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April 12, 2010
Increasing The Level Of Abstraction Of IC Design
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Gabe Moretti - Contributing Editor


by Gabe Moretti - Contributing Editor
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The recent introduction by Synopsys of Design Compiler 2010 has validated Oasys's Real Time Designer approach to IC design. When a startup company introduces a new approach to an old problem, the market always wonders whether or not the new solution will have traction among the design community. The absence of competition is a mixed blessing to startups. One the one hand, it makes it easier to sell the product to those who immediately appreciate the technological advantage and can afford to take the risk. On the other hand, because the approach is unique, many people wonder whether or not it is actually viable, and this skepticism is an obstacle to revenues.


A Difficult Problem


Since the introduction of logic synthesis as an EDA product more than twenty years ago, all synthesis tools have been built the same way: turn the RTL into gates, and then optimize that netlist to meet the placement constraints. The RTL is read in and reduced to a control/dataflow data-structure, which then is turned into gates and then the gate-level optimizer operates on the netlist until the design meets the timing constraints. At this point the netlist is ready for place and route.


This approach has worked well as long as features size on the die were large enough. But beginning at the 190 nm node, the electrical characteristics of the circuit began to significantly impact the functionality of a design that was constructed based solely on logic rules.


The physical characteristics of the constructed circuit are now so important that the number of design rules to be taken into account has increased dramatically. Physical synthesis products take rules that guide the physical representation of the circuit into account during the gate-level optimization phase and provide a netlist to the place and route tool that is more likely to function than if only logic rules had been applied. This approach presents two major problems: one is that execution time has increased considerably as circuit size increases, and the other is that almost always the verification of the placed circuit still reports rules violations.


The result is that designers have to manually complete a feedback loop between the synthesis tool and place and route tool. The size of the gates is so small now compared to the size of the connecting wires, that the electrical characteristics of the wires connecting the various logic blocks, and their placement relative to each other, are likely to change the functioning of the circuit.


The logical solution is to combine physical synthesis with place and route functions in one tool. Doing so provides both technical and financial advantages over the traditional method.


The Technical Side


It is abundantly clear that avoiding problems is more efficient than solving them. Although the design community in general has not yet shown to have fully incorporated this reality in its methods, Design Compiler 2010 appears to offer a strong tool to help designers follow the dictum. The new Synopsys tool and the Oasys Design Systems product, that has been available for a few months, allow engineers to combine logic synthesis with place and route starting at the RTL. Following a "Think Globally, Act Locally" approach, they provide designers with the ability of not only anticipate possible place and route problems that would result from a literal logic synthesis of RTL code, but to
provide directions to avoid them before running the synthesis tool. The methods and algorithms used by the two companies are quite different, but the approach is similar. Approaching the problem at a higher abstraction level lowers execution times and improves quality of results.


Methods such as this one are a step forward because they eliminate the assumptions inherent in the more generic rules used by physical synthesis tools. Physical synthesis must follow the design rules established by the foundry with little if any knowledge of the intent of the specific design. Of course, while physical synthesis tools are a good step forward, they generate what at times are very difficult problems due to design rules violations resulting from the literal application of the rules. Some of these errors can be avoided by guiding the synthesis process and thus providing more intelligence to the place and route tool. The results is less rules violations, and a time savings
(read cost savings) for designers.


Design Compiler 2010 and Real Time Designer send an important message to designers: what you do up front determines how difficult and costly your development flow will be. Although it may not be humanly possible to predict all the problems that are inherent in a particular design, all agree that designers can visualize the functional structure of a circuit even before running a synthesis tool. Design re-use, whether done with proprietary or third party blocks, aids significantly in determining the final topology of the circuit.


To be sure Synopsys has not integrated Design Compiler and IC Compiler into one product, but has significantly tighten the integration of both data and information between the two tools. It is clear that the value of Design Compiler 2010 is much greater when used with IC Compiler instead of a third party tool performing similar tasks.


Both Design Compiler 2010 and Real Time Designer are a step forward in increasing the level of abstraction and thus simplify the problem to be solved. There is of course a drawback. Designers used to a simpler set of rules for logic synthesis will now have to learn more about place and route, if they are to provide correct guidelines to a new tool.


The Financial Side


By incorporating place and route guidelines in Design Compiler 2010, Synopsys has significantly improved the advantage designer will enjoy when using the entire Synopsys flow, and not just Design Compiler as a point tool. The result, I expect, will be that more companies will decide to use the entire tool flow from Synopsys, instead of purchasing only licenses for Design Compiler. Thus the impact to the revenue flow should be greater than that of just new licenses for Design Compiler. In the same vein, Oasys avoids being "just another" alternative to synthesis, and provides an integrated path from RTL to circuit verification.


Of course both Synopsys and Oasys make another, more profound, impact to our industry: the diminishing importance of point tools. The vast majority of startups have as their most valuable asset a new point tool that improves a specific step in the design flow. The road to their success is to introduce their product in the existing design flow of some important design companies. In turn this will both gain visibility and impact the purchasing process to the extent that will cause one of the three leading EDA companies to purchase the startup. By providing a closer integration using its own tools, Synopsys is in a leadership role in demonstrating that flow integration that improves
error avoidance is more important than localized improvements. A tool that has to solve both less and simpler issues can even afford to be a bit less run time efficient than a tool that has to solve difficult and more numerous problems. Startups, then, will either need to show much greater cost savings than before in order to become viable, or will have to solve more difficult problems, like Oasys has done..


