International Symposium on Low Power Electronics and Design A Charge Pump Based Receiver Circuit to Reduce Interconnect Power Dissipation Aatmesh Shrivastava, John Lach, and Benton H. Calhoun University of Virginia, Charlottesville

2 Interconnect Power Dissipation Interconnect consumes >50% of dynamic power in a micro-processor. 90% of interconnect power is in 10% of interconnect. [1] Magen, et. al. SLIP 2004

3 Interconnect Power in the context of next generation computing A Exa-byte of data is to be transmitted per second to enable exascale computing. [2] P. Kogge et. al. DARPA/ITPO 08 State of the interconnect consumes 1-3pJ/bit/mm. A exabyte/s will need Mega-Watt Power. [2]

Outline Voltage Scaling for Interconnect –Driver –Receiver Literature Review Proposed Interconnect Receiver –Charge Pump –Complete circuit diagram –Simulation Implementing the interconnect in 4 core Alpha Results Design Comparison 4

5 Voltage Scaling for interconnects Voltage Scaling has been used to reduce interconnect power [4-10]. Logic runs at rated VDD, wires at reduced VDDI. Interconnect driver circuits are needed Key Question :- Performance overhead vs Power.

6 Interconnect Driver Two NMOS transistors are used at output stage A signal at logic level ( 1V) is converted to a signal interconnect level (0.3V) We use this driver in our proposed interconnect circuit. [4] H. Zhang, et. al. TVLSI 2000

7 Restores the signal back to the logic level. Poor performance, VDDI > V T. Differential amplifier [8-10] can be used for better performance but have higher power overhead. We propose an improved single ended receiver. VDDI ON 0 OFF ON Interconnect Receiver

8 Approx. Power-Performance-Area Prior Art In prior art either energy saving is less or performance is poor. SchemesB/W (Ghz) Swing (V) Normalized Energy Basic ( no scaling)>111 Single-ended [4,5,7]< Differential [8-10]> Capacitive [6]<

9 Delay vs Energy/bit : Prior Art Existing solutions do not address power and performance in conjunction.

10 Proposed receiver ckt Charge-pump is used. It boosts the signal to three times the interconnect swing Good performance and much lower power

11 Charge Pump in the Receiver When IN is at 0, A is precharged to 0.3V. So when IN goes high A goes to 0.6V (Ideal case). Similarly when IN is at 0.3V, A is precharged to 0V. So when IN goes low A goes to -0.3V (ideal). Total swing at A is 0.9V. C swings from V T to VDD-V T

12 Complete Circuit Diagram 0.3V 1V 0V V TL 0V V TL V TL V TL VDD-V TL V TL

13 Simulation results Reduced swing interconnect signal gets reconstructed with good performance. INOUT

14 Delay vs Energy/bit Proposed Solution gives very good performance and very low energy.

15 Energy savings in a processor Data-Bus of alpha was implemented using differential, basic and proposed interconnect circuit. Over the set of splash benchmarks, the proposed interconnect saves up to 70% of energy.

16 PPA : Power-Performance-Area Novel interconnect circuit has best in class PPA SchemesB/W (GHz) Swing (V) Norm. EnergyArea of 1 repeater Basic>1112X Single Ended [4,5,7] < X Differential [8-10] > X Capacitive [6]< NA This Work> X

17 Thank You

References 1.Nir Magen et. Al. “Interconnect-Power Dissipation in a Microprocessor” Workshop on System Level Interconnect Prediction P. Kogge, K. Bergman, S Borka, et. al, “ExaScale Computing Study: Technology Challenges in Achieving Exascale Systems” DARPA/IPTO, September E. Kusse and J.M. Rabaey, “Low-Energy Embedded FPGA Structures” IEEE International Symposium on Low Power Electronics Design, August H. Zhang, V. George and J.M. Rabaey, “Low-Swing On-Chip Signalling Techniques: Effectiveness and Robustness” IEEE Transactions on Very Large Scale Integration (VLSI), Vol-8 No-3, June J.C.G. Montesdeoca, J.A. Montiel-Nelson and S. Nooshabadi, “CMOS Driver Receiver Pair for Low Swing Signalling for Low Energy On-chip Interconnects” IEEE Transactions on Very Large Scale Integration (VLSI), Vol-17 No-2, February R. Ho, I. Ono, F. Liu, A. Chow, J. Schauar and R. Drost, “High Speed and Low Energy capacitively driven wires” IEEE International Solid State Circuits Conference, February M. Ferretti and P.A. Beere “Low Swing Signaling Using a Dynamic Diode-Connected Driver” European Solid-State Circuits Conference, September A. Narshimha, M. Kasotiya and R. Sridhar “A Low-Swing Differential signaling Scheme for on-chip Global Interconnects” International Conference on VLSI Design, January N. Tzartzanis, W.W. Walker “Differential Current Mode Sensing for Efficient On-Chip global Signaling” IEEE Journal of Solid State Circuits, Vol-40 No-11, November H. Ito, M. Kimura, K. Miyashita, T. Ishii, K. Okada and K. Masu, “A Bidirectional and Multidrop Transmission Line Interconnect for Multipoint to Multipoint On-Chip Communication” IEEE Journal of Solid State Circuits, Vol-43 No-4, April V. Alder and E.G. Friedman, “Repeater Design to Reduce Delay and Power in Resistive Interconnects”. IEEE Transactions on Circuits and Systems-II, Vol-45 No-45, May P.E. Allen and D.R. Holberg., “CMOS Analog circuit design” Oxford Press R.E. Kessler, E.J. McLellan and D.A. Webb, “The Alpha Microprocessor Architecture” International Conference on Computer Design, October N.L. Binkert, R.G. Dreslinski, L.R. Hsu, K.T. Lim, A.G. Saidi and S.K. Reinhardt, “The M5 Simulator: Modeling Networked Systems” IEEE Micro, July

19 Back Up

20 Complete Circuit Diagram When IN goes hi, A goes to 0.6V, bringing B to ground. OUT goes high completing the transition. It also brings C to VDD-V T and precharges A to 0.6V

21 Graph of A

22 Initial Condition

23 Static current

24 Voltage Sensitivity