PCB Technical - Design method of high-speed digital PCB based on signal integrity analysis

This article introduces a design method of high-speed digital signal PCB board based on signal integrity computer analysis. In this design method, the PCB board-level signal transmission model is first established for all high-speed digital signals, and then the solution space of the design is found through the calculation and analysis of the signal integrity, and finally the PCB is completed on the basis of the solution space. Board design and verification.

As the output switching speed of integrated circuits increases and the density of PCB boards increases, signal integrity has become one of the issues that must be concerned in high-speed digital PCB design. Factors such as the parameters of the components and the PCB board, the layout of the components on the PCB board, and the wiring of high-speed signals will cause signal integrity problems, resulting in unstable system operation or even no operation at all.

How to fully consider the signal integrity factors in the PCB design process and take effective control measures has become a hot topic in the PCB design industry today. The high-speed digital PCB board design method based on signal integrity computer analysis can effectively realize the signal integrity of PCB design.

1. Overview of signal integrity issues

Signal integrity (SI) refers to the ability of a signal to respond with the correct timing and voltage in the circuit. If the signal in the circuit can reach the IC with the required timing, duration and voltage amplitude, the circuit has better signal integrity. Conversely, when the signal cannot respond normally, a signal integrity problem occurs. Broadly speaking, signal integrity problems are mainly manifested in five aspects: delay, reflection, crosstalk, synchronous switching noise (SSN) and electromagnetic compatibility (EMI).

Delay means that the signal is transmitted at a limited speed on the wires of the PCB board, and the signal is sent from the sending end to the receiving end, during which there is a transmission delay. The signal delay will have an impact on the timing of the system. In a high-speed digital system, the transmission delay mainly depends on the length of the wire and the dielectric constant of the medium surrounding the wire.

In addition, when the characteristic impedance of the wires on the PCB board (referred to as transmission lines in high-speed digital systems) does not match the load impedance, a part of the energy will be reflected back along the transmission line after the signal reaches the receiving end, causing the signal waveform to be distorted or even signal The overshoot and undershoot. If the signal is reflected back and forth on the transmission line, it will produce ringing and ring oscillation.

Since there is mutual capacitance and mutual inductance between any two devices or wires on the PCB, when a device or signal on a wire changes, its change will affect other devices or inductance through mutual capacitance and mutual inductance. Wire, that is, crosstalk. The strength of crosstalk depends on the geometrical size and mutual distance of the devices and wires.

When many digital signals on the PCB board are switched synchronously (such as CPU data bus, address bus, etc.), due to the impedance of the power line and the ground line, synchronous switching noise will be generated, and ground plane bounce will occur on the ground line. Noise (referred to as ground bomb). The strength of SSN and ground bounce also depends on the IO characteristics of the integrated circuit, the impedance of the power supply layer and ground plane layer of the PCB board, and the layout and wiring of high-speed devices on the PCB board.

In addition, like other electronic devices, PCBs also have electromagnetic compatibility issues, which are mainly related to the layout and wiring of the PCB board.

2. Traditional PCB board design method

In the traditional design process, PCB design consists of circuit design, layout design, PCB production, measurement and debugging steps in turn. In the circuit design stage, due to the lack of effective methods and means to analyze the transmission characteristics of the signal on the actual PCB board, the circuit design can generally only be carried out based on the suggestions of component manufacturers and experts and past design experience. Therefore, for a new design project, it is usually difficult to make the correct selection of factors such as signal topology and component parameters according to the specific situation.

In the PCB layou t design stage, it is also difficult to make real-time analysis and evaluation of the signal performance changes caused by the PCB component layout and signal wiring, so the quality of the layout design depends more on the experience of the designer. In the PCB production stage, since the processes of each PCB board and component manufacturers are not completely the same, the parameters of the PCB board and components generally have a large tolerance range, making the performance of the PCB board more difficult to control.

In the traditional PCB design process, the performance of the PCB board can be judged by instrument measurement only after the production is completed. The problems found in the PCB board debugging stage must be modified in the next PCB board design. But what is more difficult is that some problems are often difficult to quantify into the parameters in the previous circuit design and layout design. Therefore, for more complex PCB boards, the above-mentioned process usually needs to be repeated many times to finally meet the design requirements.

