SR flip – flop is one of the most vital components in digital logic and it is also the most basic sequential circuit that is possible. The S and R in SR flip – flop means ‘SET’ and ‘RESET’ respectively. Hence it is also called Set – Reset flip – flop. The symbolic representation of the SR Flip Flop is shown below.WorkingSR flip – flop works during the transition of clock pulse either from low – to – high or from high – to – low (depending on the design) i.e. It can be either positive edge triggered or negative edge triggered.For a positive edge triggered SR flip – flop, suppose, if S input is at high level (logic 1) and R input is at low level (logic 0) during a low – to – high transition on clock pulse, then the SR flip – flop is said to be in SET state and the output of the SR flip – flop is SET to1. For the same clock situation, if the R input is at high level (logic 1) and S input is at low level (logic 0), then the SR flip – flop is said to be in RESET state and the output of the SR flip – flop is RESET to 0.The SR flip – flops can be designed by using logic gates like NOR gates and NAND gates. Unclocked or simple SR flip – flops are same as SR Latches.

• Expensive Flip Flop Brands
• D Latch Vs Flip Flop

The two types of unclocked SR flip – flops are discussed below Unclocked S-R Flip-Flop Using NAND GateSR flip flop can be designed by cross coupling of two NAND gates. It is an active low input SR flip – flop. The circuit of SR flip – flop using NAND gates is shown in below figureWorking Case 1:When both the SET and RESET inputs are high, then the output remains in previous state i.e. It holds the previous data. Case 2:When SET input is HIGH and RESET input is LOW, then the flip flop will be in RESET state.

Because the low input of NAND gate with R input drives the other NAND gate with 1, as its output is 1. So both the inputs of the NAND gate with S input are 1. This will cause the output of the flip – flop to settle in RESET state. Case 3:When SET input is LOW and RESET input is HIGH, then the flip flop will be in SET state. Because the low input of NAND gate with S input drives the other NAND gate with 1, as its output is 1.

1. In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. A flip-flop is a bistable multivibrator. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs.
2. 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.

Latch vs Flip-Flop. Latch and flip flops are basic building blocks of sequential logic circuits, hence the memory. A sequential logic circuit is a type of digital circuit which responds not only to the present inputs, but to the present state (or past) of the circuit.

Expensive Flip Flop Brands

So both the inputs of the NAND gate with R input are 1. This will cause the output of the flip – flop to settle in SET state. Case 4:When both the SET and RESET inputs are low, then the flip flop will be in undefined state. Because the low inputs of S and R, violates the rule of flip – flop that the outputs should compliment to each other. So the flip flop is in undefined state (or forbidden state). The table below summarizes above explained working of SR Flip Flop designed with the help of a NAND gates(or forbidden state).Unclocked S R Flip-Flop Using NOR GateSR flip flop can also be designed by cross coupling of two NOR gates.

It is an active high input SR flip – flop. The circuit of SR flip – flop using NOR gates is shown in below figure.The operation is same as that of NOR SR Latch. WorkingCase 1:When both the SET and RESET inputs are low, then the output remains in previous state i.e. It holds the previous data Case 2:When SET input is low and RESET input is high, then the flip flop will be in RESET state. Because the high input of NOR gate with R input drives the other NOR gate with 0, as its output is 0. So both the inputs of the NOR gate with S input are 0. This will cause the output of the flip – flop to settle in RESET state.

Case 3:When SET input is high and RESET input is low, then the flip flop will be in SET state. Because the low input of NOR gate with S input drives the other NOR gate with 1, as its output is 1. So both the inputs of the NOR gate with R input are 1. This will cause the output of the flip flop to settle in SET state. Case 4:When both the SET and RESET inputs are high, then the flip flop will be undefined state.

Because the high inputs of S and R, violates the rule of flip flop that the outputs should complement to each other. So the flip flop is in undefined state (or forbidden state).The table below summarizes above explained working of SR Flip Flop designed with the help of a NOR gate.Even though simple SR flip – flops and simple SR latches are same, both the terms are used in their respective contexts.The problem with simple SR flip – flops is that they are level sensitive to the control signal (although not shown in figure) which makes them a transparent device. In order to avoid this, Gated or Clocked SR flip – flops are introduced (whenever the term SR flip – flop is used, it usually refers to clocked SR flip – flop). Clock signal makes the device edge sensitive (and hence no transparency). Clocked SR Flip – FlopsTwo types of clocked SR flip – flops are possible: based on NAND and based on NOR. The circuit of clocked SR flip – flop using NAND gates is shown belowThis circuit is formed by adding two NAND gates to NAND based SR flip – flop.

