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An ILP-Based Automatic Bus Planner for Efficient PCB Routing

This work presents an innovative ILP-based automatic bus planner designed for dense PCBs, addressing the challenges of high pin density in electronic circuit design. The methodology involves bus decomposition, escape routing, global routing, and layer assignment. By generating candidate routes and resolving conflicts, the planner significantly reduces the manual routing time from months to hours, achieving a successful routing rate of 97.4% in just 2.5 hours. This advancement enhances productivity in PCB design and meets the growing need for automated routing solutions.

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An ILP-Based Automatic Bus Planner for Efficient PCB Routing

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  1. An ILP-based Automatic Bus Planner for Dense PCBs P. C. Wu, Q. Ma and M. D. F. Wong Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign ASPDAC 2013

  2. Outline • Introduction • Problem Formulation • Methodology • Candidate routes generation • Layer assignment • Resolving congestions • Postprocessing • Experimental Results • Conclusions

  3. Introduction • Nowadays, a dense PCB contains thousands of pins. • High pin density makes manual design of PCBs an extremely time consuming task. • An auto-router for PCBs would improve design productivity tremendously since each board takes about 2 months to route manually. • Design automation of PCB routing becomes a necessity.

  4. Introduction • Bus planning: simultaneously solve the bus decomposition, escape routing, layer assignment and global bus routing.

  5. Introduction • If the escape directions of the buses are predetermined and bus 3 is decided to escape to the left boundary, the set of buses can no longer be routed on one layer.

  6. Problem Formulation • The route of a bus is composed of the escape route part and the global route part. • If the escape routes of two buses conflict, we call it internal conflict. • If the global routes of two buses conflict, we call it external conflict. • The buses conflicting with each other have to be assigned to different layers.

  7. Problem Formulation • Input: • A number of components • A set of buses • The number of available routing layers • Objective: • Decide the bus escape routes and decomposition (if necessary) within the components • The global routes outside the components • The layer assignment of the topological routes of buses

  8. Methodology • Candidate route generation • Escape routing and bus decomposition • Global routing • Layer assignment • ILP formulation • Resolving congestion • Postprocessing

  9. Methodology

  10. Candidate Routes Generation • Generate a collection of escape routes for all buses. • A global router is applied to generate the global routes. • With a number of options for escape routes and global routes, a bus may have more than one candidate route.

  11. Candidate Routes Generation • Escape Routing and Bus Decomposition • The boundary that a pin cluster escapes to is called escaping boundary. • A bus has two pin clusters, it has 4X4=16 combinations of escaping boundary.

  12. Candidate Routes Generation • Escape Routing and Bus Decomposition • If a bus fails to be escaped, then the bus is unable to be escaped on one layer and thus has to be decomposed into two or smaller buses. • Decompose a bus by iteratively applying the escape router until all pins are escaped. • For each bus, all of its candidate escape routes can be obtained by enumerating all the combinations of the escaping boundaries and rectilinear regions.

  13. Candidate Routes Generation • Global Routing

  14. Candidate Routes Generation • Global Routing

  15. Candidate Routes Generation • Global Routing

  16. Layer Assignment • ILP Formulation • Input: • A set of buses {b1, …, bm} • A set of candidate routes {c1, …, cn} • p routing layers • Output: • A conflict free layer assignment

  17. Layer Assignment • Objective: • nci: the number of the nets of a candidate route ci • xil: candidate route ci assigned to layer l

  18. Layer Assignment • Candidate Constraints: • Conflict Constraints:

  19. Layer Assignment • Bus Decomposition Constraints: • Suppose b1 contains two bus decompositions, d1 and d2, where d1={b2, b3, b4} and d2 ={b5, b6}. • Only one bus decomposition can be selected by the ILP.

  20. Layer Assignment • Algorithm

  21. Resolving Congestions • Critical Cuts • The line segments connecting each component corner to all its visible component corners or board boundaries.

  22. Resolving Congestions

  23. Postprocessing • Improve the results by trying to reassign the unassigned buses to the routing layers. • Collect the set of all unassigned buses, try to reassign them to the first layer. • An ILP is formulated and solved. • Try to put the new set of unassigned buses to the second layer. • Such procedure is performed iteratively until no further improvement can be gained.

  24. Experimental Results

  25. Conclusions • This paper presented an automatic bus planner, including bus decomposition, escape routing, global routing and layer assignment. • The proposed planner is able to successfully route 97.4% of the nets within 2.5 hours.

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