数字逻辑设计及应用教学英文课件：Lec17-Chap 7.ppt

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1、1DigitalLogicDesignandApplicatonLecture#17LatchesandFlip-FlopsUESTC,Spring20132再谈串行输入加法器的实现QDQCXYCISiCi+1XiYiCiSCOCLK暂存XYCICOSCi+1SiXiYiCi时钟控制电平有效还是边沿有效？串行输入、串行输出注意：时钟同步IterativeVs.Sequential3IterativeVersusSequentialCircuitsC0C4X0Y0X1Y1X2Y2X3Y3S0S1S2S347.2Lat

2、chesandFlip-FlopsLatcheschangeitsoutputsatanytime(enableinputisasserted).Flip-FlopschangeitsoutputsonlywhentheclockchangesCLKPositive-EdgeRising-EdgeNegative-dgeFalling-EdgeLeveltriggeredEdge-triggered,ET57.2.5Edge-TriggeredDFlip-FlopsDQCQDQCQQQLDCLKmasterslaveQ

3、mWhenCLK=0,WhenCLK=1,QmfollowsD;Qisunchanging.Qmisunchanging,Q=QmuntilthenextrisingCLKedge1.Master/Slave,Positive-edge-triggeredQchangesonlywhenCLKrisesfrom0to16TimingDiagramformaster-slaveDF/FDCLKQQmDQCQDQCQQQLDCLKQmQ=D()7DCLKQDQCLKQLogicSymbolEdge-Triggered8DC

4、LKQDCLKQDLatchDFlip-Flop——edge-triggeredwhenCLKisasserted对信号扰动不敏感受信号扰动影响9Dflip-floptimingparametersPropagationdelay(fromCLK)tpLH(CQ)tpHL(CQ)tsetup建立时间thold保持时间Setuptime(DbeforeCLK）Holdtime(DafterCLK)DCLKQ在时钟上升边沿附近有一个时间窗口，在此窗口时间内，数据输入D不能改变，否则电路进入亚稳态。从输入信号D到来时刻至信号

5、D达到稳定所持续的时间。从输入信号D达到稳定至信号D被锁存器接受所持续的时间。102.Negative-Edge-TriggedDFlip-FlopDQCQDQCQQQNDCLKDQCLKQDQCQDQCQQQLDCLKPositive-edge-triggered3.DFlip-Flopwithpresetandclear同步(synchronous)是指与时钟同步，即时钟触发条件满足时检测清零(置位)信号是否有效，有效则在下一个时间周期的触发条件下，执行清零(置位)；异步(asynchronous)是清零(置位)信

6、号有效时，无视触发脉冲，立即清零(置位)。11123.DFlip-FlopwithpresetandclearCLKQQLDPR_LCLR_LPRDQCLKQCLRasynchronousinputsbehaviorlikethesetandresetinputsonS-Rlatch.100111110110013TimingDiagramforDflip-flopwithpresetandclearCLKPR_LCLR_LQL3.DFlip-Flopwithpresetandclearsynchronousinput

7、s:PR(preset)andCLR(cleardependontheCLK14DQCLKQDinResetCLK?R有效，Q=0R无效，Q=DinDinReset_L?154.CommercialTTLDFlip-Flops(74LS74)P535Figure7-20PR_LCLR_LCLKDQQN维持阻塞结构Smallerandfaster165.CMOSedge-triggeredDcircuitUsestransmissiongatesinfeedbackloopsQDCLKDCLKTwofeedbackloo

8、ps(masterandslavelatches)MasterSlave177.2.6DFlip-FlopwithEnable2-inputMultiplexerDQCLKQDENCLKQQLIfENisasserted,theexternalDinputisselected;ifENisnegated,thecurrentout