Faced with shrinking feature sizes and increased densities, analog designers are forced more and more to rely on layout craft and specialized place and route techniques to meet design specifications and minimize second-order parasitic effects. Modern design-rule-checking (DRC) rule decks are insufficient by themselves in terms of guaranteeing manufacturability. Traditionally, analog designers have solved this problem by engaging in some form of design-intent dialogue or communication with the back-end mask designers responsible for layout.
However, the layout artwork may be either completely unacceptable, or critical performance may be left on the table. Thus, many circuit designers are now producing their own performance-critical layouts, in an attempt to completely eliminate the failing communications loop.
Communicating design intent via word of mouth, e-mail or ad hoc design reviews is no longer sufficient. Attempts to capture critical design knowledge or intent via marked-up annotations or notes on the master schematic are falling short. When formally captured however, the EDA or CAD tool terminology for such communications is known as constraints. Constraint-aware and constraint-driven flows are becoming more popular as EDA tools advance in this area. As always, bidirectional and accurate communication across the entire flow is key.
Do
• Ensure the important circuit structures (mirrors, differential circuits, critical nets and so on) that need special layout treatment are clearly visible in your master schematics. Use appropriate schematic design styles and tools that can highlight such structures clearly. This works even better if you can highlight them in mask layouts as well.
• Ensure steady, frequent and accurate communication between circuit and mask designers by means of a firm methodology. Software that helps analyze and visualize important structures in the schematic and layout are often crucial to this communication during design reviews.
• Be consistent and accurate in your use of terminology. For example, what does "matching" mean? There are many degrees of matching, from simple symmetries to more advanced terms like interdigitated, cross-quad transistor layouts or balanced routing. Ensure consistent use of such terms in both design reviews and tools.
• Find a way to capture your "tribal knowledge" and store it with your raw design connectivity data. Formalize your constraints as a further level of design intellectual property (IP). If you store such knowledge outside of your design database, use version control tools to ensure that the constraints IP and design IP always stay in sync.
• Top notch mask designers are priceless. Ensure your design software, flow and methodology can adequately capture the knowledge from such superstars, thereby making them more effective and extending their influence at your design house. Involve them in design and reviews from the get-go, ensure their layout expertise can be used to override that of the front-end designer where appropriate and, most important, ensure no lost communications. Close that loop!
• Rely on DRC, layout-vs.-schematic and layout parasitic extraction checks alone. Today, you need to find a formal way to ensure higher-order design intent is also verified (or even better, designed-in using a correct-by-construction methodology).
Don't
• Rely on ad hoc word-of-mouth or random e-mail communication. Annotate your design and formalize your process, while ensuring correct-by-construction where possible. Small miscommunications often lead to mistakes resulting in astronomical mask redesign costs, and not all mistakes can be corrected by a metal-only fix.
• Tolerate inconsistencies across various personnel/tools/steps in your flow. Make sure they all communicate using the same languages and definitions, again to avoid those mask costs.
• Fall into trap of using "cheaper" tools that are either constraint-unaware, or don't play nice with the other tools in your flow. Getting a poor or even broken design out faster or more cheaply is a ridiculously false economy. Invest in a rigorous design methodology and use modern tools that allow you to tape out a working design faster than prior informal methods.
• Fall for the promise of "fully automatic" tools. These are typically either too slow (such that manual design is both faster and more accurate) or too cumbersome to use in practice. Auto-interactive promises to be a better compromise than fully automated in most cases. Watch out for an exponential growth in the number of constraints to be used in such tools, and ensure they can be properly managed and that the tools performance scales appropriately.
By Don O'Riordan (driordan@ cadence.com), Virtuoso platform architect, Cadence Design Systems Inc. (San Jose, Calif.)
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