The inputs are active high as the extra NAND gate inverts the inputs. A clock pulse is given as input to both the extra NAND gates.Hence the transition of the clock pulse is a key factor in functioning if this device. Assuming it is a positive edge triggered device, the truth table for this flip – flop is shown below.The same can be achieved by using NOR gates. The circuit of clocked SR flip – flop using NOR gates is shown below.The figure suggests a structure of RS flip – flop (as R is associated to the output Q), the functionality of SET and RESET remain the same i.e. When S is high, Q is set to 1 and when R is high, Q is reset to 0. ApplicationsSR flip – flops are very simple but are not widely used in practical circuits because of their illegal state where both S and R are high (S = R = 1).

Image size in a mirror mastering physics. (Assume that his feet stay at the same position.)Now what is the length (i.e., the distance from head to toe) of Joe's image2.

But they are used in switching circuits as they provide simple switching function (between Set and Reset). One such application is a Switch de – bounce circuit. The SR flip – flops are used to eliminate mechanical bounce of switches in digital circuits. Mechanical BounceMechanical switches, when pressed or released, often take some time and vibrate several times before settling down. This non – ideal behavior of the switch is called switch bounce or mechanical bounce.

This mechanical bounce will tend to fluctuate between low and high voltages which can be interpreted by digital circuit. This can result in variation of pulse signals and these series of unwanted pulses will result in the digital system to work incorrectly.For example, in this bouncing period of the signal, the fluctuations in the output voltage are very high and therefore the register counts several inputs instead of single input. To eliminate this kind of behavior of digital circuits, we use SR flip – flops. How Does S R Flip-Flop Eliminates the Mechanical BounceBased on the present state output, if the set or reset buttons are depressed then the output will change in a manner that it counts more than one signal input i.e. The circuit may receive some unwanted pulse signals and thus because of the mechanical bouncing action of machines, there is no change in outputs at Q.When the button is pressed, the contact will affect the flip flop input and there will be change in the present state and no further affects on the circuit/machine for any other mechanical switch bounces. If there is any additional input from the switch, there will be no change and SR flip – flop will reset after some small period of time.So the same switch will come to use only after an SR flip – flop executes a state change i.e. Only after receiving the single clock pulse signal.The circuit of a switch de – bouncing circuit is shown below.The input to the switch is connected to ground (logic 0).

There are two pull up resistors connected to each of the input. They ensure that flip – flop inputs S and R are always 1 when the switch is between contacts.Another circuit can be constructed with NOR SR flip – flop.The input to the switch is connected to logic 1.

There are two pull down resistors connected to each of the input. They ensure that flip – flop inputs S and R are always 0 when the switch is between the contacts a and b.Commonly used ICs for eliminating the mechanical switch bounce are MAX6816 – single input, MAX6817 – dual input, MAX6818 – octal input switch de-bouncer ICs. These ICs contain the necessary configuration with the SR flip – flops.

A digital computer needs devices which can store information. A flip flop is a binary storage device. It can store binary bit either 0 or 1. It has two stable states HIGH and LOW i.e. It has the property to remain in one state indefinitely until it is directed by an input signal to switch over to the other state. It is also called bistable multivibrator.The basic formation of flip flop is to store data.

They can be used to keep a record or what value of variable (input, output or intermediate). Flip flop are also used to exercise control over the functionality of a digital circuit i.e. Change the operation of a circuit depending on the state of one or more flip flops. These devices are mainly used in situations which require one or more of these three.Operations, storage and sequencing. Latch Flip FlopThe R-S (Reset Set) flip flop is the simplest flip flop of all and easiest to understand. It is basically a device which has two outputs one output being the inverse or complement of the other, and two inputs. A pulse on one of the inputs to take on a particular logical state.

The outputs will then remain in this state until a similar pulse is applied to the other input. The two inputs are called the Set and Reset input (sometimes called the preset and clear inputs).Such flip flop can be made simply by cross coupling two inverting gates either NAND or NOR gate could be used Figure 1(a) shows on RS flip flop using NAND gate and Figure 1(b) shows the same circuit using NOR gate. Figure 1: Latch R-S Flip Flop Using NAND and NOR GatesTo describe the circuit of Figure 1(a), assume that initially both R and S are at the logic 1 state and that output is at the logic 0 state.Now, if Q = 0 and R = 1, then these are the states of inputs of gate B, therefore the outputs of gate B is at 1 (making it the inverse of Q i.e.

