"Of course, from our childhood education, we know that we should have a thorough understanding of knowledge, the deeper the better. For any knowledge point, we can verify the high-level laws and formulas through the basic formulas and mathematical tools. I think for this point, physics in the college entrance examination has reached the extreme. In fact, physics knowledge in senior high school is not difficult, but for the purpose of selection, we combine various laws of physics and combine some brain twists and complex calculations, resulting in physics in the college entrance examination as the most difficult subject.
If we solve the hardware problems with the spirit of solving the physics problems in the college entrance examination, of course, the spirit is commendable, and it is worth encouraging this spirit of learning and exploration after work; But it's not very good for project development. After all, the work of hardware engineers is engineering development. They complete hardware projects within the specified time and budget, not at this time. What formulas and calculation results do you deduce? That's the work of scientists and that's the work of research.
An important feature of engineering development is "stepping in the footsteps of predecessors", that is, through engineering practice in the past decades and centuries, there are many empirical data and empirical methods for various situations. For example, for PCB layout, basically every company has its own design guidelines / check list, which is summarized by the company in many projects in the past, Each one can be said to have paid the price of "blood", which is for board level design; This is especially true for core chips and devices. A chip and device developed by dozens or hundreds of people from chip or device companies over several years can be thoroughly understood by hundreds of pages of datasheets.
In most cases, it is enough to know the main interfaces, parameters, functions and performance, especially the design guidelines or application notes provided by chip / device companies, which are generally the heartfelt words of chip / device engineers. From experience, generally speaking, an engineer without ten or twenty years of working experience can't write these things.
Although it looks very simple and looks like nonsense, if you analyze it carefully, combined with circuit theorem and electromagnetic law, you will find that every word is precious. Good students who have just graduated (generally speaking, they study well, like difficult problems, have strong learning ability and strong thirst for knowledge) will find that there are too many knowledge points and technical points involved in hardware design. If they want to understand this knowledge point thoroughly, they should also understand that knowledge point thoroughly. They will find that 24 hours a day is not enough, but I'm sorry, The company asks you to come here not for you to learn, but for hardware design. You will see the schematic diagram in a month. You are still holding the OrCAD manual one command by one to learn the skills of OrCAD, study why there is a serial resistance, and study whether the capacitance is 0.1uF or 0.01uF.
It's good to have a thirst for knowledge, but it's a good thing to learn from your predecessors after work and after the project. Try to use various design guidelines to complete the design work as soon as possible, record your knowledge shortcomings, study hard in your spare time, understand thoroughly, and deepen your understanding of knowledge points through design verification / test. This is the correct working method.
When I first graduated, I was glad to find that the transmission line theory was too important, so I spent a month working hard on the transmission line theory and deduced a large number of formulas for verification. In fact, it can be summarized as follows: impedance matching. If the impedance of the receiving end is greater than that of the transmitting end, what will happen to the signal; If less than, what happens to the signal; What happens if there is an open circuit; What happens if there is a short circuit? These articles will be introduced in every book on signal integrity, and there will be no complex mathematical formula derivation. Just know, and then how to balance the impedance of the transmitter, serial resistance, PCB impedance, matching impedance, etc. are simple mathematical formulas.
Therefore, for people with a strong thirst for knowledge (such as me), sometimes they must learn to "taste it", make full use of their predecessors' experience and avoid falling into the misunderstanding of technology. For example, if they don't use the formula, they have to deduce it with double integral. The main feature of hardware development is "extensive". You should know everything. A good hardware engineer should understand everything. Of course, it is better to go deep into some aspects to the principle level.
As the blog title says: you should know all kinds of technologies / standards / chips / devices about hardware design. When necessary, you can come at your fingertips, including functional performance, parameter characteristics, advantages and disadvantages.
What basic knowledge and capabilities should a communication device have
1. Ability to learn quickly:
On the one hand, communication technology, standards and chips are updated so fast that you don't have time to understand it systematically. You can only understand it through specific projects and requirements; On the other hand, for the company, the hardware products that need to be made also change rapidly. Customers need T1, E1, PDH, SDH, Ethernet, VoIP, switch and router. No one knows everything. They need to be able to understand the selected chip scheme in detail in combination with customers' needs, especially for interface protocol and electrical characteristics.
