the manufacturer has done the test for you, you still have to check this step yourself, sometimes it seems that some steps It's quite cumbersome, but it can save you a lot of time later!), in fact, whether the short circuit is not only related to the pcb, it may cause this problem in any link of production, IO short circuit generally will not cause catastrophic consequences, but the power supply short circuit. .....
Is the power network shorted?
Then see if the power output is your ideal value. For beginners, it is best to put the IC on each chip when debugging. The first thing to put on is the power chip.
Is the power network shorted?
This is more troublesome, but you have to look carefully at your schematic diagram to see if this is possible, and at the same time use the secant method to check step by step where the short circuit is. In this case), it is still a problem of assembly, or a problem of own design.
The power chip has no output?
Check whether the input of your power chip is normal. There are also enable signals, voltage divider resistors, feedback networks, etc. to be checked.
The output value of the power chip is not within the expected range?
If it exceeds, for example, 10%, then look at the voltage divider resistors first. These two voltage divider resistors generally use 1% accuracy. Have you done this? Look at the feedback network at the same time, which will also affect your output power supply. range.
Is the output transition of the power supply normal?
The output of the power supply is normal, don't be happy, if possible, take an oscilloscope to see if the output jump of the power supply is normal. That is, grab the moment when the power is turned on, and see the situation of the power supply from scratch.
The following content is in the next article, please continue to browse Hardware circuit design (2).
