lec16PCctPitfalls - Introduction to CMOS VLSI Design...

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Unformatted text preview: Introduction to CMOS VLSI Design Circuit Pitfalls Circuit Pitfalls Slide 2 CMOS VLSI Design Outline Circuit Pitfalls Detective puzzle Given circuit and symptom, diagnose cause and recommend solution All these pitfalls have caused failures in real chips Noise Budgets Reliability Circuit Pitfalls Slide 3 CMOS VLSI Design Bad Circuit 1 Circuit 2:1 multiplexer Symptom Mux works when selected D is 0 but not 1. Or fails at low V DD . Or fails in SFSF corner. X D0 Y D1 S S Principle: Solution: Circuit Pitfalls Slide 4 CMOS VLSI Design Bad Circuit 1 Circuit 2:1 multiplexer Symptom Mux works when selected D is 0 but not 1. Or fails at low V DD . Or fails in SFSF corner. X D0 Y D1 S S Principle: Threshold drop X never rises above V DD-V t V t is raised by the body effect The threshold drop is most serious as V t becomes a greater fraction of V DD . Solution: Circuit Pitfalls Slide 5 CMOS VLSI Design Bad Circuit 1 Circuit 2:1 multiplexer Symptom Mux works when selected D is 0 but not 1. Or fails at low V DD . Or fails in SFSF corner. X D0 Y D1 S S Principle: Threshold drop X never rises above V DD-V t V t is raised by the body effect The threshold drop is most serious as V t becomes a greater fraction of V DD . Solution: Use transmission gates, not pass transistors Circuit Pitfalls Slide 6 CMOS VLSI Design Bad Circuit 2 Circuit Latch Symptom Load a 0 into Q Set = 0 Eventually Q spontaneously flips to 1 Principle: Solution: D Q X Circuit Pitfalls Slide 7 CMOS VLSI Design Bad Circuit 2 Circuit Latch Symptom Load a 0 into Q Set = 0 Eventually Q spontaneously flips to 1 Principle: Leakage X is a dynamic node holding value as charge on the node Eventually subthreshold leakage may disturb charge Solution: D Q X Circuit Pitfalls Slide 8 CMOS VLSI Design Bad Circuit 2 Circuit Latch Symptom Load a 0 into Q Set = 0 Eventually Q spontaneously flips to 1 Principle: Leakage X is a dynamic node holding value as charge on the node Eventually subthreshold leakage may disturb charge Solution: Staticize node with feedback Or periodically refresh node (requires fast clock, not practical processes with big leakage) D Q X Q D X Circuit Pitfalls Slide 9 CMOS VLSI Design Bad Circuit 3 Circuit Domino AND gate Symptom Precharge gate (Y=0) Then evaluate Eventually Y spontaneously flips to 1 Principle: Solution: 1 Y X Circuit Pitfalls Slide 10 CMOS VLSI Design Bad Circuit 3 Circuit Domino AND gate Symptom Precharge gate (Y=0) Then evaluate Eventually Y spontaneously flips to 1 Principle: Leakage X is a dynamic node holding value as charge on the node Eventually subthreshold leakage may disturb charge Solution: 1 Y X Circuit Pitfalls...
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lec16PCctPitfalls - Introduction to CMOS VLSI Design...

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