What is the Difference Between Edge and Level Triggering

The main difference between edge and level triggering is that in edge triggering, the output of the sequential circuit changes during the high voltage period or low voltage period while, in level triggering, the output of the sequential circuit changes during transits from the high voltage to low voltage or low voltage to high voltage.

In a sequential circuit, the output changes depending on the triggering. There are two types of triggering as edge and level triggering. There are two levels in a clock pulse or a signal. One is a high voltage (V_H), and the other is low voltage (V_L). Furthermore, these voltage levels help to determine the triggering type.

Key Areas Covered

1. What is Edge Triggering
      – Definition, Functionality
2. What is Level Triggering
     – Definition, Functionality
3. Difference Between Edge and Level Triggering
    – Comparison of key differences

Key Terms

Edge Triggering, Level Triggering, Sequential Circuit

What is Edge Triggering

In a sequential circuit, if the output changes when the signal transits from a high level to a low level or from a low level to a high level, we call it edge triggering. Here, the edge that changes the voltage from low level to the high level is called rising edge (positive edge). And, the edge that changes the voltage from high level to the low level is called falling edge (negative edge).

Thus, when an event is triggered at the rising edge or falling edge, we call it edge triggering. For example, assume lighting an LED according to the edge triggering. In this scenario, the LED lights on every time the signal transits from low voltage to high voltage. Taking some examples; S-R flip flop, J-K flip flop and D flip flop are some common examples for flip flops with edge triggering.

What is Level Triggering

In the sequential circuit, if the output changes during the high voltage period or low voltage period, it is called level triggering. In other words, the output changes during either high voltage or low voltage period- not during the edges like in edge triggering.

Thus, when an event triggers at the clock level, we call it level triggering. Assume lighting an LED according to level triggering. LED can turn on at any time during the high voltage. In other words, the event is triggered whenever a clock level is encountered. Considering examples; SR latch and D latch are some examples for latches with level triggering. 

Difference Between Edge and Level Triggering 

Definition

Edge triggering is a type of triggering that allows a circuit to become active at the positive edge or the negative edge of the clock signal. In contrast, level triggering is a type of triggering that allows a circuit to become active when the clock pulse is on a particular level.

Functionality

In edge triggering, an event occurs at the rising edge or falling edge whereas, in level triggering, an event occurs during the high voltage level or low voltage level. Thus, this is the main difference between edge and level triggering.

Applications

Furthermore, another difference between edge and level triggering is that the flip flops work according to edge triggering, whereas Latches work according to level triggering.

Conclusion

In brief, there are two types of triggering in sequential circuits. The triggering results can change the output of the circuit. The main difference between edge and level triggering is that, in edge triggering, the output of the sequential circuit changes during the high voltage period or low voltage period while in level triggering, the output of the sequential circuit changes during transits from the high voltage to low voltage or low voltage to high voltage.

References:

1.Edge-Triggered Flip-Flops, Available here.
2.TutorialsPoint. TutorialsPoint, Available here.

Image Courtesy:

1.”Clock signal” By Dolicom – Own work (CC BY-SA 3.0) via Commons Wikimedia