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72805LB10BGGI8 Datasheet(PDF) 7 Page - Integrated Device Technology |
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72805LB10BGGI8 Datasheet(HTML) 7 Page - Integrated Device Technology |
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7 / 26 page  7 COMMERCIAL AND INDUSTRIAL TEMPERATURE RANGES IDT72805LB/72815LB/72825LB/72845LB CMOS Dual SyncFIFOTM 256 x 18, 512 x 18, 1,024 x 18, and 4,096 x 18 FUNCTIONALDESCRIPTION TIMING MODES: IDT STANDARD vs FIRST WORD FALL THROUGH (FWFT) MODE TheIDT72805LB/72815LB/72825LB/72845LBsupporttwodifferenttiming modes of operation. The selection of which mode will operate is determined duringconfigurationatReset(RS).DuringaRSoperation,theFirstLoad(FL), Read Expansion Input ( RXI)andWriteExpansionInput(WXI)pinsareused toselectthetimingmodeperthetruthtableshowninTable3.InIDTStandard Mode,thefirstwordwrittentoanemptyFIFOwillnotappearonthedataoutput lines unless a specific read operation is performed. A read operation, which consists of activating Read Enable (REN) and enabling a rising Read Clock (RCLK)edge,willshiftthewordfrominternalmemorytothedataoutputlines. InFWFTmode,thefirstwordwrittentoanemptyFIFOisclockeddirectlytothe dataoutputlinesafterthreetransitionsoftheRCLKsignal.ARENdoesnothave to be asserted for accessing the first word. Varioussignals,bothinputandoutputsignalsoperatedifferentlydepending onwhichtimingmodeisineffect. IDT STANDARD MODE In this mode, the status flags, FF, PAF, HF, PAE and EF operate in the manner outlined in Table 1. To write data into to the FIFO, Write Enable (WEN) must be LOW. Data presented to the DATA IN lines will be clocked intotheFIFOonsubsequenttransitionsoftheWriteClock(WCLK).Afterthe first write is performed, the Empty Flag (EF) will go HIGH. Subsequent writes will continue to fill up the FIFO. The Programmable Almost-Empty flag (PAE) will go HIGH after n + 1 words have been loaded into the FIFO, where n is the Empty offset value. The default setting for this value is stated in the footnote of Table 1. This parameter is also user programmable. See section on Programmable Flag Offset Loading. If one continued to write data into the FIFO, and we assumed no read operations were taking place, the Half-Full flag (HF) would toggle to LOW once the 129th (IDT72805LB), 257th (IDT72815LB), 513th (IDT72825LB), and 2,049th (IDT72845LB) word respectively was written into the FIFO. ContinuingtowritedataintotheFIFOwillcausetheProgrammableAlmost-Full flag (PAF) to go LOW. Again, if no reads are performed, the PAFwillgoLOW after(256-m)writesfortheIDT72805LB,(512-m)writesfortheIDT72815LB, (1,024-m)writesfortheIDT72825LB,and(4,096–m)writesfortheIDT72845LB. Theoffset“m”istheFulloffsetvalue.Thisparameterisalsouserprogrammable. See section on Programmable Flag Offset Loading. If there is no Full offset specified,thePAFwillbeLOWwhenthedeviceis31awayfromcompletelyfull forIDT72805LB,63awayfromcompletelyfullforIDT72815LB,and127away fromcompletelyfullfortheIDT72825LB/72845LB. WhentheFIFOisfull,theFullFlag(FF)willgoLOW,inhibitingfurtherwrite operations.Ifnoreadsareperformedafterareset,FFwillgoLOWafterDwrites totheFIFO.D=256writesfortheIDT72805LB,512fortheIDT72815LB,1,024 for the IDT72825LB, and 4,096 for the IDT72845LB, respectively. If the FIFO is full, the first read operation will cause FF to go HIGH. Subsequent read operations will cause PAF and the Half-Full flag (HF) to go HIGH at the conditions described in Table 1. If further read operations occur, without write operations, the Programmable Almost-Empty flag (PAE) will go LOW when there are n words in the FIFO, where n is the Empty offset value. If there is no Empty offset specified, the PAE will be LOW when the device