eaglecad tutorial - university of illinois...schematic capture let’s start with some of the...

EAGLECAD TUTORIAL ECE445

UniversityofIllinoisatUrbanaChampaignDepartmentofElectricalandComputerEngineering

Instructions ThisdocumentationincludesexplanationsofEagleCADfunctions/featuresandECE445-specificpointersforboththeEagleassignmentandthePCBrequiredforeachSeniorDesignproject.Thisismeanttobesupplementarymaterialtoastep-by-stepguide,liketheonelinkedintheassignment.

AppendixAhasEagle’slayerdefinitionsandAppendixBincludesPCBlayoutguidelines.

Overview Schematicsarethecoreofallelectricalengineering.Theyrepresentauniversallistofsymbolsandnotationsthatcanbefollowedbyanyoneinordertopresentsomesortofcircuitry.Infact,onecaneventhinkofsomeasasortoflanguagethatisusedtocommunicateourelectricalideas.Justlikealanguage,wecanpresentanideainaclearandspecificmanner,orwecanobfuscateitthroughadenseandcomplexlayoutofthoughts.Thegoalofthistutorialistohelpyoulearntipsandmethodstobeintheformercamp.

Whileschematicsarenicefromananalyticalstandpoint,realcircuitsdon’toftenfollowthesameshapesorlayout.Asyouhavemostlikelyseeninpreviousclasses,eventryingtoprototypeonabreadboardoftenhasonemovingpartsaroundinordertogettheproperconnections.Inthisclass,youaregoingtolearnhowtotranslatedesignsfromschematicstoarealizableprintedcircuitboard,orPCB.

EagleCADisoneofmanyavailableElectronicDesignAutomation(EDA)softwarepackages.Itallowsustoeasilydesignandtransitionbetweenschematicsandcircuitlayout.EagleCADhasalargeamountofpopularitywithhobbyists,andduetothenatureoftheprojectsinthiscourse,youwillmostlikelyfindthattobeofuse.YoucandownloadthefreewareversionofEagleCADhere:http://www.cadsoftusa.com/download-eagle/.Thelabcomputershavethefullprofessionalversionsinstalled.WARNING:DIFFERENTEAGLEVERSIONSMAYNOTBECOMPATIBLE.Thisappliesbothforwardandbackward.Weadvisestickingtooneversionfortheentiresemester.

Schematic Capture Let’sstartwithsomeofthefeaturesofEagle’sschematiccapturetool.Whenyouopen

EagleCAD,theProjectViewerscreenpopsup,andyoucancreateanewprojectbygoingtoFile->New->Project.Nameitwhateveryoulike,butmakesurethegreendotisvisiblenexttotheproject–thisindicatesitistheactiveproject.

Rightclickontheproject,andselectNew->Schematic.Thisshouldpopup:

Whileallthebuttonsavailableonthelefthavesomeusecase,we’llreallyonlybeusingafewofthemfor90%ofthework.Thenextpictureshowswhatfunctionsthesebuttonscommand.

AddingComponents:Presstheaddpartwindow,andyou’llseealistoflibrariesappear.

Theselibrariesaresuppliedfromavarietyofsources:CadSoft(thecreatorsofEagleCAD),numeroussemiconductorcompanies,andevenjustregularhobbyists.Sincethefiles

Movecomponentsontheschematicaround

Changethenameofacomponent

Drawwiresontheschematic

Labelwiresandotherconnections

Junctiontool-joinintersectionsofwires

Addacomponenttotheschematic

Changethevalueofacomponent

areverysimpletogeneratefornewcomponents(justsimpleSVGdrawings),youcanfindalibraryfornearlyanypart.Thedownsideisthattheorganizationleavesabittobedesired,butluckilythesearchfeatureissimpleenoughtouse.Fornow,we’llstartoffusingstandardlibraries.

Scrolldowntothelibrarycalledrcl–thisisyourstandardpassiveslibrary.Itcontainssymbols/footprintsforthebasicresistor,capacitor,andinductorparts.Openingitup,it’llhaveseveralsubfolders.SincethisisAmerica,openupR-US.Everythingcontainedinthatsublistisaresistor,butwithadifferentpackage.

