laboratory experiment – mastering digital design (part ii) she… · mastering digital design...

MasteringDigitalDesign DepartmentofEEEwithVerilogonFPGAs ImperialCollegeLondon

v5.0-PYKCheung,9Oct2017 Part2-1

MScCourseonAnalogueandDigitalICDesign

LaboratoryExperiment–MasteringDigitalDesign(PartII)

(webpage:http://www.ee.ic.ac.uk/pcheung/teaching/MSc_Experiment/)

PART2–CountersandFSMs

1.0LearningOutcomes

Part2oftheexperimentteachesyou:

• howtodesigndifferenttypesofcountersandtimers;• howtousetheModelsimsimulatortoverifythecorrectfunctionofyourdesignand

theuseoftestbenches;• how to predict themaximum operating clock frequency of your circuit sequential

circuits;• how to design some useful timing and counting components for later part of the

experiment.

1.1 Experiment5:DesigningaCounterStep1:Createtheprojectforan8-bitcounter

• Createinyourdirectoryafoldernamedpart_2.• Click file>New ProjectWizard, and create projectex5 and top level fileex5_top.

ThenclickFinish.• Create the Verilog file: “counter_8.v” which contains your design in Verilog. I

suggestyouuseconventionofusing“_n”toindicatethenumberofbitsinamodule.• ClickFile>New…andselectVerilogasthenewfile.Aneditwindowwillappear.

Step2:EntertheVerilogspecificationofthe8-bitbinarycounter

• EntertheVerilogmoduleasshownbelow(nextpage). Althoughyoucanmissoutthe comments, I recommend that you to retain them because the code isdeliberatelyverboseinordertoexplainthemeaningoftheVeriloglanguage.

• The line `timescale1ns / 100ps tells the systemtouse1nsas theunit timestepwithatimeresolutionof100ps.

• MakesurethatyoufullyunderstandthisVerilogcodebeforeproceedingtothenextstep.Savethefileascounter_8.v.(Irecommendthatyouusemodulenameasthefilenametoavoidconfusion.)

Step3:EntertheVerilogspecificationofthe8-bitbinarycounter

• WhileisopenedintheEditorwindow,click Project>AddCurrentFiletoProject,thenclickProject>SetasTopLevelEntity. ThiscommandtellsQuartusthatthismoduleisthetop-levelofyourdesign.

Normally we use …_top.v as the top-level module, which connects tophysical pins of the FPGA. However, for this experiment, the countermodule is verified through simulation. So we don’t need to create pinconnects.The“SetasTopLevelEntity”isveryusefulifyouwanttousethesimulatortoverifydifferentmodulesinalargedesign.Youcanmoveupanddownthemodulehierarchyandverifythemfromthelowestlevelup.



• ClickProcessing>AnalyzeCurrentFile.Thisisthefastestwaytocheckifthis.vfilehasanysyntaxerror.

• ThenClickProcessing>Start>StartAnalysisandSynthesis.ThistakesthecurrentVerilogmodule (andallothermodulesthat ituses ifany),andproducearegister-levelmodelofyourdesignreadyforregister-transferlevel(RTL)simulation.Unlikefullcompilation,thisstepdoesnotrequirepinassignmentandotherdevicespecificsteps,butissufficientforyoutosimulatethecircuitasspecifiedinVerilog.

Verilogcode:8-bitcounter(Notethatthefirstcharacteronline1before

‘timescale’isabackquote`-noteasytofindonmanykeyboards!)

Step4:Simulatethebinarycounter

• ClickTools>RunSimulationTools>RTL Simulation. This commandstarts up Modelsim simulatorprogramme as a separate process.NowyouhaveenteredtheModelsimenvironment.

• ClickSimulate>StartSimulation….Thenselectwork -> counter_8 fromthe popup window. This tellsModelsimtosimulatethismodule.

