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AR# 31732

Virtex-5 FIFO - First Word Fall Through Mode Limitations


KEYWORDS: ALMOST_EMPTY flag, EMPTY flag, ALMOST_EMPTY_OFFSET, Almost Empty flag, Almost Empty offset

When a Virtex-5 FIFO is instantiated in First Word Fall Through mode, the ALMOST_EMPTY and EMPTY flags can malfunction. The circumstance that could lead to this anomaly would be writing to an empty FIFO the number of times equal to the ALMOST_EMPTY_OFFSET before 4 read cycles have completed.


Problems can be prevented by setting the ALMOST_EMPTY_OFFSET to a value that satisfies the following equation:

ALMOST_EMPTY_OFFSET > 4 * ( WRCLK frequency / RDCLK frequency).

For example, if the read frequency is 1/2 the write frequency, the ALMOST_EMPTY_OFFSET needs to be greater than 8. This equation also means that any time the read frequency is greater than two-thirds of the write frequency, any legal value (>5) of the ALMOST_EMPTY_OFFSET will work.

An alternative solution would be to change the design to use a FIFO with FIRST_WORD_FALL_THROUGH set to "FALSE".
AR# 31732
Date 07/29/2011
Status Archive
Type General Article