Boosters for Driving Long On-chip Interconnects : Design Issues, Interconnect Synthesis and Comparison with Repeaters

1. Boosters for Driving Long On-chip Interconnects: Design Issues, Interconnect Synthesis and Comparison with Repeaters Ankireddy Nalamalpu Intel Corporation/Hillsboro Wayne Burleson UMASS/Amherst Partially Funded by SRC under research ID 766

2. Motivation • Interconnect delay will dominate DSM • Limited performance by using traditional techniques (Repeaters) for driving on-chip interconnects • Repeaters are area and power hungry • This study aims to provide • New high-performance circuit technique (Booster) for driving interconnects • Over-all Booster design methodology to integrate into automatic interconnect synthesis tools • Study/comparison Boosters with Repeaters

3. Repeater Design • Classical delay optimal repeater solution when delay of repeaters equals interconnect delay [Bakoglu85] • Repeater design solutions model short-channel effects in DSM using Alpha Power MOSFET model [Friedman98b, Nalamalpu00b]

4. Repeater Design Limitations 60 6x106 50 5x106 # Repeaters 40 4x106 [Plot from Sylvester99] Power(W) 30 Repeaters + Wire # of Repeaters 3x106 20 2x106 10 Wires Only 1x106 0 0.25 0.2 0.15 0.1 0.05 Technology Generation(m) • Limited performance with Repeaters in DSM due to non-negligible interconnect resistance • Increasing Repeater Area and Power with technology scaling [Sylvester98, Sylvester99] • 700,000 repeaters in 70nm CMOS [Cong99] • Increased design problems with repeaters driving bi-directional and multi-source buses • Inverting Polarity

5. Review of Previous Work Driver Driver Interconnect Interconnect 2,4…Inverters Receiver Receiver • Regenerative Feed-back Repeaters for driving programmable interconnections[Dobbelaere95] • Extremely sensitive to Noise • Meta-stability • Two-sided Timing Constraints • Limit in performance gain

6. Review of Previous Work • Differential, Small-swing and other design techniques[Lima95,Friedman98a] • Requires more circuit design sophistication • Cumbersome for automatic interconnect synthesis tools • Require multiple Power Supply’s in some cases • We need simple and yet high-performance circuit technique that can be integrated into automatic interconnect synthesis tools

7. Proposed Design • Our proposed circuit (Booster) differs from the existing designs in one or more of the following • High Performance • Simpler and requires fewer transistors • Noise immunity • Eliminates Meta-stability • We formulate analytical design rules for Boosters to be part of automatic interconnect synthesis tool

8. fout Full Keeper Driver Driver Input rin Driver bp P=2.0 Interconnect Interconnect bout tn N=1.0 Receiver Receiver P=2.0 Skewed Inverters P=2.0 N=1.0 N=1.0 Feed-back Booster Circuit

9. Booster Simulations • RLC 5 T-Interconnect model in 0.16m CMOS • Feedback path • Improves the speed of driver • Prevents turning-off booster prematurely thereby eliminating two-sided timing constraints • Makes circuit glitch immune Input Firing Feed-back Path Inverter Outputs

10. Booster Design • Skewed inverters respond to opposite ends of voltage transition • Driving both the inverters to feed-back path improves noise immunity • Full keeper helps noise cause • Booster firing time depends on switching thresholds of inverters • Boosters attach to the wire rather than interrupting it so can be used for bi-directional signals • Boosters don’t impact the polarity of the signal

11. Booster Design Methodology • Analytically determine number of boosters and their placements for driving given interconnect load • Consider only delay optimality • Minimize power/area impact without losing significant speed-up

12. Booster Placement 350 Delay without Booster(ps) 300 250 200 150 Delay with Booster(ps) BR1 BR2 BR3 100 50 BR1, BR2, BR3 = Boosters 0.5 1.0 1.5 2.0 2.5 3.0 Booster Transient Variation(t/T) • Boosters no good for driving very short wires due to fast transients in small RC loads (how short?) • In-order to place a booster • Firing time(T)< Time Constant(t) • Booster Transient variation (t/T) > 2.5, to minimize total number of boosters

13. Booster Analytical Model Node(a) Node(b) bp tn Length= L2 Length = L3 Length = L1 • Using simple inverter model(which will suffice)

14. Booster Analytical Model • Using more accurate alpha-power law based inverter analytical model [Nalamalpu00b] • Alpha power MOSFET law [Sakurai90] models short-channel effects • Repeater model is within 5% error of SPICE