2. Understanding of communication protocols and standards:
Communication equipment, as its name implies, is a device used to implement a variety of communication protocols (such as T1, E1, V.35, PDH, SDH / SONET, ATM, USB, VoIP, WiFi, Ethernet, TCP / IP, RS232 and other common protocols). Various circuits, PCB boards and power supplies serve the communication protocol.
Communication protocols are generally implemented by chips, either mature ASIC or self-developed FPGA / CPLD. Chip engineers or FPGA engineers are closer to the communication protocol than hardware engineers. They need to understand the communication protocol thoroughly, realize various logical state machines and meet the electrical parameter standards specified in the protocol. According to the seven layer OSI model, hardware engineers need to focus on the protocol standards of layer 1 physical layer and layer 2 data link layer. Based on the Ethernet distance, the physical layer is completed by PHY / translator chip and the data link layer is completed by MAC / switch chip. For hardware engineers engaged in Ethernet related development, they need to have a thorough understanding of phy and switch chips, From coding mode, electrical parameters, eye diagram standard, template, signal frequency to frame format, forwarding processing logic, VLAN, etc.
This is especially true for traditional PDH / SDH / SONET equipment. PDH / SDH / SONET is a more hardware equipment, that is, the main protocols are realized through ASIC, and the functions of software are mainly management, configuration, monitoring, alarm and performance. For hardware engineers, they must be familiar with the relevant protocols and interface standards used, especially for electrical specifications and eye diagram templates, In this way, we can be confident in the design verification.
3. Ability to write documents:
Just like software design, good software design requires good design documents, clear requirements, what functions to achieve and what acceptance standards to achieve. With the increase of chip integration, interface speed and board complexity, hardware design is becoming more and more complex, as well as the requirements of thermal stability, reliability, electromagnetic compatibility and environmental protection, It is no longer possible to solve the guerrilla war through Xiaomi and Bumu Cang. Every hardware project is a war, which needs good planning and analysis, so it needs to be well documented.
For hardware engineers, there are two most important documents: one is hardware design specification (HDS) and hardware test report (generally called EVT: engineering validation & test report or DVT: design validation & test report). The requirements for HDS are detailed and clear, selection of main chip / hardware initialization, CPU selection and initialization, interface chip selection / initialization / management, block diagram of connection relationship between chips, DRAM type / size / speed, flash type / size / speed, chip selection, interrupt, GPIO definition, reset logic and topology diagram, clock / crystal oscillator selection / topology, RTC use, memory map relationship, I2C device selection / topology, Interface device / line sequence definition, led size / color / drive, heat sink, fan, JTAG, power topology / timing / circuit, etc.
For DVT, the requirements are simple and complex: what interfaces, chips, main devices and circuits are on the board, especially the power supply / voltage / ripple / timing when the board works normally, eye diagram / template of service interface, signal integrity and timing of internal data bus (such as MII, rgmii, XAUI, PCIe, PCM bus, telecom bus, SerDes, UART, etc.), CPU subsystem (such as clock, reset, SDRAM / DDR, flash interface).
A good hardware engineer can clearly see what scheme and circuit the hardware system needs to use, and what the final verification test results are. The content is detailed without omitting various interfaces / circuits; Simple name, no nonsense; Both pictures and texts. When necessary, a sequence diagram and an oscilloscope snapshot can explain the problem.
4. Capability of instrument / software:
Instruments include electric soldering iron, multimeter, oscilloscope, logic analyzer, bit error meter, transmission analyzer, Ethernet tester, SmartBits / Ixia, calorimeter, attenuator, optical power meter, RF signal intensity meter, etc; Software includes office (outlook, word, Excel, PowerPoint, project, Visio), PDF, common schematic software pads or OrCAD, common PCB software pads or Allegro, Allegro viewer, circuit simulation software PSpice, signal simulation software HyperLynx, etc.
