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Floorplan-Driven Synthesis Using a Modified Form of IMPACT to Preserve Placement Information

IP.com Disclosure Number: IPCOM000036683D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 66K

Publishing Venue

IBM

Related People

Horstman, P: AUTHOR [+2]

Abstract

Detailed placement is a critical but time-consuming part of the automated design methodology. This precludes it from being applied repeatedly in the design cycle. However, without physical information logic synthesis is relegated to making crude estimates about net lengths in order to get timing estimates which it can use to guide modification of logic. A way to get around this problem is to cluster the logic into relatively few groups. Then a relative placement of these clusters can be found based on their areas and connectivity, which optimizes total area, congestion and timing delays. Once such a placement of floorplan is found, synthesis can then use the delay and congestion information to modify the logic to eliminate apparent problems.

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Floorplan-Driven Synthesis Using a Modified Form of IMPACT to Preserve Placement Information

Detailed placement is a critical but time-consuming part of the automated design methodology. This precludes it from being applied repeatedly in the design cycle. However, without physical information logic synthesis is relegated to making crude estimates about net lengths in order to get timing estimates which it can use to guide modification of logic. A way to get around this problem is to cluster the logic into relatively few groups. Then a relative placement of these clusters can be found based on their areas and connectivity, which optimizes total area, congestion and timing delays. Once such a placement of floorplan is found, synthesis can then use the delay and congestion information to modify the logic to eliminate apparent problems. This process can be repeated until all problems are eliminated at this coarse level, after which a final detailed placement can be made.

A key objection to this approach has been that once logic has been modified by synthesis, a new floorplan could be created that is radically different from the floorplan which guided the modifications synthesis made, thus producing new timing problems. However, IMPACT, a tool recently developed, can be modified to circumvent this problem.

IMPACT distributes the excess delay from timing analysis on a path through the logic among the nets of this path. Currently this distribution is based on the fanout of each net and on the sensitivity of the net source to changes in load. This distributed delay is then converted into capacitance limits on each net, which are then passed to MCXA, the simulated annealing placement program. MCXA attempts to insure that these limits are met in the final placement.

We describe a modification of IMPACT that is used to preserve information about a floorplan. The modification is that in addition to distributing delay based only on fanout and load sensitivity, the delay would also be distributed based on the bounding or steiner tree length of the nets on a path relative to the floorplan/placement from which the timing analysis was derived. Thus, everything else being equal, a net which had a longer length would get more of the excess delay than one with a shorter length, and, hence, the capacitance limits associated with nets would in effect reflect the relative lengths of these nets in the physical placement. After timing correction, only the capacitance limits of nets connected to modified logic would be changed. A subsequent score-driven...