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From the NFL Rulebook:

Section 4 Stopping the Game Clock
The game clock operator shall stop the game clock (timeout) upon a signal by any official or upon the
operator’s own positive knowledge:
(a) at the end of a down in which there is a free kick or fair-catch kick;
(b) when the kicking team recovers a scrimmage kick that has been touched by the receiving team
beyond the line of scrimmage;
(c) when the ball is out of bounds;
(d) when the ball is dead on or behind a goal line;
(e) at the end of a down during which a foul occurs;
(f) when a forward pass is incomplete;
(g) when the player who originally takes the snap is tackled behind the line of scrimmage;
Exception:��After the two-minute warning of a half, the game clock shall not be stopped.
A.R. 4.6�� �� Quarterback A1 drops back to pass and is tackled behind the line.
Ruling:��Stop the game clock until the ball can be respotted at succeeding spot. (40-second play clock starts
when timeout signal is given).
(h) at the time of a foul for which the ball remains dead or is dead immediately;
(i) when the Referee signals the two-minute warning for a half;
(j) when a down is completed during which or after there is a change of possession; or
(k) when any official signals a timeout for any other reason.