IDT72275/72285
CMOS SuperSync FIFO™ 32,768 x 18 and 65,536 x 18
COMMERCIALANDINDUSTRIAL
TEMPERATURERANGES
WhenconfiguredinIDTStandardmode,theEFandFFoutputsaredouble
register-bufferedoutputs.
FUNCTIONAL DESCRIPTION
Relevant timing diagrams for IDT Standard mode can be found in Figure
7, 8 and 11.
TIMINGMODES
IDT STANDARD VS FIRST WORD FALL THROUGH (FWFT) MODE
TheIDT72275/72285supporttwodifferenttimingmodesofoperation:IDT
Standard mode or First Word Fall Through (FWFT) mode. The selection of
whichmodewilloperateisdeterminedduringMasterReset,bythestateofthe
FWFT/SIinput.
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
16,386th word for the IDT72275 and 32,770th word for the IDT72285,
respectively was written into the FIFO. Continuing to write data into the
FIFO will cause the PAF to go LOW. Again, if no reads are performed, the
PAF will go LOW after (32,769-m) writes for the IDT72275 and (65,537-m)
writes for the IDT72285, where m is the full offset value. The default setting
for this value is stated in the footnote of Table 2.
If,atthetimeofMasterReset,FWFT/SIisLOW,thenIDTStandardmode
will be selected. This mode uses the Empty Flag (EF) to indicate whether
or not there are any words present in the FIFO. It also uses the Full Flag
function (FF) to indicate whether or not the FIFO has any free space for
writing. In IDT Standard mode, every word read from the FIFO, including
the first, must be requested using the Read Enable (REN) and RCLK.
If, at the time of Master Reset, FWFT/SI is HIGH, then FWFT mode will
be selected. This mode uses Output Ready (OR) to indicate whether or not
there is valid data at the data outputs (Qn). It also uses Input Ready (IR)
to indicate whether or not the FIFO has any free space for writing. In the
FWFTmode,thefirstwordwrittentoanemptyFIFOgoesdirectlytoQnafter
threeRCLKrisingedges,REN=LOWis notnecessary. Subsequentwords
must be accessed using the Read Enable (REN) and RCLK.
Varioussignals,bothinputandoutputsignalsoperatedifferentlydepend-
ing on which timing mode is in effect.
When the FIFO is full, the Input Ready (IR) flag will go HIGH, inhibiting
further write operations. If no reads are performed after a reset, IR will go
HIGH after D writes to the FIFO. D = 32,769 writes for the IDT72275 and
65,537 writes for the IDT72285, respectively. Note that the additional word
in FWFT mode is due to the capacity of the memory plus output register.
Ifthe FIFOis full, the firstreadoperationwillcause the IR flagtogoLOW.
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. Continuing read operations will
cause the FIFO to become empty. When the last word has been read from
theFIFO,ORwillgoHIGHinhibitingfurtherreadoperations.RENisignored
when the FIFO is empty.
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 16,385th word for IDT72275 and 32,769th word for IDT72285
respectively was written into the FIFO. Continuing to write data into the
FIFO will cause the Programmable Almost-Full flag (PAF) to go LOW.
Again, if no reads are performed, the PAF will go LOW after (32,768-m)
writes forthe IDT72275and(65,536-m)writes forthe IDT72285. The offset
“m” is the full offset value. The default setting for this value is stated in the
footnoteofTable1.This parameteris alsouserprogrammable.Seesection
on Programmable Flag Offset Loading.
When the FIFO is full, the Full Flag (FF) will go LOW, inhibiting further
write operations. If no reads are performed after a reset, FF will go LOW
after D writes to the FIFO. D = 32,768 writes for the IDT72275 and 65,536
for the IDT72285, respectively.
If the FIFO is full, the first read operation will cause FF to go HIGH.
Subsequent read operations will cause PAF and HF to go HIGH at the
conditions described in Table 1. If further read operations occur, without
write operations, PAE will go LOW when there are n words in the FIFO,
where nis the emptyoffsetvalue. Continuingreadoperations willcause the
FIFO to become empty. When the last word has been read from the FIFO,
theEFwillgoLOWinhibitingfurtherreadoperations.RENisignoredwhenthe
FIFOisempty.
When configured in FWFT mode, the OR flag output is triple register-
buffered, and the IR flag output is double register-buffered.
Relevant timing diagrams for FWFT mode can be found in Figure 9, 10
and 12.
PROGRAMMING FLAG OFFSETS
FullandEmptyFlagoffsetvaluesareuserprogrammable.TheIDT72275/
72285 has internal registers for these offsets. Default settings are stated in
the footnotes ofTable 1andTable 2. Offsetvalues canbe programmedinto
theFIFOinoneoftwoways;serialorparallelloadingmethod.Theselection
oftheloadingmethodisdoneusingtheLD(Load)pin.DuringMasterReset,
the state of the LD input determines whether serial or parallel flag offset
programming is enabled. A HIGH on LD during Master Reset selects serial
loading of offset values and in addition, sets a default PAE offset value of
3FFH (a threshold 1,023 words from the empty boundary), and a default
PAF offset value of 3FFH (a threshold 1,023 words from the full boundary).
A LOW on LD during Master Reset selects parallel loading of offset values,
and in addition, sets a default PAE offset value of 07FH (a threshold 127
words from the empty boundary), and a default PAF offset value of 07FH
(athreshold127wordsfromthefullboundary).SeeFigure3,OffsetRegister
LocationandDefaultValues.
7
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