Latches and flip-flops & their difference

This post covers Latches and Flip-flops with a set of questions and answers. The topics discussed will help to understand the concepts related to latches and flip-flops, and the difference between latches and flip-flops.


Flip-flop – definition

Flip-flops are synchronous bi-stable devices, also known as bi-stable multivibrators. In this case, the term synchronous means that the output changes state only at a specified point (leading or trailing edge) on the triggering input called the clock (CLK).

Latch – definition

The latch is a type of temporary storage device that has two stable states (bistable) and is normally placed in a category separate from that of flip-flops. Latches are similar to flip-flops because they are bi-stable devices that can reside in either of two states using a feedback arrangement, in which the outputs are connected back to the opposite inputs.

Difference between latches and flip-flops

The main difference between latches and flip-flops is in the method used for changing their state. Flip-flops are edge-triggered or edge-sensitive where as gated latches are level-sensitive.

Edge-sensitive means the flip-flop changes its state either at the positive edge (rising or leading edge) or at the negative edge (falling or trailing edge) of the clock pulse and is sensitive to its inputs only at this transition of the clock.

Level-sensitive means a gated latch changes its state during the high level of the clock pulse and is sensitive to its inputs only at this time.

Edge-triggered flip-flop

The term edge-triggered means that the flip-flop changes its state either at the positive edge (rising or leading edge) or at the negative edge (falling or trailing edge) of the clock pulse and is sensitive to its inputs only at this transition of the clock.

When a flip-flop is not edge-triggered and the input data changes dynamically, the output will be changed and we cannot obtain the desired output from a digital circuit as per requirement. To get the desired output from a digital system, flip-flops must be edge-triggered.

Difference between positive and negative edge triggering in a flip-flop

The term edge-triggered means that the flip-flop changes its state either at the positive edge (rising or leading edge) or at the negative edge (falling or trailing edge) of the clock pulse and is sensitive to its inputs only at this transition of the clock.

If the clock signal changes from low to high state and the output changes due to the inputs, it is called positive edge triggering.

When the clock signal changes from high to low state and the output changes due to the inputs, it is called negative edge triggering.

Clear and Preset | two Asynchronous inputs of a flip-flop

Most clocked Flip-flops have one or more asynchronous inputs that operate independently of the synchronous inputs (the S, R, J, K, D, and T inputs are normally referred to as synchronous inputs because their effect on the FF output is synchronized with the clock input).

These asynchronous inputs can be used to set the FF to the 1 state (known as PRESET) or reset the FF to the 0 state (known as CLEAR) at any time, regardless of the conditions at the other inputs.

Stated in another way, the asynchronous inputs are override inputs, which can be used to override all the other inputs in order to place the FF in one state or the other.

The commercially used flip-flop has two asynchronous inputs designated as PRESET and CLEAR. These are active-LOW inputs, as indicated by the bubbles on the FF symbol.

Author of this post

This post is co-authored by Professor Saraswati Saha, who is an assistant professor at RCCIIT, a renowned degree engineering college in India. Professor Saha teaches subjects related to digital electronics & microprocessors.

Latches and flip-flops & their difference
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