Patent Publication Number: US-10320369-B2

Title: Sequential circuits and operating methods thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. non-provisional application is a Continuation-in-Part of U.S. patent application Ser. No. 15/254,272 filed on Sep. 1, 2016, which claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2015-0126420 filed on Sep. 7, 2015, in the Korean Intellectual Property Office, the entire contents of all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     Example embodiments of inventive concepts relate to sequential circuits and/or operating methods thereof. 
     Description of Related Art 
     A sequential circuit (e.g., a flip-flop) is used as a data storage element in digital circuits of semiconductor integrated circuits. A sequential circuit samples an input signal at a point in time determined by a clock signal, and changes the sampled input signal into an output signal. Sequential circuits are used in semiconductor memory devices (e.g., dynamic random access memories (DRAMs)), processors, computers, etc. 
     SUMMARY 
     One or more example embodiments of inventive concepts provide relatively high-speed sequential circuits and/or operating methods thereof. 
     At least one example embodiment provides sequential circuit including: a first stage configured to charge a voltage of a first node in response to a clock, the first stage further configured to discharge the voltage of the first node in response to the clock and a logic signal; a second stage configured to charge the voltage of the second node in response to the clock, the second stage further configured to maintain a state of charge at the second node based on the logic signal, the second stage further configured to discharge the voltage of the second node in response to the clock and the logic signal; a combinational logic configured to generate the logic signal based on the voltage of the first node, the voltage of the second node, and the data; and a latch circuit configured to latch the voltage of the second node in response to the clock. The combinational logic is further configured to generate the logic signal such that (i) the voltage of the second node is not discharged when the voltage of the first node is discharged, or (ii) the voltage of the first node is not discharged when the voltage of the second node is discharged. 
     According to at least some example embodiments, the first stage may include: a first transistor configured to connect the first node to a power supply terminal in response to the clock; a second transistor configured to connect the first node to the power supply terminal in response to the voltage of the second node; and a third transistor configured to connect a first connection node to the first node in response to the clock. 
     The second stage may include: a fourth transistor configured to connect the second node to the power supply terminal in response to the clock; a fifth transistor configured to connect the second node to the power supply terminal in response to the logic signal; a sixth transistor configured to connect the second node to a second connection node in response to the logic signal; and a seventh transistor configured to connect a ground terminal to the second connection node in response to the clock. The first connection node may be connected to a gate terminal of the sixth transistor. 
     The combinational logic may include: an inverter configured to invert the voltage of the first node; and an AND-OR-Inverter (AOI) gate configured to generate the logic signal based on the voltage of the second node, the data, and an output of the inverter. 
     According to at least some example embodiments, the sequential circuit may include: a fourth transistor having a first terminal connected to the second node; and a fifth transistor configured to connect a ground terminal to a second terminal of the fourth transistor in response to the clock. 
     According to at least some example embodiments, the latch circuit may include: a first transistor configured to connect an output node to a power supply terminal in response to the voltage of the second node; a second transistor having a first terminal connected to the output node; a third transistor configured to connect a second terminal of the second transistor to a ground terminal in response to the voltage of the second node; and a buffer configured to buffer a voltage of the output node. 
     The latch circuit may further include an inverter configured to invert the voltage of the output node. 
     According to at least some example embodiments, the first stage may include: a first transistor having a first terminal connected to the first node; a second transistor configured to connect the first node to a power supply terminal in response to the voltage of the second node; a third transistor configured to connect a second terminal of the first transistor to the power supply terminal in response to a scan input; a fourth transistor configured to connect the second terminal of the first transistor to the power supply terminal in response to a scan enable signal; a fifth transistor configured to connect a first connection node to the first node in response to the clock; a sixth transistor having a first terminal connected to the first node; a seventh transistor having a first terminal connected to a second terminal of the sixth transistor of the first stage; and an eighth transistor configured to connect a second terminal of the seventh transistor to a ground terminal in response to the voltage of the second node. 
