Difference Between Latch and Flip-Flop | Latch vs Flip-Flop MCQ

In this article, we discuss both latch and flip-flop are considered as the fundamental elements of the electronic system. The crucial difference between Latch and Flip-Flop is that a latch changes its output regularly according to the change in the applied input signal when it is enabled. As against in a flip flop, the output changes with input in conjunction with the clock signal. This means the clock signal acts as the control signal to display the output according to the changed input.

Latches are something in your design that always needs attention. There are tools that help designers pinpoint the existence of latches in their design as ‘must know’ information. Thus, latches and flip flops are considered as the derived sequential circuits that are used to store information. Here we will discuss all those factors which differentiate a latch from a flip-flop.

Latch:

The latch is an electronic circuit, which changes its output immediately based on the applied input. It is used to store either 1 or 0 at any specified time. It consists of two inputs namely “SET” and RESET and two outputs, which are a complement each other.

Flip-Flop:

Flip-Flop is a basic digital memory circuit, which stores one bit of information. Flip flops are the fundamental blocks of most sequential circuits. It is also known as a bistable multivibrator or a binary or one-bit memory. Flip-flops are used as memory elements in sequential circuits.

 

Difference Between Latch and Flip-Flop

Latch	Flip-Flop
The latch is transparent – because the input is directly connected to output when enable is high. It means Latch is sensitive to pulse duration (also called soft barrier)	Flip-flop is a pair of latches (master and slave flop). Flip-flop is sensitive to pulse transition. The signal only propagates through on the rising/falling edge (also called hard barrier)
Less Area (fewer gates)	More Area (more gates) because flip-flop contains two latches.
Less Power (fewer gates)	More Power (more gates)
Fast –(The longer combinational path can be compensated by shorter path delays in the subsequent logic stages. That’s why, for higher performance, circuits designer are turning to latched-based design.)	Slow –(The delay of a combinational logic path of a design using edge-triggered flip-flops always less than the clock period except for those specified as false paths and multiple-cycle paths. Hence the longest path of a design limits the circuit performance.)
Require more tool manipulation and more hand-calculations to verify that they meet timing	Easy to check design timing using Static Timing Analysis (STA) tools
Cycle-borrowing to gain more setup time on the next register stage, as long as each loop completes in one cycle To meet the timing in the design, Designers consider latches to adjust timing mismatch.	Data launches on one rising edge, so it must set up before the next rising edge. If it arrives late, the system fails. If it arrives early, time is wasted due to hard edges in Flops
For ASICs with large clock skew, latches have substantial benefits for reducing the clock period	Even for the high-speed pulsed flip-flops with zero setup time, as they are not transparent, the impact of the clock skew is not reduced
Level-sensitive latches reduce the impact of the inaccuracy of wire load models and process variation.	Flip-flops demands the highly accurate wire load model and process
In DFT, Latches needed as a lockup state at the clock domain crossings in the scan chain to avoid unpredictable behavior	In DFT, use flops that can be scanned (controllable and observable)
In FPGA, level-sensitive transparent latches should be avoided in FPGAs	In FPGA, edge-sensitive flip-flops are used exclusively. Timings analysis is more appropriate with flops for FPGA tools
Circuit analysis is complex. You may see last minutes timing mismatch surprises at the implantation stage.	Circuit analysis is easy
High-speed microprocessor designs typically use master-slave latches instead of flip-flops so that logic can be added between the rising and falling clock edges. Most of these companies have written their own specialized STA tools to verify latch-based designs.	The most commonly used flop in the design world is the D-type flip-flop. FSM implementation mostly involves D Flip-flops due to a minimum number of logic gates and lesser cost as compared to other types of flip-flops.
For non-timing-critical configuration registers, latches work great, due to fewer gates and less power consumption	For non-power aware design, Flip flops are preferred over Latches
The latch is an asynchronous block. Therefore you must ensure that the combinational functions, which generate input signals for the latch, are race-free. Otherwise, they may generate glitches, which may be latched, causing hazards in your system.	A flip-flop, on the other hand, is edge-triggered and only changes state when a control signal goes from high to low or low to high
Latch-based design is noisy because any noise in the enable signal disrupts the latch output easily.	Flip-flop based design is robust

Digital Circuits Questions and Answers – Latch vs Flip-Flop MCQ:

1. Latches constructed with NOR and NAND gates tend to remain in the latched condition due to which configuration feature?
a) Low input voltages
b) Synchronous operation
c) Gate impedance
d) Cross-coupling

A) d

2. One example of the use of an S-R flip-flop is as ___________
a) Transition pulse generator
b) Racer
c) Switch debouncer
d) Astable oscillato

A) c

3. The truth table for an S-R flip-flop has how many VALID entries?
a) 1
b) 2
c) 3
d) 4

A) c

4. When both inputs of a J-K flip-flop cycle, the output will ___________
a) Be invalid
b) Change
c) Not change
d) Toggle

A) c

5. Which of the following is correct for a gated D-type flip-flop?
a) The Q output is either SET or RESET as soon as the D input goes HIGH or LOW
b) The output complement follows the input when enabled
c) Only one of the inputs can be HIGH at a time
d) The output toggles if one of the inputs is held HIGH

A) a

6. A basic S-R flip-flop can be constructed by cross-coupling of which basic logic gates?
a) AND or OR gates
b) XOR or XNOR gates
c) NOR or NAND gates
d) AND or NOR gates

A) c

7. The logic circuits whose outputs at any instant of time depends only on the present input but also on the past outputs are called ________________
a) Combinational circuits
b) Sequential circuits
c) Latches
d) Flip-flops

A) b

8. Whose operations are more faster among the following?
a) Combinational circuits
b) Sequential circuits
c) Latches
d) Flip-flops

A) a

9. How many types of sequential circuits are?
a) 2
b) 3
c) 4
d) 5

A) a

10. The sequential circuit is also called ___________
a) Flip-flop
b) Latch
c) Strobe
d) Adder

A) b

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