Clock domain crossing interview question

Praxisnahes Kursprojekt

• CDC Design and Analysis Project – Ensuring Robust Clock Domain Crossings
  
  Project Description:
  
  In this hands-on project, students will explore the practical aspects of designing and analyzing Clock Domain Crossing (CDC) mechanisms in digital systems. By the end of the project, participants will be able to identify CDC challenges, implement appropriate synchronizers, and analyze their robustness using simulation and theoretical techniques such as MTBF.
  
  The project involves:
  • Designing a multi-clock system with at least two asynchronous clock domains.
  • Creating CDC scenarios (e.g., control signal crossing, data bus transfer).
  • Implementing synchronizers such as dual flip-flop, pulse stretcher, and FIFO.
  • Validating the design using simulation (SystemVerilog/Verilog testbench).
  • Performing basic MTBF calculations and documenting potential improvements.
  
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  Objectives:
  • Understand and mitigate metastability and synchronization issues.
  • Apply best practices in CDC synchronizer design.
  • Gain familiarity with common CDC interview topics and design expectations.
  
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  Deliverables:
  • Verilog/SystemVerilog source files of CDC designs.
  • Testbenches for verification.
  • A short report explaining design choices, CDC risks, and mitigation strategies.
  • MTBF analysis (optional but recommended for advanced students).
  • Bonus: Include a brief section on how tools like Spyglass CDC or Questa CDC could be used in industry.
  
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  Tips for Success:
  1. Always Identify Source and Destination Domains – Before implementing any synchronizer, clearly define the clock relationship.
  2. Start Simple – Begin with a single-bit control signal before attempting multi-bit buses or handshakes.
  3. Use Waveform Debugging Tools – Tools like GTKWave or ModelSim are excellent for visualizing synchronization behavior.
  4. Review MTBF Formulas – Use the MTBF formula:
  MTBF ≈ e^(Tsu / τ) / (fclk * fdata * τ)
  Understand how setup time (Tsu) and metastability resolution time constant (τ) impact failure rates.
  5. Know the Synchronizer Types – Be ready to explain why you used a dual-flop synchronizer over a FIFO, or vice versa.
  
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  Relevant Links & Resources:
  • ASIC World – CDC Basics
  Beginner-friendly overview of CDC principles and issues.
  • Cliff Cummings – CDC and Metastability Paper (PDF)
  Industry-recognized whitepaper detailing synchronization techniques and metastability mitigation.
  • Verilog CDC Examples GitHub Repository
  Sample Verilog CDC designs and testbenches (check license before reuse).
  • Intel FPGA CDC Guidelines
  Excellent resource from Intel with design recommendations.
  • MTBF Calculation Reference (Doulos)
  Tutorial with visual explanation of MTBF and design safety.