 chips has enabled system designers to put an entire printed circuit board, and increasingly more electronic circuitry, into one chip. This type of customization began with digital circuits only. Today, however, analog circuits also are offered by ASIC vendors as part of their ASIC libraries. This move to combine analog and digital technology reflects the pressure to integrate electronic functions into real-world applications, such as automobiles, telephones, and other consumer products which typically are analog-based. Previously, such applications were handled by discrete analog circuits which received signals from sensors and then communicated an output signal to separate digital circuitry through analog/digital converters for further processing.
For digital circuitry to interface effectively with analog circuitry in present-day applications, the digital and analog circuits must be merged on a single silicon chip. However, combining analog and digital circuitry on a single silicon chip raises unique problems, especially when the circuits are to be tested. For example, the testing philosophies of analog and digital circuits are entirely different. Analog circuits produce continuous signals having various magnitudes, and they are tested not only by sensing the existence of a signal, but its magnitude, as well. On the other hand, digital circuits produce signals which are binary, and they are tested by testing the binary response of a circuit when a given combination of binary signals are applied to the inputs of the circuit. With digital circuits, the exact magnitude of each signal is relatively unimportant, as long as it is within a given set of lower and upper limits. Nonetheless, both testing philosophies must be accommodated when digital and analog circuits are combined on one chip.
Testing is further complicated by the fact that gate-to-pin ratios increase as technology advances. Since it is not feasible or practical to bring all of the circuit nodes which should be tested out to the package pins, problems of controllability and observability arise. "Controllability" in this context refers to the ability to apply a test stimulus to a circuit node. "Observability" refers to the ability to observe the circuit's response to a test condition. Since some outputs may not change states in response to a test stimulus, it is desirable to access some intermediate node in the circuit which indicates some kind of response when a test input signal is applied. If this is not possible, the chips may be untestable or, at least, are very difficult to test.
The use of analog circuitry in the chip also raises issues of completeness in testing. Since analog signals are continuous and are typically tested by sensing a signal level within the
chips has enabled system designers to place an integral printed circuit control panel, and increasingly more electronic circuitry, into one check. This type of customization began with digital circumference entirely. Today, however, analog circuit also are offered by ASIC seller as part of their ASIC libraries. This move to combine analogue and digital technology reflects the atmospheric pressure to integrate electronic functions into actual - world applications, such as automobiles, telephone, and other consumer products which typically are analog - based. Previously, such application were handled by discrete analog circuits which received signals from detector and then communicated an output signal to separate digital circuitry through analog / digital converters for further processing. 
 For digital circuitry to interface effectively with analogue circuitry in present - sidereal day lotion, the digital and analog circuits must be merged on a single silicon chip. However, combining analogue and digital circuitry on a single silicon bit raises unique problems, particularly when the circuits are to be tested. For example, the examination philosophies of analogue and digital circuits are entirely different. analogue circuits produce continuous signals having various magnitudes, and they are test not only by sensing the existence of a signal, but its magnitude, equally well. On the other hired hand, digital circuits produce signals which are binary, and they are tested by testing the binary reception of a circuit when a given combination of binary signals are apply to the remark of the circuit. With digital circuits, the accurate magnitude of each signal is relatively unimportant, as long as it is within a given set of broken and upper limits. however, both testing philosophies must be accommodated when digital and analog circuit are combined on one chip. 
 examination is further complicated by the fact that gate - to - pin ratios increase as technology advances. Since it is not feasible or practical to bring all of the circuit nodes which should be tested out to the package pin, problems of controllability and observability arise. " Controllability " in this context refers to the ability to use a test stimulus to a circuit node. " Observability " refers to the ability to observe the lap's response to a test condition. Since some outputs may not change states in reaction to a trial stimulation, it is desirable to access some average node in the circuit which indicate some kind of response when a test input signal signal is applied. If this is not possible, the chips may be untestable or, at least, are very unmanageable to test. 
 The habit of analog circuitry in the chip also raise issues of completeness in testing. Since analog signal are continuous and are typically tested by sensing a signal level within the
 chlps has enabled system dtsigners to put au entirx printsd circukt board, and increasingly moce eoectrinic circuitry, into ond chip. Thps type od cuwromization began wlch difltal eiccuits only. Todsy, however, analog circuids apso are offered by ASIC vendors as [art of tjeir ASIC librwriex. Thia move to combine analog and digitzl techiology reflects the pressure to integrate eleftronic functions lnto real-wotmd wpplications, ruch as auuokobiles, temephones, and other consumer proaucts which typucqllj are analog-uased. Ireviously, sugn applhcationx were handled by diwcrete analog circuitv which received sygnals frmm sensors and then cimmunhcatad av ougpuu smgnzl to deperate digitzl circuitrt through analog/digotwo converters ror fuwtrer processing.
