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ID:41287406
大小:704.50 KB
页数:27页
时间:2019-08-21
《数字电路7..1》由会员上传分享,免费在线阅读,更多相关内容在行业资料-天天文库。
1、逻辑电路分为两大类:combinationallogiccircuit(组合逻辑电路)sequentiallogiccircuit(时序逻辑电路)任何时刻的输出仅取决于当时的输入。任一时刻的输出不仅取决于当时的输入,还取决于过去的输入序列。电路特点:无反馈回路、无记忆元件电路特点:有反馈回路、有记忆元件1LatchesandFlip-flopsClockedSynchronousState-MachineAnalysisClockedSynchronousState-MachineDesignChapter7SequentialLogicDesignPrinciples(时序
2、逻辑设计原理)27.1BistableElements(双稳态元件)QQ_LIthastwostablestates:Q=1andQ=0——bistablecircuit(双稳电路)Whenpowerisfirstappliedtothecircuit,itrandomlycomesupinonestateortheotherandstaysthereforever.3Vin1Vout1Vin2Vout2Vout2Vin2=Vin2=Vout2stable稳态metastable亚稳态QQ_LVin1Vout1Vin2Vout2AnalogAnalysis(模拟分析)4亚稳态
3、的存在使电路的状态可能出现不确定性。稳态稳态亚稳态Randomnoisewilltendtodriveacircuitthatisoperatingatthemetastablepointtowardoneofthestableoperatingpoints.从一个“稳态”转换到另一个“稳态”需加一定宽度的脉冲(足够的驱动)。MetastableBehavior(亚稳态特性)Vin1Vout1=Vin2=Vout257.2LatchesandFlip-Flops(锁存器与触发器)Latchchangeitsoutputsaccordingtoitsinputsdirectly
4、atanytime.Flip-Flopchangeitsoutputsonlywhenaclockingsignalischanging(attheedgeofaCLKsignal).S-RLatchDLatchDFlip-FlopJ-KFlip-FlopTFlip-FlopContents:——basicbuildingblocksofmostsequentialcircuits6QQNRS(1)S=R=0Thecircuitretainspreviousstate.00QQLNORgatenotgateQ*=QQN*=QNNextstate(新态)Presentstat
5、e(原态)S-RLatchPrinciples:7QQNRS10(2)S=0,R=1a.原态:Q=0,QN=101新态:Q*=0,QN*=1b.原态:Q=1,QN=0新态:Q*=0,QN*=1锁存器清0:Q*=0QN*=1即使S,R无效(=0),锁存器仍能锁定0态.Reset10(a)QQNRS1001(b)0101S-RLatchPrinciples:8QQNRS01(3)S=1,R=0a.原态:Q=1,QN=010新态:Q*=1,QN*=0b.原态:Q=0,QN=1新态:Q*=1,QN*=0锁存器置1:Q*=1QN*=0即使S,R无效(=0),锁存器仍能锁定1态.Set0
6、1(a)QQNRS0110(b)0110S-RLatchPrinciples:9QQNRS(4)S=R=100Q*=QN*=0当S,R无效(=0)时,11QQN00状态不确定!“禁止”S-RLatchPrinciples:10…10…10S-RLatchSQRQNLogicsymbolResetSet清0置100011011SRlaststate011000QQNFunctiontableSQRQQQNRS000001010011100101110111SR01001100QQ*状态转移真值表11tpw(min)00011011SRlaststate011000Q
7、QNSRQtpLH(SQ)tpLH(RQ)SRQQNpropagationdelay禁止输入Typicaloperationminimum-pulse-width12S-RLatchS_L=R_L=111100100S_LR_Llaststate011011QQNFunctiontableretainpreviousstateS_L=1,R_L=0Q=0,QN=1S_L=0,R_L=1Q=1,QN=0S_L=R_L=0Q=QN=1,不定状态SR清0置1禁止状态SQRQLogicsymbolQQNS_LR
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