Threshold Voltage Assignment to Supply Voltage Islands in Core-based System-on-a-Chip Designs

1. Threshold Voltage Assignment to Supply Voltage Islands in Core-based System-on-a-Chip Designs Milestone 2: Steven Beigelmacher Gall Gotfried www.ece.cmu.edu/~ggall 03/23/2005

2. Overview • Review • What are we doing? • How are we doing it? • Parquet • Code overview • Closer look at the relevant portions • What have we done? • What worries us? • Demo • Milestones

3. Review • Voltage islands are regions where nearby IP blocks use a supply voltage different from the full-chip supply • We propose taking voltage islands in a core-based SoC design, and adding Vth assignments and latency measures • A coarse-grained placement problem

4. Review • Different classes of placement algorithms exist • Integer Linear Programming, Recursive, Iterative • We went with an iterative solver • Want to avoid greedy algorithms • Simulated annealing – locally bad choices can be globally good • Parquet – S. Adya, H. Chan, I. Markov • Pronunciation: pär-'kA • to make of parquetry • Parquetry -- work in the form of usually geometrically patterned wood laid or inlaid especially for floors • http://vlsicad.eecs.umich.edu/BK/parquet

5. Review • Simulated annealing loops are made up of two phases • What are the set of perturbations I can make to the current solution? (move function) • What is the relative “goodness” of that change? (cost function) • Move Function • Move block to spot (x,y), swap two blocks, block rotation, block scaling, etc

6. Review • Cost Function • Calculate a number quantifying the “goodness” of the solution • How good a solution is will depend on the area, aspect ration, wire length, etc • What are we doing? • Modifying the move function to make moves creating voltage islands • Modifying the cost function to take into account these voltage islands

7. Current State of Parquet • Code contains no comments • Only minimal support from the authors • Primary author has moved on to industry • Navigating the code has consumed most of our time • Doxygen to the rescue: http://www.ece.cmu.edu/~ggall/parquet/index.html

8. Parquet Main Call Graph

9. Parquet Annealing Graph Annealer.cxx, for the curious

10. Selecting a Move Randomly picking which move to make Making the move

11. Checking the Cost • Parquet uses different linear cost functions depending on emphasis of aspect ratio, wire length, or area AR and minWL AR minWL None Calculation of these delta components changes as t  0

12. Accepting a Move Accept good moves Else, accept bad moves with some probability that decreases as t  0 Once t equals 0, run a few more greedy iterations

13. Needed Parquet Modifications • Node objects (blocks for placement) have been modified to include list of possible supply voltages, current supply voltage • Input node file stores these voltages, which are read during the creation of the Parquet object

14. Parquet Obstacles • Voltage islands depend on adjacency of blocks • Need a way to determine of two blocks at the same Vdd can be bound without intersecting a block running at a different Vdd • This might fall out easily from sequence-pairs. We’re looking into this

15. Next Set of Modifications • Once we can detect voltage islands, need to account for them in the cost functions • Will need to examine linear vs. quadratic, convergence issues, weights, etc • Generate move functions that will push designs towards creating voltage islands • Still feasible to do Vth and latency optimizations?

16. Demo Time! • Output diagram now displays the voltage at which each block is currently running • We can’t yet detect islands, so the annealer has no way to converge on solutions with islands at this time

17. Milestones • Previous work exploration • Begin implementing algorithm for Vdd island partitioning • Deadline 02/23/2005 • Extend Algorithm • Create graphical interface to algorithm to ease understanding • Extend algorithm to parse voltage assignments and generate output graphs • Deadline 03/21/2005 • Achieve Results • Modify move and cost functions to account for supply voltage islands • Test given benchmarks with voltage assignments and examine output graphs • Extend modifications to include threshold voltages and examine output graphs • Deadline 04/11/2005

18. You Got Questions We Got Answers