Circuits and Interconnects In Aggressively Scaled CMOS

1. Circuits and InterconnectsIn Aggressively Scaled CMOS Mark Horowitz Computer Systems Laboratory Stanford University horowitz@stanford.edu VLSI Scaling

2. Device Scaling In digital CMOS design: • Only two circuit forms matter • (maybe three) • Static CMOS, and Dynamic CMOS These forms are used because: • They don’t demand much from devices • So they work with crummy transistors • Robust, especially static circuits VLSI Scaling

3. 0.36 * Ldrawn 7 5 FO4 delay (nS) 3 1 2 1.5 1 0.5 0 Feature size (mm) FO4 Inverter Delay Under Scaling • Device performance will scale • FO4 delay has been linear with tech Approximately 0.36 nS/mm*Ldrawn at TT (0.5nS/mm under worst-case conditions) • Easy to predict gate performance • We can measure them • Labs have built 0.04mm devices • Key issue is voltage scaling VLSI Scaling

4. Circuit Power • Is very much tied to voltage scaling • If the power supply scales with technology For a fixed complexity circuit • Power scales down as a^3 if you run as same frequency • Power scales down as a^2 if you run it 1/ a times faster • Power scaling is a problem because • Freq has been scaling at faster than 1/ a • Complexity of machine has been growing • This will continue to be an issue in future chips • Remember scaling the technology makes a chip lower power! VLSI Scaling

5. Voltage Scaling • Circuits performance depends on the Vdd to Vth ratio • Ideally both should scale together • If Vth scales leakage scales • If Vth does not scale, gates get slower, or Vdd can’t scale as fast and power goes up • Leakage is easier to deal with than power, transistors will leak VLSI Scaling

6. Semi-global wire resistance, 1mm long Semi-global wire capacitance, 1mm long Aggressive scaling Aggressive scaling 0.4 0.6 Conservative scaling Conservative scaling 0.3 0.4 Kohms pF 0.2 0.2 0.1 0 0 0.25 0.18 0.13 0.1 0.07 0.05 0.035 0.25 0.18 0.13 0.1 0.07 0.05 0.035 Technology Ldrawn (um) Technology Ldrawn (um) Scaling Global Wires • R gets quite a bit worse with scaling; C basically constant VLSI Scaling

7. Semi-global wire resistance, scaled length Semi-global wire capacitance, scaled length Aggressive scaling Aggressive scaling 0.4 0.6 Conservative scaling Conservative scaling 0.3 0.4 Kohms pF 0.2 0.2 0.1 0 0 0.25 0.18 0.13 0.1 0.07 0.05 0.035 0.25 0.18 0.13 0.1 0.07 0.05 0.035 Technology Ldrawn (um) Technology Ldrawn (um) Scaling Module Wires • R is basically constant, and C falls linearly with scaling VLSI Scaling

8. Architecture Scaling • Plot of IPC • Compiler + IPC • 1.5x / generation • What next? • Wider machines • Threads • Speculation • Guess answers to create parallelism • Have high wire costs • Won’t be easy VLSI Scaling

9. Clock Frequency • Most of performance comes from clock scaling • Clock frequency double each generation • Two factors contribute: technology (1.4x/gen), circuit design VLSI Scaling

10. Clock speed has been scaling faster than base technology Number of FO4 delays in a cycle has been falling Number of gates decrease 1.4x each generation Caused by: Faster circuit families (dynamic logic) Better optimization Approaching a limit: <16 FO4 is hard < 8 FO4 is very hard Gates Per Clock VLSI Scaling