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Verification Choices: Formal, Simulation, Emulation

Gabe Moretti, Senior Editor

Lately there have been articles and panels about the best type of tools to use to verify a design.  Most of the discussion has been centered on the choice between simulation and emulation, but, of course, formal techniques should also be considered.  I did not include FPGA based verification in this article because I felt to be a choice equal to emulation, but at a different price point.

I invited a few representatives of EDA companies to answer questions about the topic.  The respondents are:

Steve Bailey, Director of Emerging Technologies at Mentor Graphics,

Dave Kelf, Vice President of Marketing at OneSpin Solutions

Frank Schirrmeister, Senior Product Management Director at Cadence

Seena Shankar, Technical Marketing Manager at Silvaco

Vigyan Singhal, President and CEO at Oski Technology

Lauro Rizzatti, Verification Consultant

A Search for the Best Technique

I first wanted an opinion of what each technology does better.  Of course the question is ambiguous because the choice of tool, as Lauro Rizzatti points out, depends on the characteristics of the design to be verified.  “As much as I favor emulation, when design complexity does not stand in the way, simulation and formal are superior choices for design verification. Design debugging in simulation is unmatched by emulation. Not only interactive, flexible and versatile, simulation also supports four-state and timing analysis.
However, design complexity growth is here to stay, and the curve will only get more challenging into the future. And, we not only have to deal with complexity measured in more transistors or gates in hardware, but also measured in more code in embedded software. Tasked to address this trend, both simulation and formal would hit the wall. This is where emulation comes in to rule the day.  Performance is not the only criteria to measure the viability of a verification engine.”

Vigyan Singhal wrote: “Both formal and emulation are becoming increasingly popular. Why use a chain saw (emulation) when you can use a scalpel (formal)? Every bug that is truly a block-level bug (and most bugs are) is most cost effective to discover with formal. True system-level bugs, like bandwidth or performance for representative traffic patterns, are best left for emulation.  Too often, we make the mistake of not using formal early enough in the design flow.”

Seena Shankar provided a different point of view. “Simulation gives full visibility to the RTL and testbench. Earlier in the development cycle, it is easier to fix bugs and rerun a simulation. But we are definitely gated by the number of cycles that can be run. A basic test

exercising a couple of functional operations could take up to 12 hours for a design with a 100 million gates.

Emulation takes longer to setup because all RTL components need  to be in place before a test run can begin. The upside is that millions of operations can be run in minutes. However, debug is difficult and time consuming compared to simulation.  Formal verification needs a different kind of expertise. It is only effective for smaller blocks but can really find corner case bugs through assumptions and constraints provided to the tool.”

Steve Bailey concluded that:” It may seem that simulation is being used less today. But, it is all relative. The total number of verification cycles is growing exponentially. More simulation cycles are being performed today even though hardware acceleration and formal cycles are taking relatively larger pieces of the overall verification pie. Formal is growing in appeal as a complementary engine. Because of its comprehensive verification nature, it can significantly bend the cost curve for high-valued (difficult/challenging) verification tasks and objectives. The size and complexity of designs today require the application of all verification engines to the challenges of verifying and validating (pre-silicon) the hardware design and enabling early SW development. The use of hardware acceleration continues to shift-left and be used earlier in the verification and validation flow causing emulation and FPGA prototyping to evolve into full-fledged verification engines (not just ICE validation engines).”

If I had my choice I would like to use formal tools to develop an executable specification as early as possible in the design, making sure that all functional characteristics of the intended product will be implemented and that the execution parameters will be respected.  I agree that the choice between simulation and emulation depends on the size of the block being verified, and I also think that hardware/software co-simulation will most often require the use of an emulation/acceleration device.

Limitations to Cooperation Among the Techniques

Since all three techniques have value in some circumstance, can designers easily move from one to another?

Frank Schirrmeister provided a very exhaustive response to the question, including a good figure.

“The following figure shows some of the connections that exist today. The limitations of cooperation between the engines are often of a less technical nature. Instead, they tend to result from the gaps between different disciplines in terms of cross knowledge between them.

Figure 1: Techniques Relationships (Courtesy of Cadence)

Some example integrations include:

-          Simulation acceleration combining RTL simulation and emulation. The technical challenges have mostly been overcome using transactors to connect testbenches, often at the transaction level that runs on simulation hosts to the hardware holding the design under test (DUT) and executing at higher speed. This allows users to combine the expressiveness in simulated testbenches to increase verification efficiency with the speed of synthesizable DUTs in emulation.

