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Virtual Prototyping: A System Design Requirement
September 12, 2012 |Estimated reading time: 9 minutes
Many of today’s electronic designs incorporate multiple high-pin-count ICs, a processor or two, and large amounts of memory. Designing and laying out a PCB for this system is a complex and expensive task, and in truth, many of these boards do not work the first time. The cost of re-designing the PCB to correct a fault, no matter how small, is an expensive proposition. The days of using physical prototypes for debugging are limited!
Virtual Prototyping Advantage
Virtual prototyping, that is, simulating the system in high-speed software before building any physical parts, is rapidly becoming the standard for complex system design. There are many advantages to virtual prototyping over physical prototyping. These include product quality, product competitiveness, and reduction in design cycle time.
Product Quality –Using a prototype for design debugging cannot, in most cases, account for all corner case scenarios, resulting in errors. Nor can long-term reliability problems be discovered that may not materialize in test chambers or in the test board, as manufactured. Nothing is more expensive than one that is discovered in the field and requires extensive warranty costs or recall.
Product Competitiveness – When a design team can quickly create a sandbox design and test it virtually, there is opportunity to experiment with many “what-if” scenarios and develop a product that is functionally and performance rich. With physical experimentation, evaluating ideas is time consuming and probably will reduce the number of passes. Another advantage to performing virtual prototyping is the ability to design just to spec and not be overly conservative. Conservative design can add PCB layers, passive components, heat escape paths, etc. that all add to the cost of the product.
Reduction in Design Cycle Time – With physical prototypes, a board is manufactured and then taken to a lab for testing and evaluation. This procedure more than likely will turn up several design errors that require a re-spin through the design process. This iterative process can take several weeks per iteration and if done entirely with physical boards, it can add months to a development schedule.
Virtual prototyping does have a disadvantage: It requires the design team to perform multiple tests in software prior to building the first hardware, which often results in a longer time to the first prototype. Superficially, this seems like slower progress on the delivery schedule. In truth, that longer period much more often results in a right-the-first-time physical article.
More than Signal Integrity Analysis
Electronic circuit simulators have been around for more than five decades. The early simulators used analog simulation and were the definition of cumbersome. Today, with the extremely high operating frequencies of most systems, circuit analysis tools are a necessity.
You probably think of signal integrity first when virtual prototyping is mentioned. But virtual prototyping goes way beyond signal integrity. Power integrity, thermal management, vibration and shock, electromagnetic interference, and manufacturing for yields and reliability, to name a few, can benefit from virtual prototyping. During the design process, the pressure is on the design team to perform many types of analysis yet still meet their schedule.
Figure 1. Virtual prototyping should be used throughout the design process to reduce cycle time and cost, plus produce a reliable product.
Until recently, specialists in signal or power integrity were the only people capable of performing the complex analysis required to simulate complex circuits. Designers worked on the design and then handed it off to the specialists, who performed their highly complex analysis and suggested design changes.
There are problems with this methodology. First is the limited availability of these experts. Second, the analysis tools they used were highly technical and required very detailed knowledge to run, so only they could do an adequate job of virtual prototyping. The design might have to wait for another cycle by the specialists. All the while, the designer is limited on the progress he can make on design changes.
Another problem is the volume of design elements that needed to be analyzed. Previously, analyzing a few key high-speed nets was something a specialist could do quickly. Now, using the same example of high-speed nets, performing analysis on thousands of interconnects for signal integrity can take extensive time, even for a specialist.
Today, though, CAD suppliers like Mentor Graphics have developed easy-to-use tools for all aspects of virtual prototyping. They have focused not only on increasing the accuracy and speed of the analysis tools but also on usability. Many of the analysis tools are intuitive to use and well integrated/embedded with the design systems. Now the majority of the virtual prototyping functions can be performed by the designers themselves. This has not completely eliminated the need for the specialists, but it has significantly reduced the need for the “Pass it to the Specialist” methodology. It also affords the designers the opportunity to experiment with ”what-ifs” and produce a more competitive design.
Here are just two ways that virtual prototyping can save time and money in new product design and release.
Electrical Virtual Prototyping
With the performance and operating speed of electronic circuits constantly increasing, the effects on the interconnects on the PCB also become more complex. Correct interconnects between the components is only part of the challenge for high-speed systems; challenges include timing, crosstalk, signal integrity, power delivery design and integrity, mixed signal analysis, RF circuitry design and analysis, and electromagnetic interference. Virtual prototyping in this environment requires close collaboration between the electrical engineer and layout designer as well as the use of powerful, accurate, and fast virtual prototyping software.
