CSE 494: Electronic Design Automation

1. CSE 494: Electronic Design Automation Lecture Routing

2. Routing placement global Routing • Generates a “loose” route for each net. • Assigns a list of routing regions to each net without specifying the actual layers of wires. • Finds the actual geometric layout of each net within the assigned routing regions. Global Routing detailed Routing compaction Detailed Routing

3. Routing Constraints • 100% routing completion + area minimization, under a set of constraints: • Placement constraint : usually based on fixed placement • Number of routing layers • Geometrical constraints: must satisfy design rules • Timing constraints (performance-driven routing): must satisfy delay constraints • Crosstalk? Two-layer routing Geometrical constraint

4. graph search global steiner tree hierarchical general purpose iterative Maze detailed channel Line search switchbox restricted Left-edge over-the-cell greedy river hierarchical clock-tree specialized power & ground Classification of Routing R O U T E R S

5. Maze Router : Lee Algorithm • Lee, “An algorithm for path connection and its application”, IRE Trans. Electronic Computer, EC-10, 1961 • Discussion mainly on single-layer routing • Strengths • Guarantee to find connection between 2 terminals if it exists • Guarantee minimum path • Weakness • Requires large memory for dense layout • Slow • Applications: global routing, detailed routing

6. Lee Algorithm • Find a path from S to T by “wave propagation” Filing Retrace • Time & space complexity for an M X N grid: O(M N) (huge!)

7. Reducing Memory Requirement • Akers’s Observation (1967) • Adjacent labels for K are either k - 1 or k + 1 • Want a labeling scheme such that each label has its preceding label different from its succeeding label • Way 1: coding sequence 1,2,3,1,2,3,…; states : 1,2,3, empty, blocked (3 bits required) • Way 2: coding sequence 1,1,2,2,1,1,2,2…; states : 1,2, empty, blocked (only 2 bits) sequence 1,2,3,1,2,3,… sequence1,1,2,2,1,1,2,2…

8. Reducing Running Time Starting point selection: Choose the point farthest from the center as the starting point. Double fan out: Propagate waves from both fan out and target cells. Framing: Search inside a rectangle area 10-20% larger than the bounding box containing the source and target. --Need to enlarge the rectangle and redo if the search fails

9. Connecting Multiple Terminal Nets Step 1: Propagate wave from the source s closest target Step 2: Mark ALL cells on the path as s Step 3: Propagate wave from ALL s cells to other cells Step 4: Continue until all cells are reached Step 5: Apply heuristics to further reduce the tree cost.

10. Routing on a Weighted Grid Motivation: Finding more desirable paths Weight(grid,cells) = # unblocked grid cell segments -1

11. A Routing Example on a Weighted Grid Initialize cell weights Wave propagation First wave reaches target Finding other paths min-cost path found

12. Hadlock’s Algorithm • Hadlock, “A shortest path algorithm for grid graphs”, Networks, 1977. • Uses detour number (instead of labeling wave front in Lee’s router. • Detour number, d(P) = # of grid cells directed away from its target on path P • M D(S,T) fixed!  Minimize d(P) to find the shortest path. • Path length of P, I(P): = M D(S,T) + 2d(P) • For any cell labeled i, label its adjacent unblocked cells away from Ti+1, label i otherwise. • Time and space complexities: O(MN), but substantially reduces the # of searched cells. • Find the shortest path between S and T.

13. Hadlock’s Algorithm • D(P): # of grid cells directed away from its target on Path P. • MD(S,T): the Manhattan distance between S and T. • Path length of P, I(P): = M D(S,T) + 2d(P) • M D(S,T) fixed!  Minimize d(P) to find the shortest path. • For any cell labeled i, label its adjacent unblocked cells away from Ti+1, label i otherwise.

14. Soukup’s Algorithm • Scoukup, “Fast maze router,” DAC-78 • Combined breadth-first and depth-first search • Depth-first (line) search is first directed toward target T until an obstacle or T is reached • Breadth-first (Lee-type) search is used to “bubble” around an obstacle if an obstacle is reached • Time and space complexities: O(MN), but 10-50 times faster than Lee’s algorithm • Find A path between S and T, but may not be the shortest!

