7
IDT72255LA/72265LA CMOS SuperSync FIFO™
8,192 x 18 and 16,384 x 18
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
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 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.
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 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 HF would toggle to LOW once the 4,098th
word for the IDT72255LA and 8,194th word for the IDT72265LA, respec-
tively 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 (8,193-m) writes for the IDT72255LA and (16,385-m) writes
for the IDT72265LA, where m is the full offset value. The default setting for
this value is stated in the footnote of Table 2.
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 = 8,193 writes for the IDT72255LA and
16,385 writes for the IDT72265LA, 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 ig-
nored when the FIFO is empty.
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.
FUNCTIONAL DESCRIPTION
TIMING MODES: IDT STANDARD vs FIRST WORD FALL THROUGH
(FWFT) MODE
The IDT72255LA/72265LA 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 Re-
set, 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.
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 necessary. Subsequent
words must be accessed using the Read Enable (REN) and RCLK.
Various signals, both input and output signals operate differently depend-
ing 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 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 4,097th word for IDT72255LA and 8,193th word for IDT72265LA
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 (8,192-m)
writes for the IDT72255LA and (16,384-m) writes for the IDT72265LA.
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 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 = 8,192 writes for the IDT72255LA and
16,384 for the IDT72265LA, respectively.
