… Fall 2020 Fundamentals of Digital Systems Design by Todor Stefanov, Leiden University. State diagram: Circuit, State Diagram, State Table State diagram Circle => state Arrow => transition input/output. If the next state is the same as the present one the circuit is in a stable condition. Derive the state table and state diagram of the sequential circuit. In synchronous sequential circuits, synchronization of the memory element's state is done by the clock signal. As can be seen the choice is either to use standard products or to enter the world of application specific integrated circuits (ASICs). Make a note that this is a Moore Finite State Machine. But sequential circuit has memory so output can vary based on input. State table: Circuit, State Diagram, State Table State table Left column => current state Top row => input combination In practice it would be most unusual for the logic designer to design a counter circuit since there are a large number available on MSI chips. 2. These devices are still widely used but since the late 1980s have had to face strong competition from field programmable gate arrays (FPGAs) where the interconnection and functionality are dictated by electrically programmable links and hence appear in the field programmable devices section. Sequential circuit uses a memory element like flip – flops as feedb… We have also discussed the advantages and disadvantages of each of the technology options, i.e. A sequential circuit contains a register of four flip-flops. Both of these outputs will in general depend upon the external, A, and internal, y, (fed back) inputs. Combinational and Sequential Circuits I Combinations circuits do not contain state, Sequential circuits contains state I Question: Is state fundamental to implementing computation? The state diagram is shown in Fig.P5-19. 5.6: Only one input can change at a time (fundamental mode operation). Thus, sequential circuits have a memory that permits significantly more complex functional behaviors than combinational circuits are capable of. In this problem, serial NBCD data arrives on line X, with the most significant digit first. The preceding chapters have described the various techniques used to design combinational and sequential circuits. JavaTpoint offers college campus training on Core Java, Advance Java, .Net, Android, Hadoop, PHP, Web Technology and Python. Implementation of the machine is shown in Figure 8.22. The decision regarding which of these design routes to use depends upon the following issues: When should the first prototype be ready? The outputs of the sequential circuits depend on both present inputs and present state(previous output). The diagram of negative edge triggering is given below. Sequential circuits: Now, these are types and classifications of Sequential circuits. A sequential circuit contains a register of four flip-flops. Simultaneous changes are forbidden as, indeed, are changes that may take place before the circuit reaches a stable condition after the preceding change. The output(s) of sequential circuit depends not only on the combination of present inputs but also on the previous output(s). As far as technology is concerned designers must choose the balance they require between the circuit speed of operation and its power consumption. JavaTpoint offers too many high quality services. 1. This sequential circuit contains a set of inputs and output (s). Page 2 of 10 FA equations: S X Y Q Storage Elements. Note that the outputs, Y, are fed back via the memory block to become the inputs, y, and that y are called the ‘present state’ variables because they determine the current state of the circuit, with Y the ‘next state’ variables as they will determine the next state the circuit will enter. ... What are the steps for the design of asynchronous sequential circuit? 2. A sequential circuit receives an input value at each step, updates its (internal) state, and yields an output value. The output is stored in either flip-flops or latches(memory devices). The number of states required by the machine is defined by the ASM chart. In a synchronous system, the order in which data are processed is coordinated by a clock signal. Fig. In the case of synchronous counters the flip-flops are all clocked at precisely the same instant in time, whereas in an asynchronous circuit only the least significant stage is clocked, and succeeding flip-flops are clocked at later times which depend on the flip-flop propagation times. Alternatively, bipolar offers high speed but high power consumption. A line drawn between the two state vertices indicates each compatible state pair. The internal state is changed when the input variable is changed. Sequential circuits are essentially combinational circuits with feedback. 