For digital circuitry to pntedface effectively with qnalog circuitry in ptesent-day wpplications, the digital and analog circuits must be mxreed ok a runhle silicon chip. However, combining analog and sibinal circuitry on c single silicon cjil raises uniqug problzjs, especially when tje circoits aee to be uestec. For example, the testing pyilosophies jd analog and digitcl circuits cre enjirely different. Analog circunts prkduce contijuous sighxls having variojs kacnitudes, and they are testqd not onoy bv sensine thg existqnce of a dignal, but its magnitude, as wgll. On the other hand, digital circuits produce signals which sra bpnary, and they are tested br testing the yinary rzsponsd of a cirbuit when a given coibination of tlnary signalv are ap[liee to the inojts of the cirvuit. With digital curcuits, the exact kagvjtude of each snynql is relativelu uvim[ogtait, as nong as it iv wighiv a gixen set of ouwer and upper limits. Notethsless, both testing pmilosophigs must bq accommodatec when digital and analmg rircuiys wre combined on one chip.
Testinf is furtjer complicated by bhe sact that yate-to-pin ratios increase as technology edvances. Since it is nor feasible or practncsl to bring all jf the ciscuit nodes which shiuld be tested ouu to the package pins, pdoblemv of fontrollability and observability arise. "Controllability" in this context rwfers vo the abilitg to applj a cest stymulns to a circuit noce. "Observability" refers to the ebility to oyserve the circuit's response yo a test condijion. Since some outputs mag not chsnge states in response to a text stimulus, it is desiranle to acxess aome interoediate node in thr circbit whuch inducatcs some kind ow rrsionse when a ttst nnput signal is applied. If this iw not podsible, the chips may be untcsuable ot, at leaft, are very diffjculf to tgst.
The use of snalog circuitty in the chip also gaifes issues of xkmpleteness on testpng. Since anakog signals are continuous and are typically tested by sensing a signal level wlthin the
chips has enabled system designers to put printed board, and more electronic circuitry, of began with digital only. Today, however, circuits also are offered by ASIC as part of their ASIC libraries. This move to combine analog and digital reflects the pressure to integrate electronic functions into real-world applications, such as automobiles, and consumer which are analog-based. Previously, such applications were handled by discrete analog circuits which received signals from sensors then communicated an output signal to separate digital through analog/digital converters for processing. For digital circuitry to effectively analog circuitry present-day the and analog circuits be merged on a single silicon chip. However, combining analog and digital circuitry on a single silicon raises unique when the are be For example, the of analog and digital circuits are circuits produce continuous signals having various magnitudes, and are tested only by sensing the existence of signal, but its magnitude, as well. On the hand, digital circuits produce signals which are binary, and they are tested by testing the of a circuit when given combination of signals applied the of the With digital circuits, the exact magnitude of each signal is relatively as long as it is within a given set of upper Nonetheless, both testing must be accommodated when and circuits are combined on Testing further fact gate-to-pin increase as technology advances. it is not feasible or to bring all of be tested out to the package pins, problems controllability and observability arise. "Controllability" in this refers to the ability to apply a test stimulus to a circuit "Observability" refers ability to observe the circuit's response to a condition. Since some outputs not change states in response to a test stimulus, is to access intermediate node in circuit which indicates kind of response test input is If possible, the chips may be untestable at are very difficult to test. use circuitry in the chip also raises issues of completeness in testing. Since analog signals are are typically tested by a level within the
 chips has enabled system desiGners to put An entIre PriNtEd ciRcuiT board, and increASingLy more electronic circuiTry, inTo ONe chIP. THis tyPe of cusTOmIZAtiOn BeGan WiTH dIgitaL ciRcuits oNly. Today, hoWevEr, Analog circuiTS aLso are offeRed By ASIC vendorS as Part of ThEir asIC liBraRies. THis movE To combIne analog AnD DigitaL TechnolOGY rEfleCts the pressure to iNTeGRate electronic FunctiOnS InTO ReaL-woRld applicaTiOns, suCH as autoMObILES, teLEphones, and othEr consumer pROduCts whiCh TypICally aRe anaLoG-BasEd. PreviouslY, sucH applicatIons weRE handleD By discrEte anaLog CirCuitS WhIcH reCeIVed SIgNalS FroM sensors AnD tHen coMmunICATEd an OutPut sIgnal To separate digItaL cirCUitRy thrOugh aNaloG/dIgitaL conveRters FoR further processIng.
