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Placement and routing in full custom physical design
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Placement and routing in full custom physical design

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  • 1.Metal routes must meet minimum width and spacing “design rules” to prevent open and short circuits during fabrication. 2. Congestion can be reduced by adding blockages during floor planning. When a blockage is placed the router, routes around the blockage thereby reducing congestion.
  • Detour – Routing takes a longer route instead of a shorter one. In GR no PHYSICAL connections are made only nets are assigned to specific metal layers.
  • If TA can reduce the number of jogs and jumps in metal traces, this will generally improve timing (since each jump generally requires a via to jump to a higher or lower level metal layer). Reducing the number of vias is generally a plus for reliability and yield since their failure rate is slightly higher than that of a simple, straight metal track in a modern, planarized process.
  • The detail route doesn’t work on the entire chip at a time but instead works, box by box (using a fixed size box called Sbox) until the routing pass is complete.
  • Search and Repair divides the chip into SBoxes and works through each SBox sequentially trying to fix DRC violations by rerouting within the confines of the box. Droute – Detail Route…Sbox – Square Box.

Transcript

  • 1. Seminar Placement and Routing options in Full Custom Shankardas Deepti Bharath CGB0911002 VSD528 M. Sc. [Engg.] in VLSI System Design Module Title: Full Custom Physical Design Module Leader: Mr. Chandramohan P.
  • 2. Outline
    • Introduction
    • Full-Custom design methodology
    • Why Is Placement and Routing important?
    • Types of placement techniques in full custom design
    • Routing
    • Routing options
    • Summary
  • 3. Introduction Floor planning CTS Physical Design Partitioning Routing Placement Specification Architectural design Circuit design Physical design Test/Fabrication Logic design
  • 4. Full-Custom Design Methodology
    • Design of a chip from scratch
    • Engineers design some or all of the logic cells, circuits, and the chip layout specifically for a full-custom IC
    • Custom mask layers are created in order to fabricate a full-custom IC
    • Advantages: Complete flexibility, high degree of optimization in performance and area
    • Disadvantages: Large amount of design effort, cost escalation, time to market
  • 5. Why Is Placement and Routing Important?
    • The first phase in the VLSI design that determines the physical layout of a chip.
    • P&R is the first phase in VLSI design that determines the physical layout of a chip
    • Circuit Placement becomes very critical in today’s high performance VLSI design.
    • The quality of the attainable routing is highly determined by the placement.
    • The circuit delay, power dissipation and area are dominated by the interconnections made during routing.
  • 6.
    • For digital custom design
    • Diffusion sharing
    • Gate matrix
    • Transparent latch
    • Folding transistor
    • Tapering
    • For analog custom design
    • Fingering
    • Interdigitization
    • Common centroid
    • Folding technique
    • For mixed signal custom design
    • Shielding power supply and grounding.
    Types of placement techniques in full custom design
  • 7. (a) Diffusion sharing (b) Gate matrix layout (c) Tapering technique (f) Common centriod (e) Interdigitization (d) Fingering Analog custom design Mixed custom design Digital custom design Placement in Custom Design Figure 1. Placement techniques in custom design
  • 8. After Placement Macros Standard Cells IO Pads Corner Cells VDD rails VSS rails Power & ground straps Figure 2 Chip level placement
  • 9. Routing
    • Routing creates physical connections to a clock and signal pins through metal interconnects
      • Routed paths must meet setup and hold timing and clock skew requirements
      • Metal traces must meet physical DRC requirements
      • Clock nets are routed first followed by signal nets as it’s easier to meet skew and insertion delay targets.
    • Due to the advent of deep sub-micron technology
    • – Interconnect delay constitutes a significant part of the total net delay.
    • – Reduction in feature sizes has resulted in increased wire resistance.
    • – Increased proximity between the devices and interconnections results in increased cross-talk noise.
    • Routers should model the cross-talk noise between adjacent nets.
    • For routing high-performance circuits, techniques adopted:
    • – Buffer insertion – Wire sizing – High-performance topology generation
  • 10.
    • Global Route (GR)
    • GR assigns nets to specific metal layers and global routing
    • cells and avoids congested Gcells while minimizing detours.
    • It gives more accurate parasitic and delay estimates compared to
    • virtual route
    Global Route Track Assign Detail Route Search & Repair Figure 3 Global Routing Routing options
  • 11.
    • Track Assignment (TA)
    • Assigns each net to a specific track and lays down the actual metal
    • traces
    • It also attempts to: Make long, straight traces and Reduce the
    • number of vias.
    • TA does not check or follow physical DRC rules
    Global Route Track Assign Detail Route Search & Repair Figure 4 Track Assignment Routing options
  • 12.
    • Detail Routing
    • Detail route attempts to clear DRC violations using a fixed size Sbox.
    • Due to the fixed Sbox size, detail route may not be able to clear all
    • DRC violations.
    Global Route Track Assign Detail Route Search & Repair Figure 5 Detail Routing Routing options
  • 13.
    • Search & Repair (S&R)
    • Search & repair fixes remaining DRC violations through multiple loops using progressively larger SBox sizes left over from detail route.
    • Remaining DRC violations are addressed by another pass using a
    • larger size SBox. The larger box potentially gives more routing
    • resources to clear violations.
    Global Route Track Assign Detail Route Search & Repair Routing options Figure 6 Search and Repair
  • 14. After Routing Figure 7 Block level routing Figure 8 Magnified portion of the block
  • 15. Summary
    • Macro and Pad cell locations are defined during the Floorplanning stage, before P&R.
    • Standard cells are placed in “placement rows” during placement.
    • Placement rows are commonly abutted to reduce core area. Cell orientations in abutted rows are normally flipped.
    • Cells in a timing-critical path are placed close together to reduce routing-related delays.
    • The circuit delay, power dissipation and area are the main objective of placement.
    • Rout-ability (or congestion) is a function of the number of available tracks in a given area
    • compared to the number of signals that need to be routed through that area.
    • Routing along the timing-critical path is given priority: Creates shorter, faster connections therby reducing parasitics.
  • 16. References [1] Jon Wateresian (2002) Fabricating Printed Circuit Boards. Massachusetts: Newnes [2] Linfu Xiao, et al. , ‘ Practical Placement and Routing Techniques for Analog Circuit Designs’ , IEEE, Dept. of CSE, Chinese Univ. of Hong Kong, Shatin, China, Dec 2010. [3] Chandramohan P., Digital circuit design and layout, Full custom physical design (VSD 528), session-2 MSRSAS, Bangalore [4] Shawki Areibi and Zhen Yang (2003), ‘Congestion Driven Placement for VLSI Standard Cell, Design’ , School of Engineering, University of Guelph, Ontario, Canada, Dec 2003.
  • 17. Thank You