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chap4_2 - Principles of VLSI Design Performance Estimation...

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Principles of VLSI Design Performance Estimation CMPE 413 1 UMBC U M B C U N I V E R S I T Y O F M A R Y L A N D B A L T I M O R E C O U N T Y 1 9 6 6 Transistor Sizing So far, we have assumed that to get symmetric rise and fall times: Does this rule reduce overall delay ? Therefore, in self-loaded circuits (circuits without significant routing capaci- tance and fanouts), equal sized devices can be used to reduce power dissipa- tion and area without sacrificing performance (overall delay). W p 2 3 ( 29 W n × W p 2 W n = Assume G 1 G 2 3C eq 3C eq W p W n = Assume G 1 G 2 2C eq 2C eq t inv pair t fall t rise + = C eq = C d + C g R 1 2 + ( 29 C eq 2 R 2 --- 1 2 + ( 29 C eq + = = 6 RC eq R 1 1 + ( 29 C eq 2 R 1 1 + ( 29 C eq + = = 6 RC eq Half the resistance since p is twice as wide but twice the resistance per unit area.

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Principles of VLSI Design Performance Estimation CMPE 413 2 UMBC U M B C U N I V E R S I T Y O F M A R Y L A N D B A L T I M O R E C O U N T Y 1 9 6 6 Stage Ratio How do we drive large load capacitances, e.g. off-chip wires via the I/O pads, long buses, etc. ? By using a chain of inverters, where each successive inverter is larger than the previous one. What is the optimal value of a (the stage ratio) that both • Minimizes the delay through the chain. • Minimizes the area and power. The magic number a is e (~2.7) - see analysis in book. The optimal value may vary depending on process parameters. G 1 G 2 1 a G 3 a 2 G 4 a 3
Principles of VLSI Design Performance Estimation CMPE 413 3 UMBC U M B C U N I V E R S I T Y O F M A R Y L A N D B A L T I M O R E C O U N T Y 1 9 6 6 Power Dissipation Two components of power dissipation in CMOS circuits: Static power Dynamic power Static power dissipation: Reverse-bias leakage current through parasitic diodes formed by source/ drain diffusion and n-well diffusion. Through-current of pseudo-nMOS devices. Subthreshold conduction (current that flows when V in < V tn ). Becoming more important as power supply is scaled down.

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Principles of VLSI Design Performance Estimation CMPE 413 4 UMBC U M B C U N I V E R S I T Y O F M A R Y L A N D B A L T I M O R E C O U N T Y 1 9 6 6 Static Dissipation Reverse-bias leakage current Total Static Power dissipation: I A d I s e qV kT ------- 1 = where I s = the saturation current.
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