A closed-loop solution is a safer and more robust approach for Clock Domain Crossing (CDC) synchronization compared to the open-loop method.

In this design:

1. An enabling control signal is sent from the source (sending) clock domain.
2. The control signal is synchronized into the destination (receiving) clock domain.
3. Once recognized, an acknowledgment (ACK) signal is generated in the destination domain.
4. The ACK signal is then synchronized back through another synchronizer into the sending domain, confirming that the signal was successfully received.

This creates a feedback loop between the two clock domains.

🔄 Signal Flow Overview

✅ Advantages

• High reliability:
  The use of a synchronized feedback acknowledgment signal ensures that the original control signal has been safely received and sampled in the destination domain.

• Automatic compliance:
  The closed-loop design inherently satisfies the “three-edge” requirement, since the sender waits for confirmation before toggling the signal again.

⚠️ Disadvantages

• Increased latency:
  The control signal must be synchronized in both directions (send and acknowledge).
  This introduces considerable delay before the signal can change again.

• Lower throughput:
  Because the sender waits for acknowledgment, this method is slower compared to the open-loop solution.

🧩 Summary