11
COMMERCIALTEMPERATURERANGE
IDT72V3631/72V3641/72V3651
3.3V CMOS SYNCFIFO 512 x 36, 1,024 x 36 and 2,048 x 36
SYNCHRONOUS RETRANSMIT
The synchronous retransmit feature of these devices allow FIFO data to be
read repeatedly starting at a user-selected position. The FIFO is first put into
retransmit mode to select a beginning word and prevent ongoing FIFO write
operationsfromdestroyingretransmitdata. Datavectorswithaminimumlength
ofthreewordscanretransmitrepeatedlystartingattheselectedword. TheFIFO
can be taken out of retransmit mode at any time and allow normal device
operation.
The FIFO is put in retransmit mode by a LOW-to-HIGH transition on CLKB
when the retransmit mode (RTM) input is HIGH and OR is HIGH. The rising
CLKB edge marks the data present in the FIFO output register as the first
retransmit data. The FIFO remains in retransmit mode until a LOW-to-HIGH
transition occurs while RTM is LOW.
When two or more reads have been done past the initial marked retransmit
word, a retransmit is initiated by a LOW-to-HIGH transition on CLKB when the
read-from-mark (RFM) input is HIGH. This rising CLKB edge shifts the first
retransmit word to the FIFO output register and subsequent reads can begin
immediately. Retransmit loops can be done endlessly while the FIFO is in
retransmit mode. RFM must be LOW during the CLKB rising edge that takes
the FIFO out of retransmit mode (see Figure 11).
WhentheFIFOisputintoretransmitmode,itoperateswithtworeadpointers.
The current read pointer operates normally, incrementing each time when a
newwordisshiftedtotheFIFOoutputregister. Thisreadpointerpositionis used
by the OR and
AEflags. Theshadowreadpointerstoresthememorylocation
at the time the device is put into retransmit mode and does not change until the
deviceistakenoutofretransmitmode. Theshadowreadpointerpositionisused
by the IR and
AFflags. DatawritescanproceedwhiletheFIFOisinretransmit
mode, but
AF is set LOW by the write that stores (512-Y), (1,024 - Y), or
(2,048-Y)wordsafterthefirstretransmitwordfortheIDT72V3631,IDT72V3641,
or IDT72V3651, respectively. The IR flag is set LOW by the 512th, 1,024th,
or2,048thwriteafterthefirstretransmitwordfortheIDT72V3631,IDT72V3641,
or IDT72V3651, respectively.
When the FIFO is in retransmit mode and RFM is HIGH, a rising CLKB edge
loads the current read pointer with the shadow read-pointer value and the OR
flagreflectsthenewleveloffillimmediately. IftheretransmitchangestheFIFO
status out of the almost-empty range, up to two CLKB rising edges after the
retransmitcycleareneededtoswitch
AEhigh(seeFigure12).TherisingCLKB
NOTES:
1. When a word is present in the FIFO output register, its previous memory location is free.
2. Data in the output register does not count as a "word i n FIFO memory". Since in FWFT mode, the first words written to an empty FIFO goes unrequested to the output register (no read
operation necessary), it is not included in the memory count.
3. X is the Almost-Empty Offset for
AE. Y is the Almost-Full Offset for AF.
grammingsection). The
AEflagisLOWwhentheFIFOcontainsXorlesswords
andisHIGHwhentheFIFOcontains(X+1)ormorewords. Adatawordpresent
in the FIFO output register has been read from memory.
Two LOW-to-HIGH transitions of CLKB are required after a FIFO write for
the
AEflagtoreflectthenewleveloffill;therefore,theAEflagofaFIFOcontaining
(X+1)ormorewordsremainsLOWiftwocyclesofCLKBhavenotelapsedsince
the write that filled the memory to the (X+1) level. An
AEflagissetHIGHbythe
second LOW-to-HIGH transition of CLKB after the FIFO write that fills memory
to the (X+1) level. A LOW-to-HIGH transition of CLKB begins the first
synchronizationcycleifitoccursattimetSKEW2orgreaterafterthewritethatfills
the FIFO to (X+1) words. Otherwise, the subsequent CLKB cycle may be the
first synchronization cycle (see Figure 9).
ALMOST-FULL FLAG (
AF)
The Almost-Full flag of a FIFO is synchronized to the port Clock that writes
data to its array (CLKA). The state machine that controls an
AF flag monitors
a write-pointer and read-pointer comparator that indicates when the FIFO
memorystatusisalmost-full,almost-full-1,oralmost-full-2. Thealmost-fullstate
isdefinedbythecontentsofregisterY. Thisregisterisloadedwithapresetvalue
during a FIFO reset, programmed from port A, or programmed serially (see
Almost-EmptyflagandAlmost-Fullflagoffsetprogrammingsection). The
AFflag
isLOWwhenthenumberofwordsintheFIFOisgreaterthanorequalto(512-Y),
(1,024-Y), OR (2,048-Y) for the IDT72V3631, IDT72V3641, or IDT72V3651,
respectively. The
AF flag is HIGH when the number of words in the FIFO is
less than or equal to [512-(Y+1)], [1,024-(Y+1)], or [2,048-(Y+1)] for the
IDT72V3631, IDT72V3641, or IDT72V3651, respectively. A data word
present in the FIFO output register has been read from memory.
Two LOW-to-HIGH transitions of CLKA are required after a FIFO read for
its
AFflagtoreflectthenewleveloffill. Therefore,theAFflagofaFIFOcontaining
[512/1,024/2,048-(Y+1)]orlesswordsremainsLOWiftwocyclesofCLKAhave
notelapsedsincethereadthatreducedthenumberofwordsinmemoryto[512/
1,024/2,048-(Y+1)]. An
AF flag is set HIGH by the second LOW-to-HIGH
transition of CLKA after the FIFO read that reduces the number of words in
memory to [512/1,024/2,048-(Y+1)]. A LOW-to-HIGH transition of CLKA
begins the first synchronization cycle if it occurs at time tSKEW2 or greater after
thereadthatreducesthenumberofwordsinmemoryto[512/1,024/2,048-(Y+1)].
Otherwise, the subsequent CLKA cycle may be the first synchronization cycle
(see Figure 10).
Number of Words in the FIFO(1,2)
Synchronized
to CLKB
to CLKA
IDT72V3631(3)
IDT72V3641(3)
IDT72V3651(3)
OR
AE
AF
IR
000
L
H
1 to X
H
L
H
(X+1) to [512-(Y+1)]
(X+1) to [1,024-(Y+1)]
(X+1) to [2,048-(Y+1)]
H
(512-Y) to 511
(1,024-Y) to 1,023
(2,048-Y) to 2,047
H
L
H
512
1,024
2,048
H
L
TABLE 4 —
—
— FIFO FLAG OPERATION