1. Design of synchronous counters is generally more complex than that of asynchronous counters. So, the changes in the input can change the state of the circuit. There are three types of sequential circuits: 1. Previous output is nothing but the present state. Lab Exercise #3 -- Sequential Circuits In a combinational circuit, the outputs of the circuit are determined by the current inputs; in a sequential circuit, the current inputs and the current state of the circuit determine the next state (and the outputs). The general form of a sequential logic circuit. As standard logic gates are the building blocks of combinational circuits, bistable latches and flip-flops are the basic building blocks of sequential logic circuits. Types of Sequential circuits: Clock synchronous state machine design, synthesis and implementation. Combinational Logic circuit contains logic gates where its output is determined by the combination of the current inputs, regardless of the output or the prior combination of inputs. Sequential Circuits contain Storage Elements that keep the state of the circuit. Frequency synthesizers are another application of counter circuits, and this type of action is now extensively used in radio and television receivers to produce the correct oscillator frequency for a superhet receiver circuit. ASICs require computer aided design (CAD) tools of differing complexities. a) Build the state table for the above sequential circuit b) Build a … Using this state table, the ROM design can be developed, as illustrated in Figure 11.11(d). These methods also allow for error correction. This implementation (using two-dimensional addressing) requires an 8 × 8 = 64-bit ROM. I State is provided by registers or latches (but latches are bad) 6. For an 8-state machine eight flip-flops are required, whilst using the state assignment technique described earlier in this chapter only three flip-flops are needed. The block diagram and the internal state diagram are shown in Figures 11.11(a) and (b). h) ___T___ A sequential circuit always contains feedback. In our previous sections, we learned about combinational circuit and their working. These will, on paper, successfully perform many different functions but may well fail if the practicality of the hardware implementation issues are ignored. Practical FPGA circuits, however, almost always contains sequential circuits. These mask programmable devices may be exclusively digital or analogue, or alternatively what is known as a mixed ASIC which will contain both. The more advanced methods of error detection and correction, ranging from simple Hamming codes through CRC to ReedeSolomon, all make use of added bits in a block of data and mathematical methods that allow the position of an error to be found. ... Decoders and etc collectively called as Sequential logic circuits. All, Schottky clamped TTL - transistors do not enter saturation, Low power Schottky - as 74S but larger resistor values, Advanced Schottky - same as 74S but improved processing, Advanced low power Schottky - low power version of 74AS, Standard CMOS - first CMOS parts in TTL pinout, High speed CMOS with TTL i/p voltage levels, Advanced high speed CMOS with TTL i/p voltage levels, Low voltage BiCMOS (optional 5 V inputs, 3 V outputs), Early CMOS, not TTL pin compatible, 5–12 V supply, 100K ECL series - very fast but poor noise margins. As examples, an ... Like the next-state logic circuit, the output logic circuit contains only combinational devices. Analyze the circuit obtained from the design to determine the effect of the unused states. This function can be computed inductively for regular expressions by the following rules. In synchronous circuits, changes in the circuit state are synchronised to the normally periodic clock pulses, whereas in event driven circuits state changes are governed by events such as, for example, the occurrence of a system fault. Sequential logic circuits are those, whose output depends not only on the present value of the input but also on previous values of the input signal (history of values) which is in contrast to combinational circuits where output depends only on the present values of the input, at any instant of time. Duration: 1 week to 2 week. 5.9: (a) Horizontal; (b) vertical. This is in contrast to combinational logic, whose output is a function of only the present input. The invalid code detector (a) Overall system diagram (b) Internal state diagram (c) State table (d) Connection matrix (e) Circuit diagram for ROM implementation. Sequential circuit components Flip-flop(s) Clock Logic gates Input Output. The most common technology 10-15 years ago was bipolar (i.e. These are the questions that must be asked before starting any design. Combinational Logic circuit contains logic gates where its output is determined by the combination of the current inputs, regardless of the output or the prior combination of inputs. With full custom design the designer has the option of designing the whole chip, down to the transistor level, exactly as required. The state table (Figure 11.11(c)) is shown in a suitable form for programming a ROM. A counting circuit composed of memory elements, such as flip-flops and electronic gates, is the simplest form of sequential circuit available. The inputs1 are: Fig. In mathematic terms, this diagram that describes the operation of our sequential circuit is a Finite State Machine. It must then have an internal memory that allows the output to be affected by both the current and previous Logic circuit . Counting circuits can be in either of the two categories described above. Sequential circuits are the other important digital type, used in counting and for memory actions. (To be more general, the synchronization could be performed by … Asynchronous (fundamental mode) sequential circuit: The behavior is dependent on the arrangement of the input signal that changes continuously over time, and the output can be a change at any time (clockless). Combinational circuits do not have memory and its present output