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Voltage Island-Driven Floorplanning Considering Level shifter Placement

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Voltage Island-Driven Floorplanning Considering Level shifter Placement

J. Lin, W. Cheng, C. Lee and R. Hsu

Department of EE, NCKU, Tainan, Taiwan

ASPDAC 2012

- Introduction
- Problem Formulation
- Overview of Our Algorithm
- MSV-Driven Floorplanning Considering Level Shifter Region Allocation
- Level Shifter Placement
- Experimental Result
- Conclusion

- Low power has become a burning issue in modern VLSI design.
- One effective and practical way to reduce power consumption is Multiple Supply Voltage (MSV) technique.
- MSV technique partitions the internal logic of a chip into multiple voltage regions such that each part has its own supply voltage.
- High performance modules use high-Vdd.
- Low performance modules use low-Vdd.

- There exist two kinds of level shifters.
- High-to-low voltage level shifter:
- Signals from a cell with high supply voltage to a cell with a low voltage.
- It is usually placed in the low voltage domain.

- Low-to-high voltage level shifter:
- Signals from a cell with low supply voltage to a cell with a high voltage.
- It is better to be placed in the high voltage domain.

- High-to-low voltage level shifter:

A low-to-high level shifter

A high-to-low level shifter

- The placement of level shifters has significant impact on timing.

- The problem of voltage island-driven floorplanning considering level shifter placement can be defined as follows:
- Given a set of modules, a set of voltage islands, each island contains a subset of modules that operate at an identical voltage value.
- Determine the locations of modules under the island constraints, the locations of level shifters in an island.
- The objective is to minimize total wirelength and area of a floorplan.

- In the chip-level floorplanning stage, cluster all modules belonging to an island as a super module.
- The area of the super module should be larger than the total area of modules and level shifters assigned to it.
- By applying Defer to obtain the initial locations and shapes of super modules (a floorplan for voltage islands).

- Two methods to allocate regions for level shifters:
- Level shifter channel method and level shifter island method.

- Four cases to assign level shifters:
- (1) Assign a level shifter to one side of the island with the longest length inside the bounding box.
- (2) For the candidates from rule 1, if more than one side of the island have the same length overlapped by the bounding box, it is assigned to the side that has the shorter length in the island.

- Four cases to assign level shifters:
- (3) For the candidates from rule 2, assign level shifter to the side closest to the central line of the bounding box if two side have an identical length.
- (4) Assign a level shifter to all the rest sides.

- Once the locations and shapes of an voltage island and level shifter channels in its four boundaries are determined, start island-level floorplanning by performing fixed-outline floorplanning in the remaining region insides the island.
- After the locations of modules in each island are determined, we will come back to chip-level floorplanning stage and determine the shapes and locations of voltage islands.
- Repeat level shifter region allocation and island-level floorplanning until no level shifter channel needs to be adjusted.

- Global Level Shifter Allocation
- Roughly determine locations of level-shifters by applying the min-cost max-flow algorithm.
- Partition a level shifter region into bins B={b1, …, bm}.

Capacity=1

Capacity=|bj|

- Level shifter node set NL ={nl1, nl2, …, nln}
- Bin node set NB ={nb1, nb2, …, nbm}
- The cost F of an edge: F(nli, nbj) = Fi,j, where
- c: a user specified parameter
- Abj: the area of bin bj
- Arij: the area of the region that bj overlaps the bounding box of li
- D: the shortest Manhattan distance between the center of bj and one corner of the bounding box of li

- Detailed Level Shifter Assignment
- Determine the exact location of a level shifter in a bin.
- Apply the ILP algorithm to deal with this problem.
- Divide each bin into several grids, each grid can contain exactly one level-shifter.

- The cost si,k can be computed by the following equation:
- D: the shortest Manhattan distance between the center of grid gk and one corner of the bounding box of li.
- xck: the center coordinate of gk.

- This paper proposed a two-stage approach to place level shifters in order to reduce time complexity.
- Experimental results have shown that their algorithm can obtain better WL and smaller area if level shifters can be considered during floorplanning.