If you are exploring digital electronics, you have probably come across basic logic gates and circuits. But when it comes to storing and controlling binary data, flip-flops are essential. One of the simplest and most useful types is the Clocked SR Flip-Flop. In this blog, we will walk through what it is, how it works, and how it’s built, especially using NAND gates.
Let’s break it down in a beginner-friendly way.
What is a Clocked SR Flip-Flop?
A Clocked SR Flip-Flop (Set-Reset flip-flop) is a memory element that stores one bit of data. Unlike the basic SR latch, which continuously responds to input changes, the clocked version only updates its state when a clock signal is active. That is why it is often called a gated SR flip-flop—it has a control input (the clock) that “gates” the set and reset functions.
This design makes the clocked SR flip-flop circuit more reliable in synchronous systems, where timing matters. It’s often used in digital clocks, registers, and control units where data must change only at specific times.
Clocked SR Flip-Flop: Truth Table and Symbol
To understand how this flip-flop behaves, let’s look at its truth table. It has three inputs: S (Set), R (Reset), and Clock. The output is Q, and there's often an inverted output Q’ as well.
When the clocked RS flip flop is triggered (Clock = 1), the set/reset actions take effect. If both S and R are high, it creates an undefined or invalid state—which designers try to avoid.
The circuit symbol is similar to the SR latch, but includes an additional input for the clock signal, usually represented by a triangle or a line on the edge.
Clocked SR Flip-Flop Using NAND Gates
One of the most common ways to build a clocked SR flip-flop circuit is by using NAND gates. This approach is simple, cost-effective, and ideal for prototyping or understanding the internal working.
Here’s the general idea:
- The basic SR latch is made from two cross-coupled NAND gates.
- To make it clocked, additional NAND gates are used at the inputs, where S and R are each ANDed (via NAND logic) with the Clock.
- When the clock is low, inputs are blocked, and the output holds.
- When the clock is high, the flip-flop responds to the current S and R values. This setup ensures that the gated SR flip-flop only changes state during the clock pulse, adding control and predictability to sequential logic circuits.
Working Operation in the SR Flip-Flop Using NAND Gate
Let’s break down the operation step by step for a NAND-based clocked SR flip-flop:
1. Clock = 0 (Low):
No matter the values of S and R, the outputs of the input NAND gates are high. The flip-flop holds its last state—no change occurs.
2. Clock = 1 (High):
If S = 1 and R = 0 → Set state (Q = 1)
If S = 0 and R = 1 → Reset state (Q = 0)
If S = 0 and R = 0 → No change
If S = 1 and R = 1 → Invalid state (both outputs go low temporarily)
This simple logic makes the clocked RS flip flop predictable and easy to implement. However, the invalid condition is something you’ll need to handle, either through logic restrictions or by using a more advanced flip-flop like JK or D.
The clocked SR flip-flop is a great starting point for anyone learning about sequential circuits. It helps bridge the gap between simple logic gates and more complex memory elements. Whether you're designing your first digital circuit or just exploring how storage works in hardware, building a clocked SR flip-flop using NAND gates gives hands-on insight into the foundations of digital electronics.
As you go deeper, you’ll see how this flip-flop evolves into more robust designs. But for now, mastering the gated SR flip-flop puts you well on your way toward understanding how digital systems store and control information.
Top comments (0)