PCB Blog - Design Method of High Speed Digital PCB Board for Signal Integrity

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

With the increase in the output switching speed of integrated circuits and the increase in PCB density, signal integrity has become one of the issues that must be concerned in the design of high-speed digital PCB boards. Factors such as the parameters of components and PCB boards, the layout of components on the PCB board, and the wiring of high-speed signals can cause signal integrity problems, resulting in unstable system operation, or even no work at all. How to fully consider the signal integrity factor 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 design method based on computer analysis of signal integrity 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 a circuit. A circuit has good signal integrity if the signals in the circuit can reach the IC with the required timing, duration, and voltage amplitude. Conversely, a signal integrity problem occurs when the signal does not respond properly. Broadly speaking, signal integrity problems are mainly manifested in five areas: delay, reflection, crosstalk, simultaneous switching noise (SSN), and electromagnetic compatibility (EMI). The delay means that the signal is transmitted at a limited speed on the wires of the PCB board. The signal is sent from the sender to the receiver, and there is a transmission delay in between. The delay of the signal will have an impact on the timing of the system. In high-speed digital systems, the propagation delay is mainly determined by the length of the wire and the dielectric constant of the medium around the wire. In addition, when the characteristic impedance of the wire on the PCB (called the transmission line in the high-speed digital system) does not match the load impedance, after the signal reaches the receiving end, a part of the energy will be reflected back along the transmission line, distorting the signal waveform and even appearing the signal overshoot and undershoot. If the signal bounces back and forth on the transmission line, it can cause ringing and ringing. Since there is mutual capacitance and mutual inductance between any two devices or wires on the PCB, when the signal on one device or one wire changes, its change will affect other devices or other devices through mutual capacitance and mutual inductance. wire, that is crosstalk. The strength of crosstalk depends on the geometry and mutual distance of the device and wires.

When many digital signals on the PCB are switched synchronously (such as the data bus of the CPU, the address bus, etc.), due to the impedance on the power line and the ground line, synchronous switching noise will be generated, and the ground plane will bounce on the ground line. Noise (abbreviated to ground bounce). The strength of the SSN and the ground bounce also depends on the IO characteristics of the integrated circuit, the impedance of the power supply layer and the ground plane layer of the PCB, and the layout and wiring of the high-speed devices on the PCB. In addition, like other electronic devices, PCB boards also have electromagnetic compatibility problems, which are mainly related to the layout and wiring methods of PCB boards.

2. Traditional PCB board design method

In the traditional design process, the design of the PCB board consists of steps such as circuit design, layout design, PCB board production, measurement, and debugging. In the circuit design stage, due to the lack of effective analysis methods and means for the transmission characteristics of the signal on the actual PCB board, the circuit design can generally only be carried out according to the component manufacturers and suggestions 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 layout design stage, it is also difficult to analyze and evaluate the signal performance changes caused by the component layout and signal routing of the PCB in real-time, so the quality of the layout design is more dependent on the designer's experience. In the PCB board production stage, since the processes of each PCB board and component manufacturers are not exactly 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 board design process, the performance of the PCB board can only be judged by instrument measurement after the production is completed. 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, it is generally necessary to repeat the above process many times to finally meet the design requirements. It can be seen that with the traditional PCB board design method, the product development cycle is long, and the cost of research and development is correspondingly high.

3. PCB board design method based on signal integrity analysis

The PCB board design process based on signal integrity computer analysis is shown in Figure 2. Compared with the traditional PCB board design method, the design method based on signal integrity analysis has the following characteristics: Before the PCB board design, the signal integrity model of high-speed digital signal transmission is first established. 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 circuit design. In the process of circuit design, the design scheme is sent to the SI model for signal integrity analysis, and the tolerance range of components and PCB board parameters, the possible topology structure and parameter changes in the PCB board layout design, and other factors are integrated to calculate and analyze the design. The solution space of the scheme. After the circuit design is completed, each high-speed digital signal should have a continuous and achievable solution space. That is, when the parameters of the PCB board and components change within a certain range, the layout of the components on the PCB board and the wiring method of the signal lines on the PCB board has a certain flexibility, the signal integrity can still be guaranteed. Require. Before the layout design of the PCB board 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 board. During the PCB layout design process, the partially 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, the layout design or even the circuit design needs to be modified, which can reduce the risk of product failure due to improper design. After the PCB board design is completed, the PCB board production can be carried out. The tolerance range of the PCB manufacturing parameters should be within the solution space of the signal integrity analysis. After the PCB board is manufactured, the instrument is used to measure and debug 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, the PCB board can be finalized without or with 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 board design method based on signal integrity computer analysis, the most important part is the establishment of the PCB board-level signal integrity model, which is different 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 will determine the feasibility of this design method.

