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RTL Design Patterns/FIFOs

FIFO with Almost-Empty Flag

medium
fifoalmost-emptyflow-control

Why you need this in addition to empty

A consumer that only reacts to empty finds out it has run dry at the worst possible moment — the exact cycle it needed data. Many real consumers (a display pipeline reading from a line buffer, a streaming DSP core, a network transmit FIFO feeding a MAC) need advance warning that the well is running low so they can trigger a refill, raise an interrupt, or throttle their own output before they actually starve. almost_empty provides that low-water-mark warning.

This is the mirror image of the almost-full FIFO problem in this track — same underlying occupancy counter, opposite threshold comparison — but it’s worth building both explicitly since real designs frequently need one without the other (e.g. a write-only ingress FIFO cares only about almost_full; a read-only prefetch buffer cares only about almost_empty).

Interface

Signal Direction Description
ALMOST_EMPTY_THRESHOLD parameter Occupancy at or below which almost_empty should assert.
wr_en / wr_data input Write port, same semantics as the basic Synchronous FIFO.
rd_en / rd_data input/output Read port, same fall-through semantics as the basic Synchronous FIFO.
full output High when the FIFO holds exactly DEPTH entries.
empty output High when the FIFO holds zero entries.
almost_empty output High when occupancy <= ALMOST_EMPTY_THRESHOLD.

Correctness constraints

  • almost_empty must assert whenever occupancy is at or below ALMOST_EMPTY_THRESHOLD, including the fully-empty case (empty implies almost_empty).
  • A completely full FIFO (occupancy == DEPTH) must never report almost_empty, unless ALMOST_EMPTY_THRESHOLD >= DEPTH (a degenerate configuration that should be avoided).
  • 0 <= ALMOST_EMPTY_THRESHOLD <= DEPTH must hold.
  • The flag must be derived from registered occupancy state only, so it settles cleanly every cycle without depending combinationally on the current cycle’s rd_en/wr_en.
  • All of the base Synchronous FIFO’s correctness constraints (no full+empty overlap, dropped writes while full, dropped reads while empty, strict ordering) still apply.
← FIFO with Almost-Full Flag