IDT72275/72285 SUPERSYNC FIFO™
32,768 x 18, 65,536 x 18
COMMERCIAL TEMPERATURE RANGE
D = 32,768 writes for the IDT72275 and 65,536 for the
IDT72285, respectively.
FUNCTIONAL DESCRIPTION
If the FIFO is full, the first read operation will causeFF to go
HIGH. Subsequent read operations will cause PAFand 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 n is the empty
offset value. Continuing read operations will cause the FIFO
to become 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.
When configured in IDT Standard mode, the EF and FF
outputs are double register-buffered outputs.
Relevant timing diagrams for IDT Standard mode can be
found in Figure 7, 8 and 11.
TIMING MODES: IDT STANDARD vs FIRST WORD FALL
THROUGH (FWFT) MODE
The IDT72275/72285 support two different timing modes
of operation: IDT Standard mode or First Word Fall Through
(FWFT) mode. The selection of which mode will operate is
determined during Master Reset, by the state of the FWFT/
SI input.
If, at the time of Master Reset, FWFT/SI is LOW, then IDT
Standard mode 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.
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 transi-
tions 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 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 HFwould
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
PAFto go LOW. Again, if no reads are performed, thePAFwill
go LOW after (32,769-m) writes for the IDT72275 and
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 FWFT
mode, the first word written to an empty FIFO goes directly to
Qn after three RCLK rising edges, REN = LOW is not neces-
sary. Subsequent words must be accessed using the Read
Enable (REN) and RCLK.
Various signals, both input and output signals operate
differently depending on which timing mode is in effect.
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 into the FIFO on subsequent
transitions of the Write Clock (WCLK). After the first write is (65,537-m) writes for the IDT72285, where m is the full offset
value. Thedefaultsettingforthisvalueisstatedinthefootnote
of Table 2.
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.
ThedefaultsettingforthisvalueisstatedinthefootnoteofTable
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 PAFwill go LOW
after (32,768-m) writes for the IDT72275 and (65,536-m)
writes for the IDT72285. The offset “m” is the full 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.
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.
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. Continuing read operations
will cause the FIFO to become empty. When the last word has
been read from the FIFO, OR will go HIGH inhibiting further
read operations. REN is ignored when the FIFO is empty.
WhenconfiguredinFWFTmode,theORflagoutputistriple
register-buffered, and the IR flag output is double register-
buffered.
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.
Relevant timing diagrams for FWFT mode can be found in
Figure 9, 10 and 12.
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