Terminology » RF Components » Latch vs. Flip-Flop: Key Differences Explained

Latch vs. Flip-Flop: Key Differences Explained

latch flip-flop digital circuit logic gate clock signal

This article breaks down the differences between latches and flip-flops, two fundamental building blocks in digital circuits. We’ll explore their functionalities and highlight when to use each.

SR Latch with Enable

SR latch with enable

Fig-1: SR Latch with Enable

The diagram above shows an SR latch implemented using NAND gates and an enable input.

  • When E = 1 (Enable is active), the circuit acts like a regular SR latch using NAND gates, but with active-high S and R inputs. This means that a high signal on S will set the latch, and a high signal on R will reset it.
  • When E = 0 (Enable is inactive), the latch holds its previous state, ignoring any changes to the S and R inputs.

SR latch with enable timing diagram

The timing diagram illustrates how the latch responds to changes in the S, R, and E inputs.

  • Between t0 and t1, E = 0, so changes to the S and R inputs have no effect on the output.
  • Between t1 and t2, E = 1, so changes to S and R do affect the output.

Keep in mind that the enable input can be active-high or active-low and might be labeled as ENABLE, CLK, or CONTROL, depending on the specific circuit.

SR Flip-Flop

SR flip flop

Fig-2: SR Flip-Flop

In the SR latch, the output changes as soon as the enable signal is active. This is known as level-triggered operation.

In contrast, the SR flip-flop shown above responds only to edges of a clock signal. Essentially, we’ve replaced the enable input of the latch with a clock input. The circuit only acts as a flip-flop when the clock transitions from low to high or high to low (depending on the design). The circuit is only operational during these edge transitions.

Master-Slave SR Flip-Flop

Master Slave SR flip flop

Fig-3: Master-Slave SR Flip-Flop

Both latches and flip-flops are useful for storing data bits and for setting and resetting the bit. However, flip-flops only change their state on the active edge of a clock signal.

Like their latch counterparts (SR latches), SR flip-flops can enter an undefined state if both inputs are asserted simultaneously.

Key Differences: Latch vs. Flip-Flop

The following table summarizes the key differences between latches and flip-flops:

FeatureLatchFlip-Flop
Basic FunctionBistable device storing either 0 or 1.Bistable device storing either 0 or 1.
State ChangeChanges state as soon as the input is given; doesn’t depend on a control input or clock.Changes state only during the active edge (or level, depending on the type) of a clock signal.
Clock DependencyNo clock is present.Clock signal controls the operation.
ControlOperation is uncontrolled unless an enable signal is used. With the enable signal, the latch is often referred to as a “gated latch”.Operation is controlled by the clock signal.
TriggeringN/ATriggered by an edge (low-to-high or high-to-low transition) or, in some cases, by a pulse. Positive edge triggered or negative edge triggered.
ExamplesD latch, T latch, SR latch, JK latchD flip-flop, T flip-flop, SR flip-flop, JK flip-flop

In essence, latches are level-sensitive, while flip-flops are edge-triggered. This fundamental difference dictates their use in various digital circuits. Flip-flops provide more precise timing control and avoid race conditions, making them suitable for synchronous systems.