It can be seen that with the traditional PCB design method, the product development cycle is longer, and the cost of research and development is correspondingly higher.

3. PCB design method based on signal integrity analysis

The PCB design process based on signal integrity computer analysis is shown in Figure 2. Compared with the traditional PCB design method, the design method based on signal integrity analysis has the following characteristics:

Before PCB board design, first establish a signal integrity model for high-speed digital signal transmission.

According to the SI model, a series of pre-analysis is carried out on the signal integrity problem, and the appropriate component types, parameters and circuit topology are selected according to the results of the simulation calculation as the basis for the circuit design.

In the circuit design process, the design plan is sent to the SI model for signal integrity analysis, and the tolerance range of the components and PCB board parameters, the possible topological structure and parameter changes in the PCB layout design, and other factors are calculated and analyzed. Solution space.

After the circuit design is completed, each high-speed digital signal should have a continuous and achievable solution space. That is, when the PCB and component parameters change within a certain range, the layout of the components on the PCB board and the wiring of the signal lines on the PCB board have a certain degree of flexibility, the requirements for signal integrity can still be guaranteed .

Before the PCB layout design starts, the boundary value of each signal solution space obtained is used as the constraint condition of the layout design, which is used as the design basis for the layout and wiring of the PCB layout.

In the PCB layout design process, the partially completed or fully completed design is sent back to the SI model for post-design signal integrity analysis to confirm whether the actual layout design meets the expected signal integrity requirements. If the simulation results cannot meet the requirements, you need to modify the layout design and even the circuit design, which can reduce the risk of product failure due to improper design.

After the PCB design is completed, the PCB board can be made. The tolerance range of the PCB board manufacturing parameters should be within the range of the solution space of the signal integrity analysis.

After the PCB board is manufactured, the instrument is used for measurement and debugging to verify the correctness of the SI model and SI analysis, and use this as the basis for correcting the model.

On the basis of the correct SI model and analysis method, usually the PCB board can be finalized without or only a few repeated modifications to the design and production, which can shorten the product development cycle and reduce the development cost.

4. Signal integrity analysis model

In the PCB design method based on signal integrity computer analysis, the most core part is the establishment of the PCB board-level signal integrity model, which is the difference from the traditional design method.

The correctness of the SI model will determine the correctness of the design, and the buildability of the SI model determines the feasibility of this design method.

4.1. SI model of PCB design

There are many models that can be used for PCB board-level signal integrity analysis in electronic design. Three of the most commonly used are SPICE, IBIS and Verilog-A.

a. SPICE model

SPICE is a powerful general-purpose analog circuit simulator. Now the SPICE model has been widely used in electronic design, and two main versions have been derived: HSPICE and PSPICE. HSPICE is mainly used in integrated circuit design, while PSPICE is mainly used in PCB board and system-level design.

The SPICE model consists of two parts: Model Equations and Model Parameters. Since the model equation is provided, the SPICE model can be closely connected with the algorithm of the simulator, and better analysis efficiency and analysis results can be obtained.

When using the SPICE model to perform SI analysis at the PCB board level, it is necessary for the integrated circuit designer and manufacturer to provide a detailed and accurate description of the SPICE model of the integrated circuit I/O unit sub-circuit and the manufacturing parameters of the semiconductor characteristics. Because these materials usually belong to the intellectual property and confidentiality of designers and manufacturers, only a few semiconductor manufacturers will provide corresponding SPICE models while providing chip products.

The analysis accuracy of the SPICE model mainly depends on the source of the model parameters (that is, the accuracy of the data) and the applicable scope of the model equations. The combination of model equations with various digital simulators may also affect the accuracy of the analysis. In addition, the PCB board-level SPICE model has a large amount of simulation calculation, and the analysis is relatively time-consuming.

b. IBIS model

The IBIS model was originally developed by Intel Corporation specifically for PCB board-level and system-level digital signal integrity analysis. It is now managed by the IBIS Open Forum and has become an official industry standard (EIA/ANSI 656-A).

