Philips Semiconductors
Product specification
4-bit x 16-word FIFO register
74HC/HCT40105
LOW in order to complete the shift-in
process.
INPUT AND OUTPUTS
Data inputs (D0 to D3)
Shift-in control (SI)
Data is loaded into the input stage on
a LOW-to-HIGH transition of SI.
It also triggers an automatic data
transfer process (ripple through). If SI
is held HIGH during reset, data will be
loaded at the falling edge of the MR
signal.
With the FIFO full, SI can be held
HIGH until a shift-out (SO) pulse
occurs. Then, following a shift-out of
data, an empty location appears at
the FIFO input and DIR goes HIGH to
allow the next data to be shifted-in.
This remains at the first FIFO location
until SI goes LOW (see Fig.7).
As there is no weighting of the inputs,
any input can be assigned as the
MSB. The size of the FIFO memory
can be reduced from the 4 × 16
configuration, i.e. 3 × 16, down to
1 × 16, by tying unused data input
pins to VCC or GND.
Shift-out control (SO)
A HIGH-to-LOW transition of
SO causes the DOR flags to go LOW.
A HIGH-to-LOW transition of
SO causes upstream data to move
into the output stage, and empty
locations to move towards the input
stage (bubble-up).
Data outputs (Q0 to Q3)
Data transfer
As there is no weighting of the
outputs, any output can be assigned
as the MSB. The size of the FIFO
memory can be reduced from the
4 × 16 configuration as described for
data inputs. In a reduced format, the
unused data outputs pins must be left
open circuit.
After data has been transferred from
the input stage of the FIFO following
SI = LOW, data moves through the
FIFO asynchronously and is stacked
at the output end of the register.
Empty locations appear at the input
end of the FIFO as data moves
through the device.
Output enable (OE)
The outputs Q0 to Q3 are enabled
when OE = LOW. When OE = HIGH
the outputs are in the high impedance
OFF-state.
Master-reset (MR)
Data output
When MR is HIGH, the control
The data-out-ready flag
functions within the FIFO are cleared,
and date content is declared invalid.
The data-in ready (DIR) flag is set
HIGH and the data-out-ready (DOR)
flag is set LOW. The output stage
remains in the state of the last word
that was shifted out, or in the random
state existing at power-up.
(DOR = HIGH) indicates that there is
valid data at the output (Q0 to Q3).
The initial master-reset at power-on
(MR = HIGH) sets DOR to LOW (see
Fig.8). After MR = LOW, data shifted
into the FIFO moves through to the
output stage causing DOR to go
HIGH.
FUNCTIONAL DESCRIPTION
Data input
Following power-up, the master-reset
(MR) input is pulsed HIGH to clear the
FIFO memory (see Fig.8). The
data-in-ready flag (DIR = HIGH)
indicates that the FIFO input stage is
empty and ready to receive data.
When DIR is valid (HIGH), data
present at D0 to D3 can be shifted-in
using the SI control input.
As the DOR flag goes HIGH, data can
be shifted-out using the SO = HIGH,
data in the output stage is shifted out
and a busy indication is given by DOR
going LOW. When SO is made LOW,
data moves through the FIFO to fill
the output stage and an empty
location appears at the input stage.
When the output stage is filled DOR
goes HIGH, but if the last of the valid
data has been shifted-out leaving the
FIFO empty the DOR flag remains
LOW (see Fig.9). With the FIFO
empty, the last word that was
Status flag outputs (DIR, DOR)
Indication of the status of the FIFO is
given by two status flags,
data-in-ready (DIR) and
data-out-ready (DOR):
With SI = HIGH, data is shifted into
the input stage and a busy indication
is given by DIR going LOW.
DIR = HIGH indicates the input stage
is empty and ready to accept valid
data;
The data remains at the first location
in the FIFO until DIR is set to HIGH
and data moves through the FIFO to
the output stage, or to the last empty
location. If the FIFO is not full after the
SI pulse, DIR again becomes valid
(HIGH) to indicate that space is
available in the FIFO. The DIR flag
remains LOW if the FIFO is full (see
Fig.6). The SI use must be made
DIR = LOW indicates that the FIFO is
full or that a previous shift-in
operation is not complete (busy);
DOR = HIGH assures valid data is
present at the outputs Q0 to Q3 (does
not indicate that new data is awaiting
transfer into the output stage);
shifted-out is latched at the output
Q0 to Q3.
With the FIFO empty, the SO input
can be held HIGH until the SI control
input is used. Following an SI pulse,
DOR = LOW indicates the output
stage is busy or there is no valid data.
1998 Jan 23
4
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