When transferring multiple signals between different clock domains, simple two-flip-flop synchronizers are not sufficient to ensure safe and consistent data transfer.
A common mistake in multi-clock designs is to:
- Pass multiple related CDC bits that belong to the same data transaction,
- But synchronize each bit independently, without ensuring that all bits are sampled coherently in the receiving domain.
🔍 Why This Is a Problem
- Even if all CDC signals are synchronized to the same clock in the source domain,
small timing skews between the signals can occur. - These skews can cause the signals to be sampled on different clock edges in the receiving domain.
Contributing factors include:
- Differences in trace lengths,
- Variations in rise/fall times, and
- Process variations across the silicon die.
Even with well-matched routing, these physical variations can lead to sampling misalignment, resulting in corrupted multi-bit data in the destination clock domain.
🧠 Key Takeaway
Simple bit-by-bit synchronization is unsafe for transferring multi-bit data across clock domains.
Proper multi-bit CDC strategies must be used to avoid skewed or inconsistent sampling.
🛠️ 5.1 Multi-Bit CDC Strategies
To prevent skewed sampling of multi-bit values across clock domains, three main design strategies can be applied:
1. 🧮 Multi-Bit Signal Consolidation
- Where possible, combine multiple CDC bits into a single-bit CDC signal.
- For example, encode multiple control conditions into a single status or flag bit that can be safely synchronized.
2. ⏱️ Multi-Cycle Path Formulation
- Use a synchronized load (enable) signal to indicate when multi-bit data is valid and stable.
- The load signal is synchronized across the clock boundary, and the receiving domain captures the data only when the load pulse is asserted.
3. 🔢 Gray Code Encoding
- Transmit multi-bit values using Gray codes, where only one bit changes between successive values.
- This reduces the chance of sampling errors, since only one bit may be metastable at a time.
- Commonly used in asynchronous FIFO designs and pointer synchronization circuits.
✅ Summary
| Strategy | Description | Use Case |
|---|---|---|
| Signal Consolidation | Combine multiple bits into a single control signal | For simple control/status signaling |
| Multi-Cycle Path | Use a synchronized load signal to transfer stable data | For parallel data transfers |
| Gray Code Encoding | Encode data to ensure one-bit transitions | For counters, FIFOs, and pointers |