15. Works on line segment Features of Line-Search Algorithms Lines generated by “escape” points source target generate lines generate lines Every point is An escape point One escape point Per line segment Set A Set B Some line From B Some line From A intersect Mikami-Tabuchi Hightower Path is found • Time and space complexities: O(L), where L is the # of line segments generated

16. Mikami- Tabuchi’s Algorithm • Mikami & Tabuchi, “A computer program for optimal routing of printed circuit connectors.” IFIP, H47, 1968 • Every grid point is an escape

17. Hightower’s Algorithm • Hightower, “A solution to line-routing problem on the continuous plane,” DAC-69 • A single escape point on each line segment • If a line parallels to the blocked cells, the escape point is placed just past the endpoint of the segment

19. Comparison of Algorithms • Soukup, Mikami, and Hightower all adopt some sort of line-search operations => cannot guarantee shortest paths

20. Multilayered Routing • 3-D grid • Two Planer arrays • Neglect the weight of inter-layer connections through via. • Pins are accessible from both layers.

21. Multilayered Routing

22. Net Ordering • Net ordering greatly affects routing solutions • In the example, we should route net ‘b’ before net ‘a’

23. Net Ordering

24. Net Ordering • Order the nets in the ascending order of the number of pins within their bounding boxes • Order the nets in their ascending (or descending ?) order of their lengths • Order the nets based on their timing criticality

25. Net Ordering • A mutually intervening case

26. Rip-up and Re routing • Rip up and re routing is required if a global or a local router fails to route all nets • Approach: Manual or Automated • Two steps in rip-up and re-routing • Identify bottleneck regions, rip-off some already routed nets • Route the blocked connections, then re-route the ripped connections • Repeat the two steps until all connections are routed, or a time limit is exceeded

27. Graph Models for Global Routing: Grid Graph • Each cell is denoted by a vertex • Two vertices are joined by an edge if they are adjacent to each other • The occupied cells are represented as filled circles, while the unoccupied cells are represented by empty circles

28. Graph Model: Channel Intersection Graph • Channels are represented as edges • Channel intersections are represented as vertices • Edge weight denotes channel capacity • Extended channel intersection graph: terminals are also represented as vertices

29. Global Routing Problem • Given a netlist N={N1, N2,….,Nn } , a routing graph G = (V, E), find a Steiner tree Tifor each net Ni, 1 i  n, such that U(ej)  c(ej), ej  E and ni=1 L(Ti) is minimized, where • c(ej): capacity of edge ej; • xij = 1 if ej is in Ti ; xij = 0 otherwise; • U(ej) = ni=1 xij : # of wires that pass through the channel corresponding to the ej • L(Ti) = total length of Steiner tree Ti • For high-performance, the maximum wirelength (maxni=1 L(Ti) ) is minimized(or the longest path between two points in Ti is minimized)

30. Global Routing in different Design Styles global routing full custom standard cell gate array FPGA flexible channels most general problem flexible channels fixed feedthroughs fixed channels fixed routing tracks switchbox constraints

31. Global Routing in Standard Cell • Objective • Minimize total channel height • Assignment of feedthrough: Placement ? Global Routing? • For high performance, • Minimize the maximum wire length • Minimize the maximum path length feedthrough failed net

32. Global Routing in Gate Array • Objective • Guarantee 100% routability. • For high performance • Minimize the maximum wire length • Minimize the maximum path length Each Channel has a capacity of 2 tracks

33. Global Routing in FPGA • Objective • Guarantee 100% routability. • Consider switch-mobile architectural constraints. • For performance-driven routing, • Minimize # of switches used • Minimize the maximum wire length • Minimize the maximum path length Each Channel has a capacity of 2 tracks

34. Classification of Global-Routing Algorithm • Sequential approach: Assigns priority to nets; routes one net at a time based on its priority (net ordering?). • Concurrent approach: All nets are considered at the same time (complexity?).

35. Global Routing: Maze Routing • Routing channels can be modeled by a weighted undirected graph called channel connectivity graph • Channel = node, edge = connection between channels, capacity = weight

36. Global Routing by Integer Programming • Suppose that for each net i, there are n(i) possible trees t(i,1) , t(i,2) … t (i,n) • Constraint 1: For each net i, only one t(i,j) will be selected • The capacity of each cell boundary c(i) is not exceeded • Minimize the total tree cost • Feasible for practical problem sizes? • Key: Hierarchical approach