In political economics, both instruments and software are production tools and promote the improvement of productivity. As hardware engineers, these instruments and software are wood and artillery in their hands. A large part of the abilities of hardware engineers are reflected in the use of instruments and software, especially for the use of schematic software and oscilloscope, The use of schematic software is the concrete realization of hardware design. Through the placement of devices and the connection of net, it constitutes a very complex hardware logic software, which is the core work of the whole hardware design. The loss caused by any error and error on the schematic is huge. It is really "walking on thin ice and trembling".
In addition, the use of schematic software is also reflected in the beauty of schematic diagram. Good design is simple and clear, and the notes are clear. Whoever follows the idea can quickly find out the design intention, which needs special attention. Bad design, one device in the East and one device in the west, has no logic, has strange naming, is difficult to understand, and is quite troublesome to maintain in the future; Among all test instruments, oscilloscope is the most important for hardware engineers. Both schematic diagram and PCB are design work, but any design needs careful verification and test, especially in terms of signal. It requires a lot of oscilloscope work. If the oscilloscope is not used correctly, it is impossible to talk about correct verification, whether the grounding is well connected and the selection of test points, The selection of trigger, delay, amplitude and time all determine the test results. If the oscilloscope is used incorrectly, it will inevitably lead to wrong test results. In this case, the wrong design may be mistaken for correct, which will bring huge hidden dangers; The original correct design is mistaken for wrong, which brings a lot of time and energy waste.
5. Ability of circuit design:
With the improvement of chip integration, hardware design seems to be simpler. The first is logical connection, and then consider the selection of serial resistance and parallel capacitance required by signal integrity, power filtering and decoupling. However, for good hardware engineers, simple logical connection (the output of the same bus of this chip is connected to the input of another chip, etc.) is only the most basic skill of hardware design. Circuit is the carrier of chip function, communication protocol and various software. Without in-depth understanding of circuit, there is no in-depth understanding of hardware design, Especially for the understanding of the electrical performance parameters listed behind the chip or various parameters of discrete devices, random connection may work on the 3.3V bus, but now the working voltage has been reduced to 1V. What concept, the noise on the signal line has been so large that the sampling can be misjudged. With the increase of signal rate and the decrease of working voltage, Digital signals have become more and more analog, which requires in-depth understanding of PCB impedance, capacitive reactance, inductive reactance, discrete devices (resistance, capacitance, inductance, diode, triode, MOSFET, transformer, etc.) and ASIC interface electrical parameters, which requires in-depth study of circuit principle, analog circuit and even electromagnetic field theory. Circuit can be said to be a subset of electromagnetic field theory, Without the understanding of electromagnetic field theory, there is no understanding of capacitance, inductance, crosstalk and electromagnetic radiation.
Especially for the power circuit design, now the chip voltage is diversified, the voltage is lower and lower, and the current is larger and larger. The operators' strict requirements for the power consumption and heat dissipation of communication equipment pose more and more challenges to the power design. It can be said that for a hardware design, 40% of the work lies in the schematic / PCB design and later test verification of the power circuit. The power circuit design is the concentrated embodiment of the circuit ability of the hardware engineer. There are many parameters for various passive devices, semiconductor devices, protection devices and DC / DC conversion typical topologies. The formula needs to be considered and calculated.
6. Ability of communication and global control:
In a hardware project, the hardware engineer is generally in the role of team leader. To be fully responsible for the hardware project, it is necessary to coordinate various resources such as PCB engineer, structural engineer, signal integrity engineer and electromagnetic compatibility engineer, and closely cooperate with product manager, project manager, software engineer, production engineer and procurement engineer, To ensure that all links are step by step, it is necessary to know the whole project plan, the release time of each sub task, and the estimation and control of possible technical problems and risks.
For the outside, the hardware engineer should also deal with the chip distributor and FAE to strive for greater technical support and help; Deal well with EMC laboratories and external laboratories to obtain more flexible test time and more rectification opinions.
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