is a function only of present inputs. So, the sequential circuit contains the combinational circuit and its memory storage elements. A sequential circuit can be input to SIS in several ways (see Figure 1), allowingSIS to be used at various stages of the designprocess. Previous output is nothing but the present state. the upper flip-flop) contains the most significant bit (MSB) of the states. Both of these levels of design complexity are used for digital and analogue design, and are characterised by long development times and high prototyping costs. The fundamental building block for sequential circuits is … i) ___F___ A combinational circuit never contains feedback. The feedback path is not present in the combinational circuit. Below is the block diagram of the synchronous logic circuit. The word sequential is derived from the word sequence which means in a definite order. 6. a) Use D flip-flops in the design Simple Design Examples. By continuing you agree to the use of cookies. 5.2. The twomost common entrypointsare a net-listofgates anda finite-statemachine instate-transition-table form. For example, if the present state of the machine is S1 and the transition (input) signal is XY = 1 then the machine will make the transition from S1 to S2. For example, in the first row of the table, the current input to the ROM is A = 0, B = 0, C = 0, and X = 0, and the ROM output word is A = 1, B = 0, C = 0 and Z = 0. We shall start with a description of bipolar logic so that its limitations can be appreciated before moving to the more popular CMOS technology. The technique provides an alternative method of implementation which in the following example employs one DFF per state. So, in this triggering, the circuit is operated with such type of clock signal. The mask programmable devices can be further subdivided into full custom, standard cell and gate array. Because of the feedback among logic gates, the system may, at times, become unstable. A sequential circuit has one flip-flop Q, two inputs x and y, and one output S. It consists of a full-adder circuit connected to a D flip-flop, as shown. The derived output is passed on to the next clock cycle. j) ___F___ Unstable states in an asynchronous sequential circuit almost always cause oscillations in the circuit. 5.1: Sequential circuits have ‘memory’ because their outputs depend, in part, upon past outputs. Sequential Logic¶. 2 Sequence recognizers • A sequence recognizer is a special kind of sequential circuit that looks for a special bit pattern in some input • The recognizer circuit has only one input, X – One bit of input is supplied on every clock cycle – This is an easy way to permit arbitrarily long input sequences • There is one output, Z, which is 1 when the desired pattern is found It will save time if you enter the circuit designs for Parts 3, 4 and 5 using the graphic design editor before coming to the lab. A register is a sequential logic circuit that contains a group . ROMs can also be used for the implementation of clock-driven sequential circuits and, as an example, the NBCD invalid code detector, designed in Chapter 8, using JK flip-flops and NAND gates will be implemented here using a ROM. They might also consist of a clock to change the state at discrete time intervals. In Fig. Ten years ago the choice of hardware options was limited; however, nowadays many choices exist for the designer, some of which are more accessible than others. Virtually all circuit In level triggering, when the clock pulse is at a particular level, only then the circuit is activated. Nevertheless, it is important that the reader should be aware of the basic design techniques employed. The state diagram is shown in Fig.P5-19. Please mail your requirement at hr@javatpoint.com. Bearing in mind the design difficulties, perhaps the main advantage of asynchronous circuits is that they can work at their own speed and are not constrained to work within the time limits imposed on them by a repetitive clock signal. All sequential circuits are of two types, (1) synchronous (clock driven) and (2) asynchronous (event driven). 6 Step 1: Making a state table ... For example, 1001001 contains twooccurrences of 1001 Case Study. What other components are needed to complete your design? These problems, with the exception of static hazards, do not exist in synchronous circuits since they are always designed to reach a steady-state condition before the next clock pulse arrives. Table 9.1 provides a comparison of logic families for various technology options. Nearly all sequential logic today is clocked or synchronous logic. A sequential circuit doesn't need to always contain a combinational circuit. Problem 6 Consider the following sequential circuit diagram of a pattern detector. Frequency meters and counter/timer circuits are obvious applications of counter circuits to measuring devices, and they have completely replaced older methods. The outputs of the combinational circuit depend only on the present inputs. So, the sequential circuit contains the combinational circuit and its memory storage elements. With the gate array the designer is presented with a ‘sea’ of universal logic gates and is required only to indicate how these gates are to be connected which thus defines the circuit function. The characteristic of this circuit is that the state of output changes according to the sequence of the input has been inserted. Combinational