Local Routing
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Transcript of 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