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Encyclopedia > Flip flop (electronics)

In electronics and digital circuits, the flip-flop or bistable multivibrator is a pulsed digital circuit capable of serving as a one-bit memory. A flip-flop typically includes zero, one, or two input signals; a clock signal; and an output signal, though many commercial flip-flops additionally provide the complement of the output signal. Some flip-flops include a clear input signal, which resets the current output. Because flip-flops are implemented as integrated circuit chips, they also require power and ground connections. The field of electronics comprises the study and use of systems that operate by controlling the flow of electrons (or other charge carriers) in devices such as thermionic valves and semiconductors. ... Digital circuits are electric circuits based on a number of discrete voltage levels. ... Something that is bistable can be resting in two states. ... A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. ... Digital circuits are electric circuits based on a number of discrete voltage levels. ... A bit (binary digit) refers to a digit in the binary numeral system (base 2). ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The term input has a variety of uses in different fields. ... In synchronous digital electronics, such as most computers, a clock signal is a signal used to coordinate the actions of two or more circuits. ... // Information processing In information processing, output is the process of transmitting information by an object (verb usage). ... Look up Complement in Wiktionary, the free dictionary. ... Integrated circuit showing memory blocks, logic and input/output pads around the periphery A monolithic integrated circuit (also known as IC, microchip, silicon chip, computer chip or chip) is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) which has been manufactured in the surface... CHiPs was a US television series through MGM Studios running on NBC from September 15, 1977 to July 17, 1983. ... Electric power is the amount of work done by an electric current in a unit time. ... Ground symbols The term ground or earth usually means a common return path in electrical circuits. ...


Pulsing, or strobing, the clock causes the flip-flop to either change or retain its output signal, based upon the values of the input signals and the characteristic equation of the flip-flop. Strobing here means changing the clock; some flip-flops change output on the rising edge of the clock, and other change on the falling edge. A rising edge is, in electronics, the transition of a digital signal from low to high. ...


Flip-flops can be split into two main categories: level-triggered and edge-triggered. They can further be divided into four types that have found common applicability in clocked sequential systems: these are called the T ("toggle") flip-flop, the SR ("set-reset") flip-flop, the JK flip-flop, and the D ("data") flip-flop. The behavior of the flip-flop is described by what is termed the characteristic equation, which derives the "next" (i.e., after the next clock pulse) output, Qnext, in terms of the input signal(s) and/or the current output, Q. It has been suggested that this article or section be merged with Sequential logic. ...


The first electronic flip-flop was invented in 1919 by William Eccles and F. W. Jordan (Radio Review Dez 1919 pages 143 following). It was initially called the Eccles-Jordan trigger circuit. The name flip-flop was later derived from the sound produced on a speaker connected with one of the backcoupled amplifiers output during the trigger process within the circuit. 1919 (MCMXIX) was a common year starting on Wednesday (see link for calendar). ... Physicist William Eccles William Henry Eccles (August 23, 1875 - April 29, 1966) was a British physicist and a pioneer in the development of radio communication. ... 1919 F. W. Jordan invented together with William Henry Eccles the so called flip-flop-circuit. ...

Contents

Level-triggered flip-flops

Level-triggered flip-flops respond whenever a signal level changes.


Set-reset flip-flops (SR flip-flops)

The SR (set-reset) flip-flop has two inputs: S (set) and R (reset). If R is active, the output goes to zero. If S is active, the output goes to one. If neither is activated, the previous state is maintained. Both inputs should not be activated simultaneously; however, if they are, the typical response is for both the inverted and non inverted outputs to have the same level.


The behavior of an SR flip-flop can be written in the form of a truth table: Truth tables are a type of mathematical table used in logic to determine whether an expression is true or whether an argument is valid. ...

S R Q Qnext
0 0 0 0
0 0 1 1
0 1 X 0
1 0 X 1
1 1 X implementation dependent
An SR flip-flop timing diagram.
An SR flip-flop timing diagram.

We can implement a the SR flip-flop with a pair of either NAND or NOR gates. The NOR version is conceptually easier as it has active high inputs. However, the NAND version is more widely known and used, as NAND gates were cheaper in transistor-transistor logic. Wikipedia does not have an article with this exact name. ... Wikipedia does not have an article with this exact name. ... // Function The NAND gate is a digital logic gate that behaves according to the truth table to the right. ... // Function The NOR gate is a digital logic gate that behaves according to the truth table to the right. ... A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. ... A Motorola 68000-based computer with various TTL chips. ...


