Take the actual hardware design project as an example to discuss the basic principles and ideas of hardware development together, and welcome everyone to actively raise their own questions and opinions.
Principle one:
Fully understand the design needs of all parties and determine the right solution
Starting a hardware development project, the original driving force will come from many aspects, such as the needs of the market, based on the needs of the entire system architecture, the functional realization needs of the application software department, the need to improve the capabilities of some aspects of the system, etc., so as a hardware The designer of the system should take the initiative to understand the needs of all aspects, and put together to propose the most suitable hardware solution.
For example, the original driving force of the A project comes from a high-level software team within the company. They actually find that the original processor board IP forwarding capability cannot meet the requirements, which will cause great inconvenience to the system configuration and use. They raised the need for new hardware.
According to this goal, two high-performance network processors are selected in the hardware solution, and then need to communicate with the software designer to determine the memory size, internal structure, number and type of external interfaces and debugging interfaces. Such details, such as software personnel like to completely separate the control signaling path and the data path, should be carefully considered when determining the direction of internal data.
At the beginning of the project, many discussion meetings are needed. All relevant departments should be invited to participate. There are three advantages. The first one can fully understand the needs of everyone, so as not to miss important functions in system design. The second is to let All departments understand the situation of this project, prepare for the time and cooperation on the staff early, and the third is emotionally speaking. At the beginning of the design, various departments have participated in it. This project has become a common brain for everyone. , will get everyone's care and good cooperation, is very helpful to complete the work.
Principle 2:
Problems to be aware of in schematic design
There must be "takenism" in the schematic design. Now the chip manufacturers can generally provide the schematic diagram of the reference design, so try to use these resources as much as possible to make some of your own play on the basis of fully understanding the reference design. When the main chip is selected, the most critical peripheral design includes the power, clock and inter-chip interconnects.
The power supply is the basis for ensuring the normal operation of the hardware system. The design should be analyzed in detail: the power input that the system can provide; the power output that the board needs to produce; the current that each power supply needs to provide; the efficiency of the power supply circuit; the allowable fluctuation of each power supply. Range; power-up sequence required for the entire power system, and so on.
For example, the network processor in the A project requires 1.25V as the core voltage, the required accuracy is between +5% and -3%, and the current needs about 12A. According to these requirements, the design uses 5V power input, and uses the linear switching power supply. The controller and the IR MOSFET set up a suitable power supply circuit. The accuracy requirement determines the ESR selection of the output capacitor, and the remote feedback function is added to prevent the voltage drop caused by excessive current.
The realization of the clock circuit should take into account the jitter of the target circuit. The GE PHY device is used in the A project. At the beginning, a zero-delay clock distribution chip with a phase-locked loop is used to provide a 100MHz clock. The result is a GE link. There was a packet loss on it, and later it was replaced with a simple clock Buffer device to solve the packet loss problem. The analysis shows that the internal phase-locked loop introduces jitter.
The interconnection between the chips is to ensure the error-free transmission of data. In this respect, the high-speed differential signal lines have the characteristics of high speed, good wiring, good signal integrity, etc. The multi-chip interconnections in the A project use high-speed differential. Signal line, no problems in debugging and testing.
Principle 3:
Problems to be aware of in PCB design
The purpose of PCB design is to be clear. For important signal lines, the length of the wiring and the processing of the ground loop are very strict, and for low-speed and unimportant signal lines, it can be placed at a slightly lower wiring priority. The important parts include: power supply segmentation; memory clock lines, control line and data line length requirements; high-speed differential line wiring and so on.
In the A project, the memory chip is used to realize 1G DDR memory. The wiring for this part is very important. It is necessary to consider the topological distribution of the control line and the address line, and the length difference control of the data line and the clock line. In the process, according to the chip data manual and the actual working frequency, specific wiring rules can be obtained. For example, the length of the data lines in the same group cannot differ by more than mil, and the length between each path cannot differ by more than Mil and so on.
When these requirements are determined, the PCB designers can be clearly required to implement. If all the important wiring requirements in the design are clear, they can be converted into overall wiring constraints, and the PCB design can be realized by using the automatic routing tool software in CAD. It is also a trend in high-speed PCB design.
Principle 4:
Inspection and debugging
When preparing to debug a board, be sure to carefully check the visual inspection to see if there are visible short circuits and pin-stitching problems during the soldering process. Check if there are any component types placed incorrectly. Place errors, leak assembly, etc., and then use a multimeter to measure the resistance of each power supply to the ground to check for short circuits. This good habit can avoid damaging the board after powering up.
In the process of debugging, we must have a peace of mind. It is very normal to meet the problem. What we need to do is to do more comparison and analysis, and gradually eliminate possible causes. We must firmly believe that "everything has a solution" and "the problem arises." There are reasons for it, so that in the end it will be able to debug successfully.
Some summary words:
Now from a technical point of view, each design can be made in the end, but the success of a project depends not only on the technical implementation, but also on the time of completion, the quality of the product, and the team's cooperation. Therefore, good teamwork, transparent and honest project communication, careful research and development arrangements, and sufficient materials and staffing arrangements can ensure the success of a project.
A good hardware engineer is actually a project manager. He/she needs to get the needs of his own design from the outside world, and then summarize and analyze it into a concrete hardware implementation. Also contact with a number of chip and solution providers to select the appropriate solution. When the schematic is completed, he/she will organize colleagues to cooperate with the review and inspection, and work with the CAD engineer to complete the PCB design. .
At the same time, you should also prepare the BOM list, start purchasing and preparing materials, and contact the processing factory to complete the board placement. In the process of debugging, he/she should organize the software engineers to solve the problems together, cooperate with the test engineers to solve the problems found in the test, and wait until the products are launched to the site. If problems arise, they need to be supported in time. Therefore, a hardware designer should develop good communication skills, face the ability to adjust pressure, handle the coordination and decision-making ability of multiple affairs at the same time, and a good peace of mind.
There is also care and seriousness, because a small negligence in hardware design often causes very large economic losses. For example, when a board is encountered before, when the PCB design is complete, the wrong operation causes the power layer and the ground layer to be connected. Together, after the PCB board was manufactured, it was not checked directly on the production line. When the test was completed, the short circuit problem was found, but the components were all soldered to the board, resulting in hundreds of thousands of losses. Therefore, careful and serious inspection, responsible testing, unremitting learning and accumulation can make a hardware designer continue to make progress.
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