Xilinx Constraints Guide

Chapter 2: Entry Strategies for Xilinx Constraints
Netlist Constraints File (NCF)
Constraints Editor
Input/Output
The syntax rules for the Netlist Constraints File (NCF) are the same as those for the User
Constraints File (UCF). For more information, see User Constraints File (UCF) and
Netlist Constraints File (NCF) File Syntax.
The Constraints Editor is a graphical tool that simplifies the process of entering timing
constraints. This tool guides you through the process of creating constraints without
requiring you to understand User Constraints File (UCF) syntax. For the constraints and
devices with which Constraints Editor can be used, see Constraints Entry Methods. For
information on running Constraints Editor, see the ISE® Design Suite Help
Constraints Editor is used in the implementation phase after the translation step
(NGCBuild). Constraints Editor allows you to create and manipulate constraints
without any direct editing of the UCF. After the constraints are created or modified with
Constraints Editor, NGCBuild must be run again, using the new UCF and design source
netlist files as input and generating a new NGD file as output.
Constraints Editor requires:
• A User Constraints File (UCF)
• A Native Generic Database (NGD) file
Constraints Editor uses the NGD file to provide names of logical elements for grouping.
As output, it uses the UCF file.
After you open Constraints Editor, you must first open a UCF file. If the UCF and NGD
root names are not the same, you must select the appropriate NGD file to open. For
more information, see the Constraints Editor Help.
Upon successful completion, Constraints Editor writes out a UCF file. NGCBuild
(translation) uses the UCF file, along with design source netlists, to produce an NGD
file. The NGD file is read by the MAP program. MAP generates a physical design
database in the form of an Native Circuit Description (NCD) file and also generates
a Physical Constraints File (PCF). The implementation software uses these files to
ultimately produce a bitstream.
Note Not all Xilinx® constraints are accessible through Constraints Editor.
Starting Constraints Editor
Constraints Editor runs on personal computers and workstations. You can start
Constraints Editor:
• From ISE Design Suite
• As a Standalone Tool
• From the Command Line
Running Constraints Editor From ISE Design Suite
Within ISE Design Suite, launch Constraints Editor from the Processes window.
1. Select a design file in the Sources window.
2. Double-click Processes > Design Utilities > User Constraints > Create Timing
Constraints.
Constraints Guide
38 www.xilinx.com UG625 (v. 13.2) July 6, 2011