Youcantellapartis“through-hole”bylookingatthePCBsymbol.Ifithasgreencircles,it’sathrough-holepartasthoseindicateviasinEagleCAD.Ifithasredsquaresinstead(bottomhalfoflist),thoserepresentpads,soitisasurfacemountpart.

Toaddapart,double-clickanddropitanywhereontheschematic.Tokeepaddingcopiesofit,continueclickingontheschematic.To“letgo,”justclicktheMovetool.UsingMove,left-clicktoselecttheparttomoveandright-clicktorotateparts.

ConnectingComponents:UsetheLineTooltodrawawire(or“net”)betweenthecomponents.Don’tforgettoaddaground,orreferencenode,toeveryschematic.Youcanfindthegroundsymbolunderthesupply1orsupply2libraries.Ifyoulook,you’llseethatgrounddoesn’thaveaboardview–whichmakessense,groundisanelectricalreferencevoltage,notatangible,physicalthing.

SchematicSymbol

PCBSymbol

UsetheNameTooltogiveyournetsanappropriatename:V_REG,V_OUT,VDD_5V,etc.You’llnoticethatdoesn’treallydoanythingbydefault,weneedtoalsoshowthelabelaswell.UsetheLabelToolandclickontheconnection,thenplacethelabelsomewherenearby.Youcanchangethestyleofyourlabelviathebuttonsonthetopbar,shownbelowwhereI’vechosenbothlabelingstyles.

Creating a New Part SincetheclassassignmentusestheTPS799voltageregulator,thisistheexamplewewill

coverindetail.Youcansearchinthelibraries,butit’snotthere(partdeliberatelychosenthisway).Youcanfindthedatasheethere:http://www.ti.com/lit/ds/symlink/tps799.pdf.Forthesimplicityoftheassignment,we’regoingtousethe“FixedVoltageVersion”onthebottomofthefirstpage.NowthefirstthingtodoistocreateanewlibraryinEagleforthispart.Gototheprojectmanagerwindow,selectyourproject,andrightclickNew->Library.

LabelStyleChanger

NowisagoodtimetotalkabouthowEagleCADstoresparts.Likeyousawwhenaddingparts,componentsgenerallyhavebothaschematicandboardview.Theschematic,orsymbolview,istheagreeduponelectricalsymbol,andtheboard,orpackageview,isthephysicallayout.Someitems–suchassupplylines(e.g.+5V,GND,VSS)–onlyhaveasymbolview,whereitemslikevias,traces,andwellsonlyhaveaboardview.Whencreatinganewcomponent,wegeneratethesymbolandpackageviewsasynchronously,thencombinethemtogetherwithadeviceview.NotethatEagleCADisnotasimulationsoftwareofanykind,wecancallapartaresistorandgiveitthreeinputsandoneoutput,oracapacitorwithonlyonepin.EagleCADdoesn’tcareaboutwhatthepartiselectricallydoing,onlyphysically.

Let’sstartwithmakinganewsymbol.Inthepop-up,giveyourpartaname,presumablysomethingalongthelinesof“TPS799”.PressSymtoverifywe’recreatingasymbol,thencreateit.

Left:NewDeviceCenter:NewPackageRight:NewSymbol

Wenowhavethesymboleditorviewopen.Beginbyaddinginsomepins.Formostschematics,peoplefindlefttorighttheeasiesttoread(likeabook),sowe’llputthegroundpinonthesideratherthanthebottom.Add5pinsusingtheAddPinTool,thenrenamethemusingtheNameTool.ThegreencircleonthepinistheI/Opoint,sowe’llwantthosefacingoutwards.Again,wetendtoputinputsontheleftandoutputsontheright.Thismakesforalogicalreadingaswedecipheraschematic.

AddPin

DrawLine

ChangeName

Name

AddPin

Line

FinishupwiththeLineTooltoboxitoff.Trytocenterthepartasmuchaspossibletomakeyourlifeeasierwhenaddingitandmovingitaround.Savethesymbol(aswellasthelibraryifyouhaven’talready).

