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Reporting Timing On-Chip Variation (OCV) in Synopsys PrimeTime

ASIC-System on Chip-VLSI Design: August 2013

On-Chip Variation (OCV) is a critical factor in modern integrated circuit (IC) design, accounting for the inevitable variations that occur during the manufacturing process. These variations can significantly impact the timing performance of a chip. Synopsys PrimeTime is a widely used tool for static timing analysis (STA) that allows designers to report and analyze timing considering OCV. This comprehensive guide provides detailed instructions and commands to effectively report timing OCV in PrimeTime.

Understanding On-Chip Variation (OCV)

OCV refers to the deviations in the manufacturing process that cause variations in transistor characteristics across a chip. These variations can be categorized into several types, including:

  • Process Variations: Differences in transistor dimensions and threshold voltages.
  • Voltage Variations: Fluctuations in the supply voltage.
  • Temperature Variations: Changes in operating temperatures affecting transistor speeds.

OCV can lead to variations in signal propagation delays, which directly impact the timing performance of the circuit. Accurate reporting and analysis of timing OCV are essential to ensure that the design meets its performance specifications under all possible variations.

Prerequisites for Reporting Timing OCV in PrimeTime

Before reporting timing OCV, it is essential to set up your design environment correctly. This involves:

  • Defining Process Corners: Establishing different operating conditions that represent various OCV scenarios.
  • Setting Operating Conditions: Using the set_operating_conditions command to specify variations in process parameters.
  • Loading Technology Libraries: Ensuring that your technology libraries include the necessary OCV models.
  • Configuring Constraints: Setting timing constraints that account for OCV.

Step-by-Step Guide to Reporting Timing OCV

1. Defining Process Corners

Process corners represent different combinations of variations in manufacturing parameters. Defining accurate corners is crucial for meaningful OCV analysis. Use the set_operating_conditions command to define these corners.

set_operating_conditions -process_derate <value>

Replace <value> with the specific derate factor that corresponds to your OCV model.

2. Performing Statistical Static Timing Analysis (SSTA)

OCV analysis is inherently statistical, requiring the use of PrimeTime's SSTA capabilities. The report_timing command plays a pivotal role in this process.

To report timing with OCV, use the following command:

report_timing -ocv

This command generates timing reports that incorporate OCV effects, providing insights into how variations impact your design's timing.

3. Using the report_timing Command with OCV Options

The report_timing command can be customized with various options to tailor the OCV analysis to your specific needs.

Basic OCV Timing Report

report_timing -ocv

This command provides a basic timing report that includes OCV considerations.

Detailed OCV Timing Report

For a more comprehensive analysis, include additional parameters:

report_timing -ocv -path_type full_clock

This command extends the report to include full clock path types, offering a more detailed view of timing under OCV.

Customized OCV Timing Report

To further refine the timing report, you can limit the number of paths and control the precision of the report:

report_timing -ocv -path_type full_clock -max_paths 10 -significant_digits 3

This command limits the report to the top 10 paths with the most significant timing impacts and sets the decimal precision to three digits.

4. Adjusting Timing Derates

Tuning timing derates can help simulate the impact of OCV on your design's timing performance. The set_timing_derate command is used for this purpose.

set_timing_derate -late 1.1

This command sets the derate factor for late paths to 1.1, making them 10% slower. Adjusting derate values allows for fine-tuning based on specific OCV scenarios.

Similarly, you can adjust early paths:

set_timing_derate -early 0.9

This sets the derate factor for early paths to 0.9, making them 10% faster.

5. Reporting Timing with Derates Applied

After setting the derate values, generate the timing report to see the adjusted timing metrics:

report_timing -derate

This command provides a detailed report of the timing derate values applied to various paths, helping you understand the impact of OCV on your design's timing.

6. Analyzing Results Across Multiple Corners

To comprehensively assess OCV impacts, it's advisable to analyze timing across multiple defined corners. This involves running the report_timing command for each corner and aggregating the results.

report_timing -delay min -max_paths 10 -significant_digits 4 -nosplit -sort_by worst_slack -corner <corner_name>

Replace <corner_name> with each defined OCV corner name. This approach provides a statistical overview of timing performance under varying OCV conditions.

Best Practices for OCV Timing Analysis in PrimeTime

1. Accurate Corner Definition

Ensure that your process corners accurately reflect the expected variations in the manufacturing process. Misdefined corners can lead to misleading timing reports.

2. Consistent Operating Conditions

Maintain consistency in operating conditions across different analysis runs to ensure comparability of timing reports.

3. Comprehensive Path Analysis

Analyze critical paths comprehensively, as these are the most likely to be affected by OCV and can significantly impact overall design performance.

4. Leveraging SSTA Capabilities

Utilize PrimeTime's Statistical Static Timing Analysis (SSTA) features to gain a deeper understanding of timing variations and their impacts.

5. Regular Documentation Review

Stay updated with the latest PrimeTime documentation and Synopsys resources to utilize new features and best practices for OCV analysis.

Example Workflow for Reporting Timing OCV

Below is an example workflow demonstrating the steps and commands used to report timing OCV in PrimeTime:

Step 1: Define OCV Corners

set_operating_conditions -process_derate 1.05

set_operating_conditions -process_derate 0.95

This defines two process derate factors representing upper and lower bounds of process variations.

Step 2: Perform SSTA

report_timing -ocv

Generates a baseline timing report incorporating OCV effects based on the defined corners.

Step 3: Adjust Timing Derates

set_timing_derate -late 1.1

Adjusts late path timing derates to simulate OCV impact.

Step 4: Generate Detailed Timing Report

report_timing -ocv -path_type full_clock -max_paths 10 -significant_digits 3

Produces a detailed timing report with specific constraints for comprehensive analysis.

Step 5: Analyze Across Multiple Corners

report_timing -delay min -max_paths 10 -significant_digits 4 -nosplit -sort_by worst_slack -corner high_variation

report_timing -delay min -max_paths 10 -significant_digits 4 -nosplit -sort_by worst_slack -corner low_variation

Generates timing reports for high and low variation corners to compare performance metrics.

Troubleshooting Common Issues

1. Inaccurate Timing Reports

If your timing reports do not accurately reflect OCV impacts, ensure that:

  • All process corners are correctly defined.
  • Operating conditions are consistently set across analysis runs.
  • Technology libraries are correctly loaded with OCV models.

2. Command Syntax Errors

PrimeTime commands are sensitive to syntax. Double-check command structures and options to avoid errors.

3. Performance Issues

OCV analysis, especially when dealing with multiple corners and detailed reports, can be computationally intensive. Optimize your analysis by limiting the number of paths and controlling the precision where possible.

Additional Resources

For more detailed information and advanced configurations, refer to the official Synopsys PrimeTime documentation:

Access to some documentation may require a valid Synopsys account.

Conclusion

Reporting timing OCV in Synopsys PrimeTime is a multifaceted process that involves defining accurate process corners, leveraging statistical timing analysis, and utilizing specific commands to generate comprehensive reports. By following the outlined steps and best practices, designers can ensure that their ICs meet performance requirements even in the presence of manufacturing variations. Regularly consulting Synopsys documentation and staying updated with tool enhancements will further enhance the effectiveness of OCV analysis in your design workflow.


Last updated January 7, 2025
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