Xilinx Constraints Guide

Chapter 2: Entry Strategies for Xilinx Constraints
Physical Constraints File (PCF)
The Native Generic Database (NGD) file produced when a design netlist is read into
the Xilinx® ISE® Design Suite may contain a number of logical constraints. These
constraints originate in any of these sources.
• An attribute assigned within a schematic or Hardware Description Language (HDL)
file
• A constraint entered in a User Constraints File (UCF)
• A constraint appearing in an Netlist Constraints File (NCF) produced by a CAE
vendor toolset
Logical constraints in the NGD file are read by MAP. MAP uses some of the constraints
to map the design and converts logical constraints to physical constraints. MAP then
writes these physical constraints into a Physical Constraints File (PCF).
The PCF file is an ASCII file containing two separate sections:
• A section for those physical constraints created by the mapper
• A section for physical constraints entered by the user
The mapper section is rewritten every time you run the mapper.
Mapper-generated physical constraints appear first in the file, followed by user physical
constraints. In the event of conflicts between mapper-generated and user constraints,
user constraints are read last, and override mapper-generated constraints.
The beginning of the mapper-generated section is indicated by SCHEMATIC START.
The end of this section is indicated by SCHEMATIC END. Enter user-generated
constraints, such as timing constraints, after SCHEMATIC END.
You can write user constraints directly into the file, or you can write them indirectly (or
undo them) from within the FPGA Editor. For more information on constraints in the
FPGA Editor, see the FPGA Editor help.
Note Whenever possible, you should add design constraints to the HDL, schematic, or
UCF, instead of PCF. This simplifies design archiving and improves design role checking.
The PCF file is an optional input to PAR, FPGA Editor, TRACE, NetGen, and BitGen.
The file may contain any number of constraints, and any number of comments, in any
order. A comment consists of either a pound sign (#) or double slashes (//) followed by
any number of other characters up to a new line. Each comment line must begin with a
pound sign (#) or double slashes (//).
The structure of the PCF file is as follows.
schematic start;
translated schematic and UCF and NCF constraints in PCF format
schematic end;
user-entered physical constraints
Caution! Put all user-entered physical constraints after the schematic end statement.
Any constraints preceding this section or within this section may be overwritten or ignored.
Do not edit the schematic constraints. They are overwritten every time the mapper
generates a new PCF file.
Global constraints need not be attached to any object, but should be entered in a
constraints file.
Indicate the end of each constraint statement with a semicolon.
Constraints Guide
36 www.xilinx.com UG625 (v. 13.2) July 6, 2011