Nowwe’llhandlethepackage.Thereareseveralwaystodothis,butwe’lldoitthemostmanualwayfornow,justincaseweevergetanewpartthat’snotinEagleatall.Theparthasseveraldifferentpackages,butwe’llusethebiggestoneinthiscase,sinceifweweretoactuallycreatetheboard,itwouldbetheeasiesttouse.Packagescomeinmanydifferenttypesandnames,oftenredundantlysoasyou’llfindout.We’llbeusingtheDDCpackage,alsoknownasPlasticSmallOutline.Whiletheygivemanyviews,we’remostinterestedinthefootprint–thisishowmuchspaceweshouldleaveontheboardfortheparttositin.Dimensionsshownbelowinmillimeters(fromthedatasheet).

Let’sHitNewPackage,PAC,andnameit“TPS799”orsomethingsimilar.Thefollowingwindowshouldopen.Onceagain,verysimilartowhatwehaveseenbefore,butwithtwocriticalnewoptions.TheAddViaToolallowsustoaddaholetothefootprintthatisusedforleadsorpins.TheAddPadToolallowsustoaddapadforanysurfacemountpart.Lastly,we’ll

needtousetheGridTooltomakesurethattheminimumgridspacinglinesupwiththesmallestsignificantfigurewehave–here0.05mm.

Isuggestusinga0.5mmgrid,witha0.05mmaltsetting.Addapad,andplaceitrelativelynearthecenter.Rightclickonit,Properties,andchangeSmdSizeto“1.1x0.6”.Thisautomaticallysizesthepadtothecorrectdimensions–ifit’soffby90degrees,rotateit.NowusetheRulertogoandmeasureouttheappropriatedistanceforthenextpad.RemembertoholdALTasyoudoso–otherwisewe’llbeonthemaingriddimension,notthefinerone.I’veattachedapictureofwhattheoutputshouldlikebelow.Oncethatisdone,copyandpastethepreviouspadandplaceitalongthesameline.

AddVia

GridEdit

AddPad

Ruler

Nowthisisclearlygoingtogetprettypainfulifwewanttodothisforeverypad.Fortunatelyforus,wecangetaroundthiswithalittlemath.Deletetherulerdrawingfornow,wewon’tneedit.Addanothernewpadviacopypaste,justputtingitanywhere.Rightclick,selectPropertiesonthefirstpad,andnoteitsPosition.Nowdothesametothenewlyaddedpad,butjustmoveittotheappropriatepoint–either0.95mmup,or2.7mmout.Absolutepositiondoesn’tmeananythinghere–onlyrelative.Oncethat’sdone,gobackandnamethepads!Anypatternworks:[12345],[TopLeft,MidLeft,BotLeft,TopRight,BotRight],[NWWSWSENE],aslongasyoucantellwhichiswhich.Finalimageshouldlooklikesomethingbelow.Don’tforgettoverifyyourlayoutwitharuler–neverhurtstodouble-check.

Saveit,thengobacktothetopandopenaNewDevice.Onceagain,let’scallitTPS799.Thefollowingwindowwillshowup.Thisshouldbetheeasiestoftheprevioussteps–allwedoisconnectthedots!ThebuttonhighlightedontheleftshowstheAddSymbolTool,andthehighlightednewbuttonistheAddPackageTool.Thereshouldonlybeoneofeachinthislibrary,goaheadandaddtheschematicandpackagewecreated.

NowhitConnect,andlet’sconnectthedots(pardonthepun).Youcanfindtheorderinthedatasheet,generallyunderasectioncalled“PinMapping”or“PinConfiguration”.Gothrough,selecttheappropriatePinNameandPadName,andthenhitConnect.Proceedforallfiveoptions,andyoushouldendupwithaconfigurationshownbelow.

Finishitoffandwe’re(almost)goodtogo!Beforewefinishthis,let’sgoaheadandlearnonemorewayofdoingthis.Thelargemajorityofpartsyou’llbemakingwillprobablybeIC’s.NearlyallIC’scomeincommonpackages,sothere’snosenseinre-inventingthewheelwhenitcomestodrawingpackages.EagleCADstoresallitsreferenceICfootprintsinref-packages.lbr,orifyouaren’tcomfortablewithyoursolderingskills,ref-packages-longpad.lbr.Theonlydifferenceisthelongpadvarianthas,nosurprise,largerpadsinordertogivemoresolderingarea.

