Tools ease timing closure in FPGA design

Article By : Joe Mallett

FPGA design tools help automate the process for designers, making it easier and faster while also helping to remove import errors.

Today's engineering teams are tasked with delivering FPGA-based products under incredible schedule constraints to market windows. Closing timing constraints is still a challenge for many designers. FPGA design tools are a necessity to help define and apply the correct constraints to a design to quickly close timing and complete the project. This blog highlights the broad steps designers need to complete as they close timing and how tool automation helps to simplify the process.

Design setup

When starting a new project, designers need to set up the environment and import the IP for the design, which may come from multiple sources. FPGA design tools help automate this process for designers, making it easier and faster while also helping to remove import errors from the process. In addition to the IP import, the tools should automate the constraint import for a given block. These constraints will be shown in the FPGA Design Constraints (FDC) files within the tools, showing the correct syntax for things like clocks, I/O, and clock groups.

The FPGA designer will need to complete a first pass synthesis run with the "autoconstrain" option enabled for the new design, which should automatically create constraints for any remaining clocks. The tool also provides a report for the designer to view the initial performance of the design.

__Figure 1:__ *Synplify timing report from autoconstrain showing three inferred clocks (Source: Synopsys)*

Initial timing constraint setup

In the above example, there were no initial constraints when importing the IP, however the tool inferred three clocks based on the design and setup initial constraints for them. This is an automated way of creating and applying a set of initial constraints for the unconstrained clocks. If other constraints need to be applied to the design, the designer can enter them directly by editing the FDC file(s) or using a graphical constraints editor.

With the initial constraints defined, the designer will need to ensure all the clock constraints are correct by running the "syntax check" for the entire design. This syntax check verifies the constraints are defined correctly and any issues will be outlined in the log files for the designer to evaluate and fix quickly.

Once the "syntax check" is complete, the next step is to complete the "synthesis check" on the design. This should allow the designer to find and fix any hardware-related errors quickly.

Constraints tuning

The designer is now ready to complete a full synthesis step on the design. "Fast synthesis" and "continue on error" should be enabled in the tool, which helps to identify all the errors in a single synthesis pass. Once any errors identified by the tools are fixed, the last step is to complete a full constraint check to look for constraint setup issues with clock relationships, missing I/O constraints, and other related issues.

Once valid constraints with good syntax have been applied to the design, we can then fine-tune them for the performance requirements of the end-product. By leveraging automation, checkers, and continue-on-error capabilities, designers are able to accelerate the process of defining good quality constraints and applying them to a design.