Approaches that can ease multi-clock designs

Problem 1: Metastability
Solution: Use appropriate synchronizers. (please read my earlier articles to understand the types)

Problem 2: Reset Synchronization
Solution: Proper care is to be taken when deasserting resets as they can make sequential elements to go into metastable state. Have a seperate synchronizer for the reset signal as it enters each clock domain.

Problem 3: Glitches across clock boundaries
Solution: Signals entering synchronizers should be driven directly by a flip flop from the previous clock domain and not a combination logic.

Problem 4: Insufficient hold time in the receiving clock domain
Solution: A signal passing from a fast clock domain to a slow clock domain must be stable for multiple clock cycles in the driving domain to ensure that the slower clock domain will not miss a transition entirely.

Problem 5: Loss of signal correlation
Solution: Ways in which loss of correlation can occur are bits of buses, various copies of single signal, hanshake signals and signal convergence. So use gray code to avoid loss of correlation.

Details:

• Do extensive structural analysis of the design in RTL form to trace all clocks and resets as this can identify

1. all asynchronous clock domains in the design
2. all control and data signals crossing between clock domains
   
3. any domain crossing signals that have missing or incorrect synchronizers
4. any synchronizer that has the potential for glitches on their inputs
5. any signals that have fanouts to multiple synchronizers
6. any idependantly synchronized signals that reconverge in the receiving clock domain
7. any clock domains whose reset signals are not properly synchronized
8. any gated or derived clocks with glitch potential

The above can be automated and can be run on designs of any complexity.

Multi-clock designs are becoming more common in modern digital systems, especially in applications that require high performance, low power consumption, and interoperability with different standards. However, multi-clock designs also pose significant challenges for designers, such as clock domain crossing (CDC) verification, metastability, synchronization, and timing closure.

In this blog post, we will discuss some approaches that can ease the development and verification of multi-clock designs, such as:

- Using a common clock source and frequency for all domains, if possible, to avoid CDC issues and simplify timing analysis.

- Using clock gating techniques to reduce power consumption and dynamic timing variations in different domains.

- Using asynchronous FIFOs or dual-clock FIFOs to transfer data between domains that have different clock frequencies or phases.

- Using CDC protocols and synchronizers to ensure data integrity and avoid metastability when transferring data between domains that have different clock sources or asynchronous resets.

- Using formal methods and tools to verify CDC correctness and completeness, and to detect potential CDC errors and hazards.

- Using static timing analysis (STA) and dynamic simulation tools to verify timing constraints and functionality of multi-clock designs.

By following these approaches, designers can reduce the complexity and risk of multi-clock designs, and achieve better performance, power efficiency, and reliability.