23
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
IDT72V275/72V285 3.3V CMOS SUPERSYNC FIFO
TM
32,768 x 18 and 65,536 x 18
NOTES:
1. Use an AND gate in IDT Standard mode, an OR gate in FWFT mode.
2. Do not connect any output control signals directly together.
3. FIFO #1 and FIFO #2 must be the same depth, but may be different word widths.
OPTIONAL CONFIGURATIONS
WIDTH EXPANSION CONFIGURATION
Word width may be increased simply by connecting together the control
signals of multiple devices. Status flags can be detected from any one
device. The exceptions are the EF and FF functions in IDT Standard mode
and the IR and OR functions in FWFT mode. Because of variations in skew
between RCLK and WCLK, it is possible for EF/FF deassertion and IR/
OR assertion to vary by one cycle between FIFOs. In IDT Standard mode,
such problems can be avoided by creating composite flags, that is, ANDing
Figure 19. Block Diagram of 32,768 x 36 and 65,536 x 36 Width Expansion
DEPTH EXPANSION CONFIGURATION (FWFT MODE ONLY)
The IDT72V275 can easily be adapted to applications requiring depths
greater than 32,768 and 65,536 for the IDT72V285 with an 18-bit bus width.
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 20 shows a depth expansion
using two IDT72V275/72V285 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 tSKEW3
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.
EF of every FIFO, and separately ANDing FF of every FIFO. In FWFT mode,
composite flags can be created by ORing OR of every FIFO, and separately
ORing IR of every FIFO.
Figure 19 demonstrates a width expansion using two IDT72V275/72V285
devices. D0 - D17 from each device form a 36-bit wide input bus and Q0-Q17
from each device form a 36-bit wide output bus. Any word width can be attained
by adding additional IDT72V275/72V285 devices.
WRITE CLOCK (WCLK)
m + n m n
MASTER RESET (MRS)
READ CLOCK (RCLK)
DATA OUT
n
m + n
WRITE ENABLE (WEN)
FULL FLAG/INPUT READY (FF/IR)
PROGRAMMABLE (PAF)
PROGRAMMABLE (PAE)
EMPTY FLAG/OUTPUT READY (EF/OR) #2
OUTPUT ENABLE (OE)
READ ENABLE (REN)
m
LOAD (LD)
IDT
72V275
72V285
EMPTY FLAG/OUTPUT READY (EF/OR) #1
PARTIAL RESET (PRS)
IDT
72V275
72V285
4512 drw 22
FULL FLAG/INPUT READY (FF/IR) #2
HALF-FULL FLAG (HF)
FIRST WORD FALL THROUGH/
SERIAL INPUT (FWFT/SI)
RETRANSMIT (RT)
#1
FIFO
#2
GATE
(1)
GATE
(1)
D0 - Dm
DATA IN
Dm
+1 - Dn
Q
0 - Qm
Qm
+1 - Qn
FIFO
#1