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Lecture 10: Circuit Families

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## Lecture 10: Circuit Families

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**Outline**• Pseudo-nMOS Logic (Ratioed Logic) • Dynamic Logic • Pass Transistor Logic 10: Circuit Families**Introduction**• What makes a circuit fast? • I = C dV/dt -> tpd (C/I) DV • low capacitance • high current • small swing • Logical effort is proportional to C/I • pMOS are the enemy! • High capacitance for a given current • Can we take the pMOS capacitance off the input? • Various circuit families try to do this… 10: Circuit Families**Pseudo-nMOS**• In the old days, nMOS processes had no pMOS • Instead, use pull-up transistor that is always ON • In CMOS, use a pMOS that is always ON • Ratio issue • Make pMOS about ¼ effective strength of pulldown network 10: Circuit Families**Pseudo-nMOS**10: Circuit Families**Pseudo-NMOS VTC**10: Circuit Families**Pseudo-nMOS Design**10: Circuit Families**Pseudo-nMOS Gates**• Design for unit current on output to compare with unit inverter. • pMOS fights nMOS • Iout = 4I/3 – I/3 10: Circuit Families**Pseudo-nMOS Gates**• Design for unit current on output to compare with unit inverter. • pMOS fights nMOS 10: Circuit Families**Pseudo-nMOS Design**• Ex: Design a k-input AND gate using pseudo-nMOS. Estimate the delay driving a fanout of H • G = 1 * 8/9 = 8/9 • F = GBH = 8H/9 • P = 1 + (4+8k)/9 = (8k+13)/9 • N = 2 • D = NF1/N + P = 10: Circuit Families**Pseudo-nMOS Power**• Pseudo-nMOS draws power whenever Y = 0 • Called static power P = IDDVDD • A few mA / gate * 1M gates would be a problem • Explains why nMOS went extinct • Use pseudo-nMOS sparingly for wide NORs • Turn off pMOS when not in use 10: Circuit Families**Ratio Example**• The chip contains a 32 word x 48 bit ROM • Uses pseudo-nMOS decoder and bitline pullups • On average, one wordline and 24 bitlines are high • Find static power drawn by the ROM • Ion-p = 36 mA, VDD = 1.0 V • Solution: 10: Circuit Families**Pseudo-NMOS Design**• Pseudo-nMOS gates will not operate correctly if VOL>VIL of the driven gate. • This is most likely in the SF corner. • Conservative design requires extra weak pMOS. • Another choice is to use replica biasing. • Idea comes from analog design. • Replica biasing allows 1/3 the current ratio rather than the conservative ¼ ratio of earlier. 10: Circuit Families**Replica Biasing**10: Circuit Families**Ganged CMOS**10: Circuit Families**Ganged CMOS**10: Circuit Families**Improved Loads**10: Circuit Families**Improved Loads (2)**Differential Cascode Voltage Switch Logic (DCVSL)**DCVSL Transient Response**10: Circuit Families**Example: AND Gate**10: Circuit Families**NMOS-Only Logic**10: Circuit Families**NMOS Only Logic: Level Restoring Transistor**• Advantage: Full Swing • Restorer adds capacitance, takes away pull down current at X • Ratio problem**Restorer Sizing**10: Circuit Families**LEAP**• LEAn integration with Pass transistors • Get rid of pMOS transistors • Use weak pMOS feedback to pull fully high • Ratio constraint 10: Circuit Families**CPL**• Complementary Pass-transistor Logic • Dual-rail form of pass transistor logic • Avoids need for ratioed feedback • Optional cross-coupling for rail-to-rail swing 10: Circuit Families**Alternative CPL**10: Circuit Families**Transmission Gate**10: Circuit Families**Pass Transistor Circuits**• Use pass transistors like switches to do logic • Inputs drive diffusion terminals as well as gates • CMOS + Transmission Gates: • 2-input multiplexer • Gates should be restoring 10: Circuit Families**S**S Pass-Transistor Based Multiplexer S VDD GND In2 In1 S**Transmission Gate XOR**10: Circuit Families**Delay in Transmission Gate Networks**10: Circuit Families**Transmission Gate Full Adder**Similar delays for sum and carry**Other Pass Transistor Families**• DPTL (Differential Pass Transistor Logic) • DPL (Double Pass Transistor Logic) • EEPL (Energy Economized Pass Transistor Logic) • PPL (Push-Pull Pass Transistor Logic) • SRPL (Swing Restored Pass Transistor Logic) • DCVSPG (Differential Cascode Voltage Switch with Pass Gate Logic) 10: Circuit Families**Pass Transistor Summary**• Researchers investigated pass transistor logic for general purpose applications in the 1990’s • Benefits over static CMOS were small or negative • No longer generally used • However, pass transistors still have a niche in special circuits such as memories where they offer small size and the threshold drops can be managed 10: Circuit Families**Single Clock 2-Phase System**T/2 T 3T/2 10: Circuit Families**Shift Register**10: Circuit Families**Shift Register**• When f = 1, data move through the first transmission gate to the inverter. 10: Circuit Families**Charge Leakage**10: Circuit Families**Charge Leakage**10: Circuit Families