     The combinational logic may include: an inverter configured to invert the voltage of the first node; and an AOI gate configured to generate the logic signal based on an output of the inverter, an inverted scan enable signal, the voltage of the second node, and the data, and an output terminal of the AOI gate is connected to the first connection node. 
     According to at least some example embodiments, the first stage may include: a first transistor configured to connect the first node to a power supply terminal in response to the clock; a second transistor configured to connect the first node to the power supply terminal in response to the voltage of the second node; and a third transistor configured to connect a first connection node to the first node in response to the clock. The combinational logic may include: an inverter configured to invert the voltage of the first node; a multiplexer configured to select one of the data and a scan input in response to a scan enable signal; and an AOI gate configured to generate the logic signal based on an output of the multiplexer, the voltage of the second node, and an output of the inverter, and an output terminal of the AOI gate is connected to the first connection node. 
     According to at least some example embodiments, the first stage may include: a first transistor having a first terminal connected to the first node; a second transistor configured to connect the first node to a power supply terminal in response to the voltage of the second node; a third transistor configured to connect a second terminal of the first transistor to the power supply terminal in response to a scan input; a fourth transistor configured to connect the second terminal of the first transistor to the power supply terminal in response to a scan enable signal; a fifth transistor configured to connect a first connection ode to the first node in response to the clock; a sixth transistor having a first terminal connected to the first node; and a seventh transistor configured to connect a second connection node to a second terminal of the sixth transistor in response to the scan enable signal. 
     The combinational logic may include: an inverter configured to invert the voltage of the first node; a eighth transistor having a first terminal connected to the first connection node, and a gate configured to receive an output of the inverter; a ninth transistor configured to connect a second terminal of the eighth transistor to the power supply terminal in response to the data; a tenth transistor configured to connect the second terminal of the eighth transistor to the power supply terminal in response to an inverted scan enable signal; a eleventh transistor configured to connect the second terminal of the eighth transistor to the power supply terminal in response to the voltage of the second node; a twelfth transistor configured to connect the ground terminal to the first connection node in response to the output of the inverter; a thirteenth transistor having a first terminal connected to the first connection node; a fourteenth transistor configured to connect a second terminal of the thirteenth transistor to the second connection node in response to the inverted scan enable signal; and a fifteenth transistor configured to connect the ground terminal to the second connection node in response to the voltage of the second node. 
     At least one other example embodiment provides a sequential circuit including: a first stage; a second stage; a combinational logic; and a latch circuit. The first stage includes: a first transistor having a first terminal connected to a first node; a second transistor configured to connect the first node to a power supply terminal in response to a voltage of a second node; a third transistor configured to connect a second terminal of the first transistor to the power supply terminal in response to a scan input; a fourth transistor configured to connect the second terminal of the first transistor to the power supply terminal in response to a scan enable signal; a fifth transistor configured to connect a first connection node to the first node in response to the clock; a sixth transistor having a first terminal connected to the first node; and a seventh transistor configured to connect a second connection node to a second terminal of the sixth transistor in response to the scan enable signal. The second stage includes: an eighth transistor configured to connect the second node to the power supply terminal in response to the clock; a ninth transistor configured to connect the second node to the power supply terminal in response to a logic signal; a tenth transistor having a first terminal connected to the second node in response to the logic signal; and an eleventh transistor configured to connect a ground terminal to a second terminal of the tenth transistor in response to the clock. The combinational logic is configured to generate the logic signal based on data, the voltage of the first node, and a voltage of the second node, the combinational logic further configured to output the logic signal to the first connection node. The latch circuit is configured to latch the voltage of the second node in response to the clock. 