We can also easily add an enable input. If this is implemented in the same gates as the flip-flop, then it serves to further invert the inputs. That is, a NAND based device will now have active high inputs. This input may be regarded as a clock but the flip-flop is still unsuitable for sequential design. When the clock goes high, the signal will propagate through all flip-flops, not just from one to the next.

An SR flip-flop circuit diagram constructed from NAND gates.
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An SR flip-flop circuit diagram constructed from NAND gates.
The symbol for an unclocked SR latch.
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The symbol for an unclocked SR latch.
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A clocked SR flip-flop circuit diagram constructed from NAND gates.
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The symbol for a clocked SR flip-flop.

Image File history File links SR_Flip-flop_Diagram. ... Image File history File links SR_Flip-flop_Diagram. ... Image File history File links SR_Flip-flop. ... Image File history File links SR_Flip-flop. ... Image File history File links SR_(Clocked)_Flip-flop_Diagram. ... Image File history File links SR_(Clocked)_Flip-flop_Diagram. ... Image File history File links SR_(Clocked)_Flip-flop. ... Image File history File links SR_(Clocked)_Flip-flop. ...

D-type transparent latch

If we add two NAND gates to an unclocked SR flip-flop as in the following diagram, we get a "D-type transparent latch". When enable is active, the output follows the input (the arrangement is "transparent"); when enable is low, the output is latched at what it was when enable was last high.

 A D-type transparent latch DInput CEnable/clock QOutput Q'Inverse of Q
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A D-type transparent latch
D Input
C Enable/clock
Q Output
Q' Inverse of Q



Edge-triggered flip-flops

Edge-triggered flip-flops only change state on a particular edge (rising, falling, or very occasionally both directions) of a designated clock signal.


Toggle flip-flops (T flip-flops)

A circuit symbol for a T-type flip-flop, where > is the clock input, T is the toggle input and Q is the stored data output.
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A circuit symbol for a T-type flip-flop, where > is the clock input, T is the toggle input and Q is the stored data output.

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. This behavior is described by the characteristic equation: Image File history File links T-Type_Flip-flop. ... Image File history File links T-Type_Flip-flop. ... An equation is a mathematical statement, in symbols, that two things are the same. ...


(or, without benefit of the exclusive OR operator, the equivalent: Qnext = TQ' + T'Q ) Exclusive disjunction, also known as exclusive or and symbolized by XOR or EOR, is a logical operation on two operands that results in a logical value of true if and only if one of the operands, but not both, has a value of true. ...


and can be described in a truth table: Truth tables are a type of mathematical table used in logic to determine whether an expression is true or whether an argument is valid. ...

T Q Qnext
0 0 0
0 1 1
1 0 1
1 1 0


A toggle flip-flop composed of a single RS flip-flop becomes an oscillator, when it is clocked. To achieve toggling, the clock pulse must have exactly the length of half a cycle. While such a pulse generator can be built, a toggle flip-flop composed of two RS flip-flops is the easy solution.


JK flip-flop

JK flip-flop timing diagram
JK flip-flop timing diagram

The JK flip-flop augments the behavior of the SR flip-flop by interpreting the S = R = 1 condition as a "flip" 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 results in a D-type flip-flop. 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. Wikipedia does not have an article with this exact name. ... Wikipedia does not have an article with this exact name. ...

A circuit symbol for a JK flip-flop, where > is the clock input, J and K are data inputs, Q is the stored data output, and Q' is the inverse of Q.
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A circuit symbol for a JK flip-flop, where > is the clock input, J and K are data inputs, Q is the stored data output, and Q' is the inverse of Q.

The characteristic equation of the JK flip-flop is: Image File history File links JK_Flip-flop. ... Image File history File links JK_Flip-flop. ...



and the corresponding truth table is:

J K Q Qnext
0 0 0 0
0 0 1 1
0 1 X 0
1 0 X 1
1 1 0 1
1 1 1 0

The origin of the name for the JK flip-flop is detailed by P. L. Lindley, a JPL engineer, in a letter to EDN, an electronics newsletter. The letter is dated June 13, 1968, and was published in the August edition of the newsletter. In the letter, Mr. Lindley explains that he heard the story of the JK flip-flop from Dr. Eldred Nelson, who is responsible for coining the term while working at Hughes Aircraft. The JPL complex in Pasadena, Ca. ... June 13 is the 164th day of the year in the Gregorian calendar (165th in leap years), with 201 days remaining. ... 1968 (MCMLXVIII) was a leap year starting on Monday (the link is to a full 1968 calendar). ... Hughes logo adopted after his death Hughes developed the AIM-120 AMRAAM, one of the worlds most advanced air-to-air missiles Hughes Aircraft Company was a major defense/aerospace company founded by Howard Hughes. ...


