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Computer Organization Notes

Flip flop is the most fundamental storage device which can store one bit of binary information and is basic unit of RAM. The basic digital memory circuit is known as FLIP-FLOP. Also it is a bi-stable unit which is used in sequential digital circuits. A flip flop circuit has two outputs, one for normal value and one for complement value of the stored bit. It can be obtained by using NAND or NOR gates. Binary information can enter a flip-flop in a variety of ways and give rise to the different type of flip-flops. We have many types of flip-flops but all of these have the following common features:
1. The output Q and Q’ are always complementary to each other i.e. if Q=1 then Q’=0 and if Q=0 then Q’=1.
2. The circuit has two stable states i.e. set = 1 or reset=0.
3. If the circuit is in set(1) state, it continues to remain in this state and similarly if it is in reset(0) state, it continues to be in this state until the external signal is changed to change this state.
Basic Flip Flop Circuit: A flip flop can be obtained by using NAND or NOR gates. We shall be systematically developing a flip-flop circuit starting from the fundamental circuit as shown below in the figure.

It consists of two inverters G1 and G2. The output of G1 is connected to the input of G2 and the output of G2 is connected to the input of G1.
Since this information is locked or latched in this circuit, therefore this circuit is also referred to as a latch. The latch circuit has two states either 0 or 1.
(a) Set State: Assume that the output of G1 =1 i.e. Q=1, which is also the input of G2, therefore the output of G2 will be 0 i.e. Q’ =0, which makes A1=0 and consequently Q=1 as shown in the figure below. The circuit confirms that if the circuit is in set state, it continues to remain in that state.

(b) Reset State: In the figure below it can be shown that if Q=1 then Q’=1.
In the latch in initial figure there is no way of entering the desired digital information to be stored in it. In fact, when the power is switched on, the circuit switches to one of the stable states (Q=1 or 0) and it is not possible to predict the state. If we replace the inverters G1 and G2 with 2-input NAND Gates, the other input terminals of NAND gates can be used to enter the desired digital information. The modification circuit is shown in the figure below:
Flip flop is a sequential circuit which generally samples its inputs and changes its outputs only at particular instants of time and not continuously. Flip flop is said to be edge sensitive or edge triggered rather than being level triggered like latches. Flip-flop has come to mostly denote non-transparent devices, while the simpler transparent ones are often referred to as latches. A flip-flop is normally controlled by one or two control signals and/or a gate or clock signal. Flip-flops are constructed with basic gates from logical families. There are a number of flip-flops of various types and the difference between these flip-flops is the number if inputs they possess and in the manner in which input effects the binary state. In these devices output is feedback to the input so these devices also act as memory elements. These flip-flops are used as memory element in various sequential circuits. There are mainly 4 types of Flip-Flops : SR FF, JK FF, D FF and T FF. Some variations of these FFs are also there for e.g. Master Slave FF.
Various types of Flip-flops are described as follows.

  • RS Flip-Flops
  • JK Flip-Flops
  • D Flip-Flops
  • T Flip-Flops
  • Master Slave Flip-Flops
  • Applications of Flip flops: Flip-Flops are used in many ways in Computer Science. Few of them are listed as :
    1. A single flip-flop can be used to store one bit, or binary digit, of data.
    2. Static RAM, which is the primary type of memory used in registers to store numbers in computers and in many caches, is built out of the flip-flops. There are different types of registers which are designed with the help of flip-flops.
    3. Anyone of the flip-flop types can be used to build any other type of flip-flop.
    4. The data contained in several flip-flops may represent the state of a sequencer, the value of a counter, an ASCII character in a computer’s memory or any other piece of information.
    5. One use of flip-flops is to build finite state machines from electronic logic. The flip-flops remember the machine’s previous state, and digital logic uses that state to calculate the next state.
    6. The T-flip flop is useful for constructing various types of counters.
    7. Flip-flops are used for frequency division, a chain of T flip-flops can also work to divide an input frequency by 2n where n is the number of flip-flops used between the input and the output.
    8. D flip-flop can be used to create delay-lines which are used in digital signal processing systems. This application arises readily due to the fact that the output at the synchronous D flip-flop is nothing but the input delayed by one-clock cycle.
    All the flip-flops can be summarised in following diagram

    You can obtain Printed Copies of this material by making a request at with a nominal print charges.


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