Summary by Type - 2021.2 English

Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906)

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2021.2 English

The Summary by Type table is convenient for quickly reviewing the nature of CDC structures found in the current report. An example is shown in the following figure.

Figure 1. Summary by Type Table

The Summary by Type table includes the following columns:

  • Severity: Reports the severity of the CDC Rule as Info, Warning, or Critical.
  • ID: Unique identification number of the CDC Rule, as listed in Table 1.
  • Count: Number of occurrences of the CDC Rule in the entire report.
  • Description: Short description of the CDC Rule.

When analyzing the summary tables, it is important to start with the highest severity. Severity levels are:

  • Critical: This severity is for CDC paths with unknown or unsafe CDC Structures. You must review each individual path to either fix the structure by modifying the RTL, or waive the issue. The path details are generated by default when using the Vivado IDE, and only when -details is used with report_cdc on the command line.
    • There is some combinatorial logic on the crossing net or several source clocks are found in the fanin of the crossing net. This can degrade the Mean Time Between Failures (MTBF) characteristics.
    • There is a fanout on the crossing net to the same destination clock domain. This can lead to data coherency problems.
  • Warning: This severity is for CDC paths with known CDC Structures that are safe but non-ideal due to one of the following reasons:
    • At least one of the two first synchronizer flip-flops does not have the ASYNC_REG property set to 1 (or true)
    • The CDC structure identified requires functional correctness that the CDC engine cannot verify. These structures are Clock Enable Controlled, MUX Controlled, and MUX Data-Hold controlled CDC topologies.
  • Info: This severity indicates that CDC structures are all safe and properly constrained.