8
PropagationDelay,Pulse- beingsentthroughagroupof
only one edge were used, the
clock signal would need to be
twiceasfast.
WidthDistortionand
optocouplers,differencesin
propagationdelayswillcausethe
datatoarriveattheoutputsofthe
optocouplersatdifferenttimes.If
thisdifferenceinpropagation
delaysislargeenough,itwill
determinethemaximumrateat
whichparalleldatacanbesent
throughtheoptocouplers.
PropagationDelaySkew
Propagationdelayisafigureof
meritwhichdescribeshow
quicklyalogicsignalpropagates
throughasystem.Thepropaga-
Propagationdelayskew
representstheuncertaintyof
whereanedgemightbeafter
beingsentthroughan
tiondelayfromlowtohigh(tPLH
istheamountoftimerequiredfor
aninputsignaltopropagateto
theoutput,causingtheoutputto
change from low to high.
)
optocoupler. Figure16shows
thattherewillbeuncertaintyin
boththedataandtheclocklines.
Itisimportantthatthesetwo
areasofuncertaintynotoverlap,
otherwisetheclocksignalmight
arrivebeforeallofthedata
outputshavesettled,orsomeof
thedataoutputsmaystartto
changebeforetheclocksignal
hasarrived.Fromthese
considerations,theabsolute
minimumpulsewidththatcanbe
sentthroughoptocouplersina
parallelapplicationistwicetPSK.A
cautiousdesignshouldusea
slightlylongerpulsewidthto
ensurethatanyadditional
Propagationdelayskewisdefined
asthedifferencebetweenthe
minimumandmaximum
propagationdelays,eithertPLH or
tPHL, for any given group of
optocouplerswhichareoperating
underthesameconditions(i.e.,
thesamedrivecurrent,supply
voltage,outputload,and
Similarly,thepropagationdelay
from high to low (tPHL) is the
amountoftimerequiredforthe
inputsignaltopropagatetothe
output,causingtheoutputto
change from high to low (see
Figure7).
operatingtemperature).As
Pulse-widthdistortion(PWD)
resultswhentPLHandtPHLdifferin
value.PWDisdefinedasthe
differencebetweentPLH andtPHL
andoftendeterminesthemaxi-
mumdataratecapabilityofa
transmissionsystem.PWD can
be expressed in percent by
dividingthePWD(inns)bythe
minimumpulsewidth(inns)
beingtransmitted.Typically,PWD
on the order of 20-30% of the
minimumpulsewidthistolerable;
theexactfiguredependsonthe
particularapplication(RS232,
RS422, T-1, etc.).
illustratedinFigure15,ifthe
inputs of a group of optocouplers
areswitchedeitherONorOFFat
thesametime,tPSK isthe
differencebetweentheshortest
propagationdelay,eithertPLHor
tPHL,andthelongestpropagation
uncertaintyintherestofthe
circuit does not cause aproblem.
delay,eithertPLH ortPHL
.
ThetPSK specifiedoptocouplers
offertheadvantagesof
Asmentionedearlier,tPSK can
determinethemaximumparallel
datatransmissionrate.Figure11
isthetimingdiagramofatypical
paralleldataapplicationwithboth
theclockandthedatalinesbeing
sentthroughoptocouplers.The
figureshowsdataandclock
signalsattheinputsandoutputs
of the optocouplers. To obtain the
maximumdatatransmissionrate,
both edges of the clock signal are
being used to clock the data; if
guaranteedspecificationsfor
propagationdelays,pulse-width
distortionandpropagationdelay
skewovertherecommended
temperature,andinputcurrent,
andpowersupplyranges.
Propagationdelayskew,tPSK,is
animportantparameterto
considerinparalleldataappli-
cationswheresynchronizationof
signalsonparalleldatalinesisa
concern.Iftheparalleldatais
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