     According to at least some example embodiments, the combinational logic may include: an inverter configured to invert the voltage of the first node; a twelfth transistor having a first terminal connected to the first connection node; a thirteenth transistor configured to connect a second terminal of the twelfth transistor to the power supply terminal in response to the data; a fourteenth transistor configured to connect the second terminal of the twelfth transistor to the power supply terminal in response to an inverted scan enable signal; a fifteenth transistor configured to connect the second terminal of the twelfth transistor to the power supply terminal in response to the voltage of the second node; a sixteenth transistor configured to connect the ground terminal to the first connection node in response to the output of the inverter; a seventeenth transistor having a first terminal connected to the first connection node; an eighteenth transistor configured to connect a second terminal of the seventeenth transistor to the second connection node in response to the inverted scan enable signal; and a nineteenth transistor configured to connect the ground terminal to the second connection node in response to the voltage of the second node. 
     According to at least some example embodiments, the second stage may further include: a twelfth transistor configured to connect a first terminal of the eighth transistor to the power supply terminal in response to a reset signal, and connect a first terminal of the ninth transistor to the power supply terminal in response to the reset signal; and a thirteenth transistor configured to connect the ground terminal to the second node in response to the reset signal. 
     According to at least some example embodiments, the combinational logic may include: an inverter configured to invert the voltage of the first node; a twelfth transistor having a first terminal connected to the first connection node; a thirteenth transistor configured to connect a second terminal of the twelfth transistor to the power supply terminal in response to the data; a fourteenth transistor configured to connect the second terminal of the twelfth transistor to the power supply terminal in response to an inverted scan enable signal; a fifteenth transistor configured to connect the first connection node to the power supply terminal in response to the voltage of the second node; a sixteenth transistor configured to connect the ground terminal to the first connection node in response to an output of the inverter; a seventeenth transistor having a first terminal connected to the first connection node; an eighteenth transistor configured to connect a second terminal of the seventeenth transistor to the second connection node in response to the inverted scan enable signal; and a nineteenth transistor configured to connect the ground terminal to the second connection node in response to the voltage of the second node. 
     According to at least some example embodiments, the combinational logic may include: an inverter configured to invert the voltage of the first node; a twelfth transistor having a first terminal connected to the first connection node; a thirteenth transistor configured to connect a second terminal of the twelfth transistor to the power supply terminal in response to the data; a fourteenth transistor configured to connect the second terminal of the twelfth transistor to the power supply terminal in response to an inverted scan enable signal; a fifteenth transistor configured to connect the second terminal of the twelfth transistor to the power supply terminal in response to the voltage of the second node; a sixteenth transistor configured to connect the second connection node to the first connection node in response to an output of the inverter; a seventeenth transistor having a first terminal connected to the first connection node; a eighteenth transistor configured to connect a second terminal of the seventeenth transistor to the second connection node in response to the inverted scan enable signal; and a nineteenth transistor configured to connect the ground terminal to the second connection node in response to the voltage of the second node. 
     According to at least some example embodiments, the combinational logic may include: an inverter configured to invert the voltage of the first node; a twelfth transistor having a first terminal connected to the first connection node; a thirteenth transistor configured to connect a second terminal of the twelfth transistor to the power supply terminal in response to the data; a fourteenth transistor configured to connect the second terminal of the twelfth transistor to the power supply terminal in response to an inverted scan enable signal; a fifteenth transistor configured to connect the first connection node to the power supply terminal in response to the voltage of the second node; a sixteenth transistor configured to connect the second connection node to the first connection node in response to an output of the inverter; a seventeenth transistor having a first terminal connected to the first connection node; an eighteenth transistor configured to connect a second terminal of the seventeenth transistor to the second connection node in response to the inverted scan enable signal; and a nineteenth transistor configured to connect the ground terminal to the second connection node in response to the voltage of the second node. 
     At least one other example embodiment provides a sequential circuit including: a first stage, a second stage, a combinational logic, and a latch circuit. The first stage includes: a first transistor configured to connect a first node to a power supply terminal in response to a clock; a second transistor configured to connect the first node to the power supply terminal in response to a voltage of a second node; and a third transistor configured to connect a first connection node to the first node in response to the clock. The second stage includes: a fourth transistor configured to connect the second node to the power supply terminal in response to the clock; a fifth transistor configured to connect the second node to the power supply terminal in response to a logic signal; a sixth transistor configured to connect the second node to a second connection node in response to the logic signal; and a seventh transistor configured to connect a ground terminal to the second connection node in response to the clock. The combinational logic includes: an inverter configured to invert the voltage of the second node; and an AOI gate configured to generate the logic signal based on an output of the inverter, the voltage of the second node, and data. The latch circuit is configured to latch the voltage of the second node in response to the clock. 
