74act2708 Fairchild Semiconductor, 74act2708 Datasheet - Page 3

• Manufacturer Part Number: 74act2708
• Description: 64 X 9 First-in, First-out Memory
• Manufacturer: Fairchild Semiconductor

Functional Description
INPUTS
Data Inputs (D
Data inputs for 9-bit wide data are TTL-compatible. Word
width can be reduced by trying unused inputs to ground
and leaving the corresponding outputs open.
Reset (MR)
Reset is accomplished by pulsing the MR input LOW. Dur-
ing normal operation MR is HIGH. A reset is required after
power up to guarantee correct operation. On reset, the
data outputs go LOW, IR goes HIGH, OR goes LOW, FH
and FULL go LOW. During reset, both internal read and
write pointers are set to the first location in the array.
Shift-In (SI)
Data is written into the FIFO by pulsing SI HIGH. When
Shift-In goes HIGH, the data is loaded into an internal data
latch. Data setup and hold times need to be adhered to
with respect to the falling edge of SI. The write cycle is
complete after the falling edge of SI. The shift-in is inde-
pendent of any ongoing shift-out operation. After the first
word has been written into the FIFO, the falling edge of SI
makes HF go HIGH, indicating a non-empty FIFO. The first
data word appears at the output after the falling edge of SI.
After half the memory is filled, the next rising edge of SI
makes FULL go HIGH indicating a half-full FIFO. When the
FIFO is full, any further shift-ins are disabled.
When the FIFO is empty and OE is LOW, the falling edge
of the first SI will cause the first data word just shifted-in to
appear at the output, even though SO may be LOW.
Shift-Out (SO)
Data is read from the FIFO by the Shift-Out signal provided
the FIFO is not empty. SO going HIGH causes OR to go
LOW indicating that output stage is busy. On the falling
edge of SO, new data reaches the output after propagation
delay t
ory, OR continues to remain LOW, and the last word
shifted-out remains on the output pins.
Output Enable (OE)
OE LOW enables the 3-STATE output buffers. When OE is
HIGH, the outputs are in a 3-STATE mode.
OUTPUTS
Data Outputs (O
Data outputs are enabled when OE is LOW and in the 3-
STATE condition when OE is HIGH.
Input Ready (IR)
IR HIGH indicates data can be shifted-in. When SI goes
HIGH, IR goes LOW, indicating input stage is busy. IR
stays LOW when the FIFO is full and goes HIGH after the
falling edge of the first shift-out.
Output Ready (OR)
OR HIGH indicates data can be shifted-out from the FIFO.
When SO goes HIGH, OR goes LOW, indicating output
stage is busy. OR is LOW when the FIFO is reset or empty
and goes HIGH after the falling edge of the first shift-in.
D
. If the last data has been shifted-out of the mem-
0
–D
0
–O
8
)
8
)
3
Half-Full (HF)
This status flag along with the FULL status flag indicates
the degree of fullness of the FIFO. On reset, HF is LOW; it
rises on the falling edge of the first SI. The rising edge of
the SI pulse that fills up the FIFO makes HF go LOW.
Going from the empty to the full state with SO LOW, the
falling edge of the first SI causes HF to go HIGH, the rising
edge of the 33rd SI causes FULL to go HIGH, and the ris-
ing edge of the 64th SI causes HF to go LOW.
When the FIFO is full, HF is LOW and the falling edge of
the first shift-out causes HF to go HIGH indicating a “non-
full” FIFO.
Full Flag (FULL)
This status flag along with the HF status flag indicates the
degree of fullness of the FIFO. On reset, FULL is LOW.
When half the memory is filled, on the rising edge of the
next SI, the FULL flag goes HIGH. It remains set until the
difference between the write pointer and the read pointer is
less than or equal to one-half of the total memory of the
device. The FULL flag then goes LOW on the rising edge of
the next SO.
H
L
H
L
X
LOW Voltage Level
LOW Voltage Level
MR
Immaterial
HIGH Voltage Level
HIGH Voltage Level
H
L
HF
H
H
L
L
Inputs
SI
X
X
FULL
Status Flags Truth Table
H
H
L
L
SO
X
X
Reset Truth Table
Empty
Full
IR
32 Locations Filled
32 Locations Filled
H
X
Status Flag Condition
OR
X
L
Outputs
HF
X
L
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FULL O
X
L
0
–O
X
L
8