Asynchronous input

Also called: async input, unsynchronized input

A signal whose changes have no fixed timing relationship to the receiving clock, like a push button, so it can violate setup or hold.

An asynchronous input is any signal that can change at any moment relative to the clock that reads it. Examples:

a push button or switch;
a sensor or another chip with no shared clock;
a signal from a part of the same chip running on a different clock (a clock domain crossing).

Within a synchronous design, timing analysis guarantees that every D input is stable during its flip-flop's setup and hold window. An asynchronous input breaks that guarantee: no analysis can say when it will change. Sooner or later it changes right inside the window, and the flip-flop that samples it may go metastable.

You can't prevent that, but you can contain it:

Pass the input through a two flip flop synchronizer before any logic uses it.
Synchronize it once, then fan out the synchronized copy.

Don't confuse this with an asynchronous control input such as an asynchronous clear. That's a flip-flop pin that acts without waiting for the clock. An asynchronous input is a signal not timed by your clock.

Worked example

Example

Why the raw button is dangerous

A button feeds two flip-flops directly: one starts a counter, the other lights an LED. The button is pressed just as a clock edge arrives.

1.
Both flip-flops sample the input inside their setup-and-hold windows.
2.
Each may resolve independently: one might capture 1 and the other 0.
3.
Now the counter has started but the LED is off: an inconsistent state that the design never intended.
4.
Fix: one two-flip-flop synchronizer, whose output feeds both flip-flops.

Common mistakes

Feeding an asynchronous input to several flip-flops before synchronizing it.
Assuming a slow human press can't cause trouble. The edge of the press can still land inside a window.

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