     According to at least some example embodiments, an output terminal of the AOI gate may be connected to the first connection node. 
     The sequential circuit may further include a discharge circuit configured to discharge the voltage of the first node from the first connection node to the ground terminal. 
     The sequential circuit may further include an inverter configured to invert an output of the latch circuit. 
     At least one other example embodiment provides an operating method of a sequential circuit, the method comprising: charging at least one of a first node and a second node in response to a clock; discharging a voltage of the first node in response to the clock; and discharging a voltage of the second node in response to the clock and a logic signal. The logic signal is generated according to a logical combination of the voltage of the first node, the voltage of the second node, and data. The voltage of the second node is not discharged when the voltage of the first node is discharged, and the voltage of the first node is not discharged when the voltage of the second node is discharged. 
     According to at least some example embodiments, the method may further include: latching data corresponding to the voltage of the second node in response to the clock; inverting the latched data; forming a scan path in response to a scan enable signal; and/or resetting the voltage of the first node and the voltage of the second node in response to a reset signal. 
     At least one other example embodiment provides a processor including a first sequential circuit and a second sequential circuit. The second sequential circuit is serially connected to the first sequential circuit. Each of the first and second sequential circuits are configured to: generate a logic signal in response to a voltage of a first node, a voltage of a second node, and data; and determine, in response to the logic signal, whether to discharge the voltage of the first node or the voltage of the second node. Each of the first and second sequential circuits are further configured such that (i) the voltage of the second node is not discharged when the voltage of the first node is discharged, and (ii) the voltage of the first node is not discharged when the voltage of the second node is discharged. 
     At least one other example embodiment provides a sequential circuit including: a first stage circuit; a second stage circuit; and a combinational logic circuit coupled between a first node of the first stage circuit and a second node of the second stage circuit. The combinational logic circuit is configured to control discharging of the first node and the second node based on a voltage at the first node, a voltage at the second node, and input data, the combinational logic is further configured to control discharging of the first node and the second node such that the first node is discharged during a first time interval and the second node is discharged during a second time interval, the first and second time intervals being different and non-overlapping time intervals. 
     The sequential circuit may further include a latch circuit configured to latch the voltage at the second node in response to a clock signal. 
     The combinational logic circuit may be further configured to generate a logic signal to control the discharging of the first node and the second node, the logic signal generated based on the voltage at the first node, the voltage at the second node, and the input data. 
     The first stage circuit may be configured to charge the first node based on a clock signal. The first stage circuit may be configured to discharge the first node based on the clock signal, the voltage at the second node, and the input data. The second stage may be configured to charge the second node based on the clock signal, and the second stage may be configured to discharge the second node based on the clock signal and the logic signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will become apparent from the following description with reference to the figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified, and wherein: 
         FIG. 1  is a diagram illustrating a sequential circuit according to an example embodiment of inventive concepts; 
         FIG. 2  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 1 . 
         FIG. 3  is a circuit diagram illustrating an example embodiment of the combinational logic illustrated in  FIG. 1 ; 
         FIG. 4  is a circuit diagram illustrating an example discharge path of a first node illustrated in  FIG. 1 ; 
         FIG. 5  is a circuit diagram illustrating another example discharge path of a first node illustrated in  FIG. 1 ; 
         FIG. 6  is a circuit diagram illustrating an example embodiment of the latch circuit illustrated in  FIG. 1 ; 
         FIG. 7  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts; 
         FIG. 8  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 7 . 
         FIG. 9  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts; 
         FIG. 10  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 9 . 