15. Rule for Number of Boosters Interconnect Delay Short-circuit Power Number of Boosters • When boosters(BR1, BR2, BR3) are initially off • L1,L2 and L3 will be different for identical segment delays due to characteristics of signal propagation along RC line • Unlike Repeaters, placing non-optimal number of boosters doesn’t impact performance as much as power

16. Rule for Number of Boosters L1 L2 L3 L4 BR1 BR2 BR3 BR4 BR1, BR2, BR3, BR4 = Boosters • To Minimize number of boosters • Any down stream booster (e.g.BR2) should be fired only after improved upstream signal transient (e.g. A, BR1 is active) propagates downstream (e.g.B) • L1<L2<L3<L4 for identical segment delays

17. Booster Placement Sensitivity Block A Block B Block C No Glue Logic Realistic floor-plans will have several placement constraints • Inter block routing • Repeater staggering to reduce inductive and capacitive coupling • To ensure the design is manageable (e.g. verifiable, reusable) • To maintain datapath’s regularity

18. Booster Placement Sensitivity • Repeaters are shown to be sensitive to placement variation[Nalamalpu00a] • Worst case placement scenario’s results in performance degradation by as much as 30% • Boosters relatively insensitive to placement variation due to its dependence on transient

19. Repeaters Boosters SPICE Simulations • We used delay optimal repeater design solution obtained by using alpha-power MOSFET model[Nalamalpu00b] • Booster design rules for finding number of boosters and their placements are used to minimize design cost without losing significant speed (<5%) • CMOS 0.16 m process is used for SPICE simulations • Interconnect load is represented using RLC 5 T-model

20. SPICE Simulations nbooster nrepeater Wbooster Wrepeater Dbooster Drepeater (m) (m) (ps) (ps)

21. Boosters Vs Repeaters • Boosters shown to out-perform Repeaters by 20% for all kinds of interconnect loads (both capacitive and resistive dominated) • Boosters interconnect driving distance is 3x that of Repeaters resulting in fewer Boosters • Significant reduction in Area over Repeaters (more than 100% depending on interconnect load) • Boosters are insensitive to placement variation • Boosters don’t impact the polarity of the signal

22. Booster Applications • Uni/bi-directional interconnects • Multi-source/sink buses • Programmable Interconnections in FPGA’s Booster Off Switches On Switches Booster Booster Booster

23. Booster Applications Long AND domino gates (e.g decoders) • Precharge from top of the stack and discharge is from bottom of the stack • Bi-directional signaling can be improved using boosters

24. Booster Limitations • Boosters don’t break lines however for buffering, modularity and signal integrity reasons it is desirable to break long lines • Boosters are not well understood by CAD tools and designers

25. Conclusions • We presented analytical design solutions, both hard optimization and softer realistic design problems • We propose to combine Boosters with Repeaters in some cases to handle both modularity and signal integrity issues • Boosters find application in long dynamic ANDs, and multi-source interconnects in addition to conventional point-to-point long lines

26. Future Work • Integration into interconnect synthesis tool with Repeaters • Impact on bi-directional multi-source lines which could directly impact VLIW, FPGA, Routers, multi-processor, memory and other highly connected architectures

27. Acknowledgements Sriram Srinivasan for insightful comments Prof. Arnold Rosenberg for the initial theoretical motivations in exploring booster circuits SRC for partially supporting under Research ID 766 UMASS has filed for several patents related to Booster technology

28. References [Dobbelaere95] Dobbelaere et al , Regenerative Feed-back Repeaters for Programmable Interconnections, JSSC, 1995 [Lima95] T. Lima et al, Capacitance Coupling Immune Accelerator for Resistive Interconnects, IEEE Trans. on Electron Devices, 1995 [Friedman98a] Secareanu et al, Transparent Repeaters, GLSVLSI,1998 [Nalamalpu00a] A. Nalamalpu et al, Quantifying and Mitigating Placement Constraints, 2000 [Sakurai90] Sakurai et al , Alpha-Power MOSFET Model, JSSC,1990 [Nalamalpu00b] A. Nalamalpu et al , Repeater Ramp based Analytical Model, ISCAS,2000

29. References [Sylvester98] D.Sylvester et al, Getting to the bottom of deep sub-micron, ICCAD, 1998 [Sylvester99] D.Sylvester et al, Getting to the bottom of deep sub-micron II, ISPD, 1999 [Friedman98b] V.Adler et al, Repeater Design to Reduce Delay and Power, IEEE Trans. Circuits and System II, 1998 [Bakoglu85] Bakoglu et al, Optimal Interconnection Circuits for VLSI, IEEE Trans. Electron Devices, 1985