Answers Database

PPR 5.2: What are the SaveSig (S), eXternal (X) netflags?


Record #284

Product Family:  Software

Product Line:  FPGA Core

Problem Title:
PPR 5.2: What are the SaveSig (S), eXternal (X) netflags?


Problem Description:



Solution 1:




All of the net-based constraints supported in pre-5.0 versions of PPR
are still supported in 5.0, 5.1, and 5.2. The Save (S) flag and the External (X)
 Flag are both supported through the XNF file, and should have the same effect a
s they had for designs processed with prior versions.

The X attribute on a net tells PPR to ensure that the net is placed outside
of a CLB, rather than internal to a CLB.  If using the Xchecker to debug
your design in circuit, using the X attribute makes it more likely for your
signal to be a probable point.

The S attribute on a net tells the XACT tools to save this signal.  The
S attribute is useful if you want a net preserved in your placed and routed impl
ementation.  If you are processing a design that is incomplete and you want to p
reserve the net and the associated logic so that PPR can give you an
accurate estimate of the resource utilization, then use this attribute.  Though
you can  also use the -savesig attribute with XNFPREP to more easily
accomplish this problem.



End of Record #284