Flip-flops in use at Hughes at the time were all of the type that came to be known as J-K. In designing a logical system, Dr. Nelson assigned letters to flip-flop inputs as follows: #1: A & B, #2: C & D, #3: E & F, #4: G & H, #5: J & K. Given the size of the system that he was working on, Dr. Nelson realized that he was going to run out of letters, so he decided to use J and K as the set and reset input of each flip-flop in his system (using subscripts or somesuch to distinguish the flip-flops), since J and K were "nice, innocuous letters."


Dr. Montgomery Phister, Jr., an engineer under Dr. Nelson at Hughes, picked up the idea that J and K were the set and reset input for a "Hughes type" of flip-flop, which he then termed "J-K flip-flops," a name that he carried with him when he left for Scientific Data Systems in Santa Monica.


D flip-flop

The D flip-flop can be interpreted as a primitive delay line or zero-order hold, since the data is posted at the output one clock cycle after it arrives at the input. It is called delay flip flop since the output takes the value in the Data-in.
The term delay line has multiple meanings: In electronics and derivative fields such as telecommunications, a delay line is rigorously defined as a single-input-channel device, in which the output channel state at a given instant, t, is the same as the input channel state at the instant t... Ideally sample signal xs(t). ...


The characteristic equation of the D flip-flop is:



and the corresponding truth table is:

D Q > Qnext
0 X Rising 0
1 X Rising 1

The flip flops are very useful, as they form the basis for shift registers, which are an essential part of many electronic devices.


At the level of individual logic gates, the basic (without S and R) D-type flip-flop is made up of two D-type transparent latches arranged with a pair of inverters as below.

Starting with a low signal on the clock, C, the signal at the input, D, is transferred through the first latch, as the inverted clock signal is high.


When the clock signal is made high, the inverted signal goes low, "locking" the signal at the output of the first stage. The doubly inverted signal now makes the second latch transparent and the signal is transferred through to the output Q (and Q').


When the clock goes low again, the output from the second stage (and therefore from the whole flip-flop) is locked in place, and the signal at D is transferred through the first stage, ready for the next strobing of the clock.


The advantage of this circuit over the D-type latch is that it "captures" the signal at the moment the clock goes high, and subsequent changes of the data line do not matter, even if the signal line has not yet gone low again.


By removing the left-most inverter in the above circuit, a D-type flip flop that strobes on the falling edge of a clock signal can be obtained. This has a truth table like this:

D Q > Qnext
0 X Falling 0
1 X Falling 1

Most D-type flip-flops in ICs have the capability to be set and reset, much like an SR flip-flop. Usually, the illegal S = R = 1 condition is resolved in D-type flip-flops.

Inputs Outputs
S R D > Q Q'
0 1 X X 0 1
1 0 X X 1 0
1 1 X X 1 1

By settting S = R = 0, the flip-flop can be used as described above.


Uses

  • A single flip-flop can be used to store one bit, or binary digit, of data.
  • 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 flip-flops.
  • Any one of the flip-flop types can be used to build any of the others. The data contained in several such 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.
  • One use 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.
  • The T flip-flop is useful for constructing various types of counters. Repeated signals to the clock input will cause the flip-flop to change state once per high-to-low transition of the clock input, if its T input is "1". The output from one flip-flop can be fed to the clock input of a second and so on. The final output of the circuit, considered as the array of outputs of all the individual flip-flops, is a count, in binary, of the number of cycles of the first clock input, up to a maximum of 2n-1, where n is the number of flip-flops used. See: Counters
  • One of the problems with such a counter (called a ripple counter) is that the output is briefly invalid as the changes ripple through the logic. There are two solutions to this problem. The first is to sample the output only when it is known to be valid. The second, more widely used, is to use a different type of circuit called a synchronous counter. This uses more complex logic to ensure that the outputs of the counter all change at the same, predictable time. See: Counters
  • Frequency division: a chain of T flip-flops as described above will also function to divide an input in frequency by 2n, where n is the number of flip-flops used between the input and the output.