4.1. SI model of PCB board design

There are already a variety of models that can be used for PCB-level signal integrity analysis in electronic design. Among them, there are three commonly used ones, namely SPICE, IBIS, and Verilog-A.

a. SPICE model

SPICE is a powerful general-purpose analog circuit simulator. Now SPICE model has been widely used in electronic design, and derived two main versions: HSPICE and PSPICE, HSPICE is mainly used in integrated circuit design, and 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 equations are provided, the SPICE model and the algorithm of the simulator can be closely linked, and better analysis efficiency and analysis results can be obtained. When using the SPICE model to perform SI analysis at the PCB board level, integrated circuit designers and manufacturers are required to provide detailed and accurate SPICE models describing the sub-circuits of the integrated circuit I/O units and manufacturing parameters of semiconductor characteristics. Since these materials usually belong to the intellectual property and confidentiality of designers and manufacturers, only a few semiconductor manufacturers will provide corresponding SPICE models along with chip products. The analytical accuracy of the SPICE model mainly depends on the parameters of the model (that is, the nature of the data) and the applicable range of the model equations. The model equations can also affect the accuracy of the analysis when combined with various digital simulators. In addition, the SPICE model simulation calculation at the PCB board level is relatively large, and the analysis is time-consuming.

b. IBIS model

The IBIS model was originally developed by Intel Corporation for digital signal integrity analysis at the PCB board level and system level. It is now managed by the IBIS Open Forum and is 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 cells and pins. Because the IBIS model does not need to describe the internal design of the I/O cell and transistor manufacturing parameters, it has been welcomed and supported by semiconductor manufacturers. All major digital integrated circuit manufacturers are now able to provide the corresponding IBIS model along with the chip. The analytical precision of the IBIS model mainly depends on the number of data points and the degree of data in the I/V and V/T tables. Because the PCB board-level simulation based on the IBIS model adopts the look-up table calculation, the calculation amount 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 have been around for less than 4 years and are new standards. 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. Unlike SPICE and IBIS models, in the AMS language, it is up to the user to write the equations that describe the behavior of the 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. Since Verilog-AMS and VHDL-AMS are a 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 computational accuracy of the AMS model in PCB-level signal integrity analysis is not inferior to those of the SPICE and IBIS models.

4.2 Model selection

Since there is no unified model to complete all PCB-level signal integrity analyses, in the design of high-speed digital PCB boards, it is necessary to mix the above models to establish the transmission model of key signals and sensor signals. For discrete passive devices, the SPICE model provided by the manufacturer can be sought, or a simplified SPICE model can be established and used directly through experimental measurements. For critical digital integrated circuits, the IBIS model provided by the manufacturer must be sought. At present, most IC designers and manufacturers are able to provide the required IBIS model along with the chip through the Web site or other means. For non-critical integrated circuits, if the manufacturer's IBIS model cannot be obtained, 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 by experimental measurements. For the transmission line on the PCB board, the simplified SPICE model of the transmission line can be used in the pre-analysis of signal integrity and the solution space analysis, and in the analysis after the wiring, the complete SPICE model of the transmission line needs to be used according to the actual layout design.

5. Combining design methods with existing EDA software

At present, there is no integrated EDA software in the PCB board design industry to complete the above 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 build SPICE models for discrete, passive devices and transmission lines on PCBs, and debug and verify. Add the obtained SPICE/IBIS model of each component and transmission line to the 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 carry out the signal integrity analysis Analytical calculations. Use the database function of the SI analysis software, or use other general database software, to further organize and analyze the results of the simulation operation to search for an ideal solution space. Taking the boundary value of the solution space as the basis of PCB circuit design and the constraints of layout design, EDA software for general PCB design, such as OrCAD, Protel, PADS, PowerPCB, Allegro, and Mentor, is used to complete PCB circuit design and Layout design. When the layout design of the PCB board is completed, the parameters of the actual design circuit (such as topology, length, spacing, etc.) can be automatically or manually extracted through the above layout design software and sent back to the previous signal integrity analysis software for wiring. SI analysis to verify that the actual design meets the requirements of the solution space. When the PCB board is manufactured, the correctness of each model and simulation calculation can also be verified by the measurement of the experimental instrument.

The design method has strong practical significance for the design and development of high-speed digital PCB boards, which can not only effectively improve the performance of product design, but also greatly shorten the product development cycle and reduce development costs. It is foreseeable that with the continuous improvement and improvement of the signal integrity analysis model and the calculation analysis algorithm, the PCB board design method based on the signal integrity computer analysis will be more and more applied in the design of electronic products.