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SDAS108A − OCTOBER 1986 − REVISED SEPTEMBER 1993
DW OR N PACKAGE
(TOP VIEW)
D Asynchronous Operation
D Organized as 64 Words by 5 Bits
D Data Rates From 0 to 25 MHz
D 3-State Outputs
OE
HF
IR
SI
D0
D1
D2
D3
D4
V
CC
AF/AE
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4
5
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7
8
9
10
20
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OR
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Q1
Q2
D Package Options Include Plastic
Small-Outline Packages (DW), Plastic
J-Leaded Chip Carriers (FN), and Standard
Plastic 300-mil DIPs (N)
13 Q3
12 Q4
description
The SN74ALS235 is a 320-bit memory utilizing
advanced low-power Schottky IMPACT
technology. It features high speed with fast
fall-through times and is organized as 64 words by
5 bits.
11
GND
RST
FN PACKAGE
(TOP VIEW)
A first-in, first-out (FIFO) memory is a storage
device that allows data to be written into and read
from its array at independent data rates. The
SN74ALS235 is designed to process data at rates
from 0 to 25 MHz in a bit-parallel format, word by
word.
3
2 1 20 19
SI
SO
OR
Q0
Q1
Q2
18
17
16
15
14
4
5
6
7
8
D0
D1
D2
D3
Data is written into memory on the rising edge of
the shift-in (SI) input. When SI goes low, the first
data word ripples through to the output (see
Figure 1). As the FIFO fills up, the data words
stack up in the order they were written. When the
FIFO is full, additional shift-in pulses have no
9 10 11 12 13
effect. Data is shifted out of memory on the falling edge of the shift-out (SO) input (see Figure 2). When the FIFO
is empty, additional SO pulses have no effect. The last data word remains at the outputs until a new word falls
through or reset (RST) goes low.
Status of the SN74ALS235 FIFO memory is monitored by the output-ready (OR), input-ready (IR),
almost-full/almost-empty (AF/AE), and half-full (HF) flags. When OR is high, valid data is available at the
outputs. OR is low when SO is high and stays low when the FIFO is empty. IR is high when the inputs are ready
to receive more data. IR is low when SI is high and stays low when the FIFO is full. AF/AE is high when the FIFO
contains eight or less words (see Figure 5) or 56 or more words (see Figure 6). AF/AE is low when the FIFO
contains between nine and 55 words. HF is high when the FIFO contains 32 or more words and is low when
the FIFO contains 31 words or less (see Figure 7).
When the FIFO is empty, input data is shifted to the output automatically when SI goes low. If SO is held high
during this time, the OR flag pulses high indicating valid data at the outputs (see Figure 3).
When the FIFO is full, data can be shifted in automatically by holding SI high and taking SO low. One propagation
delay after SO goes low, IR will go high. If SI is still high when IR goes high, data at the inputs are automatically
shifted in. Since IR is normally low when the FIFO is full and SI is high, only a high-level pulse is seen on the
IR output.
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
IMPACT is a trademark of Texas Instruments Incorporated.
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Copyright 1993, Texas Instruments Incorporated
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1
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