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12
IDT723622/723632/723642 CMOS SyncBiFIFO
256 x 36 x 2, 512 x 36 x 2, 1,024 x 36 x 2
COMMERCIALTEMPERATURERANGE
almost-emptystateisdefinedbythecontentsofregisterX1for
AEBandregister
X2 for
AEA.TheseregistersareloadedwithpresetvaluesduringaFIFOreset
or programmed from port A (see Almost-Empty flag and Almost-Full flag offset
programming section). An Almost-Empty flag is LOW when its FIFO contains
X or less words and is HIGH when its FIFO contains (X+1) or more words. A
data word present in the FIFO output register has been read from memory.
Two LOW-to-HIGH transitions of the Almost-Empty flag synchronizing
clock are required after a FIFO write for its Almost-Empty flag to reflect the new
level of fill. Therefore, the Almost-Full flag of a FIFO containing (X+1) or more
words remains LOW if two cycles of its synchronizing clock have not elapsed
since the write that filled the memory to the (X+1) level. An Almost-Empty flag
issetHIGHbythesecondLOW-to-HIGHtransitionofitssynchronizingclockafter
the FIFO write that fills memory to the (X+1) level. A LOW-to-HIGH transition of
anAlmost-Emptyflagsynchronizingclockbeginsthefirstsynchronizationcycle
ifitoccursattimetSKEW2orgreaterafterthewritethatfillstheFIFOto(X+1)words.
Otherwise,thesubsequentsynchronizingclockcyclemaybethefirstsynchro-
nization cycle. (See Figures 12 and 13).
Synchronized
Number of Words in FIFO(1,2)
to CLKB
to CLKA
IDT723622(3)
IDT723632(3)
IDT723642(3)
ORB
AEB
AFA
IRA
000
L
H
1 to X1
H
L
H
(X1+1) to [256-(Y1+1)]
(X1+1) to [512-(Y1+1)]
(X1+1) to [1,024-(Y1+1)]
H
(256-Y1) to 255
(512-Y1) to 511
(1,024-Y1) to 1,023
H
L
H
256
512
1,024
H
L
full-2. From the time a word is read from a FIFO, its previous memory location
is ready to be written in a minimum of two cycles of the Input Ready flag
synchronizing clock. Therefore, an Input Ready flag is LOW if less than two
cycles of the Input Ready flag synchronizing clock have elapsed since the next
memory write location has been read. The second LOW-to-HIGH transition on
the Input Ready flag synchronizing Clock after the read sets the Input Ready
flag HIGH.
A LOW-to-HIGH transition on an Input Ready flag synchronizing clock
begins the first synchronization cycle of a read if the clock transition occurs at
timetSKEW1orgreateraftertheread.Otherwise,thesubsequentclockcyclecan
be the first synchronization cycle (see Figures 10 and 11 for timing diagrams).
ALMOST-EMPTY FLAGS (
AEA, AEB)
TheAlmost-EmptyflagofaFIFOissynchronizedtotheportclockthatreads
datafromitsarray.ThestatemachinethatcontrolsanAlmost-Emptyflagmonitors
a write pointer and read pointer comparator that indicates when the FIFO
memory status is almost-empty, almost-empty+1, or almost-empty+2. The
NOTES:
1. When a word loaded to an empty FIFO is shifted to the output register, its previous FIFO memory location is free.
2. Data in the output register does not count as a "word in FIFO memory". Since the first word written to an empty FIFO goes unrequested to the output register (no read
operation necessary), it is not included in the FIFO memory count.
3. X1 is the Almost-Empty offset for FIFO1 used by
AEB. Y1 is the Almost-Full offset for FIFO1 used by AFA. Both X1 and Y1 are selected during a reset of FIFO1 or
programmed from port A.
Synchronized
Number of Words in FIFO(1,2)
to CLKA
to CLKB
IDT723622(3)
IDT723632(3)
IDT723642(3)
ORA
AEA
AFB
IRB
000
L
H
1 to X2
H
L
H
(X2+1) to [256-(Y2+1)]
(X2+1) to [512-(Y2+1)]
(X2+1) to [1,024-(Y2+1)]
H
(256-Y2) to 255
(512-Y2) to 511
(1,024-Y2) to 1,023
H
L
H
256
512
1,024
H
L
NOTES:
1. When a word loaded to an empty FIFO is shifted to the output register, its previous FIFO memory location is free.
2. Data in the output register does not count as a "word in FIFO memory". Since the first word written to an empty FIFO goes unrequested to the output register (no read
operation necessary), it is not included in the FIFO memory count.
3. X2 is the Almost-Empty offset for FIFO2 used by
AEA. Y2 is the Almost-Full offset for FIFO2 used by AFB. Both X2 and Y2 are selected during a reset of FIFO2 or
programmed from port A.
TABLE 4 — FIFO1 FLAG OPERATION
TABLE 5 — FIFO2 FLAG OPERATION