NCF File

The constraints (attributes) specified in the HDL design or in the constraint file are written by XST into a constraint file with the same name as the output netlist and the extension ncf. The path of the .ncf file is the same as output netlist. The .ncf file is read and processed by the CPLD fitter.

The .ncf file is always created by XST, even if no constraints are specified. A previous .ncf file is not saved; it is always overwritten.

The NCF file generated by XST contains constraints on NETs.

When the equation shaping processing is applied, the .ncf file contains Keep constraints for internal signals of the design and Collapse constraints for all other AND/OR/INV primitives. So, the CPLD fitter is forced to respect the equation tailoring done by XST. An indication for the value of -pterms option of the CPLD fitter is also written (as a comment) in .ncf file:

# Minimum value for -pterms option is n

You must specify this value for -pterms option when running the CPLD fitter.