37. An Integer Programming Example • gi,j Cost of tree tij => g1,1 = 2, g1,2 = 3, g1,3 = 3, g2,1 = 2, g2,2 = 3, g2,3 = 3, g3,1 = 2, g3,2 = 2. Minimize 2x1,1 + 3x1,2 + 3x1,3 + 2x2,1 + 3x2,2 + 3x2,3 + 2x3,1 + 2x3,1 Subject to x1,1 + x1,2 +x1,3 = 1 (Constraint I : t1) x2,1 + x2,2 +x2,3 = 1 (Constraint I : t2) x3,2 +x3,2 = 1 (Constraint I : t3) x1,2 + x1,3 + x2,1 + x2,3 +x3,1 =< 2 (Constraint II : B1) x1,1 + x1,3 + x2,2 + x2,3 +x3,1 =< 2 (Constraint II : B2) x1,2 + x1,3 + x2,1 + x2,2 +x3,2 =< 2 (Constraint II : B3) x1,1 + x1,2 + x2,2 + x2,3 +x3,2 =< 2 (Constraint II : B4) xi,j = 0,1,1 =< i, j <= 3

38. Hierarchical Global Routing • Marek-Sadowska, “Router planner for custom chip design,” ICCAD, ‘86. • At each level of the hierarchy, an attempt is made to minimize the cost of nets crossing cut lines. • At the lowest level of the hierarchy, the layout surface is divided into R x R grid regions with boundary capacity equal to C tracks. • Let Rl be the # of grid regions of a given cut line l; a cut line can be divided into M = Rl/C sections. • Global routing can be formulated as a linear assignment problem: - xi,j= 1 if net i is assigned to section j; xi,j= 0 otherwise - Each net crosses the cut line exactly once: Σmj=1 xjj= 1,1 <= i <= N. - Capacity constraint of each section: Σni=1 xij<= C,1 <= i <= M. - wi,j : cost of assignment net i to section j. Minimize Σni=1 Σmj=1 wij xij

39. The Routing-Tree Problem • Problem: Given a set of net, interconnect the pins by a “routing tree.” • Minimum Rectilinear Steiner Tree (MRST) Problem: Given n points in the plane, find a minimum-length tree of rectilinear edges which connects the points. • MRST(P) = MST(P U S), where P and S are the sets of original points and Steiner points, respectively

40. Theoretic Results for MRST Problem • Hanan’s Thm: There exists an MRST with all Steiner points (set S) chosen from the intersection points of horizontal and vertical lines drawn points of P. - Hanan, “On Steiner’s problem with rectilinear distance,” SIAM J. Applied Math., 1966. • Hwang’s Thm: For any point set P,cost(MST(P))/Cost(MRST(P))<= 3/2. - Hwang, “On Steiner’s minimal tree with rectilinear distance,” SIAM J. Applied Math., 1976. • Best existing approximation algorithm: Performs bound 61/48 by Fossmeier et al. - Fossmeier et al, “Fast approximation algorithm for rectilinear Steiner problem,” Wilhelm Scickard-Institut fűr informatik, TR WSI-93-14, 93. - Zelikovsky, “An 11/6 approximation algorithm for the network Steiner problem,” Algorithmica., 1993.

41. Channel and Switchbox Routing

42. 1 3 1 4 3 2 1 2 2 (a) (b) (c) Order of Routing Regions and N-Channels • No conflicts in case of routing in the order of 1, 2, and 3 • No ordering is possible to avoid conflicts • The saturation of (b) can be resolved by using L-channels • An L-channel can be decomposed into a channel and a switchbox Switchbox 1 2 (d)

43. Routing Considerations • Number of terminals (two-terminals vs. multi-terminal nets) • Net widths (power and ground vs. signal nets) • Via restrictions (stacked vs. conventional vias) • Boundary types (regular vs. irregular) • Number of layers (two vs. three, more layers?) • Net types (critical vs. non-critical nets)

44. Routing Models • Grid-based model: • A grid is super-imposed on the routing region. • Wires follow paths along the grid lines. • Gridless model: • Any model that does not follow the gridded approach gridless grid-based

45. Models for Multi-Layer Routing • Unreserved layer model: Any net segment is allowed to be placed in any layer. • Reserved layer model: Certain types of segments are restricted to particular layer(s). • Two-layer: HV (horizontal-Vertical), VH • Three-layer: HVH, VHV 3 types of 3-layer models

46. Terminology for Channel Routing Problems • Local density at column i : total # of nets that crosses column i • Channel density: maximum local density; # of horizontal tracks required >= channel density

47. Channel RoutingProblem • Assignments of horizontal segments of nets to tracks. • Assignments of vertical segments to connect. • horizontal segments of same nets in different tracks, and • the terminals of the net to horizontal segments of the net. • Horizontal and vertical constraints must not be violated. • Horizontal constraint between two nets: The horizontal span of the two nets overlap each other. • Vertical constraint between two nets: There exists a column such that a terminal on top of the column belongs to one net and the terminal at the bottom of the column belongs to the other net. • Objective: Channel height is minimized ( i.e. channel area is minimized)