2.3.1. Recommended Initial SDC Constraints 2.3.2. SDC File Precedence 2.3.3. Iterative Constraint Modification 2.3.4. Creating Clocks and Clock Constraints 2.3.5. Creating I/O Constraints 2.3.6. Creating Delay and Skew Constraints 2.3.7. Creating Timing Exceptions 2.3.8. Example Circuit and SDC File
220.127.116.11.1. Default Multicycle Analysis 18.104.22.168.2. End Multicycle Setup = 2 and End Multicycle Hold = 0 22.214.171.124.3. End Multicycle Setup = 2 and End Multicycle Hold = 1 126.96.36.199.4. Same Frequency Clocks with Destination Clock Offset 188.8.131.52.5. Destination Clock Frequency is a Multiple of the Source Clock Frequency 184.108.40.206.6. Destination Clock Frequency is a Multiple of the Source Clock Frequency with an Offset 220.127.116.11.7. Source Clock Frequency is a Multiple of the Destination Clock Frequency 18.104.22.168.8. Source Clock Frequency is a Multiple of the Destination Clock Frequency with an Offset
2.3.3. Iterative Constraint Modification
You can iteratively modify .sdc constraints and reanalyze the timing results to ensure that you have the optimum constraints for your design.
Use the following steps to iteratively modify constraints:
- Click Tools > Timing Analyzer.
- Generate the reports you want to analyze. Double-click Report All Summaries under Macros to generate setup, hold, recovery, and removal summaries, as well as minimum pulse width checks, and a list of all the clock you define. These summaries cover all paths you constrain in your design. Whenever modifying or correcting constraints, generate the Diagnostic reports to identify unconstrained parts of your design, or ignored constraints.
- Analyze the results in the reports. When you are modifying constraints, rerun the reports to find any unexpected results. For example, a cross-domain path might indicate that you forgot to cut a transfer by including a clock in a clock group.
- Create or edit the appropriate constraints in your .sdc file and save the file.
- Double-click Reset Design in the Tasks pane. This removes all constraints from your design. Removing all constraints from your design allows rereading the .sdc files, including your changes.
- Regenerate the reports you want to analyze.
- Reanalyze the results.
- Repeat steps 4-7 as necessary.
This method performs timing analysis using new constraints, without any change to logic placement. While the Fitter uses the original constraints for place and route, the Timing Analyzer applies the new constraints. If there is any failing timing against the new constraints, this indicates a need to run place-and-route again.
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