Basic Guidelines For Hardware Development Of PCB Technology (Part I)

Below, I will take an actual hardware design project as an example to discuss the basic principles and ideas of hardware development with you, and welcome your questions and opinions.

To fully understand the design needs of all parties and determine appropriate solutions

Start a hardware development project, the original impetus will come from many aspects, such as the need of the market, based on the needs of the overall system architecture, the realization of the function of the application software department needs, improve the ability to some aspect of need, and so on, so as a hardware system designers, to actively to understand all aspects of demand, and together, the most suitable hardware solution is put forward.

For example, the original driving force of Project A comes from A high-level software team inside the company. In practice, they found that the original IP forwarding capacity of the processor board could not meet the requirements, which would cause great inconvenience to the configuration and use of the system. Therefore, they proposed the demand for new hardware.

According to this goal, the hardware scheme is pointed chose two high performance network processor, and then also need further communicate and software designers, to determine memory size, internal structure, external interfaces and debugging the number and type of the interface, and so on details, such as software people like to control signaling pathways and data path to separate completely, so in determining the internal data to be considered.

At the beginning of the project needs a lot of discussion meeting, should try to invite all related departments to participate in, there are three benefits, the first can fully understand your needs, in order to avoid missing important functions on system design, the second is to let each department to understand the situation of the project, early ready to collaboration on time and personnel, third from talking about feelings, in the design at the beginning of each department is involved in, the project becomes a common a brainchild, get everybody’s care and good cooperation, is of great help/very helpful to finish the work.

Schematic diagram design to pay attention to the problem

Schematic design should have “bring the principle”, now chip manufacturers generally can provide reference design schematic, so try to use these resources, on the basis of a full understanding of the reference design, do some of their own play.

When the main chip is selected, the most critical peripheral design includes the power supply, the clock and the interconnection between the chips.

Power is the basis to ensure the normal work of the hardware system, design to detailed analysis: the system can provide the power input; The power output that the veneer needs to generate; Each power supply needs to provide the current size; Power supply circuit efficiency; The fluctuation range that each power supply can allow; Power on sequence required by the whole power system, etc.

Such as A network processor in the items need to be as A core voltage of 1.25 V, required accuracy between + 5% – 3%, will need about 12 A current, according to these requirements, used in the design of 5 V power input, using the Linear controller and IR MOSFET switch power supply structures, the appropriate power supply circuit, the accuracy requirement determines the output capacitance of the ESR choice, and to prevent excessive current caused by the voltage drop, joined the distal feedback function.

Clock circuit to the realization of the goal of jitter requirements, such as A project involving GE PHY devices, the beginning of using an internal phase lock loop zero delay for 100 MHZ clock chip clock distribution, the results appeared on the GE link packet loss, later by A simple clock Buffer device solves the problem of packet loss, analysis up to the internal PLL jitter is introduced.

The interconnection between chips should ensure the correct transmission of data. In this regard, high-speed differential signal lines have the characteristics of high speed, easy wiring and good signal integrity. High-speed differential signal lines are adopted in the interconnection between multiple chips in Project A, and there are no problems in the debugging and testing.

Problems to be paid attention to in PCB design

In PCB design, the purpose should be clear. For important signal lines, the wiring length and processing ground loop should be strictly required, while for low speed and unimportant signal lines, the wiring priority should be placed at a slightly lower level.

Important parts include: power supply division; Memory clock line, control line and data line length requirements; High-speed differential line wiring and so on.

A project using the size of the memory chip to achieve the 1 g DDRmemory, on the part of the wiring is very critical, want to consider the topology of distribution line and address line, the length of the data and clock line difference control, etc., in the process of implementation, according to the actual working frequency of the chip data sheet and concrete rules of the wiring can draw requirements, such as cable length difference within the same group can’t more than how many mil, what difference should not exceed the length of each path between A mil, and so on.

When these requirements are determined, the PCB designer can be clearly asked to achieve them. If all the important wiring requirements are clear in the design, the overall wiring constraints can be converted. The automatic wiring tool software in CAD can be used to achieve PCB design, which is also a development trend in high-speed PCB design.

Check and debug

When ready to debug a plate must be serious to do visual inspection, first to check whether there is visible in the process of welding circuit and pin tin problems, check whether there is a component model error, the first foot placed wrong, leakage problems such as assembly, then use multimeter to measure the power supply to ground resistance, to check whether there is a short circuit, this good habit can avoid damage veneer after rushed to electricity.

In the process of debugging, we should have a peaceful state of mind, and it is very normal to meet problems. What we should do is to do more comparison and analysis, and gradually eliminate the possible causes. We should firmly believe that “everything has a way to solve” and “there must be a reason for the problem”, so that we can finally debug successfully.

Conclusion

Now from technical point of view, every design can do it, but the success of a project, not only depends on the technical implementation, also with complete time, the quality of the product, the team’s cooperate closely related, so the good team cooperation, transparent frank communication, careful meticulous research and development arrangements, abundant materials and personnel arrangement, in order to 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 of his/her design, and then summarize them and analyze them into specific hardware implementation.

He/she has to liaise with various chip and solution suppliers to select suitable solutions, and when the schematic is completed, he/she has to organize colleagues to review and review the schematic, and work with the CAD engineer to complete the PCB design.

At the same time, I should also prepare the BOM list, start purchasing and preparing materials, and contact the processing factory to complete the plate installation.

In the process of debugging, he/she shall organize software engineers to tackle the key problems and cooperate with test engineers to solve the problems found in the testing. When the products are launched to the site, if there are problems, he/she shall provide timely support.

Therefore, a hardware designer should exercise good communication skills, the ability to adjust in the face of pressure, the coordination and decision ability to deal with multiple affairs at the same time, as well as a good peaceful state of mind and so on.

And careful and serious, because a small oversight in the design of the hardware often can cause great economic loss, such as previously encountered a piece of plate made in PCB design complete the file wrong operation caused a power supply layer and stratum together, after the PCB manufacturing, and didn’t check on line directly pasted on to the test when short circuit problem is discovered, but the components have been welded onto the plate, resulting in the loss of hundreds of thousands of.

Therefore, careful and careful inspection, responsible testing, unremitting learning and accumulation, can make a hardware designer continuous progress, and then the technical industry has a small success.