FPGA_ASIC-Metastability_Overview

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1、http://www.elecfans.com电子发烧友http://bbs.elecfans.com电子技术论坛Metastability/Overview–Metastabilityofflip-flops(FFs)andlatches(D-FF,JK-FF,T-FF…)•Basics•Small-signalgainofgates•Gainandtransientbehaviorofclockedflip-flops•Timingrequirementsofclockedflip-flops•Observationofmetastability•Problemanalysisa

2、ndcharacterisation•SynchronisersState0State129/09/2005EE6471(KR)291http://www.elecfans.com电子发烧友http://bbs.elecfans.com电子技术论坛Metastability/LatchesandFlip-FlopsDataLatches:•Outputfollowsinputwhenlatchinputisasserted(level-triggered)•Dataisheldwhilelatchinputisde-assertedDataFlip-Flops:•Outputfoll

3、owsinputwhenclocked(positiveornegativeedge)•Dataisheldbetweenclockevents•Commonly:LatchesandFF’salsoprovideasynchronousreset(and/orpreset)•ExamplesynchronousFF:MM74HC74A(seebelow)DQClr29/09/2005EE6471(KR)292http://www.elecfans.com电子发烧友http://bbs.elecfans.com电子技术论坛Metastability/GainofLogicGatesD

4、igitalLogicGates:•Forsmallsignalsaroundtheirlogicthresholdslogicgatesactlikelinearamplifiers•Gatesdoprovidehighgainsaroundthethreshold(butnotethatthegainisnotinfinite!)•TTLgates/unbufferedCMOSgatesprovidearelativelylowgain(10..100)•BufferedCMOSgatesprovidehighgains(>1e3)•Inafewspecificapplicati

5、onsgatemaybeusedintheirlinearregion(e.g.oscillators)HCMOSTTL(LS)29/09/2005EE6471(KR)293http://www.elecfans.com电子发烧友http://bbs.elecfans.com电子技术论坛Metastability/TransientResponseofGatesDigitalLogicGates:•Inthelinearregionthevoltagetransferfunctionofdigitalgatesisapproximatelyfirstorder...(here,inv

6、erter~vin~voisbehavinglikeaninvertingamplifier)Vo(p)gainH(p)==Vin(p)1+pTgateVccInthelinearregionthestepresponseisapproximately...~vin~vot-vo(t)=()V-Vth×gain×1-eTgatestepVth29/09/2005EE6471(KR)294http://www.elecfans.com电子发烧友http://bbs.elecfans.com电子技术论坛Metastability/FF&LatchModelModelofF

7、F’s(andlatches):Incontrasttogates,FF’s(andlatches)behavelikesamplingamplifierswithpositivefeedback.Atclockevent,inputissampled(S1closed).Afterclockevent(S1’closed),theFFamplifier,onceslewedintothecorrectposition,merelyholdsthecirc