VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 1 What Makes a Design Difficult to Route Charles J. Alpert, Zhuo Li, Michael D. Moffitt, Gi-Joon Nam, Jarrod A. Roy, Gustavo Tellez Presented by Zhicheng Wei

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 2 What Makes a Design Difficult to Route  INTRODUCTION AND BACKGROUNDS  COMMON CONGESTION METRICS  GLOBAL ROUTING CONSTRAINTS  DETAILED ROUTING CONSTRAINTS  PLACEMENT TECHNIQUES  LOGIC SYNTHESIS TECHNIQUES  REPEATER INSERTION TECHNIQUES  CONCLUSION

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 3 What Makes a Design Difficult to Route  INTRODUCTION  Modern technology requires complex wire spacing rules and constraints  High performance routing requires multiple wire width (even same layer)  Local problems including via spacing rules, switchbox inefficiency, intra-gcell routing All of these problems make routing hard to model and lead to huge congestion issues!

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 4 What Makes a Design Difficult to Route  BACKGROUNDS  Routing problems should be considered in 3D instead of 2D  Meet congestion constraints during global routing  Try to satisfy capacity in detailed routing with a given global routing solution  Over-the-cell routing breaks traditional channel/switchbox model

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 5 What Makes a Design Difficult to Route  COMMON CONGESTION METRICS  Total Overflow  Average worst X% average worst 20% routing edges below 80% is routable  Total routed wirelength (RWL) significantly above Steiner tree may indicate routing difficulties  Number of scenic nets wirlelength/minimum Steiner tree length ratio > 1.3 is generally considered scenic  Number of nets over X% nets passing through gcells whose congestion is over X%  Number of violations  Routing runtimes

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 6 What Makes a Design Difficult to Route  COMMON CONGESTION METRICS  Total Overflow

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 7 What Makes a Design Difficult to Route GLOBAL ROUTING CONSTRAINTS  Choice of gcell size  gcell size too small large global routing space and takes more time to route  gcell size too large not able to expose congestion problems and shift burden to detail routing  Handling scenic nets  go very scenic = bad timing performance  impose scenic constrains on the router

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 8 What Makes a Design Difficult to Route DETAILED ROUTING CONSTRAINTS  Prediction failure in global routing hot sports predicted by global routing may not be open and shorts in detailed routing

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 9 What Makes a Design Difficult to Route DETAILED ROUTING CONSTRAINTS  Pin access problem certain configurations make accessing pin from higher metal layer impossible  Via modeling challenge Vias do not scale as well as device at each technology node Vias serve as routing blockages which impact local congestion Via modeling becomes non-trivial, esp with different metal pitches

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 10 What Makes a Design Difficult to Route PLACEMENT TECHNIQUES Congestion caused by time-driven placement

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 11 What Makes a Design Difficult to Route PLACEMENT TECHNIQUES Modern placement focus on minimization of HPWL Uniform placement does not always work! Uniform placement does not mean uniform wire spreading! Consider congestion-driven placement

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 12 What Makes a Design Difficult to Route PLACEMENT TECHNIQUES  Congestion Reduction by Iterated Spreading Placement (CRISP)  Selectively spreading the placement in regions with high global congestion

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 13 What Makes a Design Difficult to Route LOGIC SYNTHESIS TECHNIQUES  Logic synthesis generally ignores placement information  Create structures good for timing closure but bad for routing  Logic synthesis transforms to alleviate local congestions identify logic fan-in tree which is physically wirelength inefficient rebuild logic tree and place new synthesized gates wirelength is minimized and congestion alleviated

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 14 What Makes a Design Difficult to Route REPEATER INSERTION TECHNIQUES  Repeaters are inserted to meet timing constraints Divide long wires into small segments  Layer assignment Obtain enormous speed advantage using thick metal for most critical paths  Routing congestions caused Corona effect (congestion around corner of blockages) Aggressive layer promotion (fewer resources at higher metal layer)

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig What Makes a Design Difficult to Route 15 Aggressive Layer Promotion

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig What Makes a Design Difficult to Route 16 Corona Effect

VLSI Physical Design: From Graph Partitioning to Timing Closure Chapter 6: Detailed Routing © KLMH Lienig 17 What Makes a Design Difficult to Route CONCLUSION  Physical synthesis issues in placement, global/detail routing, logic synthesis  Advanced technologies require more complicated modeling plan  Capture more detailed routing effects in global routing stage  Estimation techniques need to be fast to optimize routing fast