R-S mk2

An SR latch, constructed from a pair of cross-coupled NOR gates.


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. It is the basic storage element in sequential logic. Flip-flops and latches are a fundamental building block of digital electronics systems used in computers, communications, and many other types of systems.

Flip-flops and latches are used as data storage elements. A flip-flop stores a single bit (binary digit) of data; one of its two states represents a "one" and the other represents a "zero". Such data storage can be used for storage of state, and such a circuit is described as sequential logic. When used in a finite-state machine, the output and next state depend not only on its current input, but also on its current state (and hence, previous inputs). It can also be used for counting of pulses, and for synchronizing variably-timed input signals to some reference timing signal.

Flip-flops can be either simple (transparent or opaque) or clocked (synchronous or edge-triggered). Although the term flip-flop has historically referred generically to both simple and clocked circuits, in modern usage it is common to reserve the term flip-flop exclusively for discussing clocked circuits; the simple ones are commonly called latches.[1][2]

Using this terminology, a latch is level-sensitive, whereas a flip-flop is edge-sensitive. That is, when a latch is enabled it becomes transparent, while a flip flop's output only changes on a single type (positive going or negative going) of clock edge.


There are three common types of Flip FLops in use today:

  • D Flip FLop
D-Type Flip-flop

D flip-flop symbol

One of the most common kinds of flip-flops (or, just flops) is the D-type flop. Like all flops, it has the ability to remember one bit of digital information. What makes the D-flop special is that it is a clocked flip-flop. [3][4]

  • T Flip FLop
T-Type Flip-flop

A circuit symbol for a T-type flip-flop

If the T input is high, the T flip-flop changes state ("toggles") whenever the clock input is strobed. If the T input is low, the flip-flop holds the previous value.When T is held high, the toggle flip-flop divides the clock frequency by two; that is, if clock frequency is 4 MHz, the output frequency obtained from the flip-flop will be 2 MHz. This "divide by" feature has application in various types of digital counters. A T flip-flop can also be built using a JK flip-flop (J & K pins are connected together and act as T) or a D flip-flop (T input XOR Qprevious drives the D input).

  • JK Flip Flop
JK Flip-flop (Simple) Symbol

A circuit symbol for a positive-edge-triggered JK flip-flop

The JK flip-flop augments the behavior of the SR flip-flop (J=Set, K=Reset) by interpreting the J = K = 1 condition as a "flip" or toggle command. Specifically, the combination J = 1, K = 0 is a command to set the flip-flop; the combination J = 0, K = 1 is a command to reset the flip-flop; and the combination J = K = 1 is a command to toggle the flip-flop, i.e., change its output to the logical complement of its current value. Setting J = K = 0 does NOT result in a D flip-flop, but rather, will hold the current state. To synthesize a D flip-flop, simply set K equal to the complement of J. Similarly, to synthesize a T flip-flop, set K equal to J. The JK flip-flop is therefore a universal flip-flop, because it can be configured to work as an SR flip-flop, a D flip-flop, or a T flip-flop.[5]


  1. Pedroni, Volnei A. (2008). Digital electronics and design with VHDL. Morgan Kaufmann. p. 329. ISBN 978-0-12-374270-4.
  2. Latches and Flip Flops (EE 42/100 Lecture 24 from Berkeley) "...Sometimes the terms flip-flop and latch are used interchangeably..."
  3. D. Sculley, Ph.D. "D Flip FLops." "Alum of Tufts University: Department of Computer Science. 2008
  4. E. J. Mastascusa . "D Flip Flop." Bucknell University . 2008.
  5. Bartlet, Terry. "The Flip Flop." 10/15/15.




Digital Electronics The JK Flip-Flop


D Flip Flops