Four principles of hardware development

Taking the actual hardware design project as an example, the Chinese PCBA manufacturer-SysPCB will discuss the basic principles and ideas of hardware development with everyone, and welcome everyone to actively raise their questions and opinions.

1. Fully understand the design needs of all parties and determine the appropriate solution

Initiating a hardware development project, the original impetus will come from many aspects, such as market needs, needs based on the entire system architecture, the needs of the application software department to achieve functions, the need to improve certain aspects of the system, etc., so as a hardware designer of the system must take the initiative to understand the needs of all aspects, and combine them to propose the most suitable hardware solution. 

For example, the original driving force of Project A came from a high-level software team within the company. They found in practice that the original IP forwarding capability of the processor board could not meet the requirements, which would cause great inconvenience to the configuration and use of the system. They put forward a demand for new hardware. According to this goal, two high-performance network processors are selected in the hardware solution, and then it is necessary to communicate with the software designer in depth to determine the memory size, internal structure, number and type of external interfaces and debugging interfaces, etc. 

And other details, such as software personnel like to completely separate the control signaling path and the data path, so that they must be carefully considered when determining the internal data direction. At the beginning of the project, it is necessary to hold a lot of discussion meetings. All relevant departments should be invited to participate as much as possible. There are three benefits. First, you can fully understand everyone's needs so as not to omit important functions in the system design. The second is to allow each department understands the situation of the project, and prepares for time and personnel collaboration in advance. The third is from the emotional aspect. At the beginning of the design, each department participated in the project, and this project became the brainchild of everyone. , Will get everyone's care and good cooperation, which is very helpful to complete the work.

2. Problems to be paid attention to in schematic design

Nowadays, chip manufacturers can generally provide the schematic diagram of the reference design, so they should use these resources as much as possible, and do some of their own play on the basis of fully understanding the reference design. When the main chip is selected, the most critical peripheral design includes power supply, clock and interconnection between chips.

The power supply is the basis to ensure 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 single board needs to generate; the current size of each power supply; the power circuit efficiency; the allowable fluctuation of each power supply scope; power-on sequence required by the entire power system, etc. 

For example, the network processor in Project A needs 1.25V as the core voltage, the accuracy is between +5%--3%, and the current needs about 12A. According to these requirements, the design adopts 5V power input and linear switching power supply. The controller and the IR MOSFET build a suitable power supply circuit. The accuracy requirements determine the ESR selection of the output capacitor. In order to prevent the voltage drop caused by the excessive current, the remote feedback function is added.

The realization of the clock circuit should take into account the jitter and other requirements of the target circuit. The GE PHY device was used in the A project. At the beginning, an internal zero-delay clock distribution chip with a phase-locked loop was used to provide a 100MHz clock. As a result, the GE link Packet loss occurred on the computer, and later replaced by a simple clock buffer device to solve the packet loss problem. Analysis is that the internal phase-locked loop introduces jitter.

The interconnection between chips must ensure error-free transmission of data. In this regard, high-speed differential signal lines have the characteristics of high speed, good wiring, and good signal integrity. The multi-chip interconnection in the A project uses high-speed differential, there has no problems during debugging and testing.

3. Problems to be paid attention to in PCB design

The purpose of PCB design should be clear. For important signal lines, the length of the wiring and the processing of the ground loop must be very strict. For low-speed and unimportant signal lines, it can be placed on a slightly lower wiring priority. The important parts include: power supply division; memory clock line, control line and data line length requirements; high-speed differential line wiring and so on.

In project A, a memory chip is used to achieve a 1G DDR memory. The wiring for this part is very critical. 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's data manual and actual operating frequency, specific wiring rules can be drawn. For example, the length of the data lines in the same group can not exceed how many mils, and the length of each path can not exceed how many mil and so on. When these requirements are determined, PCB designers can be clearly required to implement them. If all the important wiring requirements in the design are clear, they can be converted into overall wiring constraints, and the automatic wiring tool software in CAD can be used to realize the PCB design. It is also a development trend in high-speed PCB design.

4. Inspection and debugging

When you are ready to debug a board, you must first make a careful visual inspection to check if there are any visible short circuits and pin soldering faults during the soldering process, and check if there are any components placed incorrectly, first pin put the wrong place, leak assembly and other problems, and then use a multimeter to measure the resistance of each power supply to the ground to check whether there is a short circuit. This good habit can avoid damaging the board after rushing on the power. You must have a peaceful mind during the debugging process. It is very normal to encounter problems. What you need to do is to do more comparisons and analysis, and gradually eliminate possible causes. You must firmly believe that "everything can be solved" and "there must be a reason for the problem", so the debugging will be successful in the end.

5. Some conclusions

From a technical point of view, every design can finally be made, but the success of a project depends not only on the technical realization, but also closely related to the completion time, product quality, and team cooperation. Therefore, good teamwork, transparent and candid project communication, meticulous and thorough R&D arrangements, and abundant materials and personnel arrangements can ensure the success of a project.

A good hardware engineer is actually a project manager. He/she needs to communicate with the outside world to obtain the requirements for his own design, and then summarize and analyze the specific hardware implementation. It is necessary to contact numerous chip and solution suppliers to select a suitable solution. When the schematic diagram is completed, he/she has to organize colleagues to conduct cooperative review and inspection, and work with CAD engineers to complete the PCB design. 

At the same time, prepare the BOM list, start purchasing and preparing materials, and contact the processing manufacturer to complete the board placement. During the debugging process, he/she should organize the software engineer to work together to research and debug, cooperate with the test engineer to solve the problems found in the test, and wait until the product is launched to the site. If there is a problem, it needs to provide timely support. Therefore, to be a hardware designer, you must develop good communication skills, the ability to adjust to pressure, the coordination and decision-making ability to handle multiple tasks at the same time, and a good peace of mind.

There is also carefulness and seriousness, because a small negligence in the hardware design will often cause very large economic losses. For example, when the PCB design is complete and the manufacturing file is completed, the wrong operation of a board has caused the power layer and the ground layer to be connected. At the same time, after the PCB board was manufactured, it was directly mounted on the production line without inspection. The short circuit problem was discovered during the test, but the components have been soldered to the board, resulting in hundreds of thousands of losses. Therefore, careful and serious inspection, responsible testing, and unremitting learning and accumulation can make a hardware designer make continuous progress.