Multi-bit CDC

Understanding multi-bit clock-domain crossing synchronisation

This is an easy digital-design question that tests your understanding of metastability and clock-domain crossing (CDC) constraints. It probes whether you understand why a naive approach to synchronising parallel data between unrelated clock domains can introduce subtle but serious bugs.

The core issue centres on the timing relationship between multiple bits when they transition asynchronously. When each bit is synchronised independently using separate synchroniser chains, the bits can arrive at different times in the destination clock domain, even if they departed together from the source. This creates a critical window where the synchronised data is inconsistent—some bits have been synchronised while others have not yet caught up.

• Metastability and resolution in flip-flops
• Synchronisation delay variation across parallel paths
• Data coherency across clock domains
• Gray codes and handshake-based solutions for multi-bit buses