PLACEMENT IN PHYSICAL DESIGN

PragyaM.Tech -VLSI

13VLP008

AGENDA

Back end processWhat is placement and its typesPlacement problem formulationAlgorithmsSimulated based placement Partitioning based placement

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BACK END PROCESS

1. PartitioningGoal: partition of a system into number of ASIC’s(application specific integrated chip)Objective: minimise the number of external connections between each ASIC. Keep each ASIC smaller than max size.

2. Floorplanning Goal: calculate the size of blocks and assign them locations.Objective: keep highly connected blocks physically close to each other.

3. PlacementGoal: assign the interconnect areas and the locations of all the logic cells within the flexible blockObjective: minimise the ASIC area and the interconnects

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4. Global routing Goal: determine the location of all the interconnects Objective: minimise the total interconnect area.

5. Detailed routingGoal: completely route all the interconnects on the chipObjective: minimise the total interconnect length used.

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PLACEMENT AND ITS TYPES

Placement is the problem of automatically assigning correct positions to predesigned cells on the chip with no overlapping such that some objective function is optimized.

Placement is design state after logic synthesis and before routing.

Types of placement 1. Standard cell placement –Standard cells have been designed in such a way that power and clock connections run horizontally through the cell and other I/O leaves the cell from the top or bottom sides.

2. Building block placement Cells to be placed have arbitrary shape.

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Placement is done in three steps:

1. Global placement generate a rough placement that may violate some

placement constraints (e.g., there may be overlaps among modules)

2. Legalization makes the rough solution from global placement legal (no

placement constraint violation) by moving modules around locally.

3. Detailed placement further improves the legalized placement solution in an

iterative manner by rearranging a small group of modules in a local region while keeping all other modules fixed.

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Out of the three steps important one is global placement

Approaches for global placement are :Partitioning based approach (min cut partitioning) Simulated annealing approach Analytical approach (best)

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PLACEMENT PROBLEM FORMULATION

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Input:

Placement region, a set of modules, and a set of nets. The

widths and heights of the placement region and all modules

are given. The locations of I/O pins on the placement region

and on all modules are fixed.

Output:

A set of location on the chip : one location for each cell.

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•Objective:

minimize the ASIC area and the interconnects

•Goal:

Arrange all the logic cells within flexible blocks

The cells are placed to produce a routable chip that meets

timing and other constraints (e.g., low-power, noise, etc.)

•Challenge:

The number of cells in a design is very large (> 1 million).

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The placement problems for common design styles are:

Standard cell placement Gate array/FPGA placementMacro block placementMixed size placement.all about placement.pdf

Global and Detailed PlacementIn global placement , the approximatelocations for cells is decidedby placing cells in global bins.

In detailed placement, cells are Placed without over lapping.

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Good and bad placement

Good placement Bad placement

Minimize area (total wiring area)

Ensure routability Avoid signal interference Distribute heat Maximize performance

Consumes large areas Results in performance

degradation Results in difficult and

sometimes impossible tasks (Routing)

An ill-placed layout cannot be improved by high quality routing.

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PLACEMENT ALGORITHMS

Optimisation of the following is done using placement algorithm

•Total area•Total wirelength•Heuristics are used in the algorithms.

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Constructive

once the position of the cell is fixed , it can not be modified

Constructive algorithms are used to obtain aninitial placement.

Iterative

intermediate placements are modified in an attempt to improve the cost function.

The initial placement is followed by an iterative improvement phase.

Techniques for initial placement

A top-down method: min-cut partitioning and placement (bisect the circuit recursively)Min-Cut Placement method1. Cut placement area into two pieces2. Swap logic cells to minimize cut cost3.Repeat process from step 1, cutting smaller pieces until all logic cells are placed

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A bottom-up method: cluster growth (selectcells with strongest connections one by one)

Simulated Annealing Placement

•Initial placement improved through swaps and moves•Accept a swap/move if it improves the cost

Pros and cons of SAPros:•Can Reach Globally Optimal Solution (given “enough” time)•Can Optimize Simultaneously all Aspects of Physical Design•Can be Used as a End Case Placement

Cons•Extremely slow process of reaching a good solution.

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