The IBIS model uses the form of I/V and V/T tables to describe the characteristics of digital integrated circuit I/O units and pins. Since the IBIS model does not need to describe the internal design of the I/O unit and transistor manufacturing parameters, it has been welcomed and supported by semiconductor manufacturers. Now all major digital integrated circuit manufacturers can provide corresponding IBIS models while providing chips.

The analysis accuracy of the IBIS model mainly depends on the number of data points in the I/V and V/T tables and the accuracy of the data. Since the PCB board-level simulation based on the IBIS model uses table lookup calculations, the amount of calculation is small, usually only 1/10 to 1/100 of the corresponding SPICE model.

c. Verilog-AMS model and VHDL-AMS model

Verilog-AMS and VHDL-AMS appeared less than 4 years ago, and they are a new standard. As hardware behavior-level modeling languages, Verilog-AMS and VHDL-AMS are supersets of Verilog and VHDL, respectively, while Verilog-A is a subset of Verilog-AMS.

Different from SPICE and IBIS models, in the AMS language, it is up to users to write equations describing the behavior of components. Similar to the IBIS model, the AMS modeling language is an independent model format that can be used in many different types of simulation tools. AMS equations can also be written at many different levels: transistor level, I/O cell level, I/O cell group, etc.

Because Verilog-AMS and VHDL-AMS are new standards, only a few semiconductor manufacturers can provide AMS models so far, and there are fewer simulators that can support AMS than SPICE and IBIS. However, the feasibility and calculation accuracy of the AMS model in the PCB board-level signal integrity analysis are no inferior to the SPICE and IBIS models.

4.2 Selection of model

Since there is no unified model to complete all PCB board-level signal integrity analysis, in the design of high-speed digital PCB board, it is necessary to mix the above-mentioned models to establish the transmission model of key signals and sensitive signals to the greatest extent.

For discrete passive components, you can seek the SPICE model provided by the manufacturer, or directly establish and use a simplified SPICE model through experimental measurements.

For key digital integrated circuits, the IBIS model provided by the manufacturer must be sought. At present, most integrated circuit designers and manufacturers can provide the required IBIS model while providing chips through Web sites or other methods.

For non-critical integrated circuits, if the manufacturer's IBIS model is not available, a similar or default IBIS model can also be selected according to the function of the chip pins. Of course, a simplified IBIS model can also be established through experimental measurements.

For the transmission line on the PCB board, the simplified transmission line SPICE model can be used in the signal integrity pre-analysis and space-solving analysis, and the complete transmission line SPICE model needs to be used in the analysis after the wiring according to the actual layout design.

5. Combination of design method and existing EDA software

At present, there is no integrated EDA software in the PCB design industry to complete the above-mentioned design method, so it must be realized through the combination of some general software tools.

Use general SPICE software (such as PSPICE, HSPICE, etc.) to establish SPICE models for discrete and passive components and transmission lines on the PCB, and debug and verify them.

Add the SPICE/IBIS models of various components and transmission lines that have been obtained to general signal integrity analysis software, such as SPECCTRAQuest, HyperLynx, Tau, IS_Analyzer, etc., establish the SI analysis model of the signal on the PCB board, and perform the signal integrity Analysis and calculation of sex.

Use the database function that comes with the SI analysis software, or use other general-purpose database software, to further sort and analyze the results of the simulation operation, and search for the ideal solution space.

Taking the boundary value of the solution space as the basis of PCB circuit design and the constraint condition of layout design, general PCB design EDA software such as OrCAD, Protel, PADS, PowerPCB, Allegro and Mentor are used to complete PCB circuit design and layout design.

After the PCB layou t design is completed, the parameters of the actual design circuit (such as topology, length, spacing, etc.) can be extracted automatically or manually through the above layout design software, and sent back to the previous signal integrity analysis software for wiring. SI analysis to verify whether the actual design meets the requirements of the solution space.

After the PCB board is manufactured, the correctness of each model and simulation calculation can be verified through the measurement of experimental instruments.

Summary of this article:

This design method has strong practical significance for the design and development of high-speed digital PCB board s, not only can effectively improve the performance of product design, but also can greatly shorten the product development cycle and reduce development costs. It is foreseeable that with the continuous improvement and improvement of signal integrity analysis models and calculation and analysis algorithms,PCB design methods based on signal integrity computer analysis will be increasingly used in the design of electronic products.