sequential circuits, flip flops and Latches

sequential circuits, flip flops and Latches

1. Sequential logic circuits are a fundamental type of digital circuit where the output depends not only on the current inputs but also on the history of past inputs and outputs. This characteristic allows them to store information, which distinguishes them from combinational logic circuits that rely solely on current inputs. Definition Sequential circuits incorporate memory elements, enabling them to maintain a state based on previous inputs. This means the output at any given time is determined by both the present inputs and the stored state from previous operations UNIT - III Sequential circuits

2. Combinational circuit Combinational circuit: It produces an output based on present input variables only The previous state of input does not have any effect on the present state of the circuit The combinational circuit does not use any memory. Sequential circuit  produces an output based on  current input and previous input variables . 

3. Sequential circuit Sequential circuit  produces an output based on  current input and previous input variables .  A  Sequential circuit  combinational logic circuit that consists of inputs variable (X), logic gates (Combinational/Computational circuit), output variable (Z) and Memory elements.

4. Combinational Circuits Sequential Circuits 1) The outputs of the combinational circuit depend only on the present inputs. The outputs of the sequential circuits depend on both present inputs and present state(previous output). 2) The feedback path is not present in the combinational circuit. The feedback path is present in the sequential circuits. 3) In combinational circuits, memory elements are not required. In the sequential circuit, memory elements play an important role and require. 4) The clock signal is not required for combinational circuits. The clock signal is required for sequential circuits. 5) The combinational circuit is simple to design. It is not simple to design a sequential circuit. Difference between the combinational circuits and sequential circuits

5. The word “Sequential” means that things happen in a “sequence”, one after another. The clock signal actually determines about the things will happen next. Simple sequential logic circuits can be constructed from standard  Bistable  circuits such as: Flip lops, Latches  and Counters 

6. Types of Sequential Circuits There are two types of sequential circuit : Asynchronous sequential circuit  Synchronous sequential circuit  Asynchronous sequential circuit  Synchronous sequential circuit   1. Do not use a clock signal 1. Uses a clock signal 2. Faster  than synchronous sequential 2. They wait for the next clock pulse circuits because there is no clock pulse to arrive, to perform the next and change their state immediately operation, so these circuits are when there is a change in the i /p signal bit  slower  compared to async .

7. Flip-flops and latches are two kinds of memory circuits used in electronics. The main difference between them is how they react to changes. A latch changes its output whenever its input changes. A flip-flop only changes its output at specific moments, like when its control signal goes from low to high. This makes flip-flops more stable in many situations. Latches are always alert to changes, while flip-flops only act at certain times. FLIP-FLOP & LATCHES

8. Latch Flip flop Memory Elements Async . Sequential circuits Sync. Sequential circuits

9. 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 . The output is obtained in a sequential circuit from the Flip-flops   combinational circuit or flip-flop or both. The state of flip-flops changes at the active state of clock pulses and remains unaffected when the clock pulse is not active. In particular, clocked flip flops serve as memory elements in synchronous sequential Circuits and unclocked flip-flops (i.e., latches) serve as memory elements in asynchronous sequential circuits. FLIP-FLOP

10. Applications of Flip-Flops Flip-flops are edge-triggered devices that change state based on clock signals. Their applications include: Counters: Flip-flops are widely used in digital counters, which count events and display binary numbers in sequential order. Frequency Dividers: They can divide the frequency of an input signal, which is crucial in various signal processing applications. Shift Registers: Flip-flops facilitate data transfer by temporarily storing bits and allowing for series-parallel conversions. Data Storage: They act as basic memory units in digital electronics, storing binary data until changed by input signals. Digital Systems: Flip-flops are integral to digital clocks, digital door locks, and digital thermostats, where they manage and store time-related data, security codes, and temperature settings respectively. Control Logic: They are used in control systems to ensure synchronization and proper functioning of digital circuits

11. Latch  is an electronic device, 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 complement to each other. LATCH

