TNS (Total Negative Slack)

Definition: Total Negative Slack (TNS) is a metric used in static timing analysis (STA) that represents the sum of all negative slack values in a digital circuit. It provides an overall measure of the timing performance of the design, considering all the paths that fail to meet the timing requirements.

Key Points:

  • Measured in time units (e.g., nanoseconds)
  • Calculated by summing the negative slack values of all paths
  • A non-zero TNS indicates the presence of timing violations in the design
  • A TNS of 0 means that all paths meet the timing requirements
  • Complements the Worst Negative Slack (WNS) metric

Calculation: TNS = sum(Negative Slack) across all paths with negative slack

  • Negative Slack: The difference between the required time and the arrival time, when the arrival time is greater than the required time

Importance:

  • Provides a cumulative measure of the timing violations in a design
  • Helps assess the overall timing health of the design
  • Guides optimization efforts to reduce the total negative slack
  • A design with a large TNS may require significant optimizations to meet timing

Optimization Strategies:

  • Identifying and optimizing the paths contributing to the negative slack
  • Applying timing constraints and exceptions
  • Logic optimization and restructuring
  • Gate sizing and buffering
  • Clock skew optimization and clock tree synthesis

Timing Closure: Achieving timing closure involves reducing the TNS to 0, ensuring that all paths meet the timing requirements. Designers iterate through the design, analysis, and optimization process until TNS is eliminated.

Relationship with WNS: While WNS represents the most critical path, TNS provides a measure of the overall timing performance. A design may have a small WNS but a large TNS, indicating that many paths have small negative slack values. Conversely, a design may have a large WNS but a small TNS, indicating that only a few critical paths have significant negative slack.

TNS is an essential metric in the VLSI design flow, helping designers assess the overall timing health of the design and guiding optimization efforts to achieve timing closure.