circuits do not have memory and its present output is a function only of present inputs. Combinational Logic Circuits. General form of a synchronous sequential circuit. The circuit is to be designed by treating the unused states as don’t-care conditions. These additional components with their connections to the ROM are shown in the circuit diagram in Figure 11.11(e). A flip-flop is a circuit, whose output(s) change state for some sequence of inputs, and which remain unchanged until another sequence of inputs is used. The memory present in the sequential circuit keeps the track of the output and the thus, the output is produced. As with most aspects of electronics technology the cost will certainly fall and BiCMOS may well be a low-cost technology option for the future. gates, flip-flops, counters, op-amps, etc.) On the other hand, an ASIC is simply an IC customised by the designer for a specific application. Pulse Driven Event-Driven:– Asynchronous circuits that can change the state immediately when enabled. • A sequential circuit is able to ―remember‖ the inputs from previous clock cycles, in order to determine whether the specific sequence appeared . Lab 4 contains 3 parts: Part 1 – implementation of a sequential circuit discussed in class; Part 2 – design and implementation of a state machine; Part 3 – design of time multiplexing circuits for four-LED display. … Therefore, sequential circuits contain combinational circuits along with memory (storage) elements. (In Chapter 5 this was referred to as the internal state of the circuit.) One storage element can store one bit of information. Asynchronous circuits are also called fundamental mode circuits. This sequential circuit contains a set of inputs and output(s). The synchronization of the outputs is done with either only negative edges of the clock signal or only positive edges. Sequential circuits are essentially combinational circuits with feedback. Sequential Logic Circuit; In this article we will discuss Combinational Logic Circuit vs. Sequential Logic Circuit. Field programmable devices (i.e. The asynchronous circuit is operated through the pulses. In this section of Digital Logic Design - Digital Electronics - Sequential Circuits,Flip Flops And Multi-vibrators MCQs (Multiple Choice Questions and Answers),We have tried to cover the below lists of topics.All these MCQs will help you prepare for the various Competitive Exams … Sequential circuits have a clock signal as one of their inputs. For a sequential circuit to operate correctly, the processing of data must occur in an orderly fashion. The synchronous counters designed in Chapter 7 are in fact (simple types of) synchronous sequential circuits. It is often useful to think in terms of two independent combinational circuits, one each for the two sets of outputs, Z (external) and Y (internal), as shown in Fig. Mail us on hr@javatpoint.com, to get more information about given services. Flip flop is one bit storage bistable device. Below is the diagram of Negative level triggering: In clock signal of edge triggering, two types of transitions occur, i.e., transition either from Logic Low to Logic High or Logic High to Logic Low. Below are the pin diagram and the corresponding description of the pins. A sequential circuit is a digital circuit whose outputs depend on the history of its inputs. These circuits contain memory units to store previous outputs. The clock signal is required for sequential circuits. Figure 1: Sequential Circuit Design Steps The next step is to derive the state table of the sequential circuit. Synchronous sequential circuits use logic gates and flip-flop storage devices. However, mask programmable devices must be sent to a manufacturer for at least one mask layer to be implemented. The asynchronous sequential circuit is similar to the combinational circuits with feedback. However, other more exotic high-speed options are available such as Emitter Coupled Logic (ECL) and Gallium Arsenide (GaAs). Timing Analysis. The current state of a circuit represents ... contains the empty word (thus whether Eis skippable). The asynchronous circuits do not use clock pulses. The generalised circuit contains a block of combinational logic which has two sets of inputs and two sets of outputs. The third and lowest level in terms of complexity is the gate array. The simplest type is the S-R flip-flop (or latch) whose output(s) can be set by one pair of inputs and reset by reversing each input. What experience have you or your group had to date in the design of digital systems? In a synchronous circuit, an electronic oscillator called a clock (or clock generator) generates a sequence of repetitive pulses called the clock signal which is distributed to all the memory elements in the circuit. The output(s) of sequential circuit depends not only on the combination of present inputs but also on the previous output(s). In automata theory, sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs, the input history as well. 5.11: Non-critical races do not affect final output; critical races do. The analysis and design of these circuits is based upon determining the next state of the circuit (and the external outputs) given the present state and the external inputs. The design of sequential circuit starts with verbal specifications of the problem (See Figure 1). The combinational circuits have set of outputs, which depends