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UG473 (v1.11) November 12, 2014

FIFO Attributes

FIFO Almost Full/Empty Flag Offset Range

The FIFO data depth is listed in Table 2-7. The offset ranges for Almost Empty and Almost

Full are listed in Table 2-8.

The Almost Full and Almost Empty offsets are usually set to a small value of less than 10

to provide a warning that the FIFO is about to reach its limits. Because the full capacity of

any FIFO is normally not critical, most applications use the Almost Full flag not only as a

warning but also as a signal to stop writing.

Table 2-7: FIFO Data Depth

Data Width

Block RAM

FIFO Capacity

FIFO18 Mode FIFO36 Mode Standard FWFT

x4 8192 8192 8193

x4 x9 4096 4096 4097

x9 x18 2048 2048 2049

x18 x36 1024 1024 1025

x36 x72 512 512 513

Notes:

1. ALMOST_EMPTY_OFFSET and ALMOST_FULL_OFFSET for any design must be less than the total

FIFO depth.

Table 2-8: FIFO Almost Full/Empty Flag Offset Range

(1)

Data Width

ALMOST_EMPTY_OFFSET

ALMOST_FULL_OFFSET

Standard FWFT

FIFO18 Mode FIFO36 Mode Min Max Min Max Min Max

Dual-Clock (Asynchronous) – EN_SYN=FALSE

x4 5 8186 6 8187 4 8185

x4 x9 5 4090 6 4091 4 4089

x9 x18 5 2042 6 2043 4 2041

x18 x36 5 1018 6 1019 4 1017

x36 x72 5 506 6 507 4 505

Synchronous Mode – EN_SYN=TRUE

x4 1 8190 N/A N/A 1 8190

x4 x9 1 4094 N/A N/A 1 4094

x9 x18 1 2046 N/A N/A 1 2046

x18 x36 1 1022 N/A N/A 1 1022

x36 x72 1 510 N/A N/A 1 510

Notes:

1. The ranges in this table apply when RDCLK and WRCLK are at the same frequency.

1 2 ... 50 51 52 53 54 55 56 57 58 59 60 ... 85 86

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