Introduction to flipflops basic of elctronics COA.pptx

Electronic flipflops resisters

1. CS118 1 TOPIC: Introduction Flip-Flops and Its Types (Lecture 2) By – Dr Priya (Asst. Professor, CSE)

2. Types of Digital Circuits Combinational Circuits Sequential Circuits 2

3. Combinational Circuits 3 Combinational circuits are specially designed using multiple interconnected logic gates such that the output will be generated by computing the logical combinations of the present input only. No clock pulse is present here. Moreover, no previously stored value or state is taken into consideration here. The output is independent of previous states.

4. Combinational Circuits 4 In this output depends only upon present input. It’s Speed is fast. Easy designed. There is no feedback between input and output. It is time independent. Elementary building blocks are Logic gates. Used for both arithmetic and boolean operations. Combinational circuits don’t have the capability to store any state. Examples of Combinational Circuits Adders and Subtractors Multiplexers and Demultiplexers Encoders and Decoders

5. Sequential Circuits 5 The outputs of the sequential circuits depend on both present inputs and present state(previous output). The feedback path is present in the sequential circuits. In the sequential circuit, memory elements play an important role and require. The clock signal is required for sequential circuits. It is not simple to design a sequential circuit. Sequential Circuits are Flip Flops Registers Counters Latches etc.

6. Sequential Circuits 6 Sequential circuits are digital circuits that store and use the previous state information to determine their next state. Unlike combinational circuits, which only depend on the current input values to produce outputs, sequential circuits depend on both the current inputs and the previous state stored in memory elements.

7. Sequential Circuits 7 There are two types of sequential circuits: Type 1: Asynchronous sequential circuit: These circuits do not use a clock signal but uses the pulses of the inputs. These circuits are faster than synchronous sequential circuits because there is clock pulse and change their state immediately when there is a change in the input signal. We use asynchronous sequential circuits when speed of operation is important and independent of internal clock pulse. But these circuits are more difficult to design and their output is uncertain. They are used in digital systems to implement state machines and are commonly used in applications that require low power consumption or where a clock signal is not available or practical to use.

8. Sequential Circuits 8 Type2: Synchronous sequential circuit: These circuits uses clock signal and level inputs (or pulsed) (with restrictions on pulse width and circuit propagation). The output pulse is the same duration as the clock pulse for the clocked sequential circuits. Since they wait for the next clock pulse to arrive to perform the next operation, so these circuits are bit slower compared to asynchronous. Level output changes state at the start of an input pulse and remains in that until the next input or clock pulse. We use synchronous sequential circuit in synchronous counters, flip flops, and in the design of MOORE-MEALY state management machines.

9. Latches 9 Latches are digital circuits(asynchronous) that store a single bit of information and hold its value until it is updated by new input signals. They are used in digital systems as temporary storage elements to store binary information. Latches can be implemented using various digital logic gates, such as AND, OR, NOT, NAND, and NOR gates. There are two types of latches: S-R (Set-Reset) Latches: S-R latches are the simplest form of latches and are implemented using two inputs: S (Set) and R (Reset). The S input sets the output to 1, while the R input resets the output to 0. When both S and R are at 1, the latch is said to be in an “undefined” state. D (Data) Latches: D latches are also known as transparent latches and are implemented using two inputs: D (Data) and a clock signal. The output of the latch follows the input at the D terminal as long as the clock signal is high. When the clock signal goes low, the output of the latch is stored and held until the next rising edge of the clock. Latches are widely used in digital systems for various applications, including data storage, control circuits, and flip-flop circuits. They are often used in combination with other digital circuits to implement sequential circuits, such as state machines and memory elements.

10. Latches Contd.. 10 Latches are basic storage elements that operate with signal levels (rather than signal transitions). Latches controlled by a clock transition are flip-flops. Latches are level-sensitive devices. Latches are useful for the design of the asynchronous sequential circuit. Latches are sequential circuit with two stable states. These are sensitive to the input voltage applied and does not depend on the clock pulse. Flip flops that do not use clock pulse are referred to as latch.

11. S-R (Set-Reset) Latch 11 SR (Set-Reset) Latch – They are also known as preset and clear states. The SR latch forms the basic building blocks of all other types of flip-flops. SR Latch is a circuit with: (i) 2 cross-coupled NOR gate or 2 cross-coupled NAND gate. (ii) 2 input S for SET and R for RESET. (iii) 2 output Q, Q’. Q Q’ STATE 1 Set 1 Reset

12. Gated S-R Latch 12 A Gated SR latch is a SR latch with enable input which works when enable is 1 and retain the previous state when enable is 0.

