Device Architecture
AFVAL, AEVAL
The AEVAL and AFVAL pins are used to specify the almost-empty and almost-full threshold
values, respectively. They are 12-bit signals. For more information on these signals, refer to
"FIFO Flag Usage Considerations" section.
ESTOP and FSTOP Usage
The ESTOP pin is used to stop the read counter from counting any further once the FIFO is empty
(i.e., the EMPTY flag goes HIGH). Likewise, the FSTOP pin is used to stop the write counter from
counting any further once the FIFO is full (i.e., the FULL flag goes HIGH).
The FIFO counters in the Fusion device start the count at 0, reach the maximum depth for the
configuration (e.g., 511 for a 512×9 configuration), and then restart at 0. An example application
for the ESTOP, where the read counter keeps counting, would be writing to the FIFO once and
reading the same content over and over without doing another write.
FIFO Flag Usage Considerations
The AEVAL and AFVAL pins are used to specify the 12-bit AEMPTY and AFULL threshold values,
respectively. The FIFO contains separate 12-bit write address (WADDR) and read address (RADDR)
counters. WADDR is incremented every time a write operation is performed, and RADDR is
incremented every time a read operation is performed. Whenever the difference between WADDR
and RADDR is greater than or equal to AFVAL, the AFULL output is asserted. Likewise, whenever
the difference between WADDR and RADDR is less than or equal to AEVAL, the AEMPTY output is
asserted. To handle different read and write aspect ratios, AFVAL and AEVAL are expressed in terms
of total data bits instead of total data words. When users specify AFVAL and AEVAL in terms of
read or write words, the SmartGen tool translates them into bit addresses and configures these
signals automatically. SmartGen configures the AFULL flag to assert when the write address
exceeds the read address by at least a predefined value. In a 2k×8 FIFO, for example, a value of
1,500 for AFVAL means that the AFULL flag will be asserted after a write when the difference
between the write address and the read address reaches 1,500 (there have been at least 1500 more
writes than reads). It will stay asserted until the difference between the write and read addresses
drops below 1,500.
The AEMPTY flag is asserted when the difference between the write address and the read address
is less than a predefined value. In the example above, a value of 200 for AEVAL means that the
AEMPTY flag will be asserted when a read causes the difference between the write address and the
read address to drop to 200. It will stay asserted until that difference rises above 200. Note that the
FIFO can be configured with different read and write widths; in this case, the AFVAL setting is
based on the number of write data entries and the AEVAL setting is based on the number of read
data entries. For aspect ratios of 512×9 and 256×18, only 4,096 bits can be addressed by the 12 bits
of AFVAL and AEVAL. The number of words must be multiplied by 8 and 16, instead of 9 and 18.
The SmartGen tool automatically uses the proper values. To avoid halfwords being written or read,
which could happen if different read and write aspect ratios are specified, the FIFO will assert FULL
or EMPTY as soon as at least a minimum of one word cannot be written or read. For example, if a
two-bit word is written and a four-bit word is being read, the FIFO will remain in the empty state
when the first word is written. This occurs even if the FIFO is not completely empty, because in this
case, a complete word cannot be read. The same is applicable in the full state. If a four-bit word is
written and a two-bit word is read, the FIFO is full and one word is read. The FULL flag will remain
asserted because a complete word cannot be written at this point.
2-76
Preliminary v1.7
ACTEL [ Actel Corporation ]
