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Constraints are instructions placed on symbols or nets in an FPGA or CPLD schematic or textual entry file such as VHDL or Verilog. They can indicate a number of things such as placement, implementation, naming, directionality, and timing considerations. In the Xilinx development system, logical constraints (ones entered before a design is mapped to a device) are placed in a file called the UCF (User Constraints File).
The Constraints Editor is a Graphical User Interface (GUI) that eliminates the need for learning, remembering, and manually entering complicated syntax. Used in the implementation phase of the design after the translation step (NGDBuild), the Constraints Editor allows you to create and manipulate constraints without any direct editing of the UCF. After the constraints are created or modified with the Constraints Editor, NGDBuild must be run again, using the new UCF and design source netlist files as input and generating a new NGD file as output.
The Constraints Editor interface consists of a main window, three tab windows, a constraints window, an output window, and numerous dialog boxes. A brief description of the windows is included below. Window and dialog box functionality is described in the “Windows and Dialog Boxes” chapter.
The purpose of this guide is to describe the functionality and use of the Constraints Editor GUI. It does not include a detailed description of the constraints supported by the Constraints Editor. For in-depth information on specific constraints and constraints in general, please refer to the “Using Timing Constraints” chapter in the Development System Reference Guide and the “Attributes, Constraints, and Carry Logic” chapter in the Libraries Guide.