• NotethatModelsimprovidesanumberofwindowpanes.ThemostimportantistheTranscriptpane– this iswhereyouenter commands1todrive the simulator. Thewavepaneiswhereresultsaredisplayedaswaveforms.Youarerecommendedtoun-dockthispaneasshownbelowsothatitisinaseparatewindowandspansthewholewidthofyourmonitor.Finally,thereistheobjectpane,whichshowsallthesignals(objects)ofyourdesign.

1ModelsimusesascriptinglanguageknownasTclinordertocontrolhowitisdriven.YouonlyneedtolearnTclifyouwanttodoadvancestuffwithModelsimforyourpersonalinterest.



Step 5: Add waveforms to the Wave window and drivesignals

• Inthetranscriptwindow,entertwocommands:“addwave clock enable” and “add wave –hexadecimalcount”. Thiswill add these signals aswaveforms inthe wave pane and show count values ashexadecimal.

• Nowwewanttodriveclockwitha50MHzsymmetricalsignal.Todothis,enter:• Enter:“forceenable1”toenablethecounter.• Enter:“run100ns”torunthesimulatorfor5clockcycles(5x20ns=100ns).• Youwill see thewaveformpane showing the counter counting from0 to 5.Now

forceenablelowandrunforanother100ns.Thenhighagainandrunfor100ns.

• Clickonthewaveformputacursorataspecifictimeforinspectingthesignalvalues.The icons above thewaveforms (as labeled) allow you to zoom in andout of thewaveform.Trythisyourself.



Step6:CreateaTestbenchasaDO-file

• Interactively specifying the drivingsignals is very tedious and prone toerror. Therefore thepreferredmethodistocreatea“do”filewhichisatextfilecontainingasequenceofcommands(asyou have previously entered in thetranscriptwindow).

• Click File > new > source and selectnew“do”file.Thenenterthecommandlinesasshownontheright. Thensavethisas“tb_counter.do”.

• Deleteallsignalsfromthewavewindow,andentercommandvsim>restartvsim>do./tb_counter.do

• This should provide exactly the same waveform results as in step 5. However,the .do file can be reused and modified far easier than typing them into thetranscriptwindow.Itactsasasimpleformofatest-harness(ortestbench)foryourdesign. Generally speaking, you must produce testbenches for all your designsinsteadofusinginteractivemeanstotestyourcircuit.Notonlybecausethissavestime, it alsoallowsyou to change the codeandverify its correctness in the samewayforeachversionofyourdesign.

Step7:Singlestepping

• Modelsimisverypowerful.YoucanuseittodebugyourVerilogdesignalmostlikesoftware.However,dorememberthatwearedealingwithahardwaredescriptionthatoperatesinparallel. Incontrast,softwarecodesaregenerallyproceduralandoperatesequentially.

• Trythevsim>stepcommandorclickonthestep-commandpane towatchhowyoucanstepthroughyourVerilogcode. Signalvalues intheobjectandthewavewindowsareupdatedaccordingly.

• Modelsimhasmanyusefulfeaturestohelpyoudebugyourdesign.DetailsofallthecommandscanbefoundintheModelsimReferenceManual.ThisiseasilyavailableunderHelp>PDFDocumentations>ReferenceManual.Bewarethatthismanualisverythick!DONOTprintthisout.

2.0Experiment6:Implementinga16-bitcounteronDE1

Inthispartoftheexperiment,youwilltestyourcounterdesignontheDE1board.Youwillalsolearnhowtofindthemaximumclockfrequencythatyourdesignwillworkcorrectly.

Step1: Createanewprojectex6,andcopytothisdirectlyyourfilescounter_8.v.Modifycounter_8.vtocounter_16.vandmakeita16-bitcounter.Furthermore,addaresetinputtoresetthecountvaluetozerosynchronouslytotheclock.Downloadfromtheexperimentwebpagethecomponentbin2bcd_16.v,amoduleIhavedesignedtoconverta16-bitbinarynumber to 5 BCDdigits. Youwill also need the add3_ge5.vmodule. Put thesemodule inthe../mylibfolder,whichshouldalsocontainedthehex_to_7seg.vyoudesignedinPart1.

