Clock Domain Crossing

Clock Domain Crossing (CDC) — the challenge of safely transferring signals between logic driven by different clocks, where metastability can cause unpredictable failures.

The Problem

• When a signal crosses from clock domain A to clock domain B, it may change exactly when domain B's clock samples it
• Result: Metastability — the flip-flop enters an unstable state between 0 and 1
• Metastable output can propagate incorrect values downstream

Solutions

• 2-Flip-Flop Synchronizer: Signal passes through two back-to-back flip-flops in the receiving domain. First FF may go metastable, but resolves before second FF samples it. For single-bit signals
• Gray Code Counter: For multi-bit bus crossing — only one bit changes at a time. Used for FIFO pointers
• Async FIFO: Dual-clock FIFO with Gray-coded pointers crossing domains. Standard for data buses
• Handshake Protocol: REQ/ACK signaling between domains for control signals
• MUX Synchronizer: For multi-bit data with a valid/enable signal

CDC Verification

• Static CDC analysis tools identify all domain crossings
• Flag missing synchronizers, multi-bit crossings, reconvergence issues
• Tools: Synopsys SpyGlass CDC, Cadence Conformal

CDC bugs are among the hardest to detect in simulation — they depend on exact clock phase relationships and can be intermittent.