13
IDT72V255LA/72V265LA 3.3 VOLT CMOS SuperSync FIFO
8,192 x 18, 16,384 x 18
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
OCTOBER 22, 2008
WRITE ENABLE (
WEN)
When the
WEN input is LOW, data may be loaded into the FIFO RAM
array on the rising edge of every WCLK cycle if the device is not full. Data
is stored in the RAM array sequentially and independently of any ongoing
read operation.
When
WEN is HIGH, no new data is written in the RAM array on each
WCLK cycle.
To prevent data overflow in the IDT Standard mode,
FF will go LOW,
inhibiting further write operations. Upon the completion of a valid read
cycle,
FF will go HIGH allowing a write to occur. The FF is updated by two
WCLK cycles + tSKEW after the RCLK cycle.
To prevent data overflow in the FWFT mode,
IR will go HIGH,
inhibiting further write operations. Upon the completion of a valid read
cycle,
IR will go LOW allowing a write to occur. The IR flag is updated by
two WCLK cycles + tSKEW after the valid RCLK cycle.
WEN is ignored when the FIFO is full in either FWFT or IDT Standard
mode.
READ CLOCK (RCLK)
A read cycle is initiated on the rising edge of the RCLK input. Data can
be read on the outputs, on the rising edge of the RCLK input. It is permis-
sible to stop the RCLK. Note that while RCLK is idle, the
EF/OR, PAE and
HF flags will not be updated. (Note that RCLK is only capable of updating
the
HF flag to HIGH.) The Write and Read Clocks can be independent or
coincident.
READ ENABLE (
REN)
When Read Enable is LOW, data is loaded from the RAM array into the
output register on the rising edge of every RCLK cycle if the device is not
empty.
When the
REN input is HIGH, the output register holds the previous data
and no new data is loaded into the output register. The data outputs Q0-Qn
maintain the previous data value.
In the IDT Standard mode, every word accessed at Qn, including the first
word written to an empty FIFO, must be requested using
REN. When the
last word has been read from the FIFO, the Empty Flag (
EF) will go LOW,
inhibiting further read operations.
REN is ignored when the FIFO is empty.
Once a write is performed,
EF will go HIGH allowing a read to occur. The
EF flag is updated by two RCLK cycles + tSKEW after the valid WCLK
cycle.
In the FWFT mode, the first word written to an empty FIFO automatically
goes to the outputs Qn, on the third valid LOW to HIGH transition of RCLK +
tSKEW after the first write.
REN does not need to be asserted LOW. In
order to access all other words, a read must be executed using
REN. The
RCLK LOW to HIGH transition after the last word has been read from the
FIFO, Output Ready (
OR) will go HIGH with a true read (RCLK with REN
= LOW), inhibiting further read operations.
REN is ignored when the FIFO
is empty.
SERIAL ENABLE (
SEN)
The
SEN input is an enable used only for serial programming of the
offset registers. The serial programming method must be selected during
Master Reset.
SEN is always used in conjunction with LD. When these
lines are both LOW, data at the SI input can be loaded into the program
register one bit for each LOW-to-HIGH transition of WCLK. (See Figure 4.)
When
SEN is HIGH, the programmable registers retains the previous
settings and no offsets are loaded.
SEN functions the same way in both IDT
Standard and FWFT modes.
OUTPUT ENABLE (
OE)
When Output Enable is enabled (LOW), the parallel output buffers
receive data from the output register. When
OE is HIGH, the output data
bus (Qn) goes into a high impedance state.
LOAD (
LD)
This is a dual purpose pin. During Master Reset, the state of the
LD
input determines one of two default offset values (127 or 1,023) for the
PAE
and
PAF flags, along with the method by which these offset registers can be
programmed, parallel or serial. After Master Reset,
LD enables write op-
erations to and read operations from the offset registers. Only the offset
loading method currently selected can be used to write to the registers.
Offset registers can be read only in parallel. A LOW on
LD during Master
Reset selects a default
PAE offset value of 07FH (a threshold 127 words
from the empty boundary), a default
PAF offset value of 07FH (a threshold
127 words from the full boundary), and parallel loading of other offset
values. A HIGH on
LD during Master Reset selects a default PAE offset
value of 3FFH (a threshold 1,023 words from the empty boundary), a
default
PAF offset value of 3FFH (a threshold 1,023 words from the full
boundary), and serial loading of other offset values.
After Master Reset, the
LD pin is used to activate the programming
process of the flag offset values
PAE and PAF. Pulling LD LOW will begin a
serial loading or parallel load or read of these offset values. See Figure 4,
Programmable Flag Offset Programming Sequence.
OUTPUTS:
FULL FLAG (
FF/IR)
This is a dual purpose pin. In IDT Standard mode, the Full Flag (
FF)
function is selected. When the FIFO is full,
FF will go LOW, inhibiting further
write operations. When
FF is HIGH, the FIFO is not full. If no reads are
performed after a reset (either
MRS or PRS), FF will go LOW after D writes
to the FIFO (D = 8,192 for the IDT72V255LA and 16,384 for the
IDT72V265LA). See Figure 7, Write Cycle and Full Flag Timing (IDT
Standard Mode), for the relevant timing information.
In FWFT mode, the Input Ready (
IR) function is selected. IR goes LOW
when memory space is available for writing in data. When there is no
longer any free space left,
IR goes HIGH, inhibiting further write operations.
If no reads are performed after a reset (either
MRS or PRS), IR will go
HIGH after D writes to the FIFO (D = 8,193 for the IDT72V255LA and
16,385 for the IDT72V265LA) See Figure 9, Write Timing (FWFT Mode),
for the relevant timing information.
The
IR status not only measures the contents of the FIFO memory, but
also counts the presence of a word in the output register. Thus, in FWFT
mode, the total number of writes necessary to deassert
IR is one greater
than needed to assert
FF in IDT Standard mode.
FF/IR is synchronous and updated on the rising edge of WCLK. FF/IR
are double register-buffered outputs.