Step2:Createatop-levelmoduleex6_top.v inVerilogtospecifythecircuitshownbelow.MakesurethatyouhaveaddedalltherelevantVerilogmodulestotheprojectusingProject> Add/Remove Files in Project: counter_16.v, ex6_top.v and finally add hex_to_7seg.v,



add3_ge5.v and bin2bcd_16.v from your library folder ../mylib/. Go to the ex6_top.vwindowandsetthisfileasyourtop-levelmodule.

Step3:UseProcessing>AnalyzeCurrentFilecheckyournewlycreateVerilogfiles.Thisisthequickestwayto find thebasicsyntaxerrors inyourVerilogcode. Onceall thesimpleerrorsarefixed,useProcessing>StartAnalysisandElaborationtoperformfullercheckofthe “ex6_top.v” tomake sure that files are consistent and correct. There is no need tosimulatethiscircuit.

Step4: Selecting theFPGADevice–ClickAssignments>Device….andselectthecorrectCycloneVFPGA:5CSEMA5F31C6.

Step5:PinAssignment–Opentheex6_top.qsffile.Examineitscontent.Youwillfindthatnopinsarebeingassignedyet.Insertintothisfileallthepinassignments.Theeasiestwaytodothisisclickon:Edit>Insertfile..thenselect../pin_assignment.txt(youshouldhavedownloadedthisfilefromtheExperimentwebpage).Notethatyouarecurrentlynotusingall thepinsassigned in thepin_assignment.txt file. Don’tworry.Thiswillonlyproduceafewmorewarningmessages.Fullcompilationcanstillgoaheadwithouterrors.

Step6: Set clock frequency–Createanewfile“ex6_top.sdc”2whichshouldcontainonesingleline:

create_clock-name"CLOCK_50"-period20.000ns[get_ports{CLOCK_50}]

Withthis,QuartuswillknowthatthesignalCLOCK_50isa50MHzclock.

Step 7: Full Compilation – Click: Processing > StartCompilation. This will go through the entire compilationprocess.ExaminetheTaskswindowontheleftandseeallthestepsbeingtakeninordertogeneratethefinalbit-stream.

Step 8: Maximum clock frequency – As part of thecompilation process, TimeQuest timing analyzer is used topredict various timing information. In the “CompilationReport” window, you should see a list of reports resultingfrom the compilation. Double-click TimeQuest TimingAnalyzer entry, and you should see a list similar to the oneshownhere. Clickingonvariousentriesunderthiswill showthevarioustimingspecifications.Answerthefollowingquestions:

2SynopsisDelayConstraint(.sdc)filesarestandardformattedfilesintroducedbySynopsis,awell-knowncompanyspecializingonICdesignCADtools.Withthis,adesignercanspecifyvarioustimingconstraintsfortheCADtoolsthecheckagainst.Hereweareonlyusingthistodefineclockfrequency.



Whatarethepredictedmaximumfrequenciesforthiscircuitunderthehighestandlowesttemperatures?Whataretheotherinterestingtimingdatathatyoucandiscoverwiththesereports?WhyistheTimeQuestentryred,indicatingthattheremaybeaproblem?

Step9: TestyourdesignonDE1–programtheDE1andcheckthatyourdesignworks.

Step10:ExaminetheamountofFPGAresourcesbeingusedbythis16-bitcounter.Explaintheresults.

Test-yourselfTask(compulsory)–Cascadecounter

You are now required to create something yourself. In the previous exercise, the 16-bitcounteriscountinga20MHzclock.Thisismuchtoofastforustoseethecounterchanging.This part of the experiment requires you use the counter to count the number ofmillisecond elapsed. You would need to do this by having two counters cascaded (i.e.connectedinseries)witheachother.Theoverallblockdiagramisshownbelow.

The divide-by-50000 circuit generates a 1 cycle high pulse every 50,000 clock cycles.Thereforetheoutputsignaltickprovidesoneenablepulseeverymillisecond.(Seenotes.)