ThetrickypartaboutthisisthatyouhavetoknowthatmanyICfootprintsaretheexactsamephysically,buttheyhaveplentyofnamevariants.Forexample,thepartwehavehereisgiventousasDDCorR-PDSO-G5,butisfoundinref-packagesasSOT23-5L.Whileyouhaveyouroldlibraryopen,gobacktotheprojectviewer.GotoLibraries->ref-packages-longpad.lbr->SOT23-5L.Rightclick,CopytoLibrary.Thiswillcopyaversionofthefootprinttoyourlibrary,sowhenyougotomakeanewsymbol,youcanseeyounowhavetwofootprintstochoosefrom–theonewedesigned,andtheonegiventous.

Thechoiceisyoursonwhichmethodtouse.Bothhavetheirprosandcons,andunfortunatelyunlessyougetluckywiththefootprint,takealittletime.Regardless,nowthatwehaveourpartcreated,it’stimetoaddittotheschematicandmoveontothePCBlayout.

Toaddthenewlymadeparttoyourschematic,gotothetoptoolbarandhitLibrary->Use.Thelibraryyoujustcreatedshouldbeinthedefaultfolder,openitandthepartsinitwillbeavailabletous.Additinandwireitup.Don’tforgettofinishthefullvoltageregulatorschematicfortheassignment,includingthepassivecomponentsrequiredfortheTPS799towork.Seethescreenshotfromthedatasheetbelow:

Onelastimportantnoteisthegreen“junction”dotswhenwirescross.ThesearedonewiththeJunctionTool,labeledinourbuttonsfigureatthebeginning.WheneverwecrosswiresinEagleCAD,bydefaulttheseareNOTconnected.Weneedtoaddthegreendottoensuretheyareindeedtouching.It’sanicevisualaid,aswellasawaytoconfirmthatwedointendtoconnectwireswhenwecrossthem.WecancheckforthisandothersmallproblemswiththeERCTool,ElectricalRuleCheck.It’sthebottomleftmostbutton,pressitandseewhaterrorsyouget.Feelfreetoignoreanyirrelevantonesanddealwiththeimportantones.

Making the PCB PCBdesignisjustabigpuzzle.Allwetrytodoismatchourschematicbyconnectingthepadsandvias,withageneralgoalofminimizingarea.Eachboardconsistsofseverallayersto“route”(orconnect)on,seeAppendixA.ThefreeversionofEagleallowsyoutousetwolayers,thepaidversionontheEWSmachinesallowsfour.

ThefirststepistoswitchfromschematicviewtoPCBview.Pressthehighlightedbuttoninthetoolbartocreatetheboardview.You’llbepresentedwiththescreenbelow.WhatEagleCADhasdoneissavethenetlist(ornodelist)fromtheschematic,andconnectedallthepartsthatneedtobeconnectedwithyellowwires.Thewhitesquareistheworkarea,soitcanbeanarbitrarysize.TodefinetheactualsizeofthePCB,drawintheborderwithtraceswidth=0ontheDimensionlayer.

Beginbymovingsomeofthepartsintotheworkarea.TheRouteTool iswhatweusetomaketheseconnections.Whenyouselectit,you’llseeoneofthetoolbarsshowamenu

likeso .Thedropdownbarchooseswhatlayerwerouteon,andadditionally,eachlayeriscolorcodedsoyoucanalwaysvisuallyconfirmaswell.

Youcanonlyroutebetweennodes,sopickanypartwithayellowlineandbegin.Whenyouclickonepad/via,thenodesisneedstoconnecttolightup.Agreenviarepresentsaholethatgoesallthewaythroughtheboard,soyoucanconnectonanylayer.Aredorbluepadiscreatedonlyonitscorrespondingside,soyouwillneedtodrawontheappropriateside.Theselineswedrawrepresentcoppertraces,whichisaconductivematerial.Thereforeifyouoverlaptheselines,oraccidentallyoverlapalinewithapad/viathatyou’renotsupposedto,you’llbecreatinganelectricalshort.Watchoutforthat,andonlyconnectwhat’ssupposedto!

Continueaddingpartsonandconnectingthem.Everyonceinawhile,hittheRatsnet

Tool .Thistoolredrawstheyellowelectricallinestotheshortestconfigurationpossible.

Belowyou’llseeapartiallayoutwhereaviaisrequired.Theproblemhereismyclosestconnectionisonthebottom,butmypadisonthetop.

