Optimizing the Timing Constraints Signoff Flow

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Reducing Errors and Iterations with an Enhanced Timing Constraints Signoff Flow

Naveen Battu, Rimpy Chugh

Mar 26, 2024 / 3 min read

Table of Contents

Holistic Approach for Constraints Management
Preventing Silicon Bugs
Summary

They say timing is everything, and this is especially true for silicon chips. As chips grow more complex, assigning accurate constraints to various parameters��timing, in particular��becomes critical. Timing constraints have far-reaching impact, affecting everything from the power, performance, and area (PPA) of a design to its overall time to market. Unfortunately, the traditional development flow and validation process for timing constraints can be lengthy, manual, and inefficient.

For many designers, particularly those working on large, complex designs, the process to manually integrate constraints can be error-prone and difficult to validate. Shifting the process left for faster and better results calls for an automated approach that can help manage timing constraints as the chip implementation process progresses.

A Holistic Approach to Timing Constraints Management

Of a design��s three main constraints��timing, power, and area��timing is the most critical, as it relates to whether the chip will operate at its intended clock rate. Timing constraints within the Synopsys Design Constraints (SDC) file define the alignment between different clocks in a design. (The SDC file format is used to specify design intent, including timing, power, and area constraints, and is used by various EDA tools to synthesize and analyze a design.) These constraints can influence placement of design elements as well as the signal routes between these elements and, ultimately, impact the design��s performance. As chips grow more complex, the complexity of timing constraints also grows.

When implementation tools like synthesis, place and route, and signoff applied downstream are provided with conflicting clock constraints, they won��t be able to perform properly. The tools are only as good as their input. Incorrect input can ultimately lead to costly chip errors and respins.

Ideally, constraints management should be approached holistically, from the front end to the back end of the implementation process. Since the impact of timing constraints at signoff is very high, they should be addressed at RTL, when there is still time to resolve them (and do so with fewer iterations). As timing constraints are handed off from team to team during the implementation process, they go through many iterations, but their design intent should be preserved. This is where accuracy of the constraints comes into play.

Preventing Silicon Bugs and Reducing Turnaround Time

Synopsys Timing Constraints Manager, a complete solution for managing timing constraints within SDC as chip implementation progresses, provides a way to automate the process. Different IP will have different timing constraints. Accuracy��in terms of constraints translation in the right context of the final SoC��is essential for avoiding silicon bugs. Timing Constraints Manager verifies, generates, and manages SDC constraints, enabling designers to drive chip implementation using comprehensive and accurate constraints earlier in the cycle. Refer to below image illustrating various use-cases of Timing Constraints Manager.

Figure 1. Synopsys Timing Constraints Manager verifies, generates, and manages SDC constraints.

Timing Constraints Manager was part of the FishTail Design Automation portfolio. FishTail is now part of Synopsys, and its technology has been integrated into the Synopsys Digital Design Family of products.

Using Timing Constraints Manager, customers have achieved:

Improved overall turnaround time across multiple RTL releases
Reduced error-prone constraints integration and cleanup
Enhanced quality of constraints
Improved integration at SoC level, with fewer iterations and better turnaround time
Avoidance of late surprises during implementation
Ability to catch constraints bugs to ensure a robust timing signoff and prevent potential silicon issues

Summary

Constraints-related problems can lead to critical bugs in silicon chips. As chips become more complex, however, so do the constraints. Over-constraining a design can cause implementation tools to work harder to resolve conflicting requirements, leading to higher turnaround times and sub-optimal PPA. An end-to-end constraints solution that covers generation, verification, management, equivalence checking, and signoff can facilitate better results in a shorter amount of time.