Lecture Notes: Reset Domain Crossing and Its Techniques

Introduction

• Presenter: Electronics Media
• Topic: Reset Domain Crossing (RDC) and its techniques
• Previous Video: Basics of Reset Domain Crossing
  • Reset recovery
  • Set removal
  • Definition of RDC
• Objective: Explain RDC techniques

What is Reset Domain Crossing (RDC)?

• Occurs when data transitions between different reset boundaries
• Example: Transition from reset n1 to reset n2
• Potential Issue: Destination flip-flop (flop) can enter a metastable state, possibly causing chip failure

Understanding RDC with Waveforms

• Clock: Primary signal
• Reset n1: Initially de-asserted, then asserted (active low)
• Reset n2: Out of reset (de-asserted)
• Data Propagation: From flip-flop 1 to flip-flop 2 (q1 to q2)
• Metastability: When reset assertion is asynchronous and occurs near setup/removal window
  • Can cause flip-flop to sample incorrect data (0, 1, or metastable)
  • Potential chip failure if metastable data propagates

Challenges in Catching RDC

• More complex than clock domain crossing (CDC)
• Requires system-level analysis
• Consideration of reset assertion/de-assertion timing

Techniques to Mitigate RDC Issues

1. RTL Fix with Synchronizers

• Method: Insert a two-stage (double) synchronizer
• Function: Breaks path between the reset domains
  • Synchronizer aligns data with the destination reset domain
  • Minimizes metastability in data propagation

2. Reset Ordering

• System-level Strategy: Control the sequence of reset assertion
  • Ensure destination module is reset when source assertion occurs
  • Prevents data from being sampled when in metastable state

3. Clock Gating Technique

• Purpose: Control when the clock signal is active
  • Gated off during reset assertions
  • Prevents sampling of metastable data

4. Adding Waivers or Constraints

• Waivers: Manually waive off violations after analysis
  • Risky and tedious due to potential for new violations
• Constraints: Define reset order in tools like Spyglass
  • Suppresses violation alerts based on predefined reset routes

Conclusion

• Summary: Techniques for handling RDC focus on preventing metastability and ensuring reliable data propagation between reset domains
• Further Learning: Check previous video links for more detailed information
• Feedback: Questions and comments are encouraged for further clarification

Thank the audience for their attention and encourage viewers to reach out with questions.