is 31 away from completely empty for IDT72805LB, 63 away from completelyemptyforIDT72815LB,and127awayfromcompletelyemptyfor IDT72825LB/72845LB.ContinuingreadoperationswillcausetheFIFOtobe empty. When the last word has been read from the FIFO, the EF will go LOW inhibiting further read operations. REN is ignored when the FIFO is empty. FIRST WORD FALL THROUGH MODE (FWFT) In this mode, the status flags, IR, PAF, HF, PAE and OR operate in the manner outlined in Table 2. To write data into to the FIFO, WEN must be LOW. Data presented to the DATA IN lines will be clocked into the FIFO on subsequent transitions of WCLK. After the first write is performed, the Output Ready (OR) flag will go LOW. Subsequent writes will continue to fill up the FIFO. PAE will go HIGH after n + 2 words have been loaded into the FIFO, where n is the Empty offset value. The default setting for this value is stated in the footnote of Table 2. This parameter is also user program- mable. See section on Programmable Flag Offset Loading. 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 (72805LB),258th(72815LB),514th(72825LB),and2,050th(72845LB)word respectivelywaswrittenintotheFIFO.ContinuingtowritedataintotheFIFO will cause the PAF to go LOW. Again, if no reads are performed, the PAF will go LOW after (257-m) writes for the IDT72805LB, (513-m) writes for the IDT72815LB,(1,025-m)writesfortheIDT72825LB,and(4,097–m)writesfor theIDT72845LB,wheremistheFulloffsetvalue.Thedefaultsettingforthisvalue isstatedinthefootnoteofTable2. WhentheFIFOisfull,theInputReady(IR)flagwillgoHIGH,inhibitingfurther writeoperations.Ifnoreadsareperformedafterareset, IRwillgoHIGHafter D writes to the FIFO. D = 257 writes for the IDT72805LB, 513 for the IDT72815LB,1,025fortheIDT72825LB, and4,097fortheIDT72845LB.Note thattheadditionalwordinFWFTmodeisduetothecapacityofthememoryplus outputregister. If the FIFO is full, the first read operation will cause the IR flag to go LOW. Subsequent read operations will cause the PAF and HF to go HIGH at the conditions described in Table 2. If further read operations occur, without write operations, the PAE will go LOW when there are n + 1 words in the FIFO, where n is the Empty offset value. If there is no Empty offset specified, the PAE will be LOW when the device is 32 away from completely empty for IDT72805LB, 64 away from completely empty for IDT72815LB, and 128 away from completely empty for IDT72825LB/72845LB. Continuing read operationswillcausetheFIFOtobeempty.Whenthelastwordhasbeenread from the FIFO, OR will go HIGH inhibiting further read operations. REN is ignored when the FIFO is empty. PROGRAMMABLE FLAG LOADING Full and Empty flag Offset values can be user programmable. The IDT72805LB/72815LB/72825LB/72845LB has internal registers for these offsets.DefaultsettingsarestatedinthefootnotesofTable1andTable2.Offset valuesareloadedintotheFIFOusingthedatainputlinesD0-D11.Toloadthe offsetregisters,theLoad(LD)pinandWENpinmustbeheldLOW.Datapresent on D0-D11will be transferred in to the Empty Offset register on the first LOW- to-HIGHtransitionofWCLK.BycontinuingtoholdtheLDandWENpinlow,data present on D0-D11 will be transferred into the Full Offset register on the next transition of the WCLK. The third transition again writes to the Empty Offset register.Writingalloffsetregistersdoesnothavetooccuratonetime.Oneor two offset registers can be written and then by bringing the LD pin HIGH, the FIFO is returned to normal read/write operation. When the LD pin and WEN areagainsetLOW,thenextoffsetregisterinsequenceiswritten. ThecontentsoftheoffsetregisterscanbereadonthedataoutputlinesQ0- Q11 when the LD pin is set LOW and REN is set LOW. Data can then be read |
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