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AR# 544

XNFPREP 5.0: Use of 'S' SAVESIG flag does not always stop logic deletion!

Solution




PROGRAM: XNFPREP

VERSION: 5.0

SHORT DESCRIPTION:

Effect of `S' Signal Parameter (or XNFPREP savesig=true) on Logic Trimming

LONG DESCRIPTION:

PROBLEM:

Signals (and associated logic) that are tagged with the 'S' SAVESIG
flag are deleted from netlist anyway.

CAUSE:

A signal that is sourceless or loadless is normally removed by XNFPREP
during logic trimming. However, attaching an `S' parameter to such
a signal will prevent XNFPREP from removing it. (The savesig=true option
in XNFPREP can be used as an alternative to placing individual `S'
parameters on every sourceless or loadless signal in the design.)

Although an `S' parameter will stop a signal from being removed because
it is sourceless or loadless, that signal MAY still be removed for a
different reason, such as being reduced to VCC or GND. The `S' parameter
will not stop a signal from being trimming for any other reason.

The `S' parameter (and the savesig=true option) is designed to support
incremental design, in that signals to be connected in a later design
iteration can be preserved. It is NOT intended as a general-purpose way
to inhibit the logic trimming process.

SOLUTION:

Other controls over the trimming process are available.

For example:

To control whether a buffer's input signal or output signal is
preserved, attach an `X' parameter to the signal that should be kept.
This can also be used for an inverter.

To prevent a buffer or inverter from being absorbed altogether, attach
an `X' parameter to both the input and output signals. However, if this
is done to add delay to a path, be aware that the delay incurred may
decrease as process technology improves.


AR# 544
Date Created 08/31/2007
Last Updated 03/31/1997
Status Archive
Type ??????