ModifyyourcircuittoimplementthisandtestthenewcircuitontheDE1board.

3.0Experiment7:LinearFeedbackShiftRegister(LFSR)andPRBS

You will have encountered a 4-bit LFSR in my introductory talks, which implements theprimitive polynomial: 1 + X3 + X4. You are now required to implement a 7-bit LFSRimplementingthepolynomial:1+X+X7.Assumingthatyouinitializetheshiftregisterto7’d1,workoutmanuallythefirst10sequencevaluesoftheoutputsequence.(Theoutputsequence should be 127 long without repetition, is known as a pseudo-random binarysequenceorPRBS.)

ConnecttheshiftregisterclocktoKEY[3]andusethemomentarykeytocyclethroughthefirst ten valuesof thePRBS. The randomoutput shouldbedisplayed as twohexadecimaldigits.

Checkpoint:Youshouldgettothispointbytheendofthesecondweek.



4.0Experiment8(Optionalchallenge):Startinglinedelaycircuit

Thenexttwoexperimentsareoptional.Theyaredesignedtoprovideachallengetothosewhofinishearly,orforthosewhowanttolearnmoreaboutdigitaldesign,VerilogandFPGAs.Thetwoexperimentsarelinked–whatyoudesignedinExperiment8willbeusedinExperiment9.

ThegoalhereistodesignaFormula1styleofracestartinglights.Thespecificationofyourcircuitis:

1. Thecircuitistriggered(orstarted)bypressingKEY[3](don’tforgetKEY[3]islowactive);

2. The10LEDs(belowthe7-segmentdisplays)willthenstartlightingupfromlefttorightat0.5secondinterval,untilallLEDsareON;

3. Thecircuitthenwaitsforarandomperiodoftimebetween0.25and16secondsbeforeallLEDsturnOFF;

4. Youshouldalsodisplaytherandomdelayperiodinmillisecondsonfive7-segmentdisplays.

Inordertoassistyouindesigningthiscircuitwithoutspendingtoomuchtime,thefollowingoverallblockdiagramofthecircuitisprovided.Youshouldalsodownloadthesolutionbit-streamforthisexperimentfromtheexperimentwebpage(ex8sol.sof)andtryitoutbeforeattemptityourself.

Intheabovediagram,allsignalsontheleftoftheblockareinputsandthesignalsontherightareoutputs.

Thetwoclockdividercircuitsprovideclockticksonceevery1msand0.5secrespectively.EachclocktickshouldbeapositivepulselastingoneperiodofCLOCK_50(i.e.20ns).Thesystemthenusethetick_mssignalastheclockoftheremainingcircuit.

TheLFSRmoduleproducesapseudo-randombinarysequence(PRBS),whichisusedtodeterminetherandomdelayrequired.TheenablesignaltotheLFSRallowsthistocyclethroughanumberofclockcyclesbeforeitisstoppedatarandomvalue.

Thedelaymoduleistriggeredafterall10LEDsarelid,andthenprovidesadelayofNclockcycles(at1msperiod)beforeassertingthetime_outsignal(for1ms).

ThedelayvalueNisfedtothebinarytoBCDconverter,whichthendrivesthe7-segmentdisplays.

Thereareanumberofdesigndecisionstobemade:



1. HowmanybitsLFSRisrequired?2. Howmanybitsshouldyouuseinthedelaymodule?

TheFSMmoduleisthekeymoduletotheentiresystem.Youhavetodecidewhatarethestatesthatarerequired,drawthestatediagramandthenmapthattoVerilog.

5.0Experiment9(Optionalchallenge):AReactionMeter

ExtendyourcircuitinExperiment8byaddingareactioncounter.ThisshouldcountthetimebetweenalltheLEDsturningOFFandyoupressingKEY[0].Thereactiontime,insteadoftherandomdelay,shouldbedisplayedonthe7-segmentdisplaysinmilliseconds.

laboratory experiment – mastering digital design (part ii) she… · mastering digital design...

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