Enhancing RTL Power Efficiency with xReplay, FlashReplay, and Clock Gating

Innovative Solutions for Power-Efficient RTL Design and Technology

As semiconductor designs scale in complexity and power budgets tighten, early and accurate power analysis becomes critical. Simulation and power reduction are fundamental aspects of modern technology development. Cadence's Joules RTL Power Solution offers a comprehensive suite of tools to analyze and reduce power at the RTL level.

RTL-level power analysis is crucial for optimizing power consumption in modern SoCs. This blog explores advanced techniques in the Joules tool, like xReplay, FlashReplay, and sequential clock gating, to address real-time simulation and power reduction challenges.

Welcome to our comprehensive guide on RTL power optimization techniques. If you want to deepen your understanding, consider enrolling in the Joules Power Calculator v25.1 Course, where you can learn about all these methods and techniques in detail. Additionally, check out our Training Bytes, which feature videos demonstrating how to implement these methods effectively.

In this blog, we'll explore:

• The need for power reduction at the RTL level
• Overview of Joules power analysis flow
• Deep dive into xReplay, FlashReplay, and other advanced techniques
• Understand how these flows assist in identifying and reducing power hotspots

Why Does RTL Power Reduction Matter?

Traditionally, power analysis was performed in the post-layout stage. However, with increasing design complexity and tighter Power-Performance-Area (PPA) constraints, RTL-level power estimation is now essential. It enables:

• Early detection of power bottlenecks
• Informed architectural decisions
• Reduced iterations in the backend

Joules Power Analysis Flow

Joules supports both vector-based and vectorless power analysis. The typical flow includes:

1. Design elaboration: Read RTL, libraries, and power intent (CPF/UPF)
2. Stimulus processing: Read simulation data (VCD, FSDB, SHM, PHY)
3. Mapping and synthesis: Generate a prototype netlist
4. Power computation: Compute average or time-based power
5. Reporting: Generate detailed power, activity, and efficiency reports

Real-Time Problem 1: Efficient and Accurate Gate-Level Simulations

In the realm of microprocessor design, one of the most pressing challenges is conducting efficient and accurate gate-level simulations. These simulations are crucial for debugging, optimizing, and verifying complex designs. However, traditional methods often fall short due to their limited scalability and high computational costs.

xReplay and FlashReplay are groundbreaking innovations that address these challenges head-on.

xReplay: Gate-Level Simulation with RTL Stimulus

xReplay bridges the gap between RTL stimulus and gate-level power analysis. It replays RTL-generated stimulus on a gate-level netlist using the external simulator Xcelium, enabling accurate power estimation without requiring full gate-level simulation.

xReplay is a powerful feature for RTL stimulus reuse, enabling efficient simulation and debugging. It leverages existing RTL test benches and provides a seamless process for verifying design changes.

Key Features:

• Supports zero, delay, and P-delay modes
• Generates SHM, FSDB, VCD, SAIF, or TCF waveforms
• Supports vectorless simulation

Use Case:

You have RTL stimulus but want to estimate power on a gate-level netlist. xReplay enables you to reuse the RTL stimulus, saving simulation time and effort.

FlashReplay: Fast, Parallel, and Accurate

FlashReplay is a native simulation engine in Joules that replays partial or complete stimuli using graph-based algorithms. It is designed for speed and scalability, making it ideal for large SoCs. It accelerates simulation by parallelizing tasks and optimizing resource usage.

Highlights:

• Enhanced Native Simulation: There is no need for external simulators like Xcelium
• Supports zero, delay, and p-delay modes
• Can generate SAIF and TCF formats
• Supports glitch analysis and multi-threaded execution

Use Case:

You want to simulate only a portion of the design or replay partial stimulus (e.g., flip-flops, macros) and regenerate full activity for power analysis.

Practical Tips

• Use the glitch flow option to analyze and filter glitch power
• Combine the stimulus annotation option with the initial state point value option for an accurate replay.
• Use the report power collate command to merge power from multiple blocks or scenarios
• For large designs, use multi-host or LSF options to achieve parallelism

Real-Time Problem 2: High Dynamic Power Consumption

Another critical issue in modern designs is the high dynamic power consumption, particularly in clock distribution networks. This issue affects the performance, longevity, and efficiency of the devices.