The output of gate B is connected to an input of gate A so if S = 1, both inputs of gate A are at the logic 1 state. This means that the output of gate A must be 0 (as was originally specified). In other words, the 0 state at Q is continuously disabling gate B so that any change in R has no effect. Also the 1 state at is continuously enabling gate A so that any change S will be transmitted through to Q. The above conditions constitute one of the stable states of the device referred to as the Reset state since Q = 0.Now suppose that the R-S flip flop in the Reset state, the S input goes to 0. The output of gate A i.e. Q will go to 1 and with Q = 1 and R = 1, the output of gates B ( ) will go to 0 with now 0 gate A is disabled keeping Q at 1.

Consequently, when S returns to the 1 state it has no effect on the flip flop whereas a change in R will cause a change in the output of gate B. The above conditions constitute the other stable state of the device, called the Set state since Q = 1. Note that the change of the state of S from 1 to 0 has caused the flip flop to change from the Reset state to the Set state.There is another input condition which has not yet been considered. That is when both the R and S inputs are taken to the logic state 0. When this happens both Q and will be forced to 1 and will remain so far as long as R and S are kept at 0.

However when both inputs return to 1 there is no way of knowing whether the flip flop will latch in the Reset state or the Set state. The condition is said to be indeterminate because of this indeterminate state great care must be taken when using R-S flip flop to ensure that both inputs are not instructed simultaneously. Table 1: The truth table for the NAND R-S flip flop Initial ConditionsInputs (Pulsed)Final OutputQSRQ100indeterminate11110000indeterminate01101or more simply shown in Table 2 Table 2: Simple NAND R-S Flip Flop Truth Table SRQ00indeterminate01Set (1)10Reset(0)11No ChangeWhen NOR gate are used the R and S inputs are transposed compared with the NAND version. Also the stable state when R and S are both 0. A change of state is effected by pulsing the appropriate input to the 1 state. The indeterminate state is now when both R and S are simultaneously at logic 1. Table 3 shows this operation.

Table 3: NOR Gate R-S Flip Flop Truth Table SRQ00No Change01Reset (0)10Set (1)11IndeterminateClocked RS Flip FlopThe RS latch flip flop required the direct input but no clock. It is very use full to add clock to control precisely the time at which the flip flop changes the state of its output.In the clocked R-S flip flop the appropriate levels applied to their inputs are blocked till the receipt of a pulse from an other source called clock. The flip flop changes state only when clock pulse is applied depending upon the inputs. The basic circuit is shown in Figure 2. This circuit is formed by adding two AND gates at inputs to the R-S flip flop. In addition to control inputs Set (S) and Reset (R), there is a clock input (C) also.

Figure 2: Clocked RS Flip Flop Table 4: The truth table for the Clocked R-S flip flop Initial ConditionsInputs (Pulsed)Final OutputCommentQSRQ (t + 1)No Change0000No Change0010Clear Q0101Set Q011???indeterminate1001No Change1010Clear Q1101Set Q111???indeterminateThe excitation table for R-S flip flop is very simply derived as given below Table 5: Excitation table for R-S Flip Flop SRQ00No Change01Reset (0)10Set (1)11IndeterminateD Flip FlopA D type (Data or delay flip flop) has a single data input in addition to the clock input as shown in Figure 3. Figure 3: D Flip FlopBasically, such type of flip flop is a modification of clocked RS flip flop gates from a basic Latch flip flop and NOR gates modify it in to a clock RS flip flop. The D input goes directly to S input and its complement through NOT gate, is applied to the R input.This kind of flip flop prevents the value of D from reaching the output until a clock pulse occurs. The action of circuit is straight forward as follows.When the clock is low, both AND gates are disabled, there fore D can change values with out affecting the value of Q. On the other hand, when the clock is high, both AND gates are enabled. In this case, Q is forced equal to D when the clock again goes low, Q retains or stores the last value of D.

The truth table for such a flip flop is as given below in table 6. Table 6: Truth table for D Flip Flop SRQ(t + 1)11The excitation table for D flip flop is very simply derived given as under. Table 7: Excitation table for D Flip Flop SQ0011JK Flip FlopOne of the most useful and versatile flip flop is the JK flip flop the unique features of a JK flip flop are:.

