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The Language versus The Methodology

Dave Rich

Dave Rich

Posted Jul 7, 2009
3 Comments

I’ve been around simulation and synthesis languages for a while; back when you needed an NDA to see the Verilog LRM, and again with SUPERLOG, the predecessor to SystemVerilog.  It’s easy for those like me to get caught up in the features of the language and forget that any programming language is just a tool. With any technology, people pick the tools they think will get the job finished most effectively. Tools evolve to meet the challenges and requirements of their users. Verilog and VHDL have clearly evolved to become the prevailing languages for hardware design.

But before the language wars came the methodology wars. At the time when Verilog and VHDL were being introduced in the late 1980s, most hardware design was by schematic gale-level entry. We would come to our clients with our simulators and synthesis tools and try to change their design methodology by writing RTL. They would bring their best engineers to compete with our tools – and the engineer would always win by producing a design with better area and timing! However, once the productivity of synthesizing large designs with practical quality of results prevailed over the manual effort, the methodology shift was an easier sell.

Fast forward a decade – although designs have increased exponentially as predicted by Moore’s Law, RTL design has not changed in any significant way during that time. Why? Because it takes the same number of lines of RTL code to write an 8-bit adder as it does a 64-bit adder. :) OK, so that’s an over-simplification, but number of lines of RTL code written by a single design engineer has remained manageable. However, for every registered bit added to the design, the state space doubles, and the transition space for testing all permutations of the state space quadruples. Again that’s a simplification, but the correct order of magnitude.

During that period, a number of technologies have addressed the increasing complexities of verification, such as constrained random generation, coverage driven verification, and object-oriented programming. These technologies require a change in verification methodology from writing a linear set of test patterns.

Success in captivating verification engineers to these new methodologies is taking the same path of those earlier design engineers. A single verification engineer may find a single test to exercise a specific piece of functionally much quicker than it takes a constrained random test to reach that same function. But eventually, a constrained random test will exercise more functionality faster than an engineer can write individual tests. Functional Coverage fits into Constrained Random generation to help measure the quality of your tests by telling you if your constraints are working to exercise the functionality you are required to hit.  It takes the randomness out of Constrained Random generation.

Since the verification environment is more software design than hardware, Object-Oriented Programming is a technology that helps you write re-usable code, which in turn keeps your verification code manageable.

SystemVerilog has become the prevailing language that incorporates all these technologies. But does that mean the need for writing directed tests goes away. No. Designers still layout transistors or gates by hand where it’s critical to their project. Verification engineers should use the technology that is best suited to verify their design.

Firmware tests will continue to be written in C/C++ and SystemVerilog’s DPI can help link C based tests to the RTL. SystemC has become the prevailing modeling language for DSP and algorithmic based design. Most simulation tools can seamlessly link those models to RTL to either drive the test or compare results.

So next time you’re thinking about which language to choose to verify your design, step back and think about the methodology first.

Dave Rich

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Dave RichDave Rich is Verification Technologist at Mentor Graphics and is one of the authors of Mentor’s Advanced Verification Methodology cookbook. He began his career as a design and verification engineer in 1981 at Data General. In 1987, he joined Gateway Design Automation as one of the first application engineers to support Verilog-XL. At Gateway, he helped design many of the early features of the Verilog Hardware Description Language (HDL), and after Cadence acquired Gateway, helped prepare the Language Reference Manual (LRM) that would eventually be donated to the newly formed Open Verilog International. In 1995, he joined another Verilog simulation company, Frontline Design Automation as an AE manager and later as a Product Manager after it was acquired by Avant!. In 1998, he joined Ambit Design and worked as a consulting engineer for both synthesis and simulation products after it was acquired by Cadence. In 2000, he joined Co-Design Automation as Director of Application Engineering where the Superlog HDL was being developed that eventually became the basis of the Accellera SystemVerilog 3.0 standard. Co-Design Automation was acquired by Synopsys in 2002. Dave began work on numerous technical committees within Accellera and later the IEEE P1800 working group, which he continues today. Visit No Simulation

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Comments 3

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"Success in captivating verification engineers to these new methodologies is taking the same path of those earlier design engineers. "- The transition cycle continues with, again designers and also may adopt SV as their primary design language with the mature tools availability.

Sharma KS
12:59 PM Aug 14, 2009

I want to use System Verilog, but the tool chain is not as good as expected now. So, I just wait!

Zhu YH
10:07 AM Aug 21, 2009

What tools are you waiting for?

Dave Rich
4:17 AM Sep 16, 2009

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