10
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
IDT723631/723641/723651 CMOS SyncFIFO™
512 x 36, 1,024 x 36 and 2,048 x 36
LOW-to-HIGH transition of CLKA begins the first synchronization cycle if it
occurs at time tSKEW2 or greater after the read that reduces the number of
words in memory to [512/1,024/2,048-(Y+1)]. Otherwise, the subsequent
CLKA cycle may be the first synchronization cycle (see Figure 10).
SYNCHRONOUS RETRANSMIT
The synchronous retransmit feature of these devices allow FIFO data to
be read repeatedly starting at a user-selected position. The FIFO is first
put into retransmit mode to select a beginning word and prevent ongoing
FIFO write operations from destroying retransmit data. Data vectors with a
minimum length of three words can retransmit repeatedly starting at the
selected word. The FIFO can be taken out of retransmit mode at any time
and allow normal device operation.
The FIFO is put in retransmit mode by a LOW-to-HIGH transition on
CLKB when the retransmit mode (RTM) input is HIGH and OR is HIGH.
The rising CLKB edge marks the data present in the FIFO output register
as the first retransmit data. The FIFO remains in retransmit mode until a
LOW-to-HIGH transition occurs while RTM is LOW.
When two or more reads have been done past the initial retransmit
word, a retransmit is initiated by a LOW-to-HIGH transition on CLKB when
the read-from-mark (RFM) input is HIGH. This rising CLKB edge shifts the
first retransmit word to the FIFO output register and subsequent reads can
begin immediately. Retransmit loops can be done endlessly while the FIFO
is in retransmit mode. RFM must be LOW during the CLKB rising edge that
takes the FIFO out of retransmit mode.
When the FIFO is put into retransmit mode, it operates with two read
pointers. The current read pointer operates normally, incrementing each
time a new word is shifted to the FIFO output register and used by the OR
and AE flags. The shadow read pointer stores the memory location at the
time the device is put into retransmit mode and does not change until the
device is taken out of retransmit mode. The shadow read pointer is used
by the IR and AF flags. Data writes can proceed while the FIFO is in
retransmit mode, but AF is set LOW by the write that stores (512-Y), (1,024
- Y), or (2,048 - Y) words after the first retransmit word for the IDT723631,
IDT723641, or IDT723651, respectively. The IR flag is set LOW by the
512th, 1,024th, or 2,048th write after the first retransmit word for the
IDT723631, IDT723641, or IDT723651, respectively.
NOTES:
1. X is the Almost-Empty Offset for AE. Y is the Almost-Full Offset for AF.
2. When a word is present in the FIFO output register, its previous memory location is free.
3. Data in the output register does not count as a "word i n FIFO memory". Since in FWFT mode, the first words written to an empty FIFO goes unrequested to the output register
(no read operation necessary), it is not included in the memory count.
TABLE 4 — FIFO FLAG OPERATION
grammed from port A, or programmed serially (see Almost-Empty flag and
Almost-Full flag offset programming above). The AE flag is LOW when the
FIFO contains X or less words and is HIGH when the FIFO contains (X+1)
or more words. A data word present in the FIFO output register has been
read from memory.
Two LOW-to-HIGH transitions of CLKB are required after a FIFO write
for the AE flag to reflect the new level of fill; therefore, the AE flag of a FIFO
containing (X+1) or more words remains LOW if two cycles of CLKB have
not elapsed since the write that filled the memory to the (X+1) level. An AE
flag is set HIGH by the second LOW-to-HIGH transition of CLKB after the
FIFO write that fills memory to the (X+1) level. A LOW-to-HIGH transition of
CLKB begins the first synchronization cycle if it occurs at time tSKEW2 or
greater after the write that fills the FIFO to (X+1) words. Otherwise, the
subsequent CLKB cycle may be the first synchronization cycle (see Figure
9).
ALMOST-FULL FLAG (AF)
The Almost-Full flag of a FIFO is synchronized to the port Clock that
writes data to its array (CLKA). The state machine that controls an AF flag
monitors a write-pointer and read-pointer comparator that indicates when
the FIFO SRAM status is almost-full, almost-full-1, or almost-full-2. The
almost-full state is defined by the contents of register Y. This register is
loaded with a preset value during a FIFO reset, programmed from port A,
or programmed serially (see Almost-Empty flag and Almost-Full flag offset
programming). The AF flag is LOW when the number of words in the FIFO
is greater than or equal to (512-Y), (1,024-Y), OR (2,048-Y) for the
IDT723631, IDT723641, or IDT723651, respectively. The AF flag is HIGH
when the number of words in the FIFO is less than or equal to [512-(Y+1)],
[1,024-(Y+1)], or [2,048-(Y+1)] for the IDT723631, IDT723641, or
IDT723651, respectively. A data word present in the FIFO output register
has been read from memory.
Two LOW-to-HIGH transitions of CLKA are required after a FIFO read
for its AF flag to reflect the new level of fill. Therefore, the AF flag of a FIFO
containing [512/1,024/2,048-(Y+1)] or less words remains LOW if two cycles
of CLKA have not elapsed since the read that reduced the number of
words in memory to [512/1,024/2,048-(Y+1)]. An AF flag is set HIGH by
the second LOW-to-HIGH transition of CLKA after the FIFO read that
reduces the number of words in memory to [512/1,024/2,048-(Y+1)]. A
Number of Words in the FIFO
(1,2,3)
Synchronized Synchronized
to CLKB to CLKA
IDT723631 IDT723641 IDT723651 OR AE AF IR
000LLHH
1 to X 1 to X 1 to X H L H H
(X+1) to [512-(Y+1)] (X+1) to [1,024-(Y+1)] (X+1) to [2,048-(Y+1)] H H H H
(512-Y) to 511 (1,024-Y) to 1,023 (2,048-Y) to 2,047 H H L H
512 1,024 2,048 H H L L