Forthiscase,IwouldstartoffRouteToolonthetopside,selectingthepad.WhenIgettoanotcrowdedspot,withtheRouteToolstillselected,changethedropdownmenufrom“Top”to“Bottom”andfinishthetrace.Seethatsmallgreensquareinthemiddle?EagleCADautomaticallyplacedaviaformetoshort+connectmytwotraces.

Howwideshouldonemakethesetraces?Whereshouldtheywindupgoing?Thefirstruleofthumbisdrawwhatyoucanmanufacture.IfusingtheECEshop,youshouldimporttheirDRCrulesandusetheirmin-spacing/width/etcguidelines,bothfoundhere:http://eshop.ece.illinois.edu/pcbdesign/designreq/designreq.php

Problemconnection

…Someotherstuff

Afteryou’vemettheserequirements,thenyoucanreferencesomeoftherecommendedpracticeinAppendixB.

Let’sgetintoabitmoredetailaboutsomeoftheothertoolsinEagleCAD.Forone,you

can’tdeletetraces,youripthemusingtheRip-UpTool .Youcandeletepartsandholesandotheraddedcomponents,butnottraces.

MakingPlanes:Eaglealsoallowsustouseshapes/planesonourPCB.NodeslikeGNDandVDD(power)arebestdrawnasplanes,formanyreasons.Ifyou’recurious,checkwithyourTA.

Todothis,makeabox(usuallyjustinsidethePCBedge)usingtheShapeTool .Don’toverlapendpointandstartpoint,youmightgetsomeuglyerrorsifyoudo,justconnecttheendrighttothestart.Youshouldhaveanicedottedboxaroundyourcircuitlikeso.

NowusetheNameTooltonamethebox.Thenameisgoingtobewhateversignalyouwanttofillthatareawith,soyouhavetochooseanetnamethatyouassignedintheschematicview.HitRatsNesttoseetheoutcome.Alargemajorityoftheboardshouldbecomeentirelyred(iftopside)orblue(bottomside.Rightclickonthebox(theoutline,notthefilledinarea),andselectProperties,thenunderPolygon,changeIsolate.Thisistheparameterthatdetermineshowmuchseparationexistsbetweenthefillandtheothersignalsonthatlevel.Becarefulaboutmakingittoosmall,asyou’llfindithardertosolderto,aswellasincreasingtheriskofcouplingsignalstogether.Adecentvalueis50mil.

Almostdone!JustlikeintheschematichowwehadtheElectricalRuleCheck,herewe

havetheDesignRuleCheck .WhiletheERClooksourcircuitoverandseesifwehaveanythingwiredoddly,theDRCnotonlytellsusifwelaidoutanythingstrangely,italsowarnsusifwedidanythingthatisn’tpossibletobemanufactured.WhileEaglemayallowyoutoplacepartsapartwithnanometer,goodlucktryingtofindsomeonewhowillactuallymakethat!Mostservices,likeourECEshop,offeradesignrulelistingsoyoucangoinandfindoutexactlyhowwidetomakeyourtraces,orhowfaraparttospreadyourparts.Followthelinkearlierinthissectiontodownloadtheshop’sDRCrules.

PCBborder(definedonDimensionslayer)

Yourcircuit’s

PCBlayout

Thelasttoolwehaven’treallytalkedaboutistheAutoRouter .Thisisforgoodreason–ittendstosuck.PCBlayout,andEDAingeneral,isahugeunsolvedproblemintermsofautomation.Whilewithdigitalsolutionsthereisenoughreductionthattheproblemismoremanageable,analoglayoutisstillabitofanartthathasn’tbeencompletelymastered.TheAutoRouterwillmostlikelyfindasolution,butit’sfarfromoptimalmostofthetime.

Creating Gerbers Whilenotrequiredfortheassignment,it’sgoodtolearnhowtogofromtheEagleCADboardfiletogerberfiles.Thegerberformatisabinaryvectorimagefiletypethatisthecommonmediumformanufacturers.Plentyoffreeandonlinegerberviewersexistifyouwishtoviewwhatisactuallybeingoutputted,butiteffectivelyjustslicesthePCBlayerbylayerandlistseachxbyypointasaregion.

