Using Design Constraints

The Xilinx tools allow you to control the implementation of your design by entering constraints. You can enter constraints during the design and implementation phases of the design flow. During the design phase, you can enter constraints as follows.

• Add constraints to your schematic
  
  
• Add constraints to your design in your synthesis tool
  
  
• Enter constraints in the Xilinx Constraints Editor
  
  

You can apply location and timing constraints to your design. Use location constraints to control the mapping and positioning of the logic elements in the target device. The most common location constraints are pad constraints. Pad constraints are used to lock the pins of your design to specific I/O locations so that the pin placement is consistent from revision to revision. Use timing constraints to specify how fast a path must be to meet your speed requirements. You can use timing constraints for the placement and routing of your design.

Constraints entered directly in your input design are known as design constraints, and are eventually placed in your design netlist. If you want the constraints separated from your input design files, or if you want to modify your constraints without re-synthesizing your design, you can create a User Constraints File (UCF) in the Constraints Editor. This file is read by NGDBuild during the translation of your design, and is combined with an EDIF or XNF netlist into an NGD file. If a UCF file exists with the same name as the top-level netlist, it is automatically read. Otherwise, you must specify a file name for User Constraints in the Options dialog box.

Adding Constraints with the Constraints Editor

The Constraints Editor is a new graphical tool in the Xilinx Development System that allows you to enter timing constraints and pin location constraints. You can enter constraints in the graphical interface without understanding UCF file syntax. The Constraints Editor passes these constraints to the implementation tools through a UCF file.

The Constraints Editor accepts the following input files.

• A valid NGD file, which is a Xilinx logical design database file. This file serves as input to the Map program, which generates the physical design database (NCD).
  
  
• A corresponding UCF (User Constraint File), which contains logical constraints.
  
  

By default, when the NGD file is opened, an existing UCF file with the same base name as the NGD file is used. Alternatively, you can specify the name of the UCF file.

The Constraints Editor writes out a valid UCF file and a valid NGD file.These files are processed by the Map program, which generates a PCF (Physical Constraints File).

Starting the Constraints Editor

To start the Constraints Editor, select the following in the Design Manager.

Utilities Constraints Editor