Flip-Flops and Latches – A Level Electronics WJEC Revision

Flip-flops and latches are fundamental building blocks of digital electronics, providing basic memory storage capability.
They each contain a set of inputs, a set of outputs, and an internal state, which can be changed or read by the application of input signals.
Essentially, they retain their output values until new input changes are made, making them effective for storing single bits of data.

A key difference between flip-flops and latches lies in their operation and control. Latches are level-sensitive, meaning they change state in response to a high or low level of control signal.
By contrast, flip-flops are edge-sensitive; they change state only during a transition of the control signal, either from 0 to 1 (positive edge) or from 1 to 0 (negative edge).

The two main types of latches are the SR latch (set-reset) and the D latch (data).
The SR latch features two inputs, labelled S for set and R for reset. It’s noted that in an SR latch, S and R cannot be active (logic 1) at the same time.
The D latch follows the same principle as the SR latch, except it has only one input, the D input.
SR and D latches are the simplest types of latch, but there are also more complex types including the JK and T latches.

There are various types of flip-flops such as JK flip-flop, D flip-flop, T flip-flop, and SR flip-flop.
The most versatile of these is the JK flip-flop. Unlike an SR flip-flop, a JK flip-flop does not have an invalid or illegal state. This means that S and R can be active at the same time.
A D flip-flop, similar to a D latch, has only a single data input. The value on the data input is transferred to the flip-flop output on the leading edge of the clock pulse.
A T flip-flop, or toggle flip-flop, changes output on each clock cycle, given that T is high. If T is low, the flip-flop holds the previous value.

On a practical level, latches and flip-flops are implemented using logic gates. They can also be constructed out of bistable multivibrator circuits.
One major use of flip-flops and latches is the construction of digital counters, which are necessary in devices like digital clocks and computer processors.
They are also used in sequential logic circuits, where the current state and sequence of previous states determine the output.
Sequential circuits built using flip-flops and latches are critical for designing and building any kind of computing device.

Understanding the operation of flip-flops and latches often involves the use of timing diagrams, which depict signal behaviours over time.
A state diagram provides a good graphical representation of a latch or a flip-flop along with its transition states.
Timing and state diagrams are essential tools for analysing and designing digital systems that include these components.