Achronix Speedster22i User Macro Guide - Manual (Page 151)

Memories BRAM80KFIFO
Speedster22i Macro Cell Library Achronix Semiconductor Proprietary PAGE 134
Status Flags
Empty Flag
The Empty (empty) flag is asserted after the FIFO is reset or when all of the data has been read
from the FIFO. The Empty flag is synchronous to the rdclk clock domain. Further attempts to
read the FIFO when the Empty flag is asserted will depend upon the en_rd_when_empty
parameter. If the en_rd_when_empty parameter is set to 1’b0, attempts to read the FIFO when
the Empty flag is asserted will be blocked, the Read Error (read_err) flag will be set in the
following rdclk clock cycle, and the Read Pointer will remain unchanged. If the
en_rd_when_empty parameter is set to 1’b1, attempts to read the FIFO when the Empty flag is
asserted will result in a valid read operation from the FIFO with the Read Pointer
incrementing by one word value. The user should disregard the Read Error flag when the
en_rd_when_empty parameter is set to 1’b1. The use may keep track of the Read Pointer
value by monitoring the rdcount output pins.
Almost Empty Flag
The Almost Empty (almost_empty) flag is asserted when there are aempty_offset or fewer
words remaining in the FIFO (See
Table 6‐34: Condition to Assert almost_empty Flag based
on afull_offset Parameter Assignment ). The almost_empty flag may be used to determine
the number of blind reads from the FIFO that can be perfromed without monitoring the empty
flag. For example, if the aempty_offset parameter is set to 17’h0004 and the almost_empty flag
is deasseted, the user is guaranteed that there are at least five words in the FIFO. The user may
read all five words without monitoring the empty flag and be guaranteed that these words
will be read from the FIFO and the read_err flag will not be asserted. The Almost Empty flag
is synchronous with the rdclk clock input.
Full Flag
The Full (full) flag is asserted when all of the available locations of the FIFO have been written.
It is synchronous to the wrclk clock domain. Further attempts to write the FIFO when the Full
flag is asserted will depend upon the en_wr_when_full parameter. If the en_wr_when_full
parameter is set to 1’b0, attempts to write the FIFO when the Full flag is asserted will be
blocked, the Write Error (write_err) flag will be set in the following wrclk clock cycle, and the
Write Pointer will remain unchanged. If the en_wr_when_full parameter is set to 1’b1,
attempts to write the FIFO when the Full flag is asserted will result in a valid write operation
to the FIFO with the Write Pointer incrementing by one word value. The user should
disregard the Write Error flag when the en_wr_when_full parameter is set to 1’b1. The use
may keep track of the Write Pointer value by monitoring the wrcount output pins.
Almost Full Flag
The Almost Full (almost_full) flag is asserted when there are afull_offset or fewer available
locations remaining in the FIFO. The almost_full flag may be used to determine the number
of blind writes to the FIFO that can be made without monitoring the full flag. For example, if
the afull_offset parameter is set to 17’h00004 and the almost_full flag is deasserted, the user is
guaranteed that there are at least five empty locations in the FIFO. The user may write all five
words without monitoring the full flag and be guaranteed that these words will be written to
the FIFO and the write_err flag will not be asserted. The Almost Full flag is synchronous with
the wrclk clock input.
Write Error Flag
The Write Error (write_err) flag is asserted in the following wrclk clock cycle when the user
tries to write the FIFO while the Full Flag is high and the Enable Write When Full
(en_wr_when_full) parameter is set to 1’b0. The Write Error flag is undefined when the
Enable Write When Full parameter is set to 1’b1.