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Dia Nur Alamsyah/06 EK 3C

Flip-Flop

In electronics, a flip-flop or latch is a circuit that has to sta!le states an" can !e use" to store

state information# A flip-flop is a !ista!le multi$i!rator # %he circuit can !e ma"e to chan&e

state !y si&nals applie" to one or more control inputs an" ill ha$e one or to outputs# It is

the !asic stora&e element in se'uential lo&ic# Flip-flops an" latches are a fun"amental

 !uil"in& !loc( of "i&ital electronics systems use" in computers, communications, an" many

other types of systems#

Flip-flops an" latches are use" as "ata stora&e elements# A flip-flop stores a sin&le !it )!inary

"i&it* of "ata+ one of its to states represents a one an" the other represents a ero# .uch

"ata stora&e can !e use" for stora&e of state, an" such a circuit is "escri!e" as se'uential

lo&ic# hen use" in a finite-state machine, the output an" net state "epen" not only on its

current input, !ut also on its current state )an" hence, pre$ious inputs*# It can also !e use" for

countin& of pulses, an" for synchroniin& $aria!ly-time" input si&nals to some reference

timin& si&nal#

Flip-flops can !e either simple )transparent or opa'ue* or  cloc(e" )synchronous or e"&e-

tri&&ere"*# Althou&h the term flip-flop has historically referre" &enerically to !oth simple an"

cloc(e" circuits, in mo"ern usa&e it is common to reser$e the term flip-flop eclusi$ely for

"iscussin& cloc(e" circuits+ the simple ones are commonly calle" latches#12 14

5sin& this terminolo&y, a latch is le$el-sensiti$e, hereas a flip-flop is e"&e-sensiti$e# %hat

is, hen a latch is ena!le" it !ecomes transparent, hile a flip flops output only chan&es on a

sin&le type )positi$e &oin& or ne&ati$e &oin&* of cloc( e"&e#

Implementation

Flip-flops can !e either simple )transparent or asynchronous* or cloc(e" )synchronous*+ the

transparent ones are commonly calle" latches#12 %he or" latch is mainly use" for stora&e

elements, hile cloc(e" "e$ices are "escri!e" as flip-flops#14

.imple flip-flops can !e !uilt aroun" a pair of cross-couple" in$ertin& elements7 $acuum

tu!es,  !ipolar transistors, fiel" effect transistors, in$erters, an" in$ertin& lo&ic &ates ha$e all

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 !een use" in practical circuits# Cloc(e" "e$ices are specially "esi&ne" for synchronous

systems+ such "e$ices i&nore their inputs ecept at the transition of a "e"icate" cloc( si&nal

)(non as cloc(in&, pulsin&, or stro!in&*# Cloc(in& causes the flip-flop to either chan&e or

retain its output si&nal !ase" upon the $alues of the input si&nals at the transition# .ome flip-

flops chan&e output on the risin& e"&e of the cloc(, others on the fallin& e"&e#

.ince the elementary amplifyin& sta&es are in$ertin&, to sta&es can !e connecte" in

succession )as a casca"e* to form the nee"e" non-in$ertin& amplifier# In this confi&uration,

each amplifier may !e consi"ere" as an acti$e in$ertin& fee"!ac( netor( for the other

in$ertin& amplifier# %hus the to sta&es are connecte" in a non-in$ertin& loop althou&h the

circuit "ia&ram is usually "ran as a symmetric cross-couple" pair )!oth the "rain&s are

initially intro"uce" in the Eccles89or"an patent*#

Flip-Flop %ype 7

.: N;: <atch

hen usin& static &ates as !uil"in& !loc(s, the most fun"amental latch is the simple .: latch,

here . an" : stan" for set an" reset# It can !e constructe" from a pair of cross-couple"   N;: 

lo&ic &ates# %he store" !it is present on the output mar(e" =#

hile the . an" : inputs are !oth lo, fee"!ac(  maintains the = an" = outputs in a constant

state, ith = the complement of =# If . ).et* is pulse" hi&h hile : ):eset* is hel" lo, then

the = output is force" hi&h, an" stays hi&h hen . returns to lo+ similarly, if : is pulse"

hi&h hile . is hel" lo, then the = output is force" lo, an" stays lo hen : returns to

lo#

.: AND <atch

%his is an alternate mo"el of the simple .: latch hich is !uilt ith  NAND lo&ic &ates# .et

an" reset no !ecome acti$e lo si&nals, "enote" . an" : respecti$ely# ;therise, operation

is i"entical to that of the .: latch# >istorically, .:-latches ha$e !een pre"ominant "espite thenotational incon$enience of acti$e-lo inputs#

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%imin& consi"eration

Bropa&ation Delay

Another important timin& $alue for a flip-flop is the cloc(-to-output "elay )common sym!ol

in "ata sheets7 tC;* or propa&ation "elay )tB*, hich is the time a flip-flop ta(es to chan&e its

output after the cloc( e"&e# %he time for a hi&h-to-lo transition )tB><* is sometimes

"ifferent from the time for a lo-to-hi&h transition )tB<>*#

hen casca"in& flip-flops hich share the same cloc( )as in a shift re&ister *, it is important

to ensure that the tC; of a prece"in& flip-flop is lon&er than the hol" time )th* of the

folloin& flip-flop, so "ata present at the input of the succee"in& flip-flop is properly shifte"

in folloin& the acti$e e"&e of the cloc(# %his relationship !eteen tC; an" th is normally

&uarantee" if the flip-flops are physically i"entical# Furthermore, for correct operation, it is

easy to $erify that the cloc( perio" has to !e &reater than the sum tsu th#

SR latch operation[13]

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