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Floorplanning and Routing Strategies in Block Placement

Learn about the importance of floorplanning and routing in block placement, global routing, and switchbox routing. Understand the tradeoffs, block placement considerations, and the role of wiring in floorplanning. Explore different routing methods and the impact of switchboxes on routing order.

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Floorplanning and Routing Strategies in Block Placement

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Presentation Transcript

  1. Topics • Block placement. • Global routing. • Switchbox routing.

  2. Floorplanning strategies • Floorplanning must take into account blocks of varying function, size, shape. • Must design: • space allocation; • signal routing; • power supply routing; • clock distribution.

  3. Bricks-and-mortar floorplan blocks

  4. 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.

  5. Types of routing • Channel routing: • channel may grow in one dimension to accommodate wires; • pins generally on only two sides. • Switchbox routing: • cannot grow in any dimension; • pins are on all four sides, fixing dimensions of the box.

  6. Channels and switchboxes

  7. Block placement • Blocks have: • area; • aspect ratio. • Blocks may be placed at different rotations and reflections. • Uniform size blocks are easier to interchange.

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

  9. Channel definition • Channels end at block boundaries. • Several alternate channel definitions are possible:

  10. Channel definition changes with block spacing Changing spacing changes relationship between block edges:

  11. Channel graph

  12. Channel graph usage • Nodes are channels, edges placed between two channels that touch. • Channel graph shows paths between channels. • Channel graph can be used to guide global routing.

  13. Channels must be routed in order Wire out of end of one channel creates pin on side of next channel: channel B channel A constraint

  14. Windmills Can create an unroutable combination of channels with circular constraints:

  15. Slicable floorplan

  16. 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.

  17. Global routing • Goal: assign wires to paths through channels. • Don’t worry about exact routing of wires within channel. • Can estimate channel height from global routing using congestion.

  18. Line probe routing • Heuristic method for finding a short route. • Works with arbitrary combination of obstacles. • Does not explore all possible paths - not optimal.

  19. Line probe example line 1 A A line 2

  20. 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.

  21. Switchbox routing • Can’t expand a switchbox to make room for more wiring. • Switchbox may be defined by intersection of channels.

  22. Routing order and switchboxes Switchboxes frequently need more experimentation with wiring order because nets may block other nets:

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