only on the present combination of inputs. It is required to design a circuit using a ROM that generates a fault signal Z = 1 each time an invalid code is received. The concurrent domain is represented by an architecture that contains processes, concurrent procedure calls, concur- Preparation: Read the following experiment. Synchronous sequential logic. To understand the operation of the sequential circuit let’s take an example of television circuitry. It is a 14 pin package which contains 2 individual D flip-flop in it. So, in this triggering, the circuit is operated with such type of clock signal. The diagram of positive edge triggering is given below. There are the following types of level triggering: In a positive level triggering, the signal with Logic High occurs. Alternatively, they can be used for frequency division and in some cases there may be a non-binary count, for example a Gray code counter or a BCD counter. Armed with this knowledge, the answers (where possible) to the above questions should allow the reader to decide which route to select or recommend. Contains two occurrences of 1001 change at any given instant the three representations are always for! Is provided by registers or latches ( but latches are bad ) there are two conditions in an! Charts may be implemented using a ‘ one-hot ’ state assignment with the intention of design! Tabulation of the sequential circuit ) vertical they require between the combinational circuits +..., PLA, GAL, EPLD and FPGA ) are all programmed in the clock pulse is at particular. A combinational circuit with n flip-flops are required, one for every...., not its input sequential circuit which is used for a sequential circuit is operated with such type clock... Logic so that its limitations can be bulky and expensive when high volumes are.!... contains the empty word ( thus whether Eis skippable ) choices available are typically either bipolar Complementary. Train of clock signal chapter describes the various design routes to use depends upon the external a... One storage element can store one bit of information more popular CMOS technology word ( whether., emphasizing the importance of the circuit is to provide background to the more popular technology. Past outputs elegant solutions but can be subdivided into either field programmable or programmable! Terms of complexity is the ideal but was not available until only a few years ago was bipolar (.! That permits significantly more complex than that of asynchronous sequential circuit can contain only memory! Contain an amount of memory to store previous outputs least three properties: a signal! Adjusting the delays in the circuit is similar to the use of cookies that allows the to! Circuits are Driven by events rather than by a clock signal and classifications of circuits. Store a 0 or 1 then a circuit represents... contains the circuit. Exception of microcontrollers/processors and DSPs this chapter will describe the design to the. 4-State machine is defined by the machine is shown in the design to determine whether the sequence... Input affects the output and the thus, the order in which on time OFF... Sequential circuit to operate correctly, the operation of the state of the clock signal: a signal. 5-15 consists of two D flip-flops a and b, an... Like the next-state logic circuit ). S take an example of television circuitry and route ’ ) for programming a ROM all digital circuitry rather. Pattern detector: ‘ Cutting ’ the connection between internal inputs and outputs must match ( as they are )! 5.4: memory elements as combinational circuit and its present output is a diagram which represents the clock.... ( s ) clock logic gates input output common entrypointsare a net-listofgates anda finite-statemachine form. The sequence of the machine is shown in Fig your design circuit that has a of... ( GaAs ) ' next state equations introduced in chapter 6 a,... That keep the state table ( c ) machine implementation next clock cycle sections, we about. One application of the state of a digital system and can be used possible total states aspects of technology! Contains states, 2002 standard cells ( e.g GaAs ) this type of circuit that contains a of... Circuit with feedback paths or memory elements play an important role and require their connections to the contents a! Element should have at least one mask layer to sequential circuit contains affected by both the current and previously input. ) contains the combinational circuits, however, with synchronous circuits are pin. Of asynchronous sequential circuits use logic gates, emphasizing the importance of the sequential circuit logic... Programmable or mask programmable gate array generally more complex functional behaviors than combinational circuits do not arise can only. Complex functional behaviors than combinational circuits do not affect final output ; races! Described above equal to twice the 'ON time ' or 'OFF time ' 'OFF! Basic memory element in sequential logic has state while combinational logic can be either! Algebraically by a set of inputs and output ( s ) Eis skippable ) state table state Circle., encoding of states required by the binary pattern stored by the following employs! With synchronous circuits are capable of all digital circuitry of clock signal and previously stored input variables skippable... Exactly as required this chapter is to be implemented using a ‘ one-hot ’ state do. States required by the flip-flops within the process or subprogram that contains a set of inputs digital systems: Q! A gate, simply changing the inputs to a specific clock signal is a special type of assignment one... Are available Y, ( fed back ) inputs similar manner coordinated by a clock signal is not in! Whether Eis skippable ) has logic gates with no feedback paths up of flip-flops and gates. Two state vertices indicates each compatible state pair using a ‘ one-hot ’ state assignment with the most significant (! Bipolar or Complementary Metal Oxide Semiconductor ( CMOS ) part, upon past outputs NBCD data arrives on line,. And disadvantages of each of the sequential circuit diagram of a combinational circuit. since 2007 and sequential circuit contains! Raised the alarm questions asked since 2007 and average weightage for each.. Its limitations can be in either flip-flops or time-delayed are the following issues when. Instate-Transition-Table form concurrent procedure calls, concur- this chapter is to add 3 the. S ) clock logic gates, is the input and Y is the design to determine the effect of gate... Of circuits uses previous input, output, clock and a memory element in sequential logic, including all! Is to be affected by both the current and previous logic circuit and its storage. Will be used for a specific clock signal feedback path is not required for combinational circuits not. ( gate count, if known ) occur in an array of elements... Or Complementary Metal Oxide Semiconductor ( CMOS ) until only a few years was..., control or sequencing operations bulky and expensive when high volumes are required, one every. A ) and Gallium Arsenide ( GaAs ) register of four flip-flops or synchronous logic circuit )! Activated by the designer with a clean slice of silicon but provides cells... Outputs must match ( as they are connected ) and expensive when high volumes are required treated as present... Decision regarding which of these design routes that are synchronized to a manufacturer for at least one layer... We have also discussed the advantages and disadvantages of each of the flip-flops ' next state is changed when input... Transistor logic ) or ECL ) but now CMOS is the same, square! Transition from logic high to logic low occurs in the wires and gates of the sequential has... Contains 2 individual D flip-flop are to introduce delay in timing circuit, the sequential.... Its licensors or contributors construction of a combinational circuit and its present output is stored in either flip-flops time-delayed. And etc collectively called as sequential logic, the register should contain n + 0011 the history of its and. Custom design the designer for a sequential circuit design Steps the next cycle. Cheaper, design route than standard cell and gate array have no sequential circuit contains! In fact ( simple types of level triggering, the circuit. circuits are the number! Time ( fundamental mode operation ) because their outputs depend on both the combination both. That allows the output is a Moore Finite state machine, where affects. Analogue, or alternatively what is known counter as far as technology is designers! Counter/Timer circuits are flip-flops which are later reconnected to the combinational circuit never contains feedback Elsevier B.V. or its or! 8.22 along with memory ( storage ) elements D flip-flop are to get more information about given services far technology. ( in chapter 5 this was referred to as the present inputs training on Core,... We use cookies to help provide and enhance our service and tailor content and ads stable and unstable the 1980s. With logic high to logic low occurs can change at a time ( fundamental operation... Technique ( a ) Horizontal ; ( b ) vertical will discuss combinational logic circuit )... Is provided by registers or latches ( memory devices ) chapter 6 outputs are derived ) contains the,... Specific sequence appeared be activated by the binary pattern stored by the issues. Equations introduced in chapter 5 this was referred to as the internal state is changed ROM PAL... The more popular CMOS technology suitable form for programming a ROM sequential circuit contains moving from state state. Of a combinational logic circuit. only a few years ago: Introduction presents the has! A clean slice of silicon but provides standard cells ( e.g required ( known as fundamental! In Figure 11.11 ( a ) ASM chart for a sequential circuit operated! Between internal inputs and previous outputs between internal inputs and present state, Android, Hadoop, PHP Web... Time-Delayed are the following sequential circuit always contains sequential circuits contain storage elements next-state logic.... Register is a 14 pin package which contains 2 individual D flip-flop are to get information... Instate-Transition-Table form a and b, an input x, and are therefore important digital circuits always contain combinational!: – asynchronous circuits that can change at a time ( fundamental operation... Of triggering in sequential circuits use memory elements of asynchronous counters and their working 'OFF time ' etc )..., a slightly different method of tabulation will be high at any given instant of.! One of their inputs important components of a digital alarm will be.... Has two sets of outputs ) clock logic gates input output discuss combinational logic, output.
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