         FIG. 11  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts; 
         FIG. 12  is a diagram illustrating an example scan path in the sequential circuit in  FIG. 11 ; 
         FIG. 13  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 11 . 
         FIG. 14  is a diagram illustrating an exemplary scan path in the sequential circuit of  FIG. 13 . 
         FIG. 15  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts; 
         FIG. 16  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 15 . 
         FIG. 17  is a circuit diagram illustrating an example embodiment of a sequential circuit in which an AND-OR-Inverter (AOI) gate is transformed; 
         FIG. 18  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 17 . 
         FIG. 19  is a circuit diagram illustrating another example embodiment of a sequential circuit in which an AOI gate is transformed; 
         FIG. 20  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 19 . 
         FIG. 21  is a circuit diagram illustrating another example embodiment of a sequential circuit in which an AOI gate is transformed; 
         FIG. 22  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 21 . 
         FIG. 23  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts; 
         FIG. 24  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 23 . 
         FIG. 25  is a flowchart illustrating an operating method of a sequential circuit according to an example embodiment of inventive concepts; 
         FIG. 26  is a timing diagram illustrating example operating timing of a sequential circuit according to an example embodiment of inventive concepts; 
         FIG. 27  is a block diagram illustrating a solid state drive (SSD) according to an example embodiment of inventive concepts; 
         FIG. 28  is a block diagram illustrating an electronic device according to an example embodiment of inventive concepts; and 
         FIG. 29  is a block diagram illustrating a mobile device according to an example embodiment of inventive concepts. 
     
    
    
     DETAILED DESCRIPTION 
     Inventive concepts will become more readily understood by reference to the following detailed description of example embodiments and the accompanying drawings. Inventive concepts may, however, be embodied in many different forms and should not be construed as being limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete and will fully convey concept of the inventive concepts to those skilled in the art, and the inventive concepts will only be defined by the appended claims. Like reference numerals refer to like elements throughout the specification. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of inventive concepts. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on”, “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of inventive concepts. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Example embodiments are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, these example embodiments should not be construed as limited to the particular shapes of regions illustrated herein, but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of inventive concepts. 
     Although corresponding plan views and/or perspective views of some cross-sectional view(s) may not be shown, the cross-sectional view(s) of device structures illustrated herein provide support for a plurality of device structures that extend along two different directions as would be illustrated in a plan view, and/or in three different directions as would be illustrated in a perspective view. The two different directions may or may not be orthogonal to each other. The three different directions may include a third direction that may be orthogonal to the two different directions. The plurality of device structures may be integrated in a same electronic device. For example, when a device stricture (e.g., a memory cell structure or a transistor structure) is illustrated in a cross-sectional view, an electronic device may include a plurality of the device structures (e.g., memory cell structures or transistor structures), as would be illustrated by a plan view of the electronic device. The plurality of device structures may be arranged in an array and/or in a two-dimensional pattern. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Unless specifically stated otherwise, or as is apparent from the discussion, terms such as processing or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Specific details are provided in the following description to provide a thorough understanding of example embodiments. However, it will be understood by one of ordinary skill in the art that example embodiments may be practiced without these specific details. For example, systems may be shown in block diagrams so as not to obscure the example embodiments in unnecessary detail. In other instances, well-known processes, structures and techniques may be shown without unnecessary detail in order to avoid obscuring example embodiments. 
     In the following description, illustrative embodiments may be described with reference to acts and symbolic representations of operations (e.g., in the form of flow charts, flow diagrams, data flow diagrams, structure diagrams, block diagrams, etc.) that may be implemented as program modules or functional processes including routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The operations may be implemented using existing hardware in existing electronic systems (e.g., display drivers, System-on-Chip (SoC) devices, SoC systems, electronic devices, such as personal digital assistants (PDAs), smartphones, tablet personal computers (PCs), laptop computers, etc.) Such existing hardware may include one or more Central Processing Units (CPUs), digital signal processors (DSPs), application-specific-integrated-circuits (ASICs), systems-on-chip (SoCs), field programmable gate arrays (FPGAs), computers, or the like. 