More About Design Compiler 2010


To meet aggressive schedules for increasingly complex designs, engineers need an RTL synthesis solution that enables them to minimize iterations to speed up physical implementation. To address these challenges, topographical technology in Design Compiler 2010 is being extended to produce "physical guidance" to Synopsys' place-and-route solution, IC Compiler, tightening timing and area correlation to 5 percent while speeding up IC Compiler's placement phase by 1.5 times (1.5X).


A new capability allows RTL designers to perform floorplan exploration within the synthesis environment to efficiently achieve an optimal floorplan. Additionally, Design Compiler's new scalable infrastructure tuned for multicore processors yields 2X faster synthesis runtimes on four cores.


"Cutting design time and improving design performance are essential to keep our competitiveness in the marketplace," said Hitoshi Sugihara, department manager, DFM & Digital EDA Technology Development at Renesas Technology Corp. "With the new physical guidance extension to topographical technology we are seeing 5 percent correlation between Design Compiler and IC Compiler, up to 2X faster placement in IC Compiler and better design timing. We are adopting the new technology innovations in Design Compiler to minimize iterations while meeting our design goals in shorter timeframes."


To alleviate today's time-to-market pressures, Design Compiler 2010 extends topographical technology to further optimize its links with IC Compiler, tightening correlation down to 5 percent. Additional physical optimization techniques are applied during synthesis, and physical guidance is created and passed to IC Compiler, streamlining the flow and speeding up placement in IC Compiler by 1.5X. Design Compiler 2010 also provides RTL designers access to IC Compiler's floorplanning capabilities from within the synthesis environment. With the push of a button, designers can perform what-if floorplan exploration, enabling them to identify and fix floorplan issues early and achieve faster design
convergence.


"For the last few years, we have used Design Compiler's Topographical technology to find and fix design issues during synthesis to give us predictable implementation," said Shih-Arn Hwang, Deputy Director R&D Center at Realtek. "We see Design Compiler 2010 synthesis results closely correlating to physical results, while accelerating placement in IC Compiler by 1.5X. This tight correlation between synthesis and layout, along with faster runtimes, is exactly what we need for reducing iterations and significantly shortening design schedules in 65 nanometer and smaller process technologies."


Design Compiler 2010 includes a new, scalable infrastructure designed to deliver significant runtime speedup on multicore compute servers. It employs an optimized scheme of distributed and multithreaded parallelization techniques, delivering an average of 2X faster runtime on quad-core compute servers while achieving zero deviation of the synthesis results.


"We've focused Design Compiler improvements on helping designers shorten design cycles and improve productivity," said Antun Domic, senior vice president and general manager, Synopsys Implementation Group. "Since the introduction of topographical technology, the impact of logic synthesis on accelerating design closure with physical implementation has grown significantly. Design Compiler 2010 continues this trend, delivering a significant decrease in iterations and reducing run times in physical implementation. We have achieved this while dramatically updating our software infrastructure to best utilize the latest microprocessor architectures."


More About Real Time Designer


Oasys' RealTime Designer is extremely efficient memory representation, which allows enormous designs to be handled with just a modest memory footprint. The tool reads in the entire design, along with the floorplan. If there is no floorplan, often the case early in the design process, then one is generated automatically. High-level modules in the input RTL are assigned to regions if constrained by the floorplan, and then the whole RTL is partitioned, already using this coarse placement information in timing and congestion analysis. In a modern process, any timing value without an associated route is little more than a guess so it is very important to have a physical location for every
element before calculating any timing.


Hard macros - larger blocks that will not be implemented using standard cells - are also provisionally placed so that they can be considered from both a physical obstruction and a timing point of view. If there is physical hierarchy in the design, then this is honored. RTL partitions are correctly assigned to be within the physical boundaries of the appropriate partitions. At all times a fully detailed netlist of each RTL partition is available and is used to accurately time the design. This is the first breakthrough technology.


Next, to optimize the design and meet the design constraints, rather than starting to directly operate on these gates, the original RTL partitions are re-synthesized given their current physical and timing constraints. This fast constraint driven synthesis of RTL directly to library cells is the second breakthrough technology.


On top of that the RTL partitions themselves will get merged, repartitioned and replaced in order to meet timing constraints and reduce any congestion. In a final refine step all gates get a legal placement. Since the final placement already avoids excessive routing congestion, it should not subsequently get badly perturbed by the place and route tool.


The design and the placement are all fed forward to the place and route tool. The placement is so self-consistent that the place and route tool runs faster than normal, another productivity gain, and the timing resulting from the final detailed routing with all the parasitics included will be very close to the predictions from the front end.


Conclusion


For many years both the EDA and the design communities have talked about the desire to achieve RTL signoff. The drastically increase in the complexity of design rules has made significant progress to ward this goal difficult to achieve. Design Compiler 2010 and Real Time Designer give us a glimpse at how the long sought after goal may be achievable. Increasing the amount of intelligent automation used between the RTL description and the placed and route circuit may limit the creativity of designers, but improves the control of the foundry over the physical characteristics of the circuit. This in turn is bound to have a positive impact on yields.


Ever since the introduction of logic synthesis the propensity of designers to push design rules to the limit generally resulted in increasing circuit performance at the expense of yield. Given both the very high development and manufacturing costs this approach is not generally justifiable.


Experience with an integrated flow will aid in improving both the tools and the methods associated with them: the results may just be RTL signoff.


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-- Gabe Moretti, EDACafe.com Contributing Editor.


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