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

ELEC 5270/6270 Spring 2009 Low-Power Design of Electronic Circuits Pass Transistor Logic: A Low Power Logic Family. Vishwani D. Agrawal James J. Danaher Professor Dept. of Electrical and Computer Engineering Auburn University, Auburn, AL 36849 vagrawal@eng.auburn.edu

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

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  1. ELEC 5270/6270 Spring 2009Low-Power Design of Electronic CircuitsPass Transistor Logic: A Low Power Logic Family 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_Spr09/course.html ELEC6270 Spring 09, Lecture 14

  2. Low-Power Logic Styles • Pass transistor logic • Dynamic logic • Domino logic • Adiabatic and charge recovery logic • Asynchronous logic • Logic restructuring ELEC6270 Spring 09, Lecture 14

  3. Pass Transistor Logic (PTL) • Requires fewer transistors • Smaller area • Reduced capacitance • Reduced energy and power ELEC6270 Spring 09, Lecture 14

  4. CMOS AND Gate A F = AB B A F = AB B ELEC6270 Spring 09, Lecture 14

  5. Pass Transistor AND Gate B A F = AB 0 Need 4 transistors instead of 6 for CMOS AND gate. ELEC6270 Spring 09, Lecture 14

  6. CMOS OR Gate A B F = A + B A F = A + B B ELEC6270 Spring 09, Lecture 14

  7. Pass Transistor OR Gate B 1 F = A + B A Need 4 transistors instead of 6 for CMOS OR gate. ELEC6270 Spring 09, Lecture 14

  8. Reduced Voltage Swing IN Vx = VDD – Vtn OUT VDD = 2.5V n transistors, W/L = 0.5μ/0.25μ p transistors, W/L = 1.5μ/0.25μ ELEC6270 Spring 09, Lecture 14

  9. Spice Simulation 3.0 2.0 1.0 0.0 IN Vx Voltage, V OUT 0 0.5 1.0 1.5 2.0 Time, ns J. M. Rabaey, A. Chandrakasan and B. Nokolić, Digital Integrated Circuits, Upper Saddle River, New Jersey: Pearson Education, 2003. ELEC6270 Spring 09, Lecture 14

  10. Voltage Transfer Characteristic (VTC) of AND Gate B A F = AB 0 n transistors, W/L = 0.5μ/0.25μ p transistors, W/L = 1.5μ/0.25μ ELEC6270 Spring 09, Lecture 14

  11. VTC: Spice Simulation 3.0 2.0 1.0 0.0 VDD – Vtn B = VDD A = 0 → VDD F, V A = VDD, B = 0 → VDD A = B = 0 → VDD 0 0.5 1.0 1.5 2.0 2.5 Vin, V ELEC6270 Spring 09, Lecture 14

  12. Energy If this voltage is insufficient for turning the pMOS Transistor in inverter off, leakage power will be consumed. 0 → VDD Vout VDD = 2.5V i(t) CL T T E0→1 = ∫ P(t) dt = VDD ∫ i(t) dt 0 0 VDD-Vtn = VDD ∫ CL dVout = CL VDD (VDD – Vtn) < CL VDD2 0 ELEC6270 Spring 09, Lecture 14

  13. Energy: PTL vs. CMOS • PTL consumes less dynamic power than static CMOS Logic. • PTL leakage may be higher when output is low, because the reduced voltage level may be insufficient to turn the PMOS transistor in the inverter off. ELEC6270 Spring 09, Lecture 14

  14. Ways to Reduce Leakage • Level restoration • Multiple-threshold transistors • Transmission-gate logic ELEC6270 Spring 09, Lecture 14

  15. Level Restoration VDD Level restorer B=1 Vout A=1 0 CL Level restorer device should be weaker than the nMOS pass transistor. Otherwise, VDD → 0 transition at Vout will be impossible. ELEC6270 Spring 09, Lecture 14

  16. Multiple-Threshold Transistors • Use zero-threshold pass-transistors. • Use high-threshold transistors in all other gates. • This can cause leakage through multiple gates. ELEC6270 Spring 09, Lecture 14

  17. Leakage Through Zero-Threshold Transistors Zero or low-threshold transistors 1 0 Leakage current path 0 1 ELEC6270 Spring 09, Lecture 14

  18. Transmission-Gate Logic • Provides both power and ground levels. • Good design, except needs more transistors. Inverting multiplexer A S B S’A’ + SB’ ELEC6270 Spring 09, Lecture 14

