Lec-04_Speed-Optimization-a

Lec-04_Speed-Optimization-a - EE M216A .:. Fall 2010...

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EE M216A .:. Fall 2010 Lecture 4 Speed Optimization Prof. Dejan Markovi ć [email protected] Speed Optimization via Gate Sizing ± Gate sizing basics P:N ratio Complex gates Vl i t t t i Velocity saturation Tapering ± Developing intuition Number of stages vs. fanout Popular inverter chain example D. Markovic / Slide 2 ± Formal approach: logical effort ± Sizing optimization for speed EEM216A .:. Fall 2010 Lecture 4: Speed Optimization | 2
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Basic Gate Sizing Relationships ± Rise and fall delays are determined by the pull up and pull down “strength” Besides the dimensions, strength depends on µ , C OX , V T PMOS is weaker because of lower µ PMOS is weaker because of lower P Larger P network than N network ± Increasing size of gate can reduce delay Inverse (1/W) relationship with resistance (and hence delay) BUT it can slow down the gate driving it Proportional (W) relationship with Capacitance So be careful D. Markovic / Slide 3 Proportional (W) relationship with Capacitance. So be careful! EEM216A .:. Fall 2010 Lecture 4: Speed Optimization | 3 P:N Ratio for “Equal” Rise and Fall Delay ± Good to have roughly equal delays for different transitions Don’t need to worry about a worst case sequence Size P’s to compensate for mobility C V Lh l t h OX , V T , L are roughly the same Make the Pull up and Pull down resistances equal R N /R P = 1 = µ P W P / µ N W N = k β, k = mobility ratio, β = P:N ratio W P /W N = µ N / µ P W I R DRV µ / 1 / 1 D. Markovic / Slide 4 ± Approximately the same as making V THL = V DD /2 ± Easy for an inverter What about more complex gates? EEM216A .:. Fall 2010 Lecture 4: Speed Optimization | 4
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Complex Gate Sizing ± N stack series devices need N times lower resistance N × Width ± Make worst case strength of each path equal Mult input transition can result in stronger networ Multi input transition can result in stronger network ± Long series stacking is VERY bad W 2W 2W 6W 6W B A 6W E.g.: β = 2 D. Markovic / Slide 5 C W W W B A C W W W EEM216A .:. Fall 2010 Lecture 4: Speed Optimization | 5 Accounting for Velocity Saturation ± Series stacking is actually less velocity saturated If we use R no_stack = (4/3)R stack Adjust the single device size to account for velocity saturation W 2W 2W 6W 6W B A 6W E.g.: β = 2 4W/3 4W/3 D. Markovic / Slide 6 C W W W B A C W W W EEM216A .:. Fall 2010 Lecture 4: Speed Optimization | 6
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P:N Ratio for Minimum Delay ± Delay of an inverter chain (2 inverters) to include t pLH & t pHL W P W P W W in out W P W ± Let R PDRV ~ R 0 ’/W P µ P , R NDRV ~ R 0 ’/W N µ N , C G ~ C 0 (1+W P /W N ) t PD = t D1 + t D2 = R 0 ’(1/W P µ P + 1/W N µ N ) C 0 (1+W P /W N ) τ N (1+1/k β )(1+ β ) ± Min(t PD ): dt PD /d β = 0 = τ N (1 k/ β 2 ) N N N D. Markovic / Slide 7 So β = W P /W N = sqrt( µ N / µ P ) Intuition is that since NMOS has more drive for a given size, it is better to use more NMOS EEM216A .:. Fall 2010 Lecture 4: Speed Optimization | 7 FO4 Inverter Delay vs. P:N Ratio β ± Optimal β = sqrt( µ ) for minimum delay
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This note was uploaded on 10/19/2011 for the course ELECTRICLA 216A taught by Professor Marković during the Fall '10 term at UCLA.

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Lec-04_Speed-Optimization-a - EE M216A .:. Fall 2010...

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