     Further, one or more example embodiments may be (or include) hardware, firmware, hardware executing software, or any combination thereof. Such hardware may include one or more CPUs, SoCs, DSPs, ASICs, FPGAs, computers, or the like, configured as special purpose machines to perform the functions described herein as well as any other well-known functions of these elements. In at least some cases, CPUs, SoCs, DSPs, ASICs and FPGAs may generally be referred to as processing circuits, processing circuitry, processors and/or microprocessors. 
     Although a flow chart may describe operations as a sequential process, many of the operations may be performed in parallel, concurrently or simultaneously. In addition, the order of the operations may be re-arranged. A process may be terminated when its operations are completed, but may also have additional steps not included in the figure. A process may correspond to a method, function, procedure, subroutine, subprogram, etc. When a process corresponds to a function, its termination may correspond to a return of the function to the calling function or the main function. 
     As disclosed herein, the term “storage medium”, “computer readable storage medium” or “non-transitory computer readable storage medium,” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other tangible machine readable mediums for storing information. The term “computer-readable medium” may include, but is not limited to, portable or fixed storage devices, optical storage devices, and various other mediums capable of storing, containing or carrying instruction(s) and/or data. 
     Furthermore, at least some portions of example embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine or computer readable medium such as a computer readable storage medium. When implemented in software, processor(s), processing circuit(s), or processing unit(s) may be programmed to perform the necessary tasks, thereby being transformed into special purpose processor(s) or computer(s). 
     A code segment may represent a procedure, function, subprogram, program, routine, subroutine, module, software package, class, or any combination of instructions, data structures or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     As discussed herein, an “end” of a transistor may also be referred to as a “terminal” of the transistor. 
       FIG. 1  is a diagram illustrating a sequential circuit according to an example embodiment of inventive concepts. 
     Referring to  FIG. 1 , a sequential circuit  100  may include a first stage  110 , a second stage  120 , a combinational logic  130 , and a latch circuit  140 . In one example, the sequential circuit  100  may be a flip-flop. 
     The first stage  110  may include two PMOS transistors P 11  and P 12  and an NMOS transistor N 11 . The first PMOS transistor P 11  of the first stage  110  may be connected between a power supply terminal VDD and a first node NET 1  and may be turned on or off in response to a clock CLK. The second PMOS transistor P 12  of the first stage  110  may be connected between the power supply terminal VDD and the first node NET 1  and may be turned on or off in response to a voltage of a second node NET 2 . The NMOS transistor N 11  of the first stage  110  may be connected between the power supply terminal VDD and the first node NET 1  and may be turned on or off in response to the clock CLK. The first stage  110  may determine a voltage of the first node NET 1  and a voltage of a first connection node CN 1  in response to the clock CLK and a voltage of the second node NET 2 . Here, the first connection node CN 1  may be connected to an output terminal of the combinational logic  130 . 
     The second stage  120  may be complementary to the first stage  110  and may include two PMOS transistors P 21  and P 22  and two NMOS transistors N 21  and N 22 . The first PMOS transistor P 21  of the second stage  120  may be connected between the power supply terminal VDD and the second node NET 2 , and may be turned on or off in response to the clock CLK. The second PMOS transistor P 22  of the second stage  120  may be connected between the power supply terminal VDD and the second node NET 2 , and may be turned on or off in response to a voltage of the first node NET 1 . The first NMOS transistor N 21  of the second stage  120  may be connected between the second node NET 2  and a second connection node CN 2 , and may be turned on or off in response to a logic signal LS. That is, for example, the first NMOS transistor N 21  has one end connected to the second node NET 2  and other end connected the second connection node CN 2 . The second NMOS transistor N 22  of the second stage  120  may be connected between the second connection node CN 2  and a ground terminal GND and may be turned on or off in response to the clock CLK. That is, for example, the second NMOS transistor may connect the ground terminal GND to the other end of the first NMOS transistor N 21  in response to the clock CLK. 