If the J and K input are both at 1 and the clock pulse is applied, then the output will change state, regardless of its previous condition. If both J and K inputs are at 0 and the clock pulse is applied there will be no change in the output. There is no indeterminate condition, in the operation of JK flip flop i.e.

It has no ambiguous state. The circuit diagram for a JK flip flop is shown in Figure 4.Figure 4: JK Flip FlopWhen J = 0 and K = 0These J and K inputs disable the NAND gates, therefore clock pulse have no effect on the flip flop. In other words, Q returns it last value.When J = 0 and K = 1,The upper NAND gate is disabled the lower NAND gate is enabled if Q is 1 therefore, flip flop will be reset (Q = 0, =1)if not already in that state.When J = 1 and K = 0The lower NAND gate is disabled and the upper NAND gate is enabled if is at 1, As a result we will be able to set the flip flop ( Q = 1, = 0) if not already setWhen J = 1 and K = 1If Q = 0 the lower NAND gate is disabled the upper NAND gate is enabled. This will set the flip flop and hence Q will be 1. On the other hand if Q = 1, the lower NAND gate is enabled and flip flop will be reset and hence Q will be 0.

In other words, when J and K are both high, the clock pulses cause the JK flip flop to toggle. Truth table for JK flip flop is shown in table 8. Table 8: The truth table for the JK flip flop Initial ConditionsInputs (Pulsed)Final OutputQSRQ (t + 1)10The excitation table for JK flip flop is very simply derived as given in table 9. Table 9: Excitation table for JK Flip Flop SRQ00No Change01010111ToggleT Flip FlopA method of avoiding the indeterminate state found in the working of RS flip flop is to provide only one input ( the T input ) such, flip flop acts as a toggle switch. Toggle means to change in the previous stage i.e.

D Latch Vs Flip Flop

Switch to opposite state. It can be constructed from clocked RS flip flop be incorporating feedback from output to input as shown in Figure 5. Figure 5: T Flip FlopSuch a flip flop is also called toggle flip flop. In such a flip flop a train of extremely narrow triggers drives the T input each time one of these triggers, the output of the flip flop changes stage. For instance Q equals 0 just before the trigger.

Then the upper AND gate is enable and the lower AND gate is disabled. When the trigger arrives, it results in a high S input.This sets the Q output to 1. When the next trigger appears at the point T, the lower AND gate is enabled and the trigger passes through to the R input this forces the flip flop to reset.Since each incoming trigger is alternately changed into the set and reset inputs the flip flop toggles. It takes two triggers to produce one cycle of the output waveform. This means the output has half the frequency of the input stated another way, a T flip flop divides the input frequency by two. Thus such a circuit is also called a divide by two circuit.A disadvantage of the toggle flip flop is that the state of the flip flop after a trigger pulse has been applied is only known if the previous state is known.

The truth table for a T flip flop is as given table 7. Table 7: Truth table for T Flip Flop Q nTQ n + 110The excitation table for T flip flop is very simply derived as shown in Table 8.

Table 8: Excitation table for T Flip Flop TQ0Q n1nGenerally T flip flop ICs are not available. It can be constructed using JK, RS or D flip flop. Figure 6 shows the relation of T flip flop using JK flip flop. Figure 7: JK & D Flip Flop Connected as T Flip flopA D-type flip flop may be modified by external connection as a T-type stage as shown in Figure 7.

Since the Q logic is used as D-input the opposite of the Q output is transferred into the stage each clock pulse. Thus the stage having Q - 0 transistors = 1, Providing a toggle action, if the stage had Q = 1 the clock pulse would result in Q = 0 being transferred, again providing the toggle operation. The D-type flip flop connected as in Figure 6 will thus operate as a T-type stage, complementing each clock pulse. Master Slave Flip FlopFigure 8 shows the schematic diagram of master sloave J-K flip flop Figure 8: Master Slave JK Flip FlopFigure 8: Master Slave JK Flip FlopA master slave flip flop contains two clocked flip flops. The first is called master and the second slave. When the clock is high the master is active. The output of the master is set or reset according to the state of the input.

As the slave is incative during this period its output remains in the previous state. When clock becomes low the output of the slave flip flop changes because it become active during low clock period. The final output of master slave flip flop is the output of the slave flip flop. So the output of master slave flip flop is available at the end of a clock pulse.