Design Closure - Evolution of Design Constraints

Evolution of Design Constraints

The purpose of the flow is to take a design from concept phase to working chip. The complexity of the flow is a direct result of the addition and evolution of the list of design closure constraints. To understand this evolution it is important to understand the life cycle of a design constraint. In general, design constraints influence the design flow via the following five-stage evolution:

• Early warnings: Before chip issues begin occurring, academics and industry visionaries make dire predictions about the future impact of some new technology effect.
• Hardware problems: Sporadic hardware failures start showing up in the field due to the new effect. Postmanufacturing redesign and hardware re-spins are required to get the chip to function.
• Trial and error: Constraints on the effect are formulated and used to drive postdesign checking. Violations of the constraint are fixed manually.
• Find and repair: Large number of violations of the constraint drives the creation of automatic postdesign analysis and repair flows.
• Predict and prevent: Constraint checking moves earlier in the flow using predictive estimations of the effect. These drive optimizations to prevent violations of the constraint.

A good example of this evolution can be found in the signal integrity constraint. In the mid-1990s (180 nm node), industry visionaries were describing the impending dangers of coupling noise long before chips were failing. By the mid-late 1990s, noise problems were cropping up in advanced microprocessor designs. By 2000, automated noise analysis tools were available and were used to guide manual fix-up. The total number of noise problems identified by the analysis tools identified by the flow quickly became too many to correct manually. In response, CAD companies developed the noise avoidance flows that are currently in use in the industry.

At any point in time, the constraints in the design flow are at different stages of their life cycle. At the time of this writing, for example, performance optimization is the most mature and is well into the fifth phase with the widespread use of timing-driven design flows. Power- and defect-oriented yield optimization is well into the fourth phase; power supply integrity, a type of noise constraint, is in the third phase; circuit-limited yield optimization is in the second phase, etc. A list of the first-phase impending constraint crises can always be found in the International Technology Roadmap for Semiconductors (ITRS) 15-year-outlook technology roadmaps.

As a constraint matures in the design flow, it tends to work its way from the end of the flow to the beginning. As it does this, it also tends to increase in complexity and in the degree that it contends with other constraints. Constraints tend to move up in the flow due to one of the basic paradoxes of design: accuracy vs. influence. Specifically, the earlier in a design flow a constraint is addressed, the more flexibility there is to address the constraint. Ironically, the earlier one is in a design flow, the more difficult it is to predict compliance. For example, an architectural decision to pipeline a logic function can have a far greater impact on total chip performance than any amount of postrouting fix-up. At the same time, accurately predicting the performance impact of such a change before the chip logic is synthesized, let alone placed or routed, is very difficult. This paradox has shaped the evolution of the design closure flow in several ways. First, it requires that the design flow is no longer composed of a linear set of discrete steps. In the early stages of VLSI it was sufficient to break the design into discrete stages, i.e., first do logic synthesis, then do placement, then do routing. As the number and complexity of design closure constraints has increased, the linear design flow has broken down. In the past if there were too many timing constraint violations left after routing, it was necessary to loop back, modify the tool settings slightly, and reexecute the previous placement steps. If the constraints were still not met, it was necessary to reach further back in the flow and modify the chip logic and repeat the synthesis and placement steps. This type of looping is both time consuming and unable to guarantee convergence i.e., it is possible to loop back in the flow to correct one constraint violation only to find that the correction induced another unrelated violation.