9403A Datasheet, PDF (4/16 Pages) Fairchild Semiconductor – First-In First-Out (FIFO) Buffer Memory

Transfer to the Stack—The outputs of Flip-Flops F₀-F₃
feed the stack. A LOW level on the TTS input initiates a
“fall-through” action. If the top location of the stack is
empty, data is loaded into the stack and the input register is
re-initialized. Note that this initialization is postponed until
PL is LOW again. Thus, automatic FIFO action is achieved
by connecting the IRF output to the TTS input.
An RS Flip-Flop (the Request Initialization Flip-Flop shown
in Figure 10) in the control section records the fact that
data has been transferred to the stack. This prevents multi-
ple entry of the same word into the stack despite the fact
the IRF and TTS may still be LOW. The Request Initializa-
tion Flip-Flop is not cleared until PL goes LOW. Once in the
stack, data falls through the stack automatically, pausing
only when it is necessary to wait for an empty next location.
In the 9403A as in most modern FIFO designs, the MR
input only initializes the stack control section and does not
clear the data.
OUTPUT REGISTER (DATA EXTRACTION)
The Output Register receives 4-bit data words from the
bottom stack location, stores it and outputs data on a 3-
STATE 4-bit parallel data bus or on a 3-STATE serial data
bus. The output section generates and receives the neces-
sary status and control signals. Figure 3 is a conceptual
logic diagram of the output section.
FIGURE 3. Conceptual Output Section
Parallel Data Extraction—When the FIFO is empty after a
LOW pulse is applied to MR, the Output Register Empty
(ORE) output is LOW. After data has been entered into the
FIFO and has fallen through to the bottom stack location, it
is transferred into the Output Register provided the “Trans-
fer Out Parallel” (TOP) input is HIGH. As a result of the
data transfer ORE goes HIGH, indicating valid data on the
data outputs (provided the 3-STATE buffer is enabled).
TOP can now be used to clock out the next word. When
TOP goes LOW, ORE will go LOW indicating that the out-
put data has been extracted, but the data itself remains on
the output bus until the next HIGH level at TOP permits the
transfer of the next word (if available) into the Output Reg-
ister. During parallel data extraction CPSO should be LOW.
TOS should be grounded for single slice operation or con-
nected to the appropriate ORE for expanded operation
(see Expansion section).
TOP is not edge triggered. Therefore, if TOP goes HIGH
before data is available from the stack, but data does
become available before TOP goes LOW again, that data
will be transferred into the Output Register. However, inter-
nal control circuitry prevents the same data from being
transferred twice. If TOP goes HIGH and returns to LOW
before data is available from the stack, ORE remains LOW
indicating that there is no valid data at the outputs.
Serial Data Extraction—When the FIFO is empty after a
LOW pulse is applied to MR, the Output Register Empty
(ORE) output is LOW. After data has been entered into the
FIFO and has fallen through to the bottom stack location, it
is transferred into the Output Register provided TOS is
LOW and TOP is HIGH. As a result of the data transfer
ORE goes HIGH indicating valid data in the register. The 3-
STATE Serial Data Output, Q_S, is automatically enabled
and puts the first data bit on the output bus. Data is serially
shifted out on the HIGH-to-LOW transition of CPSO. To
prevent false shifting, CPSO should be LOW when the new
word is being loaded into the Output Register. The fourth
transition empties the shift register, forces ORE output
LOW and disables the serial output, Q_S (refer to Figure 3).
For serial operation the ORE output may be tied to the TOS
input, requesting a new word from the stack as soon as the
previous one has been shifted out.
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