American Football NCAA Rule 3-2-4

a. Game Clock.
Playing time shall be kept with a game clock that may be either a stopwatch operated by the line judge, back judge, field judge or side judge, or a game clock operated by an assistant under the direction of the appropriate judge. The type of game clock and the location of the game clock operator shall be determined by the game management. The game clock operator may be located on the sideline or in the traditional press box area.
If the clock operator is in the traditional press box, it is recommended the location is between the goal lines. This location shall be shielded from public interference and shall afford a direct and unobstructed view of the playing field. The game clock shall not display tenths of seconds.
b. 40-Second Clock.
1. When an official signals that the ball is dead, the play clock shall begin a 40-second count.
2. If the 40-second clock does not start or the count is interrupted for reasons beyond the control of the officials or the play-clock operator (e.g., clock malfunction), the referee shall stop the game clock and signal (both palms open in an over-the-head pumping motion) that the play clock should be reset at 40 seconds and started immediately.
3. In the event that the 40-second play clock is running and reads 25 before the ball is ready to be snapped, the referee shall declare a timeout and signal that the play clock be set at 25 seconds. When play is to be resumed, the referee will give the ready-for-play signal [S1] and the play clock shall begin the 25-second count. The game clock will
start on the snap unless it had been running when the referee declared a timeout; in that case, it will start on the referee’s signal (Rule 3-3-2- f). (A. R. 3-2-4-I and II)
c. 25-Second Clock.
If the officials signal the game clock to be stopped for any of the following reasons, the referee shall signal (one open palm in an overthe-head pumping motion) that the clock should be set at 25 seconds:
1. Penalty administration.
2. Charged team timeout.
3. Media timeout.
4. Injury timeout for a player of the offensive team only. The play clock is set to 40 seconds for an injury to a player of the defensive team.
5. Measurement.
6. Team B is awarded a first down other than a free kick.
7. After a kick down other than a free kick.
8. Score other than a touchdown (not the try).
9. Start of each period.
10. Start of a team’s possession series in an extra period.
11. Instant replay review.
12. Other administrative stoppage.
13. An offensive team player’s helmet comes completely off through play. The play clock is set to 40 seconds if the helmet comes completely off a player of the defensive team. When play is to be resumed, the referee will give the ready-for-play signal [S1] and the play clock will begin its count.
d. Device Malfunction.
If a visual 40/25-second timing device becomes inoperative, both coaches shall be notified by the referee immediately and both clocks shall be turned off.