13. Gated D Latch 13 D latch is similar to SR latch with some modifications made. Here, the inputs are complements of each other. The letter in the D latch stands for “data” as this latch stores single bit temporarily. Enable D Q(n) Q(n+1) STATE 1 x RESET 1 1 x 1 SET x x Q(n) No Change

14. Latches Contd.. 14 Advantages of Latches: Easy to Implement: Latches are simple digital circuits that can be easily implemented using basic digital logic gates. Low Power Consumption: Latches consume less power compared to other sequential circuits such as flip-flops. High Speed: Latches can operate at high speeds, making them suitable for use in high-speed digital systems. Low Cost: Latches are inexpensive to manufacture and can be used in low-cost digital systems. Versatility: Latches can be used for various applications, such as data storage, control circuits, and flip-flop circuits.

15. Latches Contd.. 15 Disadvantages of Latches: No Clock: Latches do not have a clock signal to synchronize their operations, making their behavior unpredictable. Unstable State: Latches can sometimes enter into an unstable state when both inputs are at 1. This can result in unexpected behavior in the digital system. Complex Timing: The timing of latches can be complex and difficult to specify, making them less suitable for real-time control applications.

16. Latch Circuits: Not Suitable Latch circuits are not suitable in synchronous logic circuits. When the enable signal is active, the excitation inputs are gated directly to the output Q. Thus, any change in the excitation input immediately causes a change in the latch output. The problem is solved by using a special timing control signal called a clock to restrict the times at which the states of the memory elements may change. This leads us to the edge-triggered memory elements called flip-flops .

17. Memory Elements Memory element : a device which can remember value indefinitely, or change value on command from its inputs. Characteristic table: command Memory element stored value Q Q(t) : current state Q(t+1) or Q + : next state

18. Clock Signal and Triggering 18 Clock signal A clock signal is a periodic signal in which ON time and OFF time need not be the same. When ON time and OFF time of the clock signal are the same, a square wave is used to represent the clock signal. Below is a diagram which represents the clock signal: A clock signal is considered as the square wave. Sometimes, the signal stays at logic, either high 5V or low 0V, to an equal amount of time. It repeats with a certain time period, which will be equal to twice the 'ON time' or 'OFF time'.

19. Memory Elements Two types of triggering/activation: level -triggered edge-triggered Level -triggered latches ON = 1, OFF = 0 Edge-triggered flip-flops positive edge-triggered (ON = from 0 to 1; OFF = other time) negative edge-triggered (ON = from 1 to 0; OFF = other time)

20. Types of Triggering 20 These are two types of triggering in sequential circuits: 1. Level triggering Positive level triggering Negative level triggering 2. Edge triggering Positive edge triggering Negative edge triggering Fig: 1.4 Positive Level Triggering Fig: 1.5 Negative Level Triggering Fig: 1.6 Positive Edge Triggering Fig: 1.7 Negative Level Triggering

21. Basics of Flip Flop 21 The storage elements employed in clocked sequential circuits are called flip-flops. A flip-flops is a binary cell capable of storing one bit of information. It has two outputs, one for the normal value and one for the complement value of the bit stored in it. A circuit that has two stable states is treated as a  flip flop . These stable states are used to store binary data that can be changed by applying varying inputs. There are the following types of flip flops: 1. SR Flip Flop 2. J-K Flip-flop 3. D Flip Flop 4. T Flip Flop

22. SR Flip Flop 22 The S-R flip flop  is the most common flip flop used in the digital system. In SR flip flop, when the set input "S" is true, the output Y will be high, and Y' will be low. It is required that the wiring of the circuit is maintained when the outputs are established. We maintain the wiring until set or reset input goes high, or power is shutdown.

23. J-K Flip-flop 23 The  JK flip flop   is used to remove the drawback of the S-R flip flop, i.e., undefined states. The JK flip flop is formed by doing modification in the SR flip flop. The S-R flip flop is improved in order to construct the J-K flip flop. When S and R input is set to true, the SR flip flop gives an inaccurate result. But in the case of JK flip flop, it gives the correct output. In J-K flip flop, if both of its inputs are different, the value of J at the next clock edge is taken by the output Y. If both of its input is low, then no change occurs, and if high at the clock edge, then from one state to the other, the output will be toggled. The JK Flip Flop is a Set or Reset Flip flop in the digital system.

24. D Flip Flop 24 D flip flop is a widely used flip flop in digital systems. The D flip flop is mostly used in shift-registers, counters, and input synchronization. Truth Table:

25. T Flip Flop 25 Just like JK flip-flop, T flip flop is used. Unlike JK flip flop, in T flip flop, there is only single input with the clock input. The T flip flop is constructed by connecting both of the inputs of JK flip flop together as a single input. The T flip flop is also known as  Toggle flip-flop . These T flip-flops are able to find the complement of its state . Truth Table: