( ESNUG 243 Item 3 ) ---------------------------------------------- [7/17/96]
Subject: ( ESNUG 240 #4 242 #4 ) Backannotation, Visibility, & 'Characterize'
> I don't think this is true... there's another case. What they're talking
> about is executing the set_load commands from the top level, and their
> effect not being visible when you make the current_design a lower level
> (moving DOWN the hierarchy). ... But we're backannotating lower blocks,
> then moving UP the hierarchy. I found the generalization that the set_load
> commands are only visible from the design where they were run seems
> incorrect. Rather, the tool will propagate their effects up the hierarchy,
> but not down it (use characterize for that).
From: "Russell Ray" <rray@msai.mea.com>
Hi John,
Greg states that the set_load will propagate up the hierarchy but not down.
This is true in the effect, but the only reason he gets the same timing when
at a higher level is that the set_loads are still applied to the lower design
even though he is at a higher level. This can be dangerous if someone
applies the loads at the lower level, runs timing, and later decides to
re-compile but has forgotten that the set_loads are present. Those loads
(or any other design constraint) are still present unless a reset_design is
issued.
So the reason it is "seen" up the hierarchy is the loads are applied to that
design and are therefore visible to all levels above it. The danger is if a
re-compile or other operation is issued and the intent is to not have the
loads taken into account.
- Russ Ray
Mitsubishi Semiconductor America, Inc.
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