Running X-Propagation with Low-Power Simulation

Manas Ranjan Raiguru

Nov 21, 2023 / 5 min read

In today's ever-evolving semiconductor industry, the pursuit of low-power designs has become paramount. With the increasing demand for energy-efficient devices, the need for low-power simulation methodologies has grown exponentially. One crucial aspect of low-power simulation that often remains underestimated is X-propagation analysis. In this blog, we will delve into the world of low-power simulation, focusing on why running X-propagation is key for verification closure. We will explore the challenges posed by unknown values (Xs) in digital circuits, the impact of low-power design techniques, and the critical role that X-propagation plays in ensuring reliable, power-efficient electronic systems.

The Evolution of Low-Power Design

Before delving into the specifics of X-propagation analysis, it's essential to understand the context of low-power design in the semiconductor industry. Over the years, power consumption has emerged as one of the most critical factors in the design of electronic systems, from mobile devices to data centers. This evolution can be attributed to several key factors:

  • Energy Efficiency Demands: Consumers increasingly demand devices with longer battery life and reduced energy consumption. This trend has driven the industry to prioritize low-power design.
  • Environmental Concerns: As awareness of environmental issues grows, semiconductor companies face pressure to reduce power consumption in their products to minimize their carbon footprint.
  • Heat Dissipation Challenges: High power consumption not only drains batteries quickly but also leads to excessive heat generation, which can affect device reliability and performance.

The Challenge of X-Propagation

X-values, often referred to as "unknowns," are one of the most challenging aspects of digital circuit simulation, especially in low-power designs. X-values can arise from various sources, such as uninitialized flip-flops, unconnected inputs, and asynchronous or glitch-prone signals. Understanding the significance of X-values is crucial for achieving verification closure in low-power simulations.

  • X-Values and Their Impact: X-values are neither '0' nor '1'; they represent uncertainty in the digital state of a signal. If not handled correctly, X-values can lead to erroneous simulation results and potentially mask real design issues.
  • Propagation of X-Values: X-values can propagate through the circuit, creating a ripple effect that may cause unexpected behavior. This propagation is often challenging to trace, making it a significant concern in low-power design verification.


Low-Power Design Techniques

To achieve low-power objectives, designers employ various techniques that can introduce X-values into the circuit. Some of the most common low-power design techniques include:

  • Clock Gating: Clock gating involves disabling clock signals to specific circuit elements when they are not needed. However, improper implementation can introduce X-values during clock domain crossing.
  • Power Gating/Power Shut Off: Power gating or power shut off (PSO) allows entire blocks or subsystems to be turned off when not in use. This can introduce X-values when transitioning between power states.
  • Voltage Scaling: Changing the supply voltage of a circuit can lead to issues related to X-values, especially when signals cross voltage domains.

Role of X-Propagation in Low-Power Verification

Now, let's dive into why running X-propagation analysis is crucial for achieving verification closure in low-power designs.

  • Error Detection: X-propagation analysis helps detect potential issues caused by X-values early in the design process. Identifying and addressing these issues in the verification phase is significantly more cost-effective than discovering them in Gate-level Simulations (GLS) or in silicon.
  • Accurate Power Estimation: Accurate power estimation is vital for low-power design. X-propagation analysis ensures that power consumption calculations account for all possible states, leading to more precise power estimates.
  • Bug Identification: X-propagation analysis can uncover hidden bugs that might not be apparent in regular simulations. By simulating various power states and transitions, it exposes vulnerabilities that could lead to incorrect behavior in the field.
  • Enhanced Functional Verification: Traditional simulation techniques might miss corner-case scenarios where X-values play a crucial role. X-propagation analysis provides a comprehensive view of the design's behavior, enhancing functional verification.
  • Identify missing low power cells: Although now a days static tools can easily identify any missing low power cells such as isolation, retention, power switches or level shifters, etc.,  not all verification engineers use proper hand off techniques for their designs from the design team.  Sometimes the design team can miss runing static analysis early in the design cycle. Running X-propagation with low power simulation at the RTL level helps to distinguish between the cause of X - whether it is because of a logical driver or a low power driver. Furthermore, X-propagation helps to identify any missing isolation, retention, power switches or level shifters cells in the design.

Challenges in Implementing X-Propagation Analysis

While X-propagation analysis is invaluable, it comes with its own set of challenges:

  • Runtime Overhead: Analyzing X-propagation can significantly increase simulation time and memory, which can be a concern for large and complex designs.
  • Coverage Concerns: Achieving 100% X-coverage in simulation can be challenging, and designers must make informed decisions about which signals to focus on to optimize resources.

Strategies for Successful X-Propagation Analysis

To reap the benefits of X-propagation analysis in low-power design verification, consider the following strategies:

  • Tooling and Methodology: Use industry leading advanced simulation tool such as Synopsys VCS® functional verification solutions and methodologies that offer X-optimization features and advanced analysis capabilities to streamline the process and perform accurate X-propagation analysis effectively. 
  • Selective X-Propagation: Prioritize X-propagation analysis on critical paths and signals that are most likely to introduce X-values. Use divide and conquer technique and do x-prop in block/IP level designs.
  • Iterative Approach: Perform X-propagation analysis iteratively throughout the design process, focusing on different power states and scenarios.
  • Integration with Other Verification Techniques: Combine X-propagation analysis with other verification techniques, such as formal verification using Synopsys VC Formal™ and assertion-based verification, to achieve comprehensive coverage.

Case Study: The Impact of X-Propagation on Low-Power Verification

To illustrate the importance of X-propagation analysis, let's consider a hypothetical case study involving a low-power mobile processor design.

Scenario: The design incorporates power gating to turn off unused functional units. During X-propagation analysis, it is discovered that certain control signals, when transitioning from a low-power state to an active state, produce X-values. This can lead to incorrect behavior during wake-up.

x-prop case study

Resolution: By identifying the X-propagation issue early in the verification process, the design team can modify the power gating logic and add proper reset mechanisms to ensure a clean power-up sequence. In this case the missing retention register definition can be added in the UPF. This prevents X-values from affecting the processor's behavior during wake-up, enhancing the overall reliability of the device.


In the pursuit of low-power design excellence, X-propagation analysis stands as a critical pillar for verification closure. It is no longer sufficient to focus solely on power reduction techniques; understanding the impact of X-values on the functionality and reliability of electronic systems is paramount. X-propagation analysis helps detect hidden issues, improves power estimation accuracy, and enhances functional verification.

As the semiconductor industry continues to evolve, embracing X-propagation analysis as an integral part of the low-power design verification process is essential. It not only ensures that power-efficient devices meet their energy efficiency goals but also helps deliver products with superior reliability and performance to meet the ever-growing demands of the market. In the coming years, we can expect X-propagation analysis to play an even more significant role as low-power design continues to shape the future of electronics.

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