Xilinx Constraints Guide

Chapter 2: Entry Strategies for Xilinx Constraints
UCF and NCF File Syntax
Logical constraints are found in:
• The Netlist Constraints File (NCF), an ASCII file typically generated by synthesis
programs
• The User Constraints File (UCF), an ASCII file generated by the user
Xilinx® recommends that you place user-generated constraints in the UCF or NCF files
and not in the Physical Constraints File (PCF) file.
General Rules for UCF and NCF
• UCF and NCF files are case sensitive. Identifier names (names of objects, such as
net names) must exactly match the case of the name as it exists in the source design
netlist. However, any Xilinx constraint keyword (for example, LOC, PERIOD,
HIGH, LOW) may be entered in all uppercase, all lowercase, or mixed case.
• Each statement is terminated by a semicolon (;).
• No continuation characters are necessary if a statement exceeds one line, since a
semicolon marks the end of the statement.
• Xilinx recommends that you group similar blocks, or components, as a single timing
constraint, and not as separate timing constraints.
• To add comments to the UCF and NCF files, begin each comment line with a pound
(#) sign, as in the following example.
# file TEST.UCF
# net constraints for TEST design
NET "$SIG_0 MAXDELAY" = 10;
NET "$SIG_1 MAXDELAY" = 12 ns;
• Statements need not be placed in any particular order in the UCF and NCF file.
• Enclose NET and INST names in double quotes (recommended but not mandatory).
• Enclose inverted signal names that contain a tilde (for example, ~OUTSIG1) in
double quotes (mandatory).
• You can enter multiple constraints for a given instance. For more information, see
Entering Multiple Constraints below.
Conflict in Constraints
The constraints in the UCF and NCF files and the constraints in the schematic or
synthesis file are applied equally. It does not matter whether a constraint is entered in
the schematic, HDL, UCF or NCF files. If the constraints overlap, UCF overrides NCF
and schematic/netlist constraints. NCF overrides schematic/netlist constraints.
If by mistake two or more elements are locked onto a single location, MAP detects
the conflict, issues an error message, and stops processing so that you can correct the
mistake.
Constraints Guide
32 www.xilinx.com UG625 (v. 13.2) July 6, 2011