7
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
IDT72805LB/72815LB/72825LB/72845LB CMOS Dual SyncFIFO
TM
256 x 18, 512 x 18, 1,024 x 18, and 4,096 x 18
FUNCTIONAL DESCRIPTION
TIMING MODES: IDT STANDARD vs FIRST WORD FALL THROUGH
(FWFT) MODE
The IDT72805LB/72815LB/72825LB/72845LB support two different timing
modes of operation. The selection of which mode will operate is determined
during configuration at Reset (RS). During a RS operation, the First Load (FL),
Read Expansion Input ( RXI) and Write Expansion Input (WXI) pins are used
to select the timing mode per the truth table shown in Table 3. In IDT Standard
Mode, the first word written to an empty FIFO will not appear on the data output
lines unless a specific read operation is performed. A read operation, which
consists of activating Read Enable (REN) and enabling a rising Read Clock
(RCLK) edge, will shift the word from internal memory to the data output lines.
In FWFT mode, the first word written to an empty FIFO is clocked directly to the
data output lines after three transitions of the RCLK signal. A REN does not have
to be asserted for accessing the first word.
Various signals, both input and output signals operate differently depending
on which timing mode is in effect.
IDT STANDARD MODE
In this mode, the status flags, FF, PAF, HF, PAE and EF operate in the
manner outlined in Table 1. To write data into to the FIFO, Write Enable
(WEN) must be LOW. Data presented to the DATA IN lines will be clocked
into the FIFO on subsequent transitions of the Write Clock (WCLK). After the
first write is performed, the Empty Flag (EF) will go HIGH. Subsequent
writes will continue to fill up the FIFO. The Programmable Almost-Empty
flag (PAE) will go HIGH after n + 1 words have been loaded into the FIFO,
where n is the Empty offset value. The default setting for this value is stated
in the footnote of Table 1. This parameter is also user programmable. See
section on Programmable Flag Offset Loading.
If one continued to write data into the FIFO, and we assumed no read
operations were taking place, the Half-Full flag (HF) would toggle to LOW
once the 129th (IDT72805LB), 257th (IDT72815LB), 513th (IDT72825LB),
and 2,049th (IDT72845LB) word respectively was written into the FIFO.
Continuing to write data into the FIFO will cause the Programmable Almost-Full
flag (PAF) to go LOW. Again, if no reads are performed, the PAF will go LOW
after (256-m) writes for the IDT72805LB, (512-m) writes for the IDT72815LB,
(1,024-m) writes for the IDT72825LB, and (4,096–m) writes for the IDT72845LB.
The offset “m” is the Full offset value. This parameter is also user programmable.
See section on Programmable Flag Offset Loading. If there is no Full offset
specified, the PAF will be LOW when the device is 31 away from completely full
for IDT72805LB, 63 away from completely full for IDT72815LB, and 127 away
from completely full for the IDT72825LB/72845LB.
When the FIFO is full, the Full Flag (FF) will go LOW, inhibiting further write
operations. If no reads are performed after a reset, FF will go LOW after D writes
to the FIFO. D = 256 writes for the IDT72805LB, 512 for the IDT72815LB, 1,024
for the IDT72825LB, and 4,096 for the IDT72845LB, respectively.
If the FIFO is full, the first read operation will cause FF to go HIGH.
Subsequent read operations will cause PAF and the Half-Full flag (HF) to
go HIGH at the conditions described in Table 1. If further read operations
occur, without write operations, the Programmable Almost-Empty flag
(PAE) will go LOW when there are n words in the FIFO, where n is the Empty
offset value. If there is no Empty offset specified, the PAE will be LOW when
the device is 31 away from completely empty for IDT72805LB, 63 away from
completely empty for IDT72815LB, and 127 away from completely empty for
IDT72825LB/72845LB. Continuing read operations will cause the FIFO to be
empty. When the last word has been read from the FIFO, the EF will go LOW
inhibiting further read operations. REN is ignored when the FIFO is empty.
FIRST WORD FALL THROUGH MODE (FWFT)
In this mode, the status flags, IR, PAF, HF, PAE and OR operate in the
manner outlined in Table 2. To write data into to the FIFO, WEN must be
LOW. Data presented to the DATA IN lines will be clocked into the FIFO on
subsequent transitions of WCLK. After the first write is performed, the
Output Ready (OR) flag will go LOW. Subsequent writes will continue to fill
up the FIFO. PAE will go HIGH after n + 2 words have been loaded into the
FIFO, where n is the Empty offset value. The default setting for this value
is stated in the footnote of Table 2. This parameter is also user program-
mable. See section on Programmable Flag Offset Loading.
If one continued to write data into the FIFO, and we assumed no read
operations were taking place, the HF would toggle to LOW once the 130th
(72805LB), 258th (72815LB), 514th (72825LB), and 2,050th (72845LB) word
respectively was written into the FIFO. Continuing to write data into the FIFO
will cause the PAF to go LOW. Again, if no reads are performed, the PAF will
go LOW after (257-m) writes for the IDT72805LB, (513-m) writes for the
IDT72815LB, (1,025-m) writes for the IDT72825LB, and (4,097–m) writes for
the IDT72845LB, where m is the Full offset value. The default setting for this value
is stated in the footnote of Table 2.
When the FIFO is full, the Input Ready (IR) flag will go HIGH, inhibiting further
write operations. If no reads are performed after a reset, IR will go HIGH after
D writes to the FIFO. D = 257 writes for the IDT72805LB, 513 for the
IDT72815LB, 1,025 for the IDT72825LB, and 4,097 for the IDT72845LB. Note
that the additional word in FWFT mode is due to the capacity of the memory plus
output register.
If the FIFO is full, the first read operation will cause the IR flag to go LOW.
Subsequent read operations will cause the PAF and HF to go HIGH at the
conditions described in Table 2. If further read operations occur, without
write operations, the PAE will go LOW when there are n + 1 words in the
FIFO, where n is the Empty offset value. If there is no Empty offset specified,
the PAE will be LOW when the device is 32 away from completely empty for
IDT72805LB, 64 away from completely empty for IDT72815LB, and 128
away from completely empty for IDT72825LB/72845LB. Continuing read
operations will cause the FIFO to be empty. When the last word has been read
from the FIFO, OR will go HIGH inhibiting further read operations. REN is
ignored when the FIFO is empty.
PROGRAMMABLE FLAG LOADING
Full and Empty flag Offset values can be user programmable. The
IDT72805LB/72815LB/72825LB/72845LB has internal registers for these
offsets. Default settings are stated in the footnotes of Table 1 and Table 2. Offset
values are loaded into the FIFO using the data input lines D
0-D11. To load the
offset registers, the Load (LD) pin and WEN pin must be held LOW. Data present
on D0-D11 will be transferred in to the Empty Offset register on the first LOW-
to-HIGH transition of WCLK. By continuing to hold the LD and WEN pin low, data
present on D0-D11 will be transferred into the Full Offset register on the next
transition of the WCLK. The third transition again writes to the Empty Offset
register. Writing all offset registers does not have to occur at one time. One or
two offset registers can be written and then by bringing the LD pin HIGH, the
FIFO is returned to normal read/write operation. When the LD pin and WEN
are again set LOW, the next offset register in sequence is written.
The contents of the offset registers can be read on the data output lines Q0-
Q11 when the LD pin is set LOW and REN is set LOW. Data can then be read
