UltraScale Architecture Memory Resources

Chapter 2: Built-in FIFO
X-Ref Target - Figure 2-6
Figure 2-6:
FWFT FIFO with Two Data Words in the FIFO
Flags
Empty Flag
If EMPTY is asserted, no data is available to be read from the FIFO, and any additional read
operations cause a read error (RDERR=1). The relationship of EMPTY to the output data on
the DOUT output port depends on whether the FIFO is configured as a standard FIFO or
FWFT FIFO.
When writing to an empty FIFO, the number of clock cycles required for the EMPTY output
to deassert depends on the FIFO configuration. For an independent-clocks FIFO, a write
operation is synchronized internally to the RDCLK domain before it can influence the status
of the Empty flag, resulting in a latency from the write operation to the deassertion of
EMPTY that is a combination of a few write clocks followed by a few read clocks.
The Empty flag is synchronous to the RDCLK domain and is intended as a handshaking
signal for logic reading from the FIFO.
PROGEMPTY Flag
If PROGEMPTY is asserted, the number of words in the FIFO is less than or equal to
PROG_EMPTY_THRESH.
Because of the inherent latencies in the FIFO, especially for the independent-clocks FIFO,
PROGEMPTY is always considered a pessimistic flag. This means that not all write
operations might have synchronized to the RDCLK domain, and therefore the words in the
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UG573 (v1.2) February 24, 2015