A Chip Designer’s Perspective on the Challenges of Reset Domain Crossing: Q&A with Aditya Sarda By Diana James For TechBullion

In the intricate landscape of digital hardware design, reset domain crossing (RDC) emerges as a critical consideration that engineers must navigate with precision. In multi-clock domain designs, various components may operate with distinct synchronous clock frequencies or even asynchronous clocks. When a reset signal transitions from one clock domain to another, potential hazards and metastability issues can arise, posing a threat to the stability and reliability of the system. Even within synchronous design power sequencing of the data path, reset domain crossing issues can persist. 

Aditya Sarda is a semiconductor chip design engineer with special expertise in RTL design and has over a decade of front-end design experience in low-power mixed-signal integrated circuits (ICs), SoC integration, signal processing, and computer architecture. He has led chip design for some of the leading semiconductor companies. In this Q&A, Sarda offers insights into why RDC issues are becoming more prevalent and how to address the growing complications. 

Q: What is RDC, and what repercussions can arise from it?

Sarda: RDC refers to the challenge of managing reset signals as they traverse between different clock domains within a complex integrated circuit. These blocks can be on the same clock domain or different clock domains and frequencies. Common repercussions are metastability, data corruption, reduced reliability, and functional failures, as well as timing violations, increased debugging complexity, compliance challenges, power consumption, and system instability.

Q: Why are system stability and reliability important? 

Sarda: Precision hardware is often used in applications where consistent and repeatable measurements are paramount, such as scientific instruments or medical devices. System instability can introduce variations in measurements, compromising the reliability of data and hindering the system’s trustworthiness. Reliability is directly linked to safety in certain applications, especially those involving critical systems such as aerospace, healthcare, or automotive. Hardware failures can have severe consequences, making it imperative to design systems that minimize the risk of errors, malfunctions, or unexpected behavior.

Q: What role does metastability play in RDC?

Sarda: Metastability refers to a situation where the output of a digital gate or a circuit is undefined—a voltage level that is neither a binary 0 nor 1—meaning the flip-flop or latch cannot settle into a stable state. If a gate’s input goes metastable, the output also goes metastable. This then propagates throughout the chip circuit, leading to unpredictable and potentially dangerous chip behavior and possible complete system failure.

Q: What makes the analysis and design of RDC a challenge for organizations?

Sarda: Currently, most static timing and simulation tools are unable to identify issues. Only gate level simulations can demonstrate problems, and even in those situations, the full extent of the damage revealed depends on the test scenario being run. By that point, it’s extremely late in the cycle, and major design changes are both time-consuming and very costly. Specialized tools and methodologies can model and simulate the behavior of reset signals across different clock domains, ensuring robustness and reliability in the final hardware implementation. 

There are a few testing tools out there, but they’re generic and must be programmed for each scenario, and the writing constraints are cumbersome at this time. Additionally, with the recent focus on increased chip design complexity and building bigger chips with SoC and IPs, regulations have not caught up, and there are no industry-wide standards. 

Q: What are some common missteps organizations should avoid? 

Sarda: When companies lack a sufficient understanding of RDC and its potential hazards, they don’t build RDC-safe structures or separate clock control modules upfront at the IP design stage. Other missteps include waiting too far into the design process to test the system and relying on the wrong tools to identify problems. The chip could behave unreliably without proper design techniques to safeguard against RDC issues. For instance, if the chip operating in a vehicle malfunctions, it can lead to glitches in the chip’s output, resulting in potentially life-threatening system failure. 

Q: What techniques can hardware designers employ to address RDC challenges?

Sarda: It is essential for hardware designers to employ a range of techniques, including synchronization flip-flops, handshaking protocols, and thorough analysis of the RDC paths. Ensuring reset value timing, meaning verifying the output of the circuit changes to reset values synchronously with respect to the active clock edge before receiving an asynchronous reset, is a fundamental safeguard. Many legacy design IPs don’t have this built-in protection, so sync reset pulses must be added and a handshake performed before async resetting them. For current IP designs, having a separate block enable and internally park all outputs at reset value or performing a handshake before getting an asynchronous reset from the clock control module is critical.

It’s also crucial to confirm the appropriate sequence of power up and power down blocks when they are cascaded in a chain. Proper care needs to be taken to map out which outputs of a block are feeding other blocks in the design, and a sequence of powering off needs to be designed based on this map. For instance, if the entire subsystem of a design is getting reset or powering off while the rest of the subsystems in the chip are active, it is imperative to ensure no glitches or metastability occurs at its output so the active subsystems can continue operation seamlessly. During sequencing, powering up the block at the beginning of the chain must come first and powering up the end block last. This ensures no transient power-up behavior escapes to the final output. Similarly, when powering off, the last block in the chain should be reset or powered off first, and then the rest of the blocks can be shut off together to save shutdown time.

An intricate web

As the complexity of integrated circuits continues to escalate, mastering the intricacies of RDC becomes paramount for hardware designers. A nuanced understanding of RDC safe design techniques and meticulous planning is essential to mitigate potential pitfalls, ultimately contributing to the creation of resilient and dependable digital designs.

About the Author: 

Diana James is an author and a freelance writer and editor of non-fiction and fiction works. She writes for numerous trade publications, including those in the medical, accounting, and technology industries. Connect with Diana at LinkedIn

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