The Summary by Type table helps you quickly assess the types of CDC structures identified in the report. Use this summary to prioritize your review based on the severity of the CDC rule violations.
An example is shown in the following figure.
Figure 1. Summary by Type Table
The table includes the following columns:
- Severity
- Indicates the severity level of the CDC rule. Levels include Info, Warning, or Critical.
- ID
- Displays the unique identifier for each CDC rule, as listed in the CDC Rules and Description table.
- Count
- Shows how many times the CDC rule appears in the report.
- Description
- Provides a brief explanation of the CDC rule type.
When reviewing the summary, start with the rules marked as Critical severity. The severity levels are defined as follows:
- Critical
- Indicates a CDC path with unknown or unsafe CDC structures. Review each path in
detail and either fix the issue in the RTL or waive it. You can see full path
details in the Vivado IDE or by using
report_cdc -detailson the command line.- Combinatorial logic is present on the crossing net or multiple source clocks exist in the fanin, which can reduce mean time between failures (MTBF).
- Fanout exists on the crossing net within the destination clock domain, which can cause data coherency issues.
- Warning
-
Indicates CDC paths with known synchronization structures that are considered safe but not ideal. Possible causes include:
- One or more of the first two synchronizer flip-flops lack the
ASYNC_REGproperty. - The CDC structure type requires functional correctness that the CDC engine cannot validate, such as:
- Clock enable controlled CDCs
- Multiplexer controlled CDCs
- Multiplexer data-hold CDCs
- One or more of the first two synchronizer flip-flops lack the
- Info
- Indicates that the CDC structure is both safe and correctly constrained.