Xilinx Constraints Guide

Syntax
Chapter 2: Entry Strategies for Xilinx Constraints
The UCF file supports a basic syntax that can be expressed as:
{NET|INST|PIN} "full_name" constraint;
• full_name is a full hierarchically qualified name of the object being referred to. When
the name refers to a pin, the instance name of the element is also required.
• constraint is a constraint in the same form as it would be used if it were attached as
an attribute on a schematic object. For example, LOC=P38 and FAST.
Specifying Attributes for TIMEGRP and TIMESPEC
To specify attributes for TIMEGRP, the keyword TIMEGRP precedes the attribute
definitions in the constraints files.
TIMEGRP "input_pads"=PADS EXCEPT output_pads;
Using Reserved Words
Wildcards
In all of the constraints files (NCF, UCF, and PCF), instance or variable names that match
internal reserved words may be rejected unless the names are enclosed in double quotes.
It is good practice to enclose all names in double quotes.
For example, the following entry is not accepted because the word net is a reserved word.
NET net OFFSET=IN 20 BEFORE CLOCK;
Following is the recommended way to enter the constraint.
NET "net" OFFSET=IN 20 BEFORE CLOCK;
or
NET "$SIG_0" OFFSET=IN 20 BEFORE CLOCK;
Enclose inverted signal names that contain a tilde (for example, ~OUTSIG1) in double
quotes (mandatory) as follows:
NET "~OUTSIG1" OFFSET=IN 20 BEFORE CLOCK;
You can use the wildcard characters, asterisk (*) and question mark (?) in constraint
statements as follows:
• The asterisk (*) represents any string of zero or more characters.
• The question mark (?) indicates a single character.
In net names, the wildcard characters enable you to select a group of symbols whose
output net names match a specific string or pattern. For example, the constraint shown
below increases the output speed of pads to which nets are connected with names that
meet the following patterns:
• They begin with any series of characters (represented by an asterisk [*]).
• The initial characters are followed by "AT."
• The net names end with one single character (represented by a question mark [?].
NET "*AT?" FAST;
In an instance name, a wildcard character by itself represents every symbol of the
appropriate type. For example, the following constraint initializes an entire set of ROMs
to a particular hexadecimal value, 5555.
INST "$1I3*/ROM2" INIT=5555;
Constraints Guide
UG625 (v. 13.2) July 6, 2011 www.xilinx.com 33