IDT72V205/72V215/72V225/72V235/72V245 3.3V CMOS SyncFIFOTM
256 x 18, 512 x 18, 1,024 x 18, 2,048 x 18 and 4,096 x 18
COMMERCIALANDINDUSTRIAL
TEMPERATURERANGES
72V245.Continuingreadoperations willcausetheFIFOtobeempty.When
thelastwordhasbeenreadfromtheFIFO,theEFwillgoLOWinhibitingfurther
read operations. REN is ignored when the FIFO is empty.
FUNCTIONALDESCRIPTION
TIMING MODES: IDT STANDARD vs FIRST WORD FALL THROUGH
(FWFT) MODE
FIRST WORD FALL THROUGH MODE (FWFT)
TheIDT72V205/72V215/72V225/72V235/72V245supporttwodifferent
timing modes of operation. The selection of which mode will operate is
determinedduringconfigurationatReset(RS).Duringa RSoperation,theFirst
Load(FL),ReadExpansionInput(RXI),andWriteExpansionInput(WXI)pins
areusedtoselectthetimingmodeperthetruthtableshowninTable3.InIDT
StandardMode,thefirstwordwrittentoanemptyFIFOwillnotappearonthe
dataoutputlinesunlessaspecificreadoperationisperformed.Areadoperation,
whichconsists ofactivatingReadEnable (REN)andenablinga risingRead
Clock(RCLK)edge,willshiftthewordfrominternalmemorytothedataoutput
lines.InFWFTmode,thefirstwordwrittentoanemptyFIFOisclockeddirectly
tothedataoutputlinesafter threetransitionsoftheRCLKsignal.ARENdoes
nothavetobeassertedforaccessingthefirstword.
In this mode, the status flags, IR, PAF, HF, PAE, and OR operate in the
manneroutlinedinTable2.TowritedataintototheFIFO,WENmustbeLOW.
DatapresentedtotheDATAINlineswillbeclockedintotheFIFOonsubsequent
transitionsofWCLK.Afterthefirstwriteisperformed,theOutputReady(OR)
flagwillgoLOW.SubsequentwriteswillcontinuetofilluptheFIFO.PAEwillgo
HIGHaftern+2wordshavebeenloadedintotheFIFO,wherenistheempty
offsetvalue.ThedefaultsettingforthisvalueisstatedinthefootnoteofTable2.
Thisparameterisalsouserprogrammable.SeesectiononProgrammableFlag
OffsetLoading.
If one continued to write data into the FIFO, and we assumed no read
operations were taking place, the HF would toggle to LOW once the 130th
(72V205), 258th(72V215),514th(72V225),1,026th(72V235),and2,050th
(72V245)wordrespectivelywaswrittenintotheFIFO. Continuingtowritedata
intotheFIFOwillcausethePAFtogoLOW.Again,ifnoreadsareperformed,
thePAFwillgoLOWafter(257-m)writes fortheIDT72V205,(513-m)writes
fortheIDT72V215,(1,025-m)writesfortheIDT72V225,(2,049–m)writesfor
the IDT72V235and(4,097–m)writes forthe IDT72V245, where mis the full
offsetvalue. ThedefaultsettingforthisvalueisstatedinthefootnoteofTable
Varioussignals,bothinputandoutputsignalsoperatedifferentlydepending
onwhichtimingmodeisineffect.
IDT STANDARD MODE
In this mode, the status flags, FF, PAF, HF, PAE, and EF operate in the
manneroutlinedinTable1.TowritedataintototheFIFO,WriteEnable(WEN)
mustbeLOW.DatapresentedtotheDATAINlineswillbeclockedintotheFIFO
2.
on subsequent transitions of the Write Clock (WCLK). After the first write is
performed,theEmptyFlag(EF)willgoHIGH.Subsequentwriteswillcontinue
tofilluptheFIFO.TheProgrammableAlmost-Emptyflag(PAE)willgoHIGH
aftern+1wordshavebeenloadedintotheFIFO,wherenistheemptyoffset
value.ThedefaultsettingforthisvalueisstatedinthefootnoteofTable1.This
parameter is also user programmable. See section on Programmable Flag
OffsetLoading.
WhentheFIFOisfull,theInputReady(IR)flagwillgoHIGH,inhibitingfurther
writeoperations. Ifnoreadsareperformedafterareset,IRwillgoHIGHafter
DwritestotheFIFO. D=257writesfortheIDT72V205,513fortheIDT72V215,
1,025 for the IDT72V225, 2,049 for the IDT72V235 and 4,097 for the
IDT72V245.NotethattheadditionalwordinFWFTmodeisduetothecapacity
ofthememoryplusoutputregister.
