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1. Energy Efficient CMOS Optical I/O Samuel Palermo
2. Outline Introduction
90nm CMOS 16Gb/s optical I/O transceiver
Integrated CMOS photonics
Conclusion 2
3. 3 Increasing I/O Bandwidth Demand Single ? Multi ? Many-Core ?Processors
Tera-scale many-core processors will aggressively drive aggregate I/O rates
4. Electrical I/O Channels Frequency dependent loss
Caused by dispersion & reflections
Exponential with channel distance
Equalization circuitry required
Power & area overhead
Typical commercial electrical I/O xcvr ~20mW/Gb/s at 10Gb/s
5. Chip-to-Chip Optical Interconnects Optical interconnects remove many channel limitations
Reduced complexity and power consumption
Potential for high information density with wavelength-division multiplexing (WDM) 5
6. 90nm CMOS 16Gb/s Optical Transceiver Architecture 6
7. 7 VCSEL Bandwidth vs Reliability Mean Time to Failure (MTTF) is inversely proportional to current density squared
[2]
Steep trade-off between bandwidth and reliability
8. 8 Multiplexing FIR Circuit Implementation
9. 9 Tap Mux & Output Stage 5:1 multiplexing predriver uses 5 pairs of complementary clock phases spaced by a bit time
Tunable delay predriver compensates for static phase offsets and duty cycle error
10. 10 VCSEL TX Optical Testing
11. 11 VCSEL 16Gb/s Optical Eye Diagrams
12. 12 Optical RX Scaling Issues Traditionally, TIA has high RT and low Rin
13. 13 Integrating Receiver Block Diagram
14. 14 Demultiplexing Receiver Demultiplexing with multiple clock phases allows higher data rate
Data Rate = #Clock Phases x Clock Frequency
Gives sense-amp time to resolve data
Allows continuous data resolution
15. 15 Clocked Sense Amplifier Offset cancelled with digitally adjustable PMOS capacitors
Step=2.3mV, Range=±70mV
Kickback charge can corrupt adjacent bits
Need high common-mode input for adequate speed
16. 16 1V Modified Integrating Receiver Differential Buffer
Fixes sense-amp common-mode input for improved speed and offset performance
Reduces kickback charge
Cost of extra power and noise
Input Range = 0.6 – 1.1V
17. 17 Optical Transceiver Testing
18. 18 Receiver Sensitivity Test Conditions
8B/10B data patterns (variance of 6 bits)
Long runlength data (variance of 10 bits)
BER < 10-10
19. 19 Transceiver Power Consumption Power at 16Gb/s = 129mW (8.1mW/Gb/s)
Power scales with data rate
Mostly CMOS circuitry
Integrating RX sensitivity increases at lower data rates
20. 20 Optical vs Electrical XCVRPerformance Comparisons Compares favorably due to simple equalization circuitry
Should scale well
Better VCSEL technology
Lower capacitance photodetectors
Higher data rates ? More equalization for electrical channels
21. Further Improving Optical I/O Energy Efficiency Leverage ultra-low-power electrical link design techniques
Adaptive supply scaling, Source-synchronous clocking
Optical component research
Longer wavelength VCSEL (850nm -> 1.3mm)
Higher speed VCSEL (> 20Gbps)
Co-optimization of optical device design and packaging
Target 1-2mW/Gbps
Monolithically integrated CMOS photonics
Waveguide, ring-resonator modulator & detector suitable for CMOS on-die integration
Target <1mW/Gbps
22. Integrated CMOS Photonics 22 Tight integration allows reduced parasitics & improved energy efficiency
CPD ˜ 1fF, Cmod ˜ 10fF
Challenges
Integrating optical devices w/min yield impact
Low loss waveguides with high coupling efficiency
23. Future Photonic CMOS Chip 23 High Bandwidth Interconnect with DWDM Optical Routing
Core to Core, Chip to Chip, Core to Memory
24. Conclusion Optical interconnects provide a path to reducing the I/O bandwidth problem
Proposed optical interconnect architecture is suitable for large scale integration in current/future CMOS technologies
VCSEL TX equalizer allows low current operation
Low voltage integrating receiver
Integrated CMOS photonics provides potential for unified intra/inter-chip interconnect with ultimate energy efficiency
25. 25 Backup Slides
26. 26 Receiver Sensitivity Analysis
27. 27 Transceiver Performance Summary