54
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
IDT72T1845/55/65/75/85/95/105/115/125 2.5V TeraSync™ 18-BIT/9-BIT FIFO
2Kx18/4Kx9, 4Kx18/
8Kx9, 8Kx18/16Kx9, 16Kx18/32Kx9, 32Kx18/64Kx9, 64Kx18/128Kx9, 128Kx18/256Kx9, 256Kx18/512Kx9, 512Kx18/1Mx9
FEBRUARY 10, 2009
DEPTH EXPANSION CONFIGURATION (FWFT MODE ONLY)
The IDT72T1845 can easily be adapted to applications requiring depths
greater than 2,048 when the x18 Input or x18 Output bus Width is selected, 4,096
for the IDT72T1855, 8,192 for the IDT72T1865, 16,384 for the IDT72T1875,
32,768 for the IDT72T1885, 65,536 for the IDT72T1895, 131,072 for the
IDT72T18105, 262,144 for the IDT72T18115 and 524,288 for the
IDT72T18125. When both x9 Input and x9 Output bus Widths are selected,
depths greater than 4,096 can be adapted for the IDT72T1845, 8,192 for the
IDT72T1855, 16,384 for the IDT72T1865, 32,768 for the IDT72T1875,
65,536 for the IDT72T1885, 131,072 for the IDT72T1895, 262,144 for the
IDT72T8105, 524,288 for the IDT72T18115 and 1,048,576 for the
IDT72T18125. In FWFT mode, the FIFOs can be connected in series (the data
outputs of one FIFO connected to the data inputs of the next) with no external
logic necessary. The resulting configuration provides a total depth equivalent
to the sum of the depths associated with each single FIFO. Figure 37 shows
a depth expansion using two IDT72T1845/72T1855/72T1865/72T1875/
72T1885/72T1895/72T18105/72T18115/72T18125 devices.
Care should be taken to select FWFT mode during Master Reset for all FIFOs
in the depth expansion configuration. The first word written to an empty
configuration will pass from one FIFO to the next ("ripple down") until it finally
appears at the outputs of the last FIFO in the chain – no read operation is
necessary but the RCLK of each FIFO must be free-running. Each time the data
word appears at the outputs of one FIFO, that device's OR line goes LOW,
enabling a write to the next FIFO in line.
For an empty expansion configuration, the amount of time it takes for OR of
the last FIFO in the chain to go LOW (i.e. valid data to appear on the last FIFO's
outputs) after a word has been written to the first FIFO is the sum of the delays
for each individual FIFO:
(N – 1)*(4*transfer clock) + 3*TRCLK
where N is the number of FIFOs in the expansion and TRCLK is the RCLK period.
Note that extra cycles should be added for the possibility that the tSKEW1
specification is not met between WCLK and transfer clock, or RCLK and transfer
clock, for the OR flag.
The "ripple down" delay is only noticeable for the first word written to an empty
depth expansion configuration. There will be no delay evident for subsequent
words written to the configuration.
The first free location created by reading from a full depth expansion
configuration will "bubble up" from the last FIFO to the previous one until it finally
moves into the first FIFO of the chain. Each time a free location is created in one
FIFO of the chain, that FIFO's IR line goes LOW, enabling the preceding FIFO
to write a word to fill it.
For a full expansion configuration, the amount of time it takes for IR of the first
FIFO in the chain to go LOW after a word has been read from the last FIFO is
the sum of the delays for each individual FIFO:
(N – 1)*(3*transfer clock) + 2 TWCLK
where N is the number of FIFOs in the expansion and TWCLK is the WCLK
period. Note that extra cycles should be added for the possibility that the tSKEW1
specification is not met between RCLK and transfer clock, or WCLK and transfer
clock, for the IR flag.
The Transfer Clock line should be tied to either WCLK or RCLK, whichever
is faster. Both these actions result in data moving, as quickly as possible, to the
end of the chain and free locations to the beginning of the chain.
Figure 37. Block Diagram of Depth Expansion
For the x18 Input or x18 Output bus Width:
4,096 x 18, 8,192 x 18, 16,384 x 18, 32,768 x 18, 65,536 x 18, 131,072 x 18, 262,144 x 18, 524,288 x 18 and 1,048,576 x 18
For both x9 Input and x9 Output bus Widths:
8,192 x 9, 16,384 x 9, 32,768 x 9, 65,536 x 9, 131,072 x 9, 262,144 x 9, 524,288 x 9, 1,048,576 x 9 and 2,097,152 x 9
Dn
INPUT READY
WRITE ENABLE
WRITE CLOCK
WEN
WCLK
IR
DATA IN
RCLK
READ CLOCK
RCLK
REN
OE
OUTPUT ENABLE
OUTPUT READY
Qn
Dn
IR
GND
WEN
WCLK
OR
REN
OE
Qn
READ ENABLE
OR
DATA OUT
TRANSFER CLOCK
5909 drw41
n
n n
FWFT/SI FWFT/SI
FWFT/SI
IDT
72T1845
72T1855
72T1865
72T1875
72T1885
72T1895
72T18105
72T18115
72T18125
RCS
READ CHIP SELECT
RCS
IDT
72T1845
72T1855
72T1865
72T1875
72T1885
72T1895
72T18105
72T18115
72T18125
