Combinational Vs Sequential Logic Combinational circuit. Sequential Circuit ? F. CS221: Digital Design Sequential Logic Design (Latch & FF)

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1 /5/25 22: Digil Design equenil Logic Design (Lch & FF) A. hu Dep of omp. c. & Engg. Indin Insiue of Technology Guwhi Ouline ominionl Vs equenil Logic Design Design flip flop, h sores one i lch ilizing lch: Level ensiive locked Lch : Flip Flop Edge ensiive D, JK, T flip flops hrcerizion Tle nd Equion, D, JK nd T Flip flop 3. ominionl Vs equenil Logic ominionl circui Oupu depends on presen inpu Exmples: F (A,B,), FA, HA, Muliplier, Decoder, Muliplexor, Adder, Prioriy Encoder ominionl Digil ircui F = F (,) Propgion dely (+ pd )=F((), ()) ominionl Vs equenil Logic equenil circui Oupu depends no jus on presen inpus Bu Bulso onpssequence sequence ofinpus (e) ores is, lso known s hving se ominionl Vs equenil Logic imple exmple: circui h couns up in inry equenil digil circui? F Mus know sequence of ps inpus o know oupu () = F ((),(), H) H is Hisory/equence/e Inpu clk equenil ircui ominionl logic s s Hisory/ equence/e n n x oupu () = F ((), (), H) H is Hisory/equence/e Where o ore his Hisory (Memory Elemen)

2 /5/25 Exmple Needing Bi orge Exmple: Fligh endn cll uon Press cll: ligh urns on ys onfer uon relesed Press Presscncel: ligh urns off How o design circui for his? Exmple Needing Bi orge ll uon ncel uon ll uon ncel uon ll uon ncel uon Bi orge. ll uon pressed ligh urns on Bi orge 2. ll uon relesed ligh sys on Bi orge 3. ncel uon pressed ligh urns off Blue ligh Blue ligh Blue ligh Exmple Needing Bi orge Fligh endn cll uon Press cll: ligh urns on ys onfer uon relesed Press Presscncel: ligh urns off How o design circui for his? Exmple Needing Bi orge Fligh endn cll uon Press cll: ligh urns on : ys onfer uon relesed Press cncel: ligh urns off Logicgecircuioimplemenhis? ge circui o implemen his? ll ncel Doesn work. = when ll=, u doesn sy when ll reurns o Need some form of feedck in he circui Firs emp Bi orge We need some sor of feedck Does circui his circui do wh we wn? Firs emp Bi orge No: Once ecomes (when =), sys forever no vlue of cn ring ck o Once ecomes (when =), sys forever no vlue of cn ring ck o 2

3 /5/25 Bi orge Using n Lch Wih cross coupled NO ges Does he circui o he righ? Do wh we wn? (se) lch (rese) es! How did someone come up wih h circui? Mye jus ril nd error, i of insigh... = Bi orge Using n Lch = = = = = = = (se) Funcion Tle of Lch lch Exmple Using Lch for Bi orge (rese) e : mke ou pu ese : mke ou pu Afer=, = Afer =, = Foridden ll uon ncel uon Bi orge Blue ligh ll ncel Blue ligh lch cn serve s i sorge of fligh endn cll uon : Gre ll= : ses o sys even fer ll= ncel= : reses o Bu, here s prolem... Lch wih NAND Ges + ** (Unpredicle) Opposie o Lch wih NO Ges e will do = nd ese will = Prolem wih Lch: = Prolem If = nd = simulneously, we don know wh vlue willke (Unpredicle) ll ce ondiion : ncel Who will win or? Evenully, wh will e he vlue of? Blue ligh 3

4 /5/25 = = Prolem wih Lch = = = = oluion o ce ondiion We ry o void = nd = y he following circui lch my oscille. Then, ecuse one ph will e slighly longer hn he oher, will evenully sele o or u we don know which. n we ensure vlue = nd = will no hppen he sme ime? Prolem wih Lch Prolem no jus one of user pressing wo uons sme ime n lso occur even if inpus come from circui h supposedly never ses = nd = sme ime Bu does, due o differen delys of differen phs = nd = se eserdy i ws my miske Prolem wih Lch Arirry ircui = = lch The longer ph from o hn o cuses = for shor ime could e long enough o cuse oscillion Fil oluion: Level ensiive Lch Add enle inpu or En Only le nd chnge when = : Ensure circui in fron of never ses =, excep riefly due o ph delys hnge o only fer sufficien ime for nd o e sle When ecomes, he sle nd vlue psses hrough he wo AND ges o he lch s inpus. Level-sensiive lch Levelsensiive lch symol Level ensiive: Lch wih NAND Ge Level-sensiive lch Level-sensiive lch symol 4

5 /5/25 Ensure = nd = should no hppened o Level ensiive Lch Level-sensiive lch Though = riefly... oluion: Ensure, ilize, ore Level-sensiive lch En... never = En Ensure ge Never Hppens = ilize ge When = ilize nd Use when = ore ge ore i Thnks 5