Vishwani D. Agrawal James J. Danaher Professor Dept. of Electrical and Computer Engineering

1. ELEC 5270/6270 Spring 2011Low-Power Design of Electronic CircuitsPseudo-nMOS, Dynamic CMOSand Domino CMOS Logic Vishwani D. Agrawal James J. Danaher Professor Dept. of Electrical and Computer Engineering Auburn University, Auburn, AL 36849 vagrawal@eng.auburn.edu http://www.eng.auburn.edu/~vagrawal/COURSE/E6270_Spr11/course.html ELEC6270 Spring 11, Lecture 9

2. Static CMOS: Pros and Cons • Advantages: Static (robust) operation, low power, scalable with technology. • Disadvantages: • Large size: An N input gate requires 2N transistors. • Large capacitance: Each fanout must drive two devices. • Alternatives: Pass-transistor logic (PTL), pseudo-nMOS, dynamic CMOS, domino CMOS. ELEC6270 Spring 11, Lecture 9

3. A Pseudo-nMOS Gate VDD VDD PUN Output Output PDN PDN Inputs Inputs CMOS Gate Pseudo-nMOS Gate ELEC6270 Spring 11, Lecture 9

4. Pseudo-nMOS NOR VDD Output Input 3 Input 2 Input 1 ELEC6270 Spring 11, Lecture 9

5. Pseudo-nMOS NAND VDD Output Input 1 Input 2 ELEC6270 Spring 11, Lecture 9

6. Pseudo-nMOS Inverter VDD Output Input ELEC6270 Spring 11, Lecture 9

7. Inverter Characteristics 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Nominal device: W 0.5μ ── = ──── = 2 Ln 0.25μ W/Lp = 4 Output voltage, V W/Lp = 0.5 W/Lp = 1 W/Lp = 2 W/Lp = 0.25 0.0 0.5 1.0 1.5 2.0 2.5 Input voltage, V ELEC6270 Spring 11, Lecture 9

8. Performance of Inverter J. M. Rabaey, A. Chandrakasan and B. Nokolić, Digital Integrated Circuits, Upper Saddle River, New Jersey: Pearson Education, 2003, page 262. ELEC6270 Spring 11, Lecture 9

9. Negative Aspects of Pseudo-nMOS • Output 0 state is ratioed logic. • Faster gates mean higher static power. • Low static power means slow gates. ELEC6270 Spring 11, Lecture 9

10. A Dynamic CMOS Gate VDD Precharge transistor Output PDN CL Inputs Evaluate transistor CK ELEC6270 Spring 11, Lecture 9

11. Two-Phase Operation in a Vector Period ELEC6270 Spring 11, Lecture 9

12. 4-Input NAND Dynamic CMOS Gate VDD CK A B C D CK Output = CK’ + (ABCD)’∙ CK CL tL→H ≈ 0 ELEC6270 Spring 11, Lecture 9

13. Characteristics of Dynamic CMOS • Nonratioed logic – sizing of pMOS transistor is not important for output levels. • Smaller number of transistors, N+2 vs. 2N. • Larger precharge transistor reduces output fall time, but increases precharge power. Faster switching due to smaller capacitance. • Static power – negligible. • Short-circuit power – none. • Dynamic power • no glitches – following precharge, signals can either make transitions only in one direction, 1→0, or no transition, 1→1. • only logic transitions – all nodes at logic 0 are charged to VDD during precharge phase. ELEC6270 Spring 11, Lecture 9

14. Switching Speed and Power • Fewer transistors mean smaller node capacitance. • No short-circuit current to slow down discharging of capacitance. • Only dynamic power consumed, but can be higher than CMOS. ELEC6270 Spring 11, Lecture 9

15. Logic Activity • Probability of 0 → 1 transition: • Static CMOS, p0 p1 = p0(1 – p0) • Dynamic CMOS, p0 ≥ p0 p1 • Example: 2-input NOR gate • Static CMOS, Pdyn = 0.1875 CLVDD2fCK • Dynamic CMOS, Pdyn = 0.75 CLVDD2fCK p1 = 0.5 p1 = 0.25 p0 = 0.75 p1 = 0.5 ELEC6270 Spring 11, Lecture 9

16. Charge Leakage CK Precharge Evaluate VDD Output A’ CK A=0 CK A’ CL Ideal Actual Time J. M. Rabaey, A. Chandrakasan and B. Nokolić, Digital Integrated Circuits, Upper Saddle River, New Jersey: Pearson Education, 2003. ELEC6270 Spring 11, Lecture 9

17. Bleeder Transistor VDD VDD CK A B C D CK CK A B C D CK Output Output CL CL ELEC6270 Spring 11, Lecture 9

18. A Problems With Dynamic CMOS VDD VDD CK A B C prech. evaluate CK A=0→1 CK CK CK B C J. M. Rabaey, A. Chandrakasan and B. Nokolić, Digital Integrated Circuits, Upper Saddle River, New Jersey: Pearson Education, 2003. ELEC6270 Spring 11, Lecture 9

19. Remedy • Set all inputs to gates to 0 during precharge. • Since precharge raises all outputs to 1, inserting inverters between gates will do the trick. ELEC6270 Spring 11, Lecture 9

20. Domino CMOS VDD VDD CK A B C prech. evaluate CK A=0→1 CK CK CK C B R. H. Krambeck, C. M. Lee and H.-F. S. Law, “High-Speed Compact Circuits with CMOS,” IEEE J. Solid-State Circuits, vol. SC-17, no. 3, pp. 614-619, June 1982. ELEC6270 Spring 11, Lecture 9

21. Bleeder in Domino CMOS VDD CK A B C D CK Output CL ELEC6270 Spring 11, Lecture 9

22. Logic Mapping for Noninverting Gates AND A B C D E F G H ABC X Y G+H AND/OR ABC D E F G+H OR Y ELEC6270 Spring 11, Lecture 9

23. Selecting a Logic Style • Static CMOS: most reliable and predictable, reasonable in power and speed, voltage scaling and device sizing are well understood. • Pass-transistor logic: beneficial for multiplexer and XOR dominated circuits like adders, etc. • For large fanin gates, static CMOS is inefficient; a choice can be made between pseudo-nMOS, dynamic CMOS and domino CMOS. ELEC6270 Spring 11, Lecture 9