Local Routing

8/8/2019 Local Routing

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LOCAL ROUTING


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Local routing is usually done using maze-

search algorithm or line-search algorithm.

In channel routing, a no. between 0 and N can

be assigned to each terminal, unused terminalsare numbered 0.

Terminals with same no. are interconnected.


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Minimum two levels of connect are assumed

in routing.

In single-metal technology, one layer ispolysilicon or polysilicide and other layer is

metal.

A set of routes is obtained using Manhattangeometry

Manhattan geometry constraints:-

Different nets should not intersect on samelayer.

Critical routing should be of minimal length.

Channel width should be reduced.


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Manhattan geometry assumes that vertical and

horizontal tracks are on different metal layers.

In a system of N nodes, max no. of possible

interconnections is N(N-1)/2, for no. of

combinations of N items taken two at a time.

If N is large, max possible no. of interconnects

increases as square of the no. of nodes.


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Lee-Moore Maze running algorithm:

Finds the shortest path between any two

points.

Divides the layout into a no. of grids of nodes.

Weighs each node by its distance from source

of the wire to be routed.

Counts the no. of squares from one end of bus

line to the other end.

The correct answer is the route that requires

less no. of squares.


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The wavefront fans out

in diamond shape from

the point of origin.

Fig.Maze running approach with

wavefront propagating from source S.


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Fig.Maze running approach Completed

routing

The shortest route

between S and T is of

length 11 squares.

Requires excessive

amount of storage for

large designs whichmay contain millions

of grid points.

Wavefront extends in

all directions, socomputation is very

slow.


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Fig.Maze running approach with a

wavefront propagating from both the

source and the target.

Wavefronts fanning out

from S and T intersect

on the count of 5.

Computation time is

reduced.

Computation time is

further reduced by

assigning penalties tomoves away from the

target.


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Line-search algorithm:

Vertical and horizontal lines are drawn from

source S and target T.

Further, horizontal and vertical lines that

intersect the original lines are drawn.

This step is repeated until nodes S and T are

connected.


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There are two solutions:

S-G-L-T consisting ofthree line segments and

17 squares long.

S-J-H-K-T consisting of

four line segments and13 squares long.

So the shortest route is

S-J-H-K-T which is 13

squares long.Fig. Line search approach to connecttwo nodes.


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Refinements on Maze-routing:

To improve routing,

metal layer is increased

by replacing vertical

wiring in poly-silicon by vertical wiring in

metal.

Fig. Layouts before and after metallization


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Plowing:

When a module must be relocated or when

another line must be added, the

interconnections need to be maintained.

Plowing allows relocation while maintaining

the integrity of routed nets.


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Plowing : (a)Before (b)After relocation of a module


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Using programmable functional blocks:

Most layouts have fixed terminals and cannot

be easily interchanged. So routing may result

in a large no. of horizontal tracks.

PLAs, PROMs, PALs and other programmable

functional blocks have interchangeable

terminals.

This minimizes the no. of horizontal tracks

required, which in turn reduces the channel

width and also shortens the line lengths.


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Fig.M

odules with two rows of terminals (a) Unaligned , (b) Aligned

Local Routing

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