IDT72281/72291
CMOS SuperSync FIFO™ 65,536 x 9 and 131,072 x 9
COMMERCIALANDINDUSTRIAL
TEMPERATURERANGES
When configured in IDT Standard mode, the EF and FF outputs are
double register-buffered outputs.
FUNCTIONALDESCRIPTION
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 IDT72281/72291 support two different timing modes of operation:
IDT Standard mode or First Word Fall Through (FWFT) mode. The selec-
tion of which mode will operate is determined during Master Reset, by the
state of the FWFT/SI input.
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 32,770th
word for the IDT72281 and 65,538th word for the IDT72291, 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 (65,537-m) writes for the IDT72281 and (131,073-m) writes for the
IDT72291, where m is the full offset value. The default setting for this value
is stated in the footnote of Table 2.
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.
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 = 65,537 writes for the IDT72281 and
131,073 writes for the IDT72291, 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.
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 32,769th word for IDT72281 and 65,537th word for IDT72291
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 (65,536-m)
writes for the IDT72281 and (131,072-m) writes for the IDT72291. 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 = 65,536 writes for the IDT72281 and 131,072 for the
IDT72291, 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 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 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
Full and Empty Flag offset values are user programmable. The IDT72281/
72291 has internal registers for these offsets. Default settings are stated in
the footnotes of Table 1 and Table 2. Offset values can be programmed into
the FIFO in one of two ways; serial or parallel loading method. The selec-
tion of the loading method is done using the LD (Load) pin. During Master
Reset, 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 (a
threshold 127 words from the full boundary). See Figure 3, Offset Register
Location and Default Values.
7
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