SN74ACT7200L, SN74ACT7201LA, SN74ACT7202LA
256
×
9, 512
×
9, 1024
×
9
ASYNCHRONOUS FIRST-IN, FIRST-OUT MEMORIES
SCAS221A – FEBRUARY 1993 – REVISED SEPTEMBER 1995
Terminal Functions
TERMINAL
NAME
D0 – D8
EF
I/O
I
O
Data inputs
Empty-flag output. EF is low when the read pointer is equal to the write pointer, inhibiting any operation initiated by a read
cycle. When the FIFO is empty, a data word can be read automatically at Q0 – Q8 by holding R low when loading the data
word with a low-level pulse on W.
Full-flag output. FF is low when the write pointer is one location less than the read pointer, indicating that the device is
full and inhibiting any operation initiated by a write cycle. FF goes low when the number of writes after reset exceeds the
number of reads by 256 for the SN74ACT7200L, 512 for the SN74ACT7201LA, and 1024 for the SN74ACT7202LA.
When the FIFO is full, a data word can be written automatically into memory by holding W low while reading out another
data word with a low-level pulse on R.
First-load/retransmit input. FL /RT performs two separate functions. When cascading two or more devices for word-depth
expansion, FL /RT is tied to ground on the first device in the daisy chain to indicate that it is the first device loaded and
unloaded; it is tied high on all other devices in the depth-expansion chain.
FL /RT
I
A device is not used in depth expansion when its expansion (XI) input is tied to ground. In that case, FL /RT acts as a
retransmit enable. A retransmit operation is initiated when FL/RT is pulsed low. This sets the internal read pointer to the
first location and does not affect the write pointer. R and W must be at a high logic level during the low-level FL /RT
retransmit pulse. Retransmit should be used only when less than 256/512/1024 writes are performed between resets;
otherwise, an attempt to retransmit can cause the loss of unread data. The retransmit function can affect XO/HF
depending on the relative locations of the read and write pointers.
Ground
O
Data outputs. Q0 – Q8 are in the high-impedance state when R is high or the FIFO is empty.
Read-enable input. A read cycle begins on the falling edge of R if EF is high. This activates Q0 – Q8 and shifts the next
data value to this bus. The data outputs return to the high-impedance state as R goes high. As the last stored word is
read by the falling edge of R, EF transitions low but Q0 – Q8 remain active until R returns high. When the FIFO is empty,
the internal read pointer is unchanged by a pulse on R.
Reset input. A reset is performed by taking RS low. This initializes the internal read and write pointers to the first location
and sets EF low, FF high, and HF high. Both R and W must be held high for a reset during the window shown in Figure 7.
A reset is required after power up before a write operation can take place.
Supply voltage
I
Write-enable input. A write cycle begins on the falling edge of W if FF is high. The value on D0 – D8 is stored in memory
as W returns high. When the FIFO is full, FF is low, inhibiting W from performing any operation on the device.
Expansion-in input. XI performs two functions. XI is tied to ground to indicate that the device is not used in depth
expansion. When the device is used in depth expansion, XI is connected to the expansion-out (XO) output of the previous
device in the depth-expansion chain.
Expansion-out/half-full-flag output. XO/HF performs two functions. When the device is not used in depth expansion (i.e.,
when XI is tied to ground), XO/HF indicates when half the memory locations are filled. After half of the memory is filled,
the falling edge on W for the next write operation drives XO/HF low. XO/HF remains low until a rising edge of R reduces
the number of words stored to exactly half of the total memory.
When the device is used in depth expansion, XO/HF is connected to XI of the next device in the daisy chain. XO/HF drives
the daisy chain by sending a pulse to the next device when the previous device reaches the last memory location.
DESCRIPTION
FF
O
GND
Q0 – Q8
R
I
RS
VCC
W
I
XI
I
XO/HF
O
6
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