A bit (binary digit) refers to a digit in the binary numeral system (base 2). ... Static Random Access Memory (SRAM) is a type of semiconductor memory. ... In computer architecture, a processor register is a small amount of very fast computer memory used to speed the execution of computer programs by providing quick access to commonly used values—typically, the values being in the midst of a calculation at a given point in time. ... A Lego RCX Computer is an example of an embedded computer used to control mechanical devices. ... In general, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred often in relationship to a clock signal. ... There are 95 printable ASCII characters, numbered 32 to 126. ... Fig. ... Digital circuits are electric circuits based on a number of discrete voltage levels. ... This article is about the term counter used in electronics and computing. ... Binary coding is the term used to describe how information, normally numbers, are stored in binary, radix-2 form. ... This article is about the term counter used in electronics and computing. ... This article is about the term counter used in electronics and computing. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...

Timing and metastability

A flip-flop in combination with a Schmitt trigger can be used for the implementation of an arbiter in asynchronous circuits. In electronics, a Schmitt (or Schmidt) trigger is a comparator circuit that incorporates positive feedback. ... Arbiters are used in asynchronous circuits to order computational activities for shared resources to preventing concurrent incorrect operations. ... An asynchronous circuit is a circuit in which the parts are largely autonomous. ...


Clocked flip-flops are prone to a problem called metastability, which happens when a data or control input is changing at the instant of the clock pulse. The result is that the output may behave unpredictably, taking many times longer than normal to settle to its correct state, or even oscillating several times before settling. Theoretically it can take infinite time to settle down. In a computer system this can cause corruption of data or a program crash. Metastability in electronics is the ability of a non-equilibrium electronic state to persist for a long period of time (see asynchronous circuit). ... A Lego RCX Computer is an example of an embedded computer used to control mechanical devices. ...


In many cases, metastability in flip-flops can be avoided by ensuring that the data and control inputs are held constant for specified periods before and after the clock pulse, called the setup time (tsu) and the hold time (th) respectively. These times are specified in the data sheet for the device, and are typically between a few nanoseconds and a few hundred picoseconds for modern devices.


Unfortunately, it is not always possible to meet the setup and hold criteria, because the flip-flop may be connected to a real-time signal that could change at any time, outside the control of the designer. In this case, the best the designer can do is to reduce the probability of error to a certain level, depending on the required reliability of the circuit. One technique for suppressing metastability is to connect two or more flip-flops in a chain, so that the output of each one feeds the data input of the next, and all devices share a common clock. With this method, the probability of a metastable event can be reduced to a negligible value, but never to zero. The probability of metastability gets closer and closer to zero as the number of flip-flops connected in series is increased.


So-called metastable-hardened flip-flops are available, which work by reducing the setup and hold times as much as possible, but even these cannot eliminate the problem entirely. This is because metastability is more than simply a matter of circuit design. When the transitions in the clock and the data are close together in time, the flip-flop is forced to decide which event happened first. However fast we make the device, there is always the possibility that the input events will be so close together that it cannot detect which one happened first. It is therefore logically impossible to build a perfectly metastable-proof flip-flop.


Another important timing value for a flip-flop is the clock-to-output delay (common symbol in data sheets: tCO) or propagation delay (tP), which is the time the flip-flop takes to change its output after the clock edge. The time for a high-to-low transition (tPHL) is sometimes different from the time for a low-to-high transition (tPLH). In computer science, the propogation delay is the amount of time starting from when the input to a logic gate becomes stable and valid to the time that the output of that logic gate is stable and valid. ...


When connecting flip-flops in a chain, it is important to ensure that the tCO of the first flip-flop is longer than the hold time (tH) of the second flip-flop, otherwise the second flip-flop will not receive the data reliably. The relationship between tCO and tH is normally guaranteed if both flip-flops are of the same type.


Flip-Flop integrated circuits

Integrated circuit (ICs) can be found with one or two Flip-flop circuits on board. For example, the 7473 Dual JK Master-Slave Flip-flop or the 74374, an octal D Flip-flop, in the 7400 series. Integrated circuit showing memory blocks, logic and input/output pads around the periphery A monolithic integrated circuit (also known as IC, microchip, silicon chip, computer chip or chip) is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) which has been manufactured in the surface... The 7400 chip, containing four NANDs. ...


See also

A monostable multivibrator is an electronic circuit that has two states, only one of which is stable. ... An astable multivibrator is an electronic circuit that has two states, neither one of which is stable. ... Please wikify (format) this article or section as suggested in the Guide to layout and the Manual of Style. ...

External links

Wikimedia Commons has media related to:
Category:Flip-flops
  • Summary of flip-flop types
  • Another summary of flip-flop types
  • Explanation of How Flip Flops Work with Logic Gate Diagrams
  • Flip-flops interactive (requires Java)

 
 

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