Modern SoC designs demand aggressive power optimization strategies to meet performance and thermal constraints. The Joules platform offers a suite of techniques to identify and reduce unnecessary power consumption at the RTL level.

Solution: Advanced Power Reduction Techniques

Techniques such as ODC-based Sequential Clock Gating and STB-based Sequential Clock Gating offer practical solutions to mitigate this issue.

Dynamic Power Consumption with Unnecessary Clock Activity in Idle Logic

Solved by ODC-based Sequential Clock Gating Technique.

• Observability Don't Care (ODC) analysis identifies clock gating opportunities by detecting when a register's output does not affect the primary outputs or memory elements.
• It also identifies areas in a design where the output of registers is not utilized downstream, allowing for strategic clock gating. This method minimizes unnecessary toggles, resulting in significant power savings.
• This technique is especially effective in reducing switching activity in control logic and datapaths that are idle for significant portions of time.
• It includes command flows for computing, reporting, and implementing ODC, and shows how to verify changes using Jasper Sequential Equivalence Checking (SEC).

Dynamic Power Consumption in Stable Logic

Solved by STB-based Sequential Clock Gating Technique

• Stability Don't Care (STB) analysis targets registers that hold constant values over time. Joules identifies these and gates their clocks to avoid unnecessary toggling.
• Stability Don't Care (STB) analysis focuses on the stability of register inputs. By analyzing conditions where inputs remain stable, STB enables clock gating to prevent redundant operations.
• This technique complements clock gating by reducing unnecessary combinational logic activity.
• It describes how to compute and report STB opportunities and implement gating.

Verification with Jasper SEC

• After applying ODC/STB optimizations, use Cadence Jasper SEC to verify the RTL changes and ensure correctness.

Conclusion

xReplay and FlashReplay are powerful features in Cadence’s Joules ecosystem that enable early, accurate, and efficient power analysis. Whether you're working with full RTL stimulus or partial activity data, these flows help you identify power hotspots, optimize clock gating, and reduce dynamic power—all before tape-out.

Power reduction at the RTL level is no longer optional—it's essential. Joules empowers designers with automated, verifiable techniques to identify and eliminate wasted power. By leveraging ODC and STB-based gating, logic gating, and memory optimization, teams can achieve significant power savings early in the design cycle.

These techniques in Joules provide a comprehensive solution for addressing power challenges in modern SoCs.

Thank you for reading this blog on RTL power optimization techniques. To further enhance your skills and knowledge, we highly recommend enrolling in the Joules Power Calculator v25.1 Course. This course offers in-depth training on all the methods and techniques discussed in this blog. Additionally, don't miss our Training Bytes, which feature engaging videos demonstrating practical implementations of these techniques. Take the next step in your professional development and unlock the full potential of RTL power optimization!

Training Bytes on Joules RTL Power Solutions
What is Joules FlashReplay?
How Does FlashReplay Flow Work Under the Hood in Genus/Innovus?
What are the Different Types of FlashReplay Flows?
Understanding xReplay Flow.
How to Run xReplay in Joules?
What Happens During xReplay Flow in Genus and Innovus?
What is ODC-based Sequential Clock Generation Flow?
Understanding ODC/STB Analysis/Implementation Flow in Joules RTL Power Solution.
How to Apply Observability Don't Care (ODC) Technique in Joules?
What Is ODC-Based Data Gating Flow (Apply Data Gating Flow)?
Sample Script for Stability Don't Care (STB) Analysis.

Upon completion, you will be awarded prestigious digital badges, as shown below.

These badges are not only a testament to your expertise but also highly shareable, allowing you to showcase your skills. Achieving these credentials will significantly boost your professional growth and recognition.

To learn about additional courses, please check out Learning Maps, which enhance your skills in all areas of the chip design process using Cadence EDA Tools.
If you have not yet registered in the Cadence ASK portal, please visit the link to register and enjoy learning courses.