  19. Transmission-Gate XOR B AB’+A’B A ELEC6270 Spring 09, Lecture 14

  20. A Logic Library ELEC6270 Spring 09, Lecture 14

  21. Synthesis of PTL Shannon’s expansion: Z = AB + BC + AC = A(B+BC+C) + A’(BC) = A(B+C) + A’BC = A[B+B’C] + A’[BC] 1 C C 0 1 0 1 0 B 1 0 A Z ELEC6270 Spring 09, Lecture 14

  22. Pass-Transistor Cell ELEC6270 Spring 09, Lecture 14

  23. Synthesis of Z = A’B + B’C + A’C’ A 0 B C = 1, Z = A’B + B’ B = 1, Z = A’ B = 0, Z = 1 C = 0, Z = A’ C Z ELEC6270 Spring 09, Lecture 14

  24. Synthesis of Z = A’ + BC’ + B’C C C’ 0 B B’ A A’ A = 1, Z = BC’ + B’C B = 1, Z = C’ B = 0, Z = C A = 0, Z = 1 Z ELEC6270 Spring 09, Lecture 14

  25. Synthesis of Z = AB’C’ + A’B’C C’ C 1 A = 1, Z = B’C’ B = 1, Z = 0 B = 0, Z = C’ A = 0, Z = B’C B = 1, Z = 0 B = 0, Z = C A’ A B’ B Z ELEC6270 Spring 09, Lecture 14

  26. CPL: Complementary Pass-Transistor Logic • Every signal and its complement is generated. • Gates are static, because the output is connected to either VDD or GND. • Design is modular; same cell can produce various gates by simply permuting the input signals. • Also called differential pass-transistor logic (DPL) ELEC6270 Spring 09, Lecture 14

  27. A CPL Cell ELEC6270 Spring 09, Lecture 14

  28. CPL Cell Used As AND/NAND B B’ ABA’ B’ Z = AB Z’ = (AB)’ ELEC6270 Spring 09, Lecture 14

  29. CPL Cell Used As OR/NOR B’ B ABA’ B’ Z = A + B Z’ = (A + B)’ ELEC6270 Spring 09, Lecture 14

  30. CPL Cell Used As XOR/XNOR B’ B AA’A’ A Z = AB’ + A’B Z’ = AB + A’B’ ELEC6270 Spring 09, Lecture 14

  31. CPL vs. CMOS • CPL requires fewer transistors. • Useful for modular (array) circuits like adders, multipliers, barrel shifter, etc. • CPL operation can be faster and energy efficient. • Following example is taken from: • M. E. Elrabaa, I. S. Abu-Khater, and M. I. Elmasry, Advanced Low-Power Digital Circuit Techniques, Springer, 1997, Chapter 2. ELEC6270 Spring 09, Lecture 14

  32. Example: 4-Bit Carry Select Adder A_1 B_1 Adder cell S1’ S0’ C0’ C0 C1’ C1 A_3 B_3 Adder cell S1’ S0’ C0’ C0 C1’ C1 A_2 B_2 Adder cell S1’ S0’ C0’ C0 C1’ C1 A_4 B_4 Adder cell S1’ S0’ C0’ C0 C1’ C1 M M M M M M M M M M M M M M M M M M M M C’_2 C_4 C’_4 C’_0 C_0 S_2 S_1 S_3 S_4 ELEC6270 Spring 09, Lecture 14

  33. CMOS Carry-Select Adder Cell Ai Bi S1’ S0’ C0’ C0 C1’ C1 ELEC6270 Spring 09, Lecture 14

  34. CPL Adder Cell Ai Bi S1’ S0’ C0’ C0 C1’ C1 ELEC6270 Spring 09, Lecture 14

  35. CPL Multiplexer Cell in2 in1 M Ci Ci out Ci’ Ci’ in2 in1 ELEC6270 Spring 09, Lecture 14

  36. 32-Bit Adders in 0.8μ, 3.3V ELEC6270 Spring 09, Lecture 14

  37. References • G. R. Cho and T. Chen, “On the Impact of Technology Scaling on Mixed PTL/Static Logic,” Proc. IEEE Int. Conf. Computer Design, 2002. • R. Zimmermann and W. Fichtner, “Low-Power Logic Styles: CMOS Versus Pass-Transistor Logic,” IEEE J. Solid State Circuits, vol. 32, no. 7, pp. 1079-1090, July 1997. ELEC6270 Spring 09, Lecture 14

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