SelecttheCAMProcessor(highlightedabove)togetstarted.SelectFile,Open,Jobandselectgerber274x.Thewindowshouldpopulatewithseveraltabs,eachrepresentinganewoutputfile.Eachoutputfileisanew“slice”oftheboard.PressingProcessJobatthebottomwillcreateallofthesefilesinthesamedirectoryasyourproject.

Whyaretheresomanyfilesforatwolayerboard?Wellyoucanseewhateach“slice”actuallyisinthelayertabontheright.Wehaveourtopandbottomsolder,aswellassoldermask,silkscreen,andadrillfiletonotatealltheholesintheboard.Fancierboardsrequireevenmorelayers,withitemslikegluepointsandsolderstoppoints.

Appendix A

EAGLECAD LAYER REFERENCE

ECE445UniversityofIllinoisatUrbanaChampaign

DepartmentofElectricalandComputerEngineering

Manylayersareprefixedwithat<X>forTop<X>orb<X>forBottom<X>.Thelayer

LayerNumber(s)

Name Purpose

1-16 Top->Bottom

ThewireroutingmetallayersforthePCB.Alsocalled“traces”ontheboard.Fortwolayerboards,onlyusetheTop(red)andBottom(blue)layerstodrawconnections.Alsoshowsthesurfacemountpadsforrespectivesides.

17 Pads Somewhatconfusingly,EagleCADdefinespadsaseverythrough-holeconnectionofapart.Whileweoftencallthisa“via”,thosehaveaseparatedefinition.

18 Vias Whenatracefromlayers1to16switchestoanotherlayer(ie,from[1]Topto[16]Bottom),agreenviaiscreatedtoverticallyshorttheselayers.Bydefault,thisisafullboard(toptobottom)short.Whileitispossibletodoblindvias(shortonly[3]to[5],forexample),thecostisextremelyprohibitive.

19 Unrouted Thisistheairwirelayerthatshowsallthe“birds-eyeconnections”foralltheparts.AfinishedPCBdesignshouldhavenothingonthislayer.TheRatsnestbuttonwilltellyouhowmanyconnectionsareleft.

20 Dimension Thissetsthephysicalsizeoftheboard.Anythingoutsideofthisshape(usuallyrectangular,butcanbeanything)willnotbemanufactured.

21-22 Place Thisisadecorativelayerforshowingpartsphysicalprofiles.Thislayerisdrawnwithsilkscreenandisgenerallyusedtomakesurepartsdonotphysicallyoverlap.

23-24 Origins Theoriginisthesmallcross(+)signifyingthecenterofthepart.Thisismostlyusedtohelptheuserselectparttheyintendtointhedesignphase,andisrarelymanufactured.

25-26 Name Self-explanatory.Canbemovedwiththe“Smash”tool.27-28 Value Self-explanatory.Canbemovedwiththe“Smash”tool.29-30 Stop Forserviceshopboards,thislayerdoesnothing.Butwhen

makingaboardataprofessionalmanufacturer,theentireboardiscoveredinsoldermaskbydefault.Thislayerspecificsthesoldermaskfreesections(soldermaskSTOP),andeffectivelyshowsthefreetosoldertoareas.Thisisingeneralslightlylargerthanthepadorviaitself.

31-32 Cream Forserviceshopboards,thislayerdoesnothing.Ifyou’remakingastencilfortheboard,thislayerdefinesthestenciloutline.Thisshouldbethesameasallthetopandbottompads.

33-34 Finish Forserviceshopboards,thislayerdoesnothing.Forhighqualityboards,thissetstheplatingandfinishused.

35-36 Glue Forserviceshopboards,thislayerdoesnothing.Whenusingadvancedsurfacemountsolderingtechniques,suchaswavesoldering,manufacturersoftenspecifyplacinggluetokeepsomepartsinplaceduringtheextremeheatstress.Thisisgenerallyplacedinthecenterofparts.

37-38 Test Forserviceshopboards,thislayerdoesnothing.Thisisforestablishingautomatedtestingpoints.

39-40 Keepout Thislayerisusedwithpolygons.Ifpartsareplacedwithinthisshape,theDRCwillthrowerrors.

41-43 Restrict Thislayerisusedwithpolygons.Iftracesareplacedwithinthisshape,theDRCwillthrowerrors.

44 Drills Thislayershowsallconducting(plated)holesintheboard.45 Holes Thislayershowsallnon-conductingholesintheboard.

Generallyformechanicalformounting.