FOr digital CirCuItrY tO inteRFace efFecTivEly with Analog cIRcuItRY IN pResent-day applicatiOnS, THe Digital aNd analOG cIrCUits must Be MerGed oN A SinglE silICoN chip. HowEver, coMBiNiNg analoG aNd digiTaL ciRcuItry oN A sinGle silIcon chip RaiseS Unique problems, ESpecially when THe CIRcUIts aRe tO be tested. FoR exaMPle, tHe teSTiNg pHIlosoPhies Of ANaLOg and digital circuitS aRe entiRely dIfferent. AnaloG circuits pRODUce contiNuouS SiGNals having variOus maGnitudes, anD They are tEsted Not only bY sensing tHE ExistencE of A siGnaL, buT ITs Magnitude, as weLL. on thE oTher hanD, diGital ciRcuIts ProDucE sIgnals whiCh are binArY, aNd ThEy aRe tesTEd by testInG thE bInaRy resPOnse of A circUit wHeN a GIveN combinATiON Of biNaRy SignAls ArE applIed tO The Inputs oF the circuIt. WITh diGiTaL circuiTs, the exact magNiTude of each SiGnaL is relATIvely uniMportant, as long as it is witHIn a giveN seT of loWer aNd upper liMitS. NonetHelESs, both TestinG philOsOphIES must BE AcComMoDated when dIGItaL and aNaLog cIrcuits Are combined on one chIP.
TeSting is furtheR coMpliCATeD by THe FAct ThAT gaTE-To-pin ratios incrEase as techNoLOgY advances. SINce It Is not feAsible oR pracTIcal to bRing all of The circuiT nOdes WHIch Should be teSted out tO the packaGE pins, PRoBlems Of cOntrolLaBilIty anD obserVAbiLity aRise. "CoNtRollabIlity" In This contExt refers to the ability to Apply a Test sTimUlus to a ciRcuIT noDe. "ObservaBiliTy" refers to The AbiLity tO obSErve tHe ciRCuIt's REsponSe to A Test condiTIoN. SiNCE sOme outputs mAY NOt cHange StaTEs in reSponSe to a test stimulus, IT is desirable to AcceSS SomE inTErmeDiAte node in the ciRcuIt WHIch indicAtEs some kind oF responsE wHEn a teSt inpuT signaL is applIED. IF This is Not pOssIble, the chIps MaY Be untesTaBlE Or, at leAst, aRe Very diFficulT To teST.
the use of analog ciRcuitRY In the CHip Also rAiSes issuES of cOmpletenesS in testing. SInce anAlog SignaLs are coNtInuous And ArE typically TEsted by seNsing A signal LeVel wIthIn the
 chips has enabled systemdesignersto pu t a n e nt ireprin ted circuit bo a rd,and increasingly moreelect ro n ic c i rc uitry , intoo ne c hip .Th isty p eof cu sto mizatio n began wi thdi gital circui t sonly. Toda y,however, ana log  circu it s a l so ar e o ffere d by A S IC ven dors as p ar t  of th e ir ASIC l ib rari es. This move toc om b ine analog and  digit al te c h nol ogy  reflectsth e pre s sure to in t e g rat e  electronic f unctions in t o r eal-wo rl d a p plicat ions,  s u chas automobi les,  telephon es, an d  otherc onsumer  produ cts  wh icht yp ic all ya rea na log - bas ed. Prev io us ly, s ucha p p l icat ion s we re ha ndled by disc ret e an a log  circ uitswhic hrecei ved si gnals  f rom sensors and  the n communi cat ed  an  o utput signal  to  se paratedigital cir cu i t r ythrough analog/dig it a l  c onverter s forf ur th e r proces si ng. 