-          At this point, we even have enabled hot-swap between simulation and emulation. For example, we can run gate-level netlists without timing in emulation at faster speeds. This allows users to reach a point of interest at a later point of the execution that would take hours or days in simulation. Once the point of interest is reached, users can switch (hot swap) back into simulation, adding back the timing and continue the gate-level timing simulation.

-          Emulation and FPGA-based prototyping can share a common front-end, such as in the Cadence System Development Suite, to allow faster bring-up using multi-fabric compilation.

-          Formal and simulation also combine nicely for assertions, X-propagation, etc., and, when assertions are synthesizable and can be mapped into emulation, formal techniques are linked even with hardware-based execution.

Vigyan Singhal noted that: “Interchangeability of databases and poorly architected testbenches are limitations. There is still no unification of coverage database standard enabling integration of results between formal, simulation and emulation. Often, formal or simulation testbenches are not architected for reuse, even though they can almost always be. All constraints in formal testbenches should be simulatable and emulatable; if checkers and bus functional models (BFMs) are separated in simulation, checkers can sometimes be used in formal and in emulation.”

Dave Kelf concluded that: “the real question here is: How do we describe requirements and design specs in machine-readable forms, use this information to produce a verification plan, translate them into test structures for different tools, and extract coverage information that can be checked against the verification plan? It is this top-down, closed-loop environment generally accepted as ideal, but we have yet to see it realized in the industry. We are limited fundamentally by the ability to create a machine-readable specification.”

Portable Stimulus

Accellera has formed a study group to explore the possibility of developing a portable stimulus methodology.  The group is very active and progress is being made in that direction.  Since the group has yet to publish a first proposal, it was difficult to ask any specific questions, although I thought that a judgement on the desirability of such effort was important.

Frank Schirrmeister wrote: “At the highest level, the portable stimulus project allows designers to create tests to verify SoC integration, including items like low-power scenarios and cache coherency. By keeping the tests as software routines executing on processors that are available in the design anyway, the stimulus becomes portable between the different dynamic engines, specifically simulation, emulation, and FPGA prototyping. The difference in usage with the same stimulus then really lies in execution speed – regressions can run on faster engines with less debug – and on debug insight once a bug is encountered.”

Dave Kelf also has a positive opinion about the effort. “Portable Stimulus is an excellent effort to abstract the key part of the UVM test structures such that they may be applied to both simulation and emulation. This is a worthy effort in the right direction, but it is just scraping the surface. The industry needs to bring assertions into this process, and consider how this stimulus may be better derived from high-level specifications”

SystemVerilog

The language SystemVerilog is considered by some the best language to use for SoC development.  Yet, the language has limitations according to some of the respondents.

Seena Shankar answered the question “Is SystemVerilog the best we can do for system verification? as follows: “Sort of. SystemVerilog encapsulates the best features from Software and hardware paradigms for verification. It is a standard that is very easy to follow but may not be the best in performance. If the performance hit can be managed with a combination of system C/C++ or Verilog or any other verification languages the solution might be limited in terms of portability across projects or simulators.”

Dave Kelf wrote: “One of the most misnamed languages is SystemVerilog. Possibly the only thing this language was not designed to do was any kind of system specification. The name was produced in a misguided attempt to compete or compare with SystemC, and that was clearly a mistake. Now it is possible to use SystemVerilog at the system level, but it is clear that a C derived language is far more effective.
What is required is a format that allows untimed algorithmic design with enough information for it to be synthesized, virtual platforms that provide a hardware/software test capability at an acceptable level of performance, and general system structures to be analyzed and specified. C++ is the only language close to this requirement.”

And Frank Schirrmeister observed: “SystemVerilog and technologies like universal verification methodology (UVM) work well at the IP and sub-system level, but seem to run out of steam when extended to full system-on-chip (SoC) verification. That’s where the portable stimulus project comes in, extending what is available in UVM to the SoC level and allowing vertical re-use from IP to the SoC. This approach overcomes the issues for which UVM falls short at the SoC level.”

Conclusion

Both design engineers and verification engineers are still waiting for help from EDA companies.  They have to deal with differing methodologies, and imperfect languages while tackling ever more complex designs.  It is not surprising then that verification is the most expensive portion of a development project.  Designers must be careful to insure that what they write is verifiable, while verification engineers need to not only understand the requirements and architecture of the design, but also be familiar with the characteristics of the language used by developers to describe both the architecture and the functionality of the intended product.  I believe that one way to improve the situation is for both EDA companies and system companies to approach a new design not just as a piece of silicon but as a product that integrates hardware, software, mechanical, and physical characteristics.  Then both development and verification plans can choose the most appropriate tools that can co-exist and provide coherent results.

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