The world of signal integrity is changing rapidly. Up until recently, interconnect buses were multiple nets running in parallel at frequencies of several hundreds of megahertz and what might be considered fast edge rates. Now, high-speed serial busses such as PCI Express, DDR2/3, and Serial ATA, running at frequencies from several hundred megahertz to beyond a gigahertz, require very tight timing margins. The silicon geometry required for these speeds makes for extremely fast edge rates (hundreds of picoseconds). All of this is compounded by the fact that in many high-speed systems, up to 90% of interconnects are running at these extraordinarily high speeds.
The virtual prototyping software tools to simulate these high-speed nets must be not only efficient and easy to use, but also must allow easy and effective communication and collaboration throughout the entire design process (Figure 2). It starts with engineers defining the constraints for all of the high-speed nets and supported by analysis software that virtually simulates the characteristics of the signals and the PCB board physical stack-up. These constraints are entered into a management system that will feed into the rest of the design process, controlling the layout designer’s routing, and finally in the post-layout verification of the design’s most critical nets.
Figure 2. A complete virtual prototyping system for high-speed designs covers the whole design process, from definition through layout to verification.
Thermal Effects Virtual Prototyping
Faster circuits dissipate more heat…it’s pure physics. Compounding the dissipation problem, increasing densities of those ICs on the PCB and in the system mean that there is less real estate over which to distribute that heat. If ICs exceed their maximum temperature, two quality issues can surface: First, it reduces performance. The ICs literally slow down and the performance of the system suffers. In addition, timing errors that result in functionality failures often occur after an IC has been thermally stressed. The second quality issue is reliability itself. Above a critical temperature, the reliability of the IC decreases exponentially and may result in a long-term failure and warranty costs to the product. This second effect may not be discovered through physical prototyping as the failure may take months or years to materialize and building/testing a physical prototype may not run long enough to cause the failure.
Proper thermal analysis of a system is absolutely necessary. Employing virtual prototyping during the design process is a multi-step process requiring the both electrical and mechanical designers as depicted in Figure 3.
Figure 3. Using thermal virtual prototyping, hot spots can be located long before a prototype is built, saving a lot of money and time compared to building actual physical prototypes.
For virtual prototyping thermal effects, the analysis software calls on the thermal model generally supplied by the IC maker. As a baseline, the software tool will analyze the standalone PCB as the design is being performed. A PCB designer typically wants an analysis of his active component placements to determine if they are creating a section of the board that will be difficult to dissipate heat. However, this requires more than a rough estimate of the board with the component dissipations and locations. Since the heat dissipation paths are many (heat sinks, copper in the inner layers of the board, convection, conduction and radiation), the data passed from the PCB design system to thermal analysis must be complete.
True virtual prototyping must account for the PCB or multiple PCBs in the final product enclosure in the end-user’s probable environment. This analysis is typically accomplished in the mechanical design domain where the MCAD system possesses the complete physical definition of the product: enclosure, mounting methods, heat sinks and rails, the PCBs, etc.
The PCB designer must transfer the design data for the PCB(s) to the mechanical designer, where they can be inserted into the enclosure. The MCAD system must have a full 3D physical definition and thermal properties of the components, their leads, etc. and all elements of the complete product. The mechanical designer then uses software employing computational fluid dynamics to perform a combination of convection, radiation, and conduction analysis to determine if the ICs exceed maximum temperatures and may result in a reliability or performance problem.
New Best Practice
Using virtual prototyping is a change from what many companies have been doing their entire existence. Now, software makes virtual prototyping possible, but this is not enough. The company has to commit to this new methodology:
- Commit to making the change: The initial design process will take longer than it used to take before you have a piece of hardware in your hands. Don’t forget that your goal is to cut the TOTAL design process time and virtual prototyping can make that possible. Management and the design team members have to be sold and commit to the change.
- Construct a new best practice to incorporate virtual prototyping: Virtual prototyping must be an integral component, with functionality from start to finish of development. Making virtual prototyping continuous from design conception right through to manufacturing delivery and not an afterthought will enable you to truly shorten your design cycle times and get better product to market faster.
- Educate the design team: Educate the design team in the use of the virtual prototyping software. Engineers and designer will be stepping out of their normal job responsibilities and need proper training in methodology and specific tool use.
- Put the right virtual prototyping software in place: Understand that these tools must not only be useable by a few company specialists, but also by the PCB designers themselves. The software has to be easy to learn and use, seamlessly integrated with the product design functions, fast and accurate.
John Isaac has worked in the EDA industry with PCB and IC technology for over 40 years. His career started with IBM where he managed the development of EDA systems for IBM's internal design of their high-end ICs and PCBs. He then joined Mentor Graphics where he has held marketing positions in both PCB and IC product areas. He is currently responsible for worldwide market development for the Systems Design Division.