Asynchronous FIFO

When to use it

When a stream has to cross from one clock domain to another, a plain register won't do: a multi-bit value sampled mid-change can be garbage. A dual-clock FIFO is the standard answer, the writer pushes on its clock, the reader pops on theirs, and the FIFO handles the crossing.

How it works

Two pointers walk a shared memory, one per clock. The clever part is that each pointer is passed to the other domain as Gray code, so the synchronized value is only ever off by one step, never a corrupt in-between. wfull and rempty are registered comparisons of the local pointer against the synchronized remote one. (This is the classic Cummings design.)

Gotchas

• The pointers must be Gray-coded and passed through a two-flop synchronizer, that's the whole reason this is safe. Don't shortcut it.
• Depth is 2^AW. Size it for the burst you need to absorb (see the FIFO depth calculator).
• full and rempty are deliberately conservative; they may lag by a clock, but they never lie in the dangerous direction.