SRAM and FIFO Memories in Actel's Low-Power Flash Devices
6- 14
v1.1
While the RESET signal is active, read and write operations are disabled. As with any asynchronous
RESET signal, care must be taken not to assert it too close to the edges of active read and write
clocks.
WD
This is the input data bus and is 18 bits wide. Not all 18 bits are valid in all configurations. When a
data width less than 18 is specified, unused higher-order signals must be grounded (
Table 6-7).
RD
This is the output data bus and is 18 bits wide. Not all 18 bits are valid in all configurations. Like the
WD bus, high-order bits become unusable if the data width is less than 18. The output data on
ESTOP, FSTOP
ESTOP is used to stop the FIFO read counter from further counting once the FIFO is empty (i.e., the
EMPTY flag goes HIGH). A HIGH on this signal inhibits the counting.
FSTOP is used to stop the FIFO write counter from further counting once the FIFO is full (i.e., the
FULL flag goes HIGH). A HIGH on this signal inhibits the counting.
FULL, EMPTY
When the FIFO is full and no more data can be written, the FULL flag asserts HIGH. The FULL flag is
synchronous to WCLK to inhibit writing immediately upon detection of a full condition and to
prevent overflows. Since the write address is compared to a resynchronized (and thus time-
delayed) version of the read address, the FULL flag will remain asserted until two WCLK active
edges after a read operation eliminates the full condition.
When the FIFO is empty and no more data can be read, the EMPTY flag asserts HIGH. The EMPTY
flag is synchronous to RCLK to inhibit reading immediately upon detection of an empty condition
and to prevent underflows. Since the read address is compared to a resynchronized (and thus time-
delayed) version of the write address, the EMPTY flag will remain asserted until two RCLK active
edges after a write operation removes the empty condition.
AFULL, AEMPTY
These are programmable flags and will be asserted on the threshold specified by AFVAL and
AEVAL, respectively.
When the number of words stored in the FIFO reaches the amount specified by AEVAL while
reading, the AEMPTY output will go HIGH. Likewise, when the number of words stored in the FIFO
reaches the amount specified by AFVAL while writing, the AFULL output will go HIGH.
AFVAL, AEVAL
The AEVAL and AFVAL pins are used to specify the almost-empty and almost-full threshold values.
They are 12-bit signals. For more information on these signals, refer to the
"FIFO Flag UsageTable 6-7
Input Data Signal Usage for Different Aspect Ratios
D×W
WD/RD Unused
4k×1
WD[17:1], RD[17:1]
2k×2
WD[17:2], RD[17:2]
1k×4
WD[17:4], RD[17:4]
512×9
WD[17:9], RD[17:9]
256×18
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