In a Multi-Cycle Path (MCP) formulation with feedback, the synchronized enable signal from the receiving clock domain is passed back to the sending clock domain as an acknowledge (ACK) signal.
This creates a closed-loop handshake ensuring that new data is only sent after the previous transfer has been safely received.

⚙️ How It Works

As shown in Figure 21 (conceptually), the system consists of:

• a_clk — sending clock domain
• b_clk — receiving clock domain
• asend — control signal sent from the sender to initiate a data transfer
• b_ack — acknowledge signal generated in the receiving domain
• aack — synchronized version of b_ack in the sending domain
• aready — a ready signal indicating the sender can send new data again
• adatain — multi-bit data word being transferred

🔁 Step-by-Step Operation

1. Sender initiates a transfer:

   • The sending domain asserts asend along with valid data on adatain.

2. Receiver captures data:

   • The asend toggle is synchronized into the receiving domain through the MCP synchronizer.
   • The receiving domain then captures the unsynchronized data once the synchronized enable pulse is detected.

3. Receiver sends back acknowledgment:

   • After successfully capturing the data, the receiver toggles or asserts an acknowledge signal (b_ack).

4. Sender receives acknowledgment:

   • The b_ack signal is sent back through another synchronizer to the sending domain, producing aack.

5. Sender updates ready signal:

   • A small READY–BUSY FSM in the sending domain uses aack to set a aready flag high.
   • aready = 1 indicates the receiver is ready for the next data word.

6. Next data transfer:

   • The sender waits until aready = 1 before changing adatain and re-asserting asend for the next transfer.

🧠 Key Principle

The sender must never update adatain until the receiver acknowledges that the previous word has been captured.

This ensures data integrity and prevents overwriting or metastable sampling.

✅ Advantages

⚠️ Design Notes

• The READY–BUSY FSM in the sender typically has one active state (BUSY) between sending and acknowledgment.
• This design assumes the receiver is always ready for the next transfer (i.e., no back-pressure mechanism needed).
• The technique can be extended with additional logic if the receiver needs to throttle or pause transfers.

🪄 Typical Sequence Diagram

🧩 Summary

• MCP with feedback adds a synchronization loop to confirm successful data transfer.
• It’s ideal for event-driven or sporadic multi-bit transfers across unrelated clock domains.
• Combines the speed of MCP (multi-bit unsynchronized transfer) with the safety of a closed-loop handshake.

Would you like me to include a timing diagram (like Figure 21) to visualize the handshake sequence between asend, adatain, b_ack, and aack? It can make this closed-loop concept much clearer.