Appendix B

GUIDELINES FOR LAYOUT ECE445

UniversityofIllinoisatUrbanaChampaignDepartmentofElectricalandComputerEngineering

1. FOLLOWTHEPCBSHOP’sRULES:http://eshop.ece.illinois.edu/pcbdesign/designreq/designreq.php

2. Figureoutifyoursignalishigh-speed.Agoodruleofthumbisdescribedbelow:a. FigureouttheTriseofyourfastestsignal(usuallyaclock)b. CalculatetheTdelayofthatsignal–distancetraveleddividedbyvelocity(velocitywill

dependonyourdielectricmaterial-aninternetsearchwillusuallyturnupcommoneffectivedielectricconstants,!!.YoucanapproximateFR-4tobe4.7,althoughthisvarieswithfrequency.Velocity= !!!

,c=speedoflight)

c. RatioofTrise/Tdelay>6:yourcircuitswillbehaveasexpectedfromyourschematic(i.e.wirescanbetreatedasshorts).Goto#10.

d. RatioofTrise/Tdelay<2.5:youwillneedtothinkofyourwireasatransmissionline,goto#3.

e. Ifyourratioissomewhereinbetween,youshoulddiscusswithyourTA.Tobesafeyoucouldstillabidebythepracticesforhigh-speedlayouts.

3. Youwouldwanttocontrolimpedance,usuallyto50ohmifsingle-ended.YoucoulddothisbyfiguringoutthethicknessofthePCBdielectricanditsmaterial,andthenusinganonlinecalculatorfordeterminingthewidthofyourtrace.TalktothePCBshoptoletthemknowyouneedcontrolledimpedance-anyGNDallaroundthetracewillmatter.

4. IdeallyyouwantsolidGNDplaneunderneaththeentirelengthofthetracefromtransmittertoreceiver.

5. Giveyourselfplentyofseparationbetweenthetraces(especiallyifyouhaveothernoise-sensitivecircuits).Acoupleofmmminimum,greaterspacewherepossible.

6. Avoidchangingmaterialsbetweentransmitterandreceiver(trytostayononePCB,trynottosolderwiresin-betweenorgothroughaconnector).

7. Avoidviatransitionswherepossible.Ifyouhavetodivebetweenlayersmakesurethereturnpathhasawaytofollowthesignal.

8. Terminateyourlines-usuallyaseriesresistorclosetothetransmitterisused.TalktoyourTAaboutthepropervalueinyourapplication.

9. Designyourpowerdeliveryverycarefully-youneedplentyofbulkandhighfrequency(usuallysmaller-value)capacitorsinparallelfordecoupling.TheseneedtobeclosetoVDDpinsonyourchipsspewingouthigh-speedsignals.

10. Fornoise-sensitivecircuits(analogaudio,senselines,anythingextremelylowvoltageorhighimpedance,etc),grouptheseasfaraspossibleawayfromdigitallogicwithhighclockrates.

11. ConsiderisolatedGNDplanesifyouhaveananalogportionandadigitalportion.Trueelectricalisolationcanbedonewithoptocouplersandtransformers,butasimplersolutionisaGNDmoat,wherethequieteranalogGNDisacopperislandenclosedbya“moat”ofno-fill,separatingitfromthesurrounddigitalGND.Theseareconnectedatonepointonly,topreventtoomuchDCvoltagedrift.Allanalogsignals(ontheirlayeraboveorbelowtheplane)arereferencedtotheanalogGNDandviceversafordigitalsignals-nosignalscrossoverthemoat.TalktoyourTAabouttheimplementation.

12. Considerusingseparatepowersourcesforthesensitivecircuits-anLDOisagoodoption.

13. Ifyouhaveapartthat’sinasmallordifficulttosolderpackage,tryusingasocketormakingabreakoutboardtosolderintoyourmainboard.Thisway,ifyoumessupsolderingthesmallconnections,youwon’tbreakyourmainboard.

14. Ifyouhavetheroom,considermakingpowerplanes.CreateshapesjustlikefortheGNDplane,butthistime,namethemyourVDDnetnames.EachVDDwithsubstantialcurrentconsumption(DCortransient)shouldideallyhaveitsownplane,withplentyofbulkdecouplingattheoutputofitsownregulator.