     The second stage  120  may determine a voltage of the second node NET 2  in response to the clock CLK, a voltage of the first node NET 1 , and the logic signal LS. In at least one example embodiment, the voltage of the first node NET 1  may be complementary to the voltage of the second node NET 2  during a set time period and/or interval (e.g., in a specific duration). 
     An example embodiment of inventive concepts is exemplified in  FIG. 1  with regard to the first NMOS transistor N 21  and the second NMOS transistor N 22  being serially connected to each other. However, the scope and spirit of inventive concepts should not be limited thereto. For example, an NMOS transistor associated with the clock CLK may be connected to the second node NET 2 , and an NMOS transistor associated with the logic signal LS may be connected to the ground terminal GND. 
     The combinational logic  130  may receive the voltage of the first node NET 1 , the voltage of the second node NET 2 , and data D and may internally perform a logical operation using the voltage of the first node NET 1 , the voltage of the second node NET 2 , and data D. Accordingly, the combinational logic  130  may be implemented to generate the logic signal LS. The combinational logic  130  may be implemented with various types of logic circuits. 
     The latch circuit  140  may be implemented to latch the voltage of the second node NET 2  in response to the clock CLK. The latch circuit  140  may output the latched data Q or /Q. 
     The conceptual operation of the sequential circuit  100  according to an example embodiment of inventive concepts is described below. When the clock CLK is at a low level, the first node NET 1  and the second node NET 2  may be charged with a voltage (e.g., VDD) of a high-level. That is, for example, when the clock CLK is at a low level, the first node NET 1  and the second node NET 2  may be charged regardless (or, alternatively, independent) of the data D. Afterwards, whether to discharge the first node NET 1  or whether to discharge the second node NET 2  may be determined based on the data D and the clock CLK, and data corresponding to the voltage of the second node NET 2  may be latched according to the determination result in the latch circuit  140 . 
     In at least one example embodiment, a discharge operation about the first node NET 1  may be complementary to a discharge operation about the second node NET 2 . For example, an output value Q or /Q) of the sequential circuit  100  may be determined by discharging one of the first node NET 1  and the second node NET 2  based on a state of the data D. 
     In at least one example embodiment, when one node (e.g., NET 2 ) is discharged, a discharge operation about the other node (e.g., NET 1 ) may be suppressed, prevented and/or blocked (e.g., completely blocked), and vice versa. For example, when one of the first and second nodes NET 1  and NET 2  is discharged, the combinational logic  130  may suppress, prevent and/or block the discharging of the other thereof until the discharged node is again charged. At this time, one of the PMOS transistor P 12  and the PMOS transistor P 22  may be turned on, and may connect the other node to the power supply terminal VDD. 
     Conventionally, a pulse-based sequential circuit may charge a node in response to a clock having a low state, and may determine whether to keep (or maintain) voltage state of a node or whether to discharge the node based on a pulse generated using a clock, a clock of a high state, and a data state. The pulse-based sequential circuit may operate relatively rapidly. However, the pulse-based sequential circuit may be prone to clock changes. 
     On the other hand, the sequential circuit  100 , according to at least one example embodiment of inventive concepts, may determine (e.g., complementarily determine) whether to discharge the nodes NET 1  and NET 2  based on the logic signal LS, which is determined according to voltages of the nodes NET 1  and NET 2  and a state of the data D, and the clock CLK. Accordingly, the sequential circuit  100  according to at least one example embodiment of inventive concepts may reduce and/or minimize influence due to a clock change as compared with conventional sequential circuits. 
     Moreover, a conventional pulse-based sequential circuit may be implemented with NMOS transistors having a 3-stack structure to discharge a node. On the other hand, the sequential circuit  100 , according to at least one example embodiment, ray be implemented with the NMOS transistors N 21  and N 22  having a 2-stack structure to discharge the second node NET 2 . Accordingly, compared with a conventional sequential circuit, the sequential circuit  100  according to at least one example embodiment of inventive concepts may