Question

D-Latch is a simple clocked memory element in which the output is equal to the stored state inside the element.

In D-Latch the state is changed whenever the appropriate inputs change and the clock is asserted. A D-Latch has two inputs and two outputs.

The inputs are the data value to be stored and a clock signal that indicates when the latch should read the value on the data input and store it. The outputs are simply the value of the internal state and its complement.

When the clock input is asserted, the latch is said to be open, and the value of the output becomes the value of the data input. When the clock input is de-asserted, the latch is said to be closed, and the value of the output is whatever value was stored the last time the latch was open.

What is the difference between DFF and D-Latch?

Can one chip be used for constructing the other? Explain.

Answer 1

D-latch:-

Latch is an electronic device that can be used to store one bit of information. The D latch is used to capture, or 'latch' the logic level which is present on the Data line when the clock input is high. If the data on the D line changes state while the clock pulse is high, then the output, Q, follows the input, D. When the CLK input falls to logic 0, the last state of the D input is trapped and held in the latch.

D Flip Flop:-

The working of D flip flop is similar to the D latch except that the output of D Flip Flop takes the state of the D input at the moment of a positive edge at the clock pin (or negative edge if the clock input is active low) and delays it by one clock cycle. That's why, it is commonly known as a delay flip flop. The D FlipFlop can be interpreted as a delay line or zero order hold. The advantage of the D flip-flop over the D-type "transparent latch" is that the signal on the D input pin is captured the moment the flip-flop is clocked, and subsequent changes on the D input will be ignored until the next clock event.

Difference between DFF and D-Latch:-

Can one chip be used for constructing the other?

1. D-latch is a level Triggering device while D Flip Flop is an Edge triggering device.
2. The disadvantage of the D FF is its circuit size, which is about twice as large as that of a D latch. That's why, delay and power consumption in Flip flop is more as compared to D latch.

3. Latches are used as temporary buffers whereas flip flops are used as registers.

4. Flip flop can be considered as a basic memory cell because it stores the value on the data line with the advantage of the output being synchronized to a clock.

Concepts	Latch	Flip-Flop
What is it?	A Latch is a circuit element that alters the output based on the current input, previous input, and previous output.	The flip-flops are built from latches and it includes an additional clock signal apart from the inputs used in the latches.
Types	There are four types of latches namely SR Latch, D Latch, JK latch, and T Latch.	There are four types of flip flops namely SR Flip-flop, D Flip-flop, JK Flip-flop, and T Flip-flop.
Built from	They are built from logic gates to form sequential circuits.	They are built from latches with an additional clock signal to form sequential circuits.
Output changes	When there is a change in the input during the continuous input checking process.	Of course, the outputs are computed based on the inputs during the continuous input checking process but they are computed only when the time signal is ‘+ve’.
Sensitive to?	It is sensitive to the input switch and we can transmit data as long as it is ‘On’.	It is sensitive to the clock signals and it never alters the output until there is a change in the input clock signal.
How do they work?	It works purely based on the binary inputs.	It works based on the binary inputs as well as on the clock signal.
Trigger Type	It is level triggered as the output gets changed based on the binary levels ‘0’ or ‘1’.	It is edge triggered as the output gets changed based on the ’+’ or ‘-’ clock signals.
Can be used as a register?	No. As registers need more sophisticated electronic circuits where time plays an important role. Here we miss the clock or time signals and hence it cannot be used as a register.	Yes. It includes clock signals in its inputs and therefore, cascaded flip-flops can be used as registers.
Synchronous?	No. It is asynchronous as it never works based on the time signals.	Yes. It is synchronous as it works based on the clock signals.

Modern day electronics require up to date information in the most case and hence the use of flip flops are inevitable. But we cannot build a flip-flop without the basic concept of latches. Therefore, the operation of flip-flops is dependent on the mechanism of latches and, in turn, the latter uses logic gates for its functioning. Though we have pointed out many differences between the two, the basic difference is the timely output. With that as the base, the other differences arise automatically.

Latches are built from logic gates to form sequential circuits. It never bothers about the clock or timely input. But in the case of flip flops, they are built from latches with an additional clock signal to form sequential circuits. The timely input is given much importance in flip-flops and the output gets changed from time to time.

So it is clear that D-latches are used for construction of D Flip Flop.