For d igita l ci r cu itry tointerf a ce  e ffectiv el y with  a nal ogcircu i tryin pre sent-day  appl i cations, the d i gital and ana l og c ir c uits  mu st be merge d on a si ngle si lic o n chi p. Ho we v er ,  combining analog a nd  digit al ci rcuitry on asingle sil i c o n chip r aise s  u n ique problems,  espe cially whe n  the cir cuits  are tobe tested . For exam ple , t hetes t i ng  philosophies o f an al og anddig ital ci rcu its  ar e e nt irely dif ferent.An al og  c irc uitsp roduce c on tin uo ussigna l s havi ng va riou sma g nit udes, a n dt h ey a re  t este d n ot  only  bys ens ing the  existenc e o f  a s ig na l, butits magnitude ,as well. O nthe  other h and, dig ital circuits produce s i gnals w hic h are  bin ary, andthe y aretes t ed bytestin g the  b ina r y  resp o n se  of  a  circuit w h e n a  give ncomb ination  of binary signals are  applied to t heinpu t s  o f t h ec irc ui t . W i t h digital circu its, the e xa c tmagnitudeo f e ac h signa l is re lativ e ly unim portant,as long a sit i s wit hin a give n set of  lower an d  uppe r  l imits . N onethe le ss,  both  testi n g p hilos ophies  m ust be  acco mm odated w hen digital and analogcircui ts ar e c ombined o n o n e c hip.
Test ingis further  co mpl icate d b y  thefact th atg ate-t o-pi n  ratios i n cr eas e as  technology a d van ces.Sin c e it i s no t feasible or pra c tical to bring  all o f t hec ircu it  nodes which s hou ld b e tested  o ut to the p ackage p in s , pro blemsof con trollab i l it y  and o bser vab ility ari se.  " C ontroll ab il i ty" in  thi scontex t refe r s to t he ability to ap ply a t est s t imu lus t oa circu i t no de. "Obser vability" r efersto t he ab ility t oobserv e t he  circuit's responseto atest co nd itio n.Sincesome o utput s ma ynot  change s t a te s  i nr esp onse  to a  t eststimulus, it is de sir a ble toac ces s some i n te r m ediate nod e i n the c ircuit whi c h in d ic a tes s ome ki nd ofrespons e  wh en  a test  in p u t signalis applie d . I fthis  is notpo ssib le , t he  chips may be u ntestabl e or, at l east,are very diffic ul t to test.
T he us eo f analo g cir cuitryi n t hechi p al sorais e s issues of completeness  intesti n g.Since  anal og  signals arecontinuo u s and are typ ically tested b y  se n sing  as ig na l level w ith i n the 
 chips_has enabled_system designers to put_an entire_printed_circuit board,_and_increasingly more electronic_circuitry, into one_chip. This type of_customization began with_digital_circuits only. Today, however, analog circuits also are offered by ASIC vendors as part_of_their ASIC_libraries._This_move to combine analog and_digital technology reflects the pressure_to integrate_electronic functions into real-world applications, such as automobiles,_telephones,_and other consumer_products which typically are analog-based. Previously, such applications were_handled by discrete analog circuits which_received signals from_sensors_and_then communicated an output_signal to separate digital circuitry through_analog/digital converters for further processing.
For digital_circuitry to interface effectively with analog circuitry_in present-day applications, the digital and_analog circuits must be merged_on a_single silicon chip. However, combining_analog and digital_circuitry on_a single silicon_chip raises unique problems, especially when_the circuits are_to be tested. For example, the_testing_philosophies of analog_and_digital_circuits are_entirely different. Analog_circuits_produce continuous_signals_having various magnitudes, and they are_tested_not only by sensing the existence of_a signal, but its_magnitude,_as well. On the_other hand, digital circuits produce_signals which are binary, and they_are tested_by testing_the binary response of a circuit when a given combination of_binary signals are applied to the_inputs of the circuit._With digital_circuits,_the exact magnitude_of_each signal_is relatively unimportant, as long as it_is within_a given set of lower and_upper limits. Nonetheless, both_testing_philosophies must be accommodated when digital_and analog circuits are combined on_one chip.
Testing is further complicated_by_the_fact that gate-to-pin ratios increase_as technology advances. Since it is_not feasible or_practical to bring all of the circuit_nodes_which should be tested out to_the_package pins, problems of controllability and_observability_arise._"Controllability" in this context refers_to the ability to apply a_test stimulus to a circuit node. "Observability" refers to_the ability to_observe the circuit's response to_a_test_condition. Since some outputs may not change states in response_to a_test stimulus, it_is desirable to access some intermediate node in the circuit_which indicates some kind of response when_a test input signal is applied. If this is not possible,_the chips may be untestable or, at least,_are very difficult to test.
The use of_analog circuitry in the_chip_also raises issues of completeness in testing._Since_analog_signals_are continuous and_are typically tested_by sensing a_signal level_within the