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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Start by determining whether the problem is a synthesis failure, an inaccurate timing model, or a real timing violation on a path the design must meet. Check the constraints and timing summary before changing RTL: a clean violation list is not proof of timing success if important paths are unconstrained.
First identify what failed
Synthesis errors and timing violations are different problems. A synthesis error prevents the tool from completing some part of synthesis; a timing violation means timing analysis found that a constrained path misses its requirement. Warnings can also indicate that a constraint or object was not applied as intended. Use the synthesis log and the reports from the matching tool release to establish what happened; there is no universal RTL fix for a message without its context.
- Synthesis did not complete: Find the first relevant error in the synthesis log, then check the referenced source, object, or constraint in the installed tool’s documentation. Later errors may be consequences of the first one.
- Synthesis completed, but timing fails: Confirm that clocks and interface delays represent the board and application, then use the timing summary and failing-path reports to locate the cause.
- No violations are reported: Check that the relevant paths are constrained. Unconstrained paths are not evidence of timing closure.
Use a reliable troubleshooting sequence
- Establish the timing requirements. Verify that clock periods, generated clocks, input and output delays, and clock relationships reflect the actual design and its interfaces. AMD’s Vivado constraints guidance warns against both over-constraining and under-constraining: either can undermine analysis or make timing closure harder. Relevant constraint types include
create_clock,create_generated_clock,set_input_delay,set_output_delay,set_clock_groups,set_false_path,set_max_delay, andset_multicycle_path. Use only those that match the design’s real behavior. - Check constraint coverage and application. Confirm that constraints target the intended clocks and design objects. In Vivado, define clocks before constraints that refer to them; a reference to an undeclared clock can cause the related constraint to be ignored. Check XDC dependencies and file order as well. Wildcard patterns can match unintended objects, so inspect what each constraint actually covers.
- Review the timing summary. In Vivado, AMD describes Report Timing Summary as the signoff overview and a starting point for more detailed analysis. When timing fails—or constraints appear to be missing—use the summary to identify affected path groups or checks, then narrow follow-up reports to those paths.
- Inspect the failing paths. Determine whether delay is concentrated in logic or routing, and examine the path’s fanout, topology, and optimization constraints. The report should guide whether the issue is in the timing model or implementation; do not assume every failure needs an RTL change.
- Check clocks, crossings, and exceptions. Verify clock definitions and relationships, identify genuine asynchronous crossings, and confirm that exceptions match the intended paths. Review whether exceptions are active, ignored, or overridden before relying on them.
- Make one justified change and rerun analysis. Change the constraint model only when it was inaccurate; otherwise address the implementation problem indicated by the path report. Recheck the relevant reports after each change.
Decide whether the constraint model or implementation is at fault
A timing failure is meaningful only when the analyzed path and its requirement represent the design’s actual operation. Use the distinction below to choose the next investigation rather than masking a failure.
| What the reports suggest | What to investigate | Appropriate next move |
|---|---|---|
| A relevant path is unconstrained or a clock or interface requirement is missing | Constraint coverage, clock definitions, I/O delays, and whether the target objects were found | Correct the timing model to represent the actual board and application, then rerun timing analysis. |
| A required, correctly constrained path fails | The path report’s logic delay, fanout, routing, and optimization constraints | Address the cause shown by the report; do not hide a required path with an unjustified exception. |
| A path is asynchronous or has a genuine multicycle requirement | Whether the clock relationship and exception reflect actual design behavior and target the right objects | Model the crossing or multicycle behavior accurately, then verify the exception’s coverage and status. |
| No violations appear, but timing coverage is uncertain | Whether all required clocks and paths are constrained and analyzed | Resolve coverage gaps before treating the result as timing closure. |
Read critical paths for their actual cause
Do not choose a fix from the headline slack alone. Follow the reported path and identify where delay accumulates. AMD’s Vivado design-analysis guidance notes that high logic delay can point to many logic levels or constraints and attributes that limit optimization, including DONT_TOUCH and MARK_DEBUG. Intel’s timing-closure guidance identifies high-fanout control signals, suboptimal use of global networks, long local routes without pipelining, and missed register duplication as possible contributors to large timing failures. These are diagnostic possibilities, not automatic prescriptions.
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- Delay concentrated in logic: Check path depth and whether synthesis or implementation constraints are limiting optimization.
- Delay associated with fanout or topology: Inspect the net and route characteristics rather than assuming the RTL expression alone explains the result.
- Unclear cause: Compare the detailed path report with the timing summary and the exact constraints applied to that path.
Validate CDC paths and timing exceptions
Clock-domain crossings and exceptions deserve special scrutiny because a broad or misplaced exception can make reports look better without making the design safe. Intel AN 584, Timing Closure Methodology for Advanced FPGA Designs (document 683145, published 2021-10-08), states that the Timing Analyzer does not analyze unconstrained paths and treats paths as valid single-cycle paths unless they are identified as false or multicycle paths. Its guidance also cautions that wildcard patterns can target unintended objects.
For a suspected asynchronous crossing, verify the clock relationship and that the crossing is synchronized and constrained intentionally. For an exception, confirm all three points:
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- It describes real design behavior rather than being used to suppress an inconvenient failure.
- Its target objects are the intended paths, not unintended matches from a wildcard.
- The timing tool has not ignored or overridden it.
In Vivado, report_exceptions shows active exceptions as well as exceptions that are ignored or overridden. Check its results rather than assuming an exception took effect. Vivado timing methodology checks also cover clock definitions and relationships, CDC, I/O delays, setup and hold issues, and exception usage.
Use vendor tools without assuming commands are release-independent
For Vivado, AMD’s Timing Constraints Wizard analyzes the synthesized or implemented netlist and can recommend missing clocks, I/O delays, and clock-domain constraints. It does not repair inappropriate constraints already present in the source XDC files, so review those files if timing checks remain problematic. AMD’s cited constraint and design-analysis guides are Vivado 2026.1 documents; the guides list UG903’s release date as 2026-07-01 and UG906’s as 2026-06-23.
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For Intel Quartus, use the timing guidance for the edition and release installed on your system. The Quartus Prime Pro timing guide cited here is dated 2025-09-29; AN 584 is older, dated 2021-10-08. Constraint syntax, checks, and exception precedence can vary by tool and release, so confirm them against the documentation matching your installation rather than copying a command blindly.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to record when escalating a failure
If the reports do not explain the failure, preserve enough context for someone else to reproduce the analysis. Include the exact tool edition and release, the first relevant synthesis-log error or warning, the timing summary, the detailed failing-path report, and the constraints applied to the path. Note whether the path is synchronous, generated-clock-related, or an intentional asynchronous crossing, and state what changed between runs. This makes it possible to separate a tool diagnostic from a constraint-model problem or an implementation failure.
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