The timing diagram
 
Timing diagrams are used to show changes and their relationship to clock times. It provides a visual representation of objects changing state and interacting over time.  
Timing diagrams can be used for defining hardware-driven or embedded software components.
The X-axis of the timing diagram normally has the time units with the Y-axis showing the objects and their states. States are normally changed by some sort of event that causes the state change.
Timing diagrams can be drawn from a value or time based point of view.
Example: Time-based timing diagram

graphic

Example: Value-based timing diagram

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