Answers Database

XC3000/XC4000/XC52000: XACT 5.2- XNFMERGE: S attribute attached to OBUF output at top level gets dropped


Record #1567

Product Family:  Software

Product Line:  FPGA Core

Problem Title:
XC3000/XC4000/XC52000: XACT 5.2- XNFMERGE: S attribute attached to OBUF
output at top level gets dropped



Problem Description:
Keywords: obuf, xnfmerge

Urgency: Standard

Description

XNFMERGE may drop S attributes attached to the output
signal of a sourceless OBUF, causing the component to
be trimmed out by XNFPREP.

When the SAVE flag (S attribute) is attached to a
signal, all this tells XNFPREP is that it should
not remove the signal on its first trimming pass
just because its pins are sourceless or loadless.
However, if any logic related to the signal's source
or destination is optimized to VCC or GND, the signal
*will* get removed.  This should not depend on whether
the signal is an actual output port or not, and the
fact that the property is dropped by XNFMERGE is a
bug (reference #30671).


Solution 1:

The workaround is to attach the S property to
the INPUT of the OBUF rather than the output.



End of Record #1567