SN74ACT7808
2048 × 9
STROBED FIRST-IN, FIRST-OUT MEMORY
SCAS205B - FEBRUARY 1991 - REVISED SEPTEMBER 1995
features
- Load Clocks and Unload Clocks Can Be Asynchronous or
Coincident
- 2048 Words by 9 Bits
- Low-Power Advanced CMOS Technology
- Fast Access Times of 15 ns With a 50-pF Load
- Programmable Almost-Full/Almost-Empty Flag
- Expansion Logic for Depth Cascading
- Empty, Full, and Half-Full Flags
- Fall-Through Time of 20 ns Typ
- Data Rates From 0 to 50 MHz
- 3-State Outputs
- Available in 44-Pin PLCC (FN), Space-Saving 64-Pin Thin Quad
Flat (PM), and Reduced-Height 64-Pin Quad Flat (PAG) Packages
description
A FIFO memory is a storage device that allows data to be written
into and read from its array at independent data rates. The
SN74ACT7808 is a 2048-word by 9-bit FIFO designed for high speed and
fast access times. It processes data at rates up to 50 MHz and access
times of 15 ns in a bit-parallel format.
Data is written into memory on a low-to-high transition at the
load-clock (LDCK) input and is read out on a low-to-high transition
at the unload-clock (UNCK) input. The memory is full when the number
of words clocked in exceeds the number of words clocked out by 2048.
When the memory is full, LDCK signals have no effect on the data
residing in memory. When the memory is empty, UNCK signals have no
effect.
Status of the FIFO memory is monitored by the full (
), empty (
), half-full (HF), and
almost-full/almost-empty (AF/AE) flags. The output is low when the memory is
full and high when the memory is not full. The output is low when the memory is
empty and high when it is not empty. The HF output is high when the
FIFO contains 1024 or more words and is low when it contains 1023 or
less words. The AF/AE status flag is a programmable flag. The first
one or two low-to-high transitions of LDCK after reset can be used to
program the almost-empty offset value (X) and the almost-full offset
value (Y) if program enable (
) is low. The AF/AE flag is high when the FIFO contains
X or less words or (2048 - Y) or more words. The AF/AE flag is low
when the FIFO contains between (X + 1) and (2047 - Y) words.
A low level on the reset (
) input resets the internal stack pointers and sets high, AF/AE high, HF low, and low. The Q outputs are not reset
to any specific logic level. The FIFO must be reset upon power up.
The first word loaded into empty memory causes to go high and the data to appear
on the Q outputs. It is important to note that the first word does
not have to be unloaded. Data outputs are noninverting with respect
to the data inputs and are in the high-impedance state when the
output-enable (OE) input is low. OE does not affect the output flags.
Cascading is easily accomplished in the word-width and word-depth
directions. When not using the FIFO in depth expansion, cascade
enable (
) must be tied
high.
The SN74ACT7808 is characterized for operation from 0°C to
70°C.