Introduction#
We often want to limit when a digital circuit can respond to its inputs.
Definition: Active Circuit
A digital circuit is active during a time period \(\Delta t\), if its outputs can be affected by its inputs during \(\Delta t\).
Definition: Inactive Circuit
A digital circuit is inactive during a time period \(\Delta t\), if its outputs cannot be affected by its inputs during \(\Delta t\).
The process of turning an inactive circuit into an active circuit is known as triggering. This is usually done by adding an additional input to the circuit, known as a trigger, which controls when the circuit is active.
Level Triggering#
Definition: Level Triggering
A digital circuit is level-triggered if it is active only during the time when its trigger is in a specific, stable state.
In other words, a level-triggered digital circuit is active when its trigger is either \(0\) or\(1\), but is inactive when its trigger is transitioning from \(0\) to \(1\) or vice versa.
We have two types of level triggering:
Definition: Positive Level Triggering
A level-triggered digital circuit has positive level triggering if it is active only when its trigger is set to a stable set of \(1\).
Definition: Negative Level Triggering
A level-triggered digital circuit has negative level triggering if it is active only when its trigger is set to a stable set of \(0\).
Edge Triggering#
Definition: Edge Triggering
A digital circuit is edge-triggered if it is active only during the time when its trigger is transitioning between two stable states.
In other words, an edge-triggered digital circuit is active when its trigger is in the process of transitioning from \(0\) to \(1\) or vice versa, but is inactive when its trigger is in a stable state of \(0\) or \(1\).
We have two types of edge triggering:
Definition: Positive Edge Triggering
An edge-triggered digital circuit has positive edge triggering if it is active only when its trigger is transitioning from \(0\) to \(1\).
Definition: Negative Edge Triggering
An edge-triggered digital circuit has negative edge triggering if it is active only when its trigger is transitioning from \(1\) to \(0\).
In practice, ensuring the correct functioning of an edge triggered circuit requires that its inputs are held stable for a particular time period before as well as after each trigger.
Definition: Setup Time
The setup time \(t_{\text{setup}}\) of an edge triggered circuit is the minimum duration for which its inputs need to be stable before each trigger to ensure proper function.
Definition: Hold Time
The hold time \(t_{\text{hold}}\) of an edge triggered circuit is the minimum duration for which its inputs need to be stable after each trigger to ensure proper function.