What the hardware engineers need to do, these five aspects must not be known

Time flies, and it has been three years since I first painted the first circuit. Just as you started to touch the board, like everyone else, you are full of doubts and some excitement.

A lot of experience and knowledge about hardware circuits on the Internet is dazzling. Like signal integrity, EMI, PS design will confuse you.

Don't worry, everything should come slowly. What a hardware engineer needs to do, after reading this article, I believe you will understand.

In fact, hardware is mainly reflected in these aspects, of course, this is a summary of 俺, for your reference:

1) The overall idea. Design hardware circuits, big frameworks and architectures to figure out, but it's not easy to do this. Some big frameworks may have their own bosses and teachers already thinking about it. They just implement the ideas concretely; but some have to design their own frameworks, then you have to figure out what functions to implement, and then find out whether you can achieve the same or similar functions. Reference board.

2) Understand the circuit. If you find a reference design, congratulations, you can save a lot of time (including pre-design and post-debugging). Copy now? NO, or understand and understand first, on the one hand can improve our circuit understanding ability, and can avoid errors in the design.

3) Didn't find the reference design? It's ok. First determine the large IC chip, look for the datasheet, see if its key parameters meet your own requirements, which are the key parameters that you need, and whether you can understand these key parameters, are the embodiment of the capabilities of hardware engineers, which also requires long-term The ground slowly accumulates. During this period, you should be good at asking questions. Because you don't understand things, others can wake you up in one sentence, especially hardware design.

4) The hardware circuit design is mainly three parts, schematic, pcb, bill of materials (BOM) table. Schematic design is to convert the previous ideas into circuit schematics.

Pcb involves the actual circuit board, which is based on the schematic diagram converted from the netlist (the netlist is the bridge between the communication schematic and the pcb), and the specific component package is placed on the circuit board, and then according to the fly The line connects its electrical signal. After completing the PCB layout and routing, which components should be used, we will use the BOM.

5) What tools are used? Protel, which is altium easy to use, is also popular in China. It is enough for general work, suitable for beginner designers.

In addition, in fact, whether using simple protel or complex cadence tools, the hardware design is the same (protel operation is similar to windwos, post-command type; and cadence product concept & allegro is pre-command type Used to protel, suddenly turned to cadence tools, will not be used to this reason).

There must be a big part of the design:

1) Schematic design.

2) pcb design.

3) Make a BOM.

Now briefly talk about the design process (steps):

1) The principle library is established. To put a new component on the schematic, we have to build a library of components. You should be clear about the differences between ic body, ic pins, input pin, output pin, analog pin, digital pin, power pin.

2) After having a sufficient library, you can draw on the schematic, and connect the relevant components through wire according to the datasheet and system design requirements.

3) After this step, we can generate netlist, this netlist is the bridge between the schematic and pcb.

4) Get the netlist and draw the pcb right away? Don't worry, do ERC first. ERC is an abbreviation for electrical rule checking. It can troubleshoot some basic design errors of the schematic, such as multiple outputs connected together.

5) I got a pcb from the netlist, a bunch of dense components, and countless flying lines that made you startled? Oh, don't worry, you have to come slowly.

6) Determine the size of the board. Draw a frame in the keepout area (or the mechanism area), which will limit the area of ​​your wiring. It is necessary to consider the length of the board and the width of the board according to the demand (sometimes, it is necessary to consider the thickness of the board).

After the board is finalized, the component layout (placement) is made, and the layout step is extremely critical. It often determines the difficulty of post-wiring. Which components should be placed on the front and which components should be placed on the back should be considered.

But these are all problems that benevolent and benevolent, and that wise men see wisdom; they can be placed differently from different angles. In fact, I have drawn the schematic diagram, understand all the components, and naturally have a clear understanding of the component placement (if you let a person who is not drawing a schematic to place the component, the result will often surprise you ^_^). For the first entry, pay attention to the analog components, the isolation of the digital components, and the placement of the mechanical position, while paying attention to the topology of the power supply.

The next step is wiring. This is often interactive with the layout. Experienced people often see where they can be successfully routed at the beginning. Check with DRC first, this is a must check. The DRC will mark the coverage of the wiring and the violation of the rules, and correct it according to this one-by-one troubleshooting.

To get down, you need to assemble the pcb and prepare the bom table. Generally, you can export it directly from the schematic. However, it should be noted that in the schematic diagram, which parts of the components should be on and which components should not be on, it is necessary to have a psychological number. For small batches or research boards, it is also convenient to manage yourself with excel.

For the novice, the first version is not recommended to be directly handed over to the assembly plant or the welding factory to weld all the bom materials, which is not convenient for troubleshooting. The best way is to prepare the components yourself according to the bom table. After the board comes, step by step on the components and debug.

Talk about debugging

1) Get the first step of the board, do not rush to see the power supply function, hardware debugging can not be completed in one step. First take a multimeter to see if the key network is not normal, mainly to see if there is a short circuit between the power supply and the ground (although the manufacturer has already done the test for you, this step still has to look at it yourself, sometimes it seems that some steps It is very cumbersome, but it can save you a lot of time!), in fact, the short circuit is not only related to pcb, it may cause this problem in any part of production and production, IO short circuit generally does not cause catastrophic consequences, but the power supply is short-circuited.

2) The power supply network is not short-circuited? So good, then look at whether the power output is its ideal value. For beginners, it is best to use one chip on the IC when debugging. The first one is the power chip.

3) Is the power supply network shorted? This is more troublesome, but it is necessary to take a closer look at whether the schematic diagram of the schematic is possible. At the same time, combined with the method of secant, step by step to check where the bottom is short-circuited, it is a problem of pcb (the general bad pcb factory may appear This situation), or the problem of assembly, or the problem of own design. There are some tips on checking for shorts, which will be logged out in the future.

4) The power chip has no output? Check to see if your power chip input is normal. Also check the place where there is an enable signal, a voltage divider resistor, and a feedback network.

5) The power chip output value is not in the expected range? If it is very outrageous, such as 10%, then look at the voltage divider resistors, these two voltage divider resistors generally use 1% accuracy, this you did not, and look at the feedback network, it will also affect you The range of output power.

6) The power output is normal, don't be happy, if you have the conditions, take a look at the oscilloscope and see if the output jump of the power supply is normal. That is to grasp the moment of power-on, look at the situation of the power supply from scratch (as to why you have to look at it, oh... Professionals still have to see ~)

In addition, to remind everyone that high-speed look at the signal edge, not the clock frequency.

1) Generally speaking, when the clock frequency is high, the rising edge of the signal is fast, so generally we regard them as high-speed signals; but the reverse is not necessarily true, the clock frequency is low, if the rising edge of the signal is still fast, the same should be taken Treated as a high speed signal.

According to the signal theory, the rising edge of the signal contains high-frequency information (using Fourier transform, we can find the quantitative expression), so once the rising edge of the signal is steep, we should deal with the high-speed signal, the design is not good, it is likely to rise. The edge is too slow, there is overshoot, undershoot, and ringing. For example, the I2C signal, in ultra-fast mode, has a clock frequency of 1MHz, but its specification requires that the rise or fall time does not exceed 120ns! There are indeed a lot of boards I2C can't pass!

2) Therefore, we should pay more attention to the signal bandwidth. According to the empirical formula, the relationship between bandwidth and rise time (10%~90%)

The above is some of the experience sharing, I hope to help you.