集成电路设计-系统设计

1集成电路设计第十章

系统设计（1）

Sequentialmachinedesign2outlineCounter01stringrecognizerTrafficlightcontrollerStateassignment3SequentialmachinedesignTwowaystospecifysequentialmachine:structure:interconnectionoflogicgatesandmemoryelements.function:Booleandescriptionofnext-stateandoutputfunctions.Bestwaydependsontypeofmachinebeingdescribed.4CounterEasytospecifyasone-bitcounter.Hardertospecifyn-bitcounterbehavior.

Canspecifyn-bitcounterasstructuremadeof1-bitcounters.5One-bitcounterTruthtable:CountCin next Cout0 0 0 00 1 1 01 0 1 01 1 0 16One-bitcounterimplementation7One-bitcounteroperationAlloperationsareperformedas

’sedge.XORcomputesnextvalueofthisbitofcounter.NAND/invertercomputecarry-out.8n-bitcounterstructure9Statetransitiongraphs/tablesBasicfunctionaldescriptionofFSM.Symbolictruthtablefornext-state,outputfunctions:nostructureoflogic;noencodingofstates.Statetransitiongraphandtablearefunctionallyequivalent.1001stringrecognizerBehaviorofmachinewhichrecognizes“01”incontinuousstreamofbits:time 0 1 2 3 4 5input 0 0 1 1 0 1state bit1 bit2 bit2 bit1 bit1 bit2next bit2 bit2 bit1 bit1 bit2 bit1output 0 0 1 0 0 11101recognizeroperationWaitsfor0toappearinstatebit1.Goesintoseparatestatebit2when0appears.If1appearsimmediatelyafter0,can’thavea01onnextcycle,socangobacktowaitfor0instatebit1.12StatetransitiontableSymbolicstatetransitiontable:input present next output0 bit1 bit2 01 bit1 bit1 00 bit2 bit2 01 bit2 bit1 113StatetransitiongraphEquivalenttostatetransitiontable:14StateassignmentMustfindbinaryencodingforsymbolicstates—stateassignment.Choiceofstateassignmentdirectlyaffectsboththenext-stateandoutputlogic:area;delay.Mayalsoencodesomemachineinputs/outputs.1501recognizerencodingChoosebit1=0,bit2=1:input present next output0 0 1 01 0 0 00 1 1 01 1 0 116LogicimplementationAfterencoding,truthtablecanbeimplementedingates:17TrafficlightcontrollerIntersectionoftworoads:highway(busy);farm(notbusy).Wanttogivegreenlighttohighwayasmuchaspossible.Wanttogivegreentofarmwhenneeded.Mustalwayshaveatleastoneredlight.18Trafficlightsystemhighwayfarmroadsensortrafficlight19SystemoperationSensoronfarmroadindicateswhencarsonfarmroadarewaitingforgreenlight.Mustobeyrequiredlengthsforgreen,yellowlights.20TrafficlightmachineBuildcontrolleroutoftwomachines:sequencerwhichsetscolorsoflights,etc.timerwhichisusedtocontroldurationsoflights.Separatecounterisolateslogicaldesignfromclockperiod.Separatecountergreatlyreducesnumberofstatesinsequencer.21Sequencerstatetransitiongraphhwy-greenfarm-greenhwy-yellowfarm-yellow(cars&long)’/0greenredcars&long/1yellowredshort’/0yellowredshort/1redgreencars&long’/0greenredcars’&long/1redyellowshort’/0redyellowshort/1greenred22Stateassignmentinn-space0s1code=111s2code=11011123UnreachablestatesStateassignmentmaycausesomestatestobeunreachable.Asaresult,itmaynotbepossibletoapplysomerequiredtestvalues.24Unreachablestateexample