First-Word Fall-Through (FWFT) Read Operation

When looking at Xilinx Kintex-7 FPGAs memory resources, you’ll find that its FIFO generators support two modes of read options - standard read operating and FWFT read operation. What is FWFT?

FWFT is a acronym for First-Word Fall-Through. It is generally used to describe a FIFO operation in the memory. The FWFT feature provides the ability to look-ahead to the next word available from the FIFO without issuing a read operation. When data is available in the FIFO, the first word falls through the FIFO and appears automatically on the output bus (dout).

Once the first word appears on dout, empty is deasserted indicating one or more readable words in the FIFO, and VALID is asserted, indicating a valid word is present on dout. Below figure shows a FWFT read access.

Unlike the standard read mode, the FWFT empty flag is asserted after the last data is read from the FIFO. When empty is asserted, VALID is deasserted. In the standard read mode, when empty is asserted, VALID is asserted for 1 clock cycle. The FWFT feature also increases the effective read depth of the FIFO by two read words. The FWFT feature adds two clock cycle latency to the deassertion of empty, when the first data is written into a empty FIFO.

FWFT is useful in applications that require Low-latency access to data and to applications that require throttling based on the contents of the read data.