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APPLICATIONINFORMATION
V
CC1
V
CC1
V
CC2
系统-1 系统-2
1
2
3
4
8
7
6
5
dir ctrl
i/o-1
pullup/pulldown
或者 总线 支撑
(1)
V
CC2
i/o-2
pullup/pulldown
或者 总线 支撑
(1)
EnableTimes
SN74AVC2T45
双-bitdual-supplybustransceiver
withconfigurablevoltagetranslationand3-stateoutputs
SCES531F–DECEMBER2003–REVISEDMAY2005
Figure13showsthesn74avc2t45usedinabidirectionallogiclevel-shiftingapplication.sincethe
sn74avc2t45doesnothaveanoutput-使能(oe)管脚,thesystemdesignershouldtakeprecautionstoavoid
buscontentionbetweensystem-1andsystem-2whenchangingdirections.
followingisasequencethatillustratesdatatransmissionfromsystem-1tosystem-2andthenfrom
系统-2tosystem-1.
statedirctrli/o-1i/o-2description
1houtinsystem-1datatosystem-2
系统-2isgettingreadytosenddatatosystem-1.i/o-1andi/o-2are
2hhi-zhi-zdisabled.
thebus-linestatedependsonpulluporpulldown.
(1)
dirbitisflipped.i/o-1andi/o-2stillaredisabled.
3lhi-zhi-z
thebus-linestatedependsonpulluporpulldown.
(1)
4loutinsystem-2datatosystem-1
(1)系统-1andsystem-2mustusethesameconditions,i.e.,bothpulluporbothpulldown.
figure13.bidirectionallogiclevel-shiftingapplication
calculatetheenabletimesforthesn74avc2t45usingthefollowingformulas:
•
t
PZH
(dirtoa)=t
PLZ
(dirtob)+t
PLH
(btoa)
•
t
PZL
(dirtoa)=t
PHZ
(dirtob)+t
PHL
(btoa)
•
t
PZH
(dirtob)=t
PLZ
(dirtoa)+t
PLH
(atob)
•
t
PZL
(dirtob)=t
PHZ
(dirtoa)+t
PHL
(atob)
inabidirectionalapplication,theseenabletimesprovidethemaximumdelayfromthetimethedirbitis
switcheduntilanoutputisexpected.forexample,ifthesn74avc2t45initiallyistransmittingfromatob,然后
thedirbitisswitched;thebportofthedevicemustbedisabledbeforepresentingitwithaninput.aftertheb
porthasbeendisabled,aninputsignalappliedtoitappearsonthecorrespondingaportafterthespecified
propagationdelay.
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