Flip-Flop: Triggering Methods

Let's start this article with some basics of flip-flops before moving onto discuss their triggering methods.

Flip-Flops in Digital Electronics

In digital circuits, a flip-flop is a sequential logic circuit which is used to store 1-bit of information. A flip-flop has one or more inputs and two outputs. It also has an input for provide a clock signal. Flip-flop is a fundamental building block of digital circuits. Hence, flip-flop is also referred to as an elementary memory element in digital systems. Flip-flops find their applications in computer memory, registers, counters, and many other digital systems and circuits.

A flip-flop has two stable states specified as “set” and “reset”. The state of the flip-flop updates according to the inputs applied when it is triggered by applying an appropriate clock pulse.

Depending on the input, there are several types of flip-flops exist such as S-R flip-flop, J-K flip-flop, D flip-flop, and T flip-flop.

After getting insights into the basics of flip-flop, let now us discuss the triggering methods of flip-flop.

Triggering Methods of Flip-Flop

As mentioned above that a flip-flop responds to the inputs and updates its output states when an appropriate clock pulse applied, i.e. when the flip-flop is properly triggered by a clock signal. There are several types of flip-flop triggering methods exist. Some most common flip-flop triggering methods are described below.

Asynchronous Triggering of Flip-Flop

When a flip-flop is triggered by the input signals itself and these inputs are not synchronized with the clock pulse, then it is called asynchronous triggering of the flip-flop.

In the case of asynchronous triggering, the flip-flop is triggered by the input signals and updates its output states when the inputs are applied regardless of the clock pulse. The asynchronous triggering method is also known as direct triggering method.

Synchronous Triggering of Flip-Flop

When operation of the flip-flop on the application of the input signals is synchronized with the clock pulse, then it is called synchronous triggering of flip-flop. In the synchronous triggering method, the flip-flop responds to the inputs and updates its output state only when a clock pulse exits, otherwise, the flip-flop maintains its previous state of output.

The synchronous triggering method of flip-flop is further classified into the following two types −

Level Triggering Method

In the level triggering method, the flip-flop responds to the inputs and updates its output state when the clock pulse is in either high level or low level. When the flip-flop is triggered to update its output state when the clock pulse is in the logic high level, then it is called positive level triggering. When the flip-flop is triggered to update the output state when the clock pulse is in the logic low level, then it is called negative level triggering.

Edge Triggering Method

When the flip-flop is triggered to update its output state on the rising or falling edge of the clock pulse, then it is called edge triggering method. If the flip-flop is triggered when the clock pulse goes from low to high (positive edge), then it is called positive edge-triggering method. If the flip-flop is triggered to update its output when the clock pulse goes from high to low (negative edge), then it is called negative edge- triggering method.

Pulse Triggering Method

The pulse triggering method is a type of triggering method of flip-flop in which the flip-flop is triggered by a spike (a short pulse) of the input signal. In the pulse triggering method, the clock pulse is set to high and the input signal is then allowed to trigger the flip-flop on positive or negative edge of the clock pulse. Therefore, the pulse triggering method can only be used to trigger the flip-flops having a clock enable input.

Master-Slave Triggering Method

Master-slave triggering method, is also known as two-stage triggering method. In the case of master slave triggering method, two flip-flops are connected in series, where first flipflop is called master flip-flop and the second flip-flop is called the slave flip-flop.

In the master-slave triggering method, the positive clock pulse is used to trigger the master flip-flop, while the inverted or negative clock pulse is used to trigger the slave flip-flop. Master-slave triggering methods is used to avoid race-around condition in the flip-flop.

Gated Triggering Method

When triggering of a flip-flop is done by the input signals which are gated by a control signal, it is called gated triggering method of flip-flop. The gated triggering method is mainly used to trigger flip-flop having a clock enable input or a clock disable input.

In this method, when the control signal is high, then the flip-flop can be triggered by the input signals on the rising or falling edge of the clock pulse. On the other hand, if the control signal is low, the flip-flop is disabled and hence the inputs cannot trigger it.

Conclusion

This is all about the triggering methods of flip-flops. In conclusion, the triggering method is the way of activating a flip-flop so that it can respond to the inputs and can update its output state as per the input data.