1. Directional and Single-Driver Wires in FPGA Interconnect Guy Lemieux Edmund LeeMarvin TomAnthony Yu Dept. of ECE, University of British Columbia Vancouver, BC, Canada

2. Outline Motivation Bidirection vs. Directional –New detailed routing architecture –Which is better? Tristate vs. Single-Driver –HSPICE results –Which is better? Place & Route Results Conclusions

3. Motivation: Bidirectional Wires Logic Interconnect

4. Motivation: Bidirectional Wires Problem Half of Tristate Buffers Left Unused Buffers Dominate Size of Device

5. Bidirectional vs. Directional Wiring

6. Bidirectional vs Directional

10. 2-Dimensional FPGA aka Island-Style or Mesh

11. Bidirectional Switch Block

12. Directional Switch Block

13. Bidirectional vs Directional Switch Element: Same Quantity and Type of Circuit Elements (twice the wiring) Switch Block: Directional has Half as Many Switch Elements

14. Building up Long Wires Start with One Switch Element Wire ends for straight connections.

15. Building up Long Wires Connect MUX Inputs Extend MUX inputs

16. Building up Long Wires Connect MUX Inputs TURN UP from wire-ends to mux

17. Building up Long Wires Connect MUX Inputs TURN DOWN from wire-ends to mux

18. Building up Long Wires Add +2 More Wires (4 total) Add LONG WIRES, turning UP and DOWN.

19. Building up Long Wires Add +2 More Wires (6 total) Add LONG WIRES, turning UP and DOWN

20. Building up Long Wires Single Layout Tile !!! Add wire twisting

21. Long Wires! 123 NOTICE: One switch element holds 6 wires #Wires := WireLength x NumDirections = 3 x 2 = 6 No “partial” switch elements with fewer wires

22. Small Switch Block One L3 Switch Element

23. Bigger Switch Block Two L3 Switch Elements NOTICE Switch element design forces quantization of channel width Bidirectional One quantum = 1*L Directional One quantum = 2*L

24. Summary Directional wiring –Good Potential area savings –Bad Big input muxes, slower Bigger quantum size (2*L) Detailed-routing architecture is different (need new switch block) –Need to evaluate!

25. Tristate vs. Single-driver Wiring

26. Bidirectional Wiring Outputs are Tristates Multi-driver Wiring!!! Bidir Architecture Fanout increases delay

27. Directional Wiring Outputs can be Tristates Dir-Tri Architecture Multi-driver Wiring!!! Fanout increases delay

28. Directional Wiring Outputs can use switch block muxes Dir Architecture Single-driver Wiring!!! New connectivity constraint

29. Tristate HSPICE Model

31. Single-driver HSPICE Model

32. HSPICE Delays Includes Switch + Wire TSMC 0.18um

33. AREA * HSPICE Delay TSMC 0.18um

34. Summary Single-driver wiring –Good Same delay as tristate No delay increases caused by fanout Fewer wire loads: 27% lower capacitance –Bad Directional only (by necessity) Area-delay product “seems” worse, but isn’t

35. Place and Route Results

36. Channel Width

37. Area (Transistor Count)

38. Delay

39. Area-Delay Product

40. Results Summary Average improvements using single-driver wiring 0% channel width 9% delay 14% tile length of physical layout 25% transistor count 32% area-delay product 37% wiring capacitance

41. Conclusions No more tristates! –Eliminates need for pass transistors –No “Vt” loss signal degradation –Better signal reliability, better drive strength Significant savings in all metrics –Any reasons left to use bidirectional wiring ??? Savings INCREASES with circuit size –Because interconnect dominates big circuits