Intel® Quartus® Prime Standard Edition User Guide: Design Recommendations

ID 683323
Date 9/24/2018
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3.1.1. Synchronization Register Chains

A synchronization register chain, or synchronizer, is defined as a sequence of registers that meets the following requirements:
  • The registers in the chain are all clocked by the same clock or phase-related clocks.
  • The first register in the chain is driven asynchronously or from an unrelated clock domain.
  • Each register fans out to only one register, except the last register in the chain.
The length of the synchronization register chain is the number of registers in the synchronizing clock domain that meet the above requirements. The figure shows a sample two-register synchronization chain.
Figure 26. Sample Synchronization Register Chain


The path between synchronization registers can contain combinational logic if all registers of the synchronization register chain are in the same clock domain. The figure shows an example of a synchronization register chain that includes logic between the registers.
Figure 27. Sample Synchronization Register Chain Containing Logic


The timing slack available in the register-to-register paths of the synchronizer allows a metastable signal to settle, and is referred to as the available settling time. The available settling time in the MTBF calculation for a synchronizer is the sum of the output timing slacks for each register in the chain. Adding available settling time with additional synchronization registers improves the metastability MTBF.