12. Latches are level-sensitive devices that maintain their state until the input changes. Their applications include: Data Storage: Latches serve as memory elements that store binary information, making them essential in various digital circuits. Power Gating: Used to manage power consumption in electronic devices by controlling the power supply to different components based on the state of the latch. Asynchronous Systems: D latches are particularly useful in systems where data needs to be stored without a clock signal, such as input/output ports. Two-Phase Synchronous Systems: Latches help avoid transition counts in data storage, ensuring smooth data flow during operations. Pulse Latches: These latches behave similarly to flip-flops and are used for generating quick responses in digital circuits Applications of Latches

13. Storage Elements A storage element in a digital circuit can maintain a binary state as long as power is delivered to the circuit It maintains state until directed by an input signal to switch states. Bistable circuit: A circuit which has two stable states . . . . . A B . . X Y . .

14. Latches Latches are basic storage elements that operate with signal levels Latches are useful for the design of the asynchronous sequential circuit. Latches are level-sensitive/ Level triggered devices. Triggering means making a circuit active. In  level triggering  the circuit will become active when the gating / input is on a particular  level .  Types of Latches: RS Latch D latch

15. RS Filpflop Reset - Set flipflop The SR (or) RS latch is a circuit with two cross-coupled NOR gates or two cross-coupled NAND gates Two inputs labeled S for set and R for reset. RS latch using NAND gates RS latch using NOR gates

16. RS Flipflop  using NOR gates: Possible RS combinations are R S Q p Qp ’ 1 1 1 1 A B Y 1 1 1 1 1 NOR truth table Qp is the Output (Present state) Qp ’ is the compl . of output (Present state)

17. Truth table for SR (or) RS latch using NOR gates S R Q p Qp ’ State Q p Qp ’ Memory / No change state 1 1 RESET 1 1 SET 1 1 - - Invalid/ Forbidden state

18. RS Flipflop  using NAND gates: A B Y 1 1 1 1 1 1 1 NAND truth table S R Q p State Q p Invalid/ Forbidden state 1 1 SET 1 RESET 1 1 - Memory / No change state

19. J K Flip-Flop JK flip – flop is named after Jack Kilby , the electrical engineer who invented IC. A JK flip – flop is called a Universal Programmable flip – flop because, using its inputs J (Preset), K (Clear) , function of any other flip – flop can be imitated. It is the modification of SR flip – flop with no Invalid state .

20. JK Flip-Flop In this the J input is similar to the SET input of SR flip – flop and the K input is similar to the RESET input And outputs are one is main output represented by Q and the other is complement of Q represented by Q’. The symbol of JK flip – flop is shown below.

21. Construction & Logic Circuit JK Flip-Flop Using NOR gates JK Flip-Flop Using NAND gates

22. NAND Truth table J=0 , K=1 Q=0 Q’=1 J=0 , K=0 Q= 0 Q’= 1 J=1 , K=0 Q= 1 Q’= 0 J=0 , K=0 Q= 1 Q’= 0 J=1 , K=1 Q= T Q’= T A B Y 1 1 1 1 1 1 1

23. Clk J K Q Q’ State ↑ » 1 Q Q’ NC ↑ » 1 1 1 RESET ↑ » 1 1 1 SET ↑ » 1 1 1 T T Toggle » 0 X X - - - JK Flip-Flop Truth Table Toggle means  switch between two states . Conditions for toggle in JK- flip flop : Both J and K should be 1. Clock should be present( Here I have considered +ve clock/rising edge)

24. Timing Diagram

25. Review A J-K flip-flop is nothing more than an S-R flip-flop with an added layer of feedback. This eliminates the invalid condition. When both J and K inputs are activated (J=1,K=1), and the clock input is applied then the circuit will  toggle  from a set state to a reset state or vice versa. The limitation of JK Latch is Race Around Condition (in JK Latch) Race around condition means  toggling is happening at the output many a time within a single clock period .