IftheFIFOisfull,thefirstreadoperationwillcausethe IRflagtogoLOW.
Subsequent read operations will cause the PAF and HF to go HIGH at the
conditionsdescribedinTable2.Iffurtherreadoperationsoccur,withoutwrite
operations,thePAEwillgoLOWwhentherearen+1wordsintheFIFO,where
nistheemptyoffsetvalue.Ifthereisnoemptyoffsetspecified,thePAEwillbe
LOWwhenthedeviceis 32awayfromcompletelyemptyforIDT72V205,64
awayfromcompletelyemptyforIDT72V215,and128awayfromcompletely
emptyforIDT72V225/72V235/72V245.Continuingreadoperationswillcause
the FIFOtobe empty. Whenthe lastwordhas beenreadfromthe FIFO,OR
willgoHIGHinhibitingfurtherreadoperations.RENisignoredwhentheFIFO
isempty.
If one continued to write data into the FIFO, and we assumed no read
operationsweretakingplace,theHalf-FullFlag(HF)wouldtoggletoLOWonce
the129th(72V205), 257th(72V215),513th(72V225),1,025th(72V235),and
2,049th(72V245)wordrespectivelywaswrittenintotheFIFO. Continuingto
writedataintotheFIFOwillcausetheProgrammableAlmost-FullFlag(PAF)
togoLOW.Again,ifnoreadsareperformed, thePAFwillgoLOWafter(256-m)
writesfortheIDT72V205,(512-m)writesfortheIDT72V215,(1,024-m)writes
fortheIDT72V225,(2,048-m)writesfortheIDT72V235and(4,096–m)writes
fortheIDT72V245. Theoffset“m”isthefulloffsetvalue. Thisparameterisalso
userprogrammable.SeesectiononProgrammableFlagOffsetLoading. Ifthere
isnofulloffsetspecified,thePAFwillbeLOWwhenthedeviceis31awayfrom
completelyfullforIDT72V205,63awayfromcompletelyfullforIDT72V215,and
127awayfromcompletelyfullfor theIDT72V225/72V235/72V245.
WhentheFIFOisfull,theFullFlag(FF)willgoLOW,inhibitingfurtherwrite
operations. Ifnoreadsareperformedafterareset,FFwillgoLOWafterDwrites
totheFIFO. D=256writesfortheIDT72V205,512fortheIDT72V215,1,024
for the IDT72V225, 2,048 for the IDT72V235 and 4,096 for the IDT72V245,
respectively.
If the FIFO is full, the first read operation will cause FF to go HIGH.
SubsequentreadoperationswillcausePAFandtheHalf-FullFlag(HF)togo
HIGHattheconditionsdescribedinTable1.Iffurtherreadoperationsoccur,
withoutwriteoperations,theProgrammableAlmost-EmptyFlag(PAE)willgo
LOWwhenthere are nwords inthe FIFO, where nis the emptyoffsetvalue.
Ifthereisnoemptyoffsetspecified,thePAEwillbeLOWwhenthedeviceis31
awayfromcompletelyemptyforIDT72V205,63awayfromcompletelyempty
forIDT72V215,and127awayfromcompletelyemptyforIDT72V225/72V235/
PROGRAMMABLEFLAGLOADING
FullandEmptyflagoffsetvaluescanbeuserprogrammable.TheIDT72V205/
72V215/72V225/72V235/72V245 has internal registers for these offsets.
DefaultsettingsarestatedinthefootnotesofTable1andTable2.Offsetvalues
areloadedintotheFIFOusingthedatainputlines D0-D11.Toloadtheoffset
registers,theLoad(LD)pinandWENpinmustbeheldLOW.Datapresenton
D0-D11willbetransferredintotheEmptyOffsetregisteronthefirstLOW-to-HIGH
transitionofWCLK.BycontinuingtoholdtheLDandWENpinlow, datapresent
onD0-D11willbetransferredintotheFullOffsetregisteronthenexttransition
oftheWCLK.ThethirdtransitionagainwritestotheEmptyOffsetregister. Writing
alloffsetregistersdoesnothavetooccuratonetime.Oneortwooffsetregisters
canbe writtenandthenbybringingtheLD pinHIGH, the FIFOis returnedto
normalread/writeoperation.WhentheLDpinandWENareagainsetLOW,
thenextoffsetregisterinsequenceiswritten.
6
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