VLSI System Design Methodologies

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VLSI System Design Methodologies

• Md. Shabiul Islam
• Lecture No H
• MULTIMEDIA UNIVERSITY

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Topics

• Block placement.
• Global routing.
• Switchbox routing.

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Floorplanning strategies

• Floorplanning is chip-level layout design. When
  designing a leaf cell, we used transistors and
  vias as our basic components floorplanning uses
  the adders, registers, and FSMs as the building
  blocks.
• The fundamental difference between floorplanning
  and leaf-cell design is that fllorplanning works
  with components that are much larger than the
  wires connecting them.

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• Floorplanning must take into account blocks of
  varying function, size, shape.
• We call the layout cells blocks during
  floorplanning because we use them like building
  blocks to construct the floorplan.In bricks-and
  mortal style layout, the cells may have radically
  different sizes and shapes.

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• The layout program must place the component on
  the chip by position and orientation, leaving
  sufficient space between the componnets for the
  necessary wires.
• Must design
• space allocation
• signal routing
• power supply routing
• clock distribution.

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Bricks-and-mortar floorplan
RAM
blocks
std cell
data path
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Purposes of floorplanning

• Early in design
• Prepare a floorplan to budget area, wire
  area/delay. Tradeoffs between blocks can be
  negotiated.
• Late in design
• Make sure the pieces fit together as planned.
• Implement the global layout.

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Types of routing

• Channel routing
• channel may grow in one dimension to accommodate
  wires
• pins generally on only two sides.
• Switchbox routing A switchbox is a routing area
  with connections anywhere along its four sides,
  which means that it cannot grow in either
  direction.

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• Switching boxes are useful in connecting abutting
  channels. To take wiring into account during
  placement, we must define routing channels and
  switchboxes, then assign nets to paths through
  those channels/switchboxes.
• cannot grow in any dimension
• pins are on all four sides, fixing dimensions of
  the box.

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Channels and switchboxes
channel
switchbox
switchbox pins
channel
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Block placement

• A first cut at fllorplanning is to ignore wiring
  and arrange the blocks to minimize wasted space.
  A good way to manually experiment with floorplans
  is to draw the blocks on graph, cut them out, and
  arrange them on another block of graph paper.
• A block is characterized by its area and its
  aspect ratio.

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• Blocks may be placed at different rotations and
  reflections.
• Uniform size blocks are easier to interchange.
  Interchangeability makes wiring optimization
  easier.
• Experimentation with floorplans may suggest that
  a change to the shape or size of a block would
  make the floorplanner s job easier.

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Blocks and wiring

• Cannot ignore wiring during block placementlarge
  wiring areas may force rearrangement of blocks.
• Wiring plan must consider area and delay of
  critical signals.
• Blocks divide wiring area into routing channels.

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Channel definition

• The first job is to define the routing channels
  and switchboxes. We want to break up the space
  between the blocks into rectangular regions for
  simplicity during detailed routing this step is
  known as channel definition.
• Channels end at block boundaries.
• Several alternate channel definitions are
  possible

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Channel definition changes with block spacing

• Channel definition has no single solution for two
  different reasons. First, by moving the lighting
  to change the shadows cast, we can change the
  channel definition. There is no way to choose the
  optimum division of the chip into touting
  regions, through it is probably best to use
  fewer, larger channels than to break the chip
  into many small regions.

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• Second, we can change the way shadows are cast by
  changing the distance between the blocks, as
  shown in next Fig.
• Changing spacing changes relationship between
  block edges

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Channel graph
C
A
B
D
E
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Channel graph usage

• The full geometric description of the channels
  and switchboxes is cumbersome for global
  routing-all we really need is the topology of the
  paths between blocks.
• The channel graph reduces the floorplan to a
  description of the routes between blocks in
  previous Fig.
• Nodes are channels, edges placed between two
  channels that touch.

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• Channel graph shows paths between channels.
• Channel graph can be used to guide global
  routing.

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Channels must be routed in order

• Channel ordering is a problem for placement and
  global routing because some channel graphs dont
  have any feasible routing order.
• As a result, there is no channel we can route
  first and guarantee that the complete structure
  can be successfully routed.
• Wire out of end of one channel creates pin on
  side of next channel

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Windmills

• The best solution to windmills is to avoid them
  completely. If the floorplan is a slicing
  structure in Fig. It has no windmills.
• A slicing structure can be recursively sliced
  down to its blocks- a slice is a straight cut
  through the routing region that separates the
  chip into two sections.
• Can create an unroutable combination of channels
  with circular constraints

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Windmills

• Each section forms a smaller floorplan that can
  be cut again.

B
A
D
C
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Slicable floorplan
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Slicability property

• A slicable floorplan can be recursively cut in
  two without cutting any blocks.
• A slicable floorplan is guaranteed to have no
  windmills, therefore guaranteed to have a
  feasible order of routing for the channels.
• Slicability is a desirable property for
  floorplans.

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Global routing

• Goal assign wires to paths through channels.
• Dont worry about exact routing of wires within
  channel.
• Can estimate channel height from global routing
  using congestion.

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Line probe routing

• A good algorithm for hand routing is the
  line-probe method introduced by Mikami and
  Tabuchi and by Hightower.
• Line-probe routing can work on arbitrary-shaped
  routing regions.
• Heuristic method for finding a short route.
• Does not explore all possible pathsnot optimal.

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Line probe example
line 1
A
A
line 2
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Channel utilization

• Want to keep all channels about equally full to
  minimize wasted area.
• Important to route time-critical signals first.
• Shortest path may not be best for global wiring.
• In general, may need to rip-up wires and reroute
  to improve the global routing.

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Switchbox routing

• Cant expand a switchbox to make room for more
  wiring.
• Switchbox may be defined by intersection of
  channels.

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Routing order and switchboxes

• Switchboxes frequently need more experimentation
  with wiring order because nets may block other
  nets

A
B
A
B
B blocks A