CDC Analysis Skill

Overview

Expert skill for Clock Domain Crossing (CDC) analysis and synchronizer design, ensuring metastability-safe multi-clock FPGA designs.

Capabilities

• Identify all clock domain crossings in RTL
• Design 2FF and 3FF synchronizers with ASYNC_REG
• Implement Gray code counters for async FIFOs
• Design handshake protocols (req-ack, valid-ready)
• Calculate MTBF for synchronizers
• Generate CDC constraints (set_false_path, set_max_delay)
• Detect CDC violations (reconvergence, data stability)
• Support Xilinx CDC-aware design flows

Target Processes

• cdc-design.js
• reset-strategy.js
• clock-network-design.js
• timing-constraints.js

Usage Guidelines

Synchronizer Types

• 2FF Synchronizer: Standard single-bit synchronization (MTBF > 100 years typical)
• 3FF Synchronizer: High-reliability applications
• Pulse Synchronizer: Edge detection across domains
• Handshake Synchronizer: Multi-bit data with control signals

FIFO Design

• Use Gray code for pointer crossing
• Ensure proper empty/full flag generation
• Consider almost-empty/almost-full for flow control
• Apply correct FIFO depth calculation

Constraint Guidelines

• set_false_path for 2FF synchronizer paths
• set_max_delay for data bus with valid synchronization
• set_clock_groups for asynchronous clocks
• Apply ASYNC_REG attribute to synchronizer flip-flops

CDC Violations to Detect

• Combinational logic between synchronizer stages
• Fan-out from unsynchronized signals
• Reconvergence of synchronized signals
• Data stability violations

Dependencies

• CDC analysis tool integration
• Vendor-specific CDC rule knowledge
• Metastability theory understanding