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cmos delay calculation
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HowtoextractMOStransistoreffective resistance used in DelayeffectiveresistanceusedinDelay
calculationsD.W.Parent
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AnistheeffectiveresistanceofanNMOSTransistor.
AMI06Parameters
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Simple Example of a Delay calculationSimpleExampleofaDelaycalculation
Howdoesthismatchaspicesimulationwithallthepropermobilityandcapacitancemodels?
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Delay is 161ps, not 11.6ps!Delayis161ps,not11.6ps!
W 3Wn=3umWp=2xWnRisetime50psoninput
Vdd=5VVdd 5V
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Why?Why?AnhastobetakenoverthehowdischargecycletomodelthemobilitychangesintheChannelvs.thegateanddrainvoltages.
WehavenottakenintoaccounttheoutputcapacitanceoftheCMOSdriver.(DraintobodyCapacitancesofthedraindiodesofthepmos andnmos transistorsaswellasthegatetoDrain overlap of the nmos and pmos transistorsDrainoverlapofthenmos andpmos transistors.
WecanextractAn,andtheoutputcapacitancebysweepingCL(Cexternal)andmeasuringdelay.
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Set up an Parametric AnalysisSetupanParametricAnalysis
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Wewanttoextractdelayllautomatically.
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In the wave form viewer click on calculator.Inthewaveformviewerclickoncalculator.
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InthecalculatorclickonoptionsandsettoSetAl b iAlgebraic.
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ClickonSpecialFunctions..Delayp yIfyoudonotseetheGetBufferbuttonsClickonoptionsincalculatorAnd set to Set AlgebraicAndsettoSetAlgebraic.
Back in calculator click on family and then in the waveform click on the A inputBackincalculatorclickonfamilyandtheninthewaveformclickontheAinput
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BackinThresholddelayclickWf1:GetBuffery
Gobackintocalculator,eraseA,andsubstituteY.
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Gobacktothresholddelayandclickf ffonWf2:GetBuffer
Filloutthepopupwithathresholdvalueof2.5
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In threshold delay click on apply.Inthresholddelayclickonapply.
l k lClickonerplot.
Changethexaxistolinear,doubleclickingonCL.13
Use A and B to get slopeUseAandBtogetslope
StillOff!
NewAn: 14
ByCalibratingAnwegotmuchclosertomatchingourh d l l t d d l b t till ffhandcalculateddelaybutwearestilloff.
We need Coutput!WeneedCoutput!
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Thisseemstomatchquitewell,thetroubleisthatwewouldhavetodothisextractionof Coutput every time we changed Wn and WpofCoutput everytimewechangedWn andWp.
Weneed:Cout asafunctionofWn andWp!
OnlyafunctionofWp!
BothfunctionsofWn
SweepWp tochange AssumeCoutput varieslinearlywithWp16
DoasweepofWp keepingWn andCexternalfi dfixed.
Slopeisins/m 17
HowdowegetCn?
SweepCexternal andmeasureThefallingoutputdelay
mxb
Make sure to change the edgesMakesuretochangetheedgesTomeasurethefallingpropagationdelay
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Find Ap from the slopeFindAp fromtheslope
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Extract CNExtractCN
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HowdoesthisCoutput compare?p p
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Double Check DelayDoubleCheckDelay
Within5%error.ItlooksliketheextractionofCoutput istheproblem.
Can we use this for design?Canweusethisfordesign?
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HowtousetheseequationsforDesign
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DesignEquationcontinued
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DesignEquationcontinued
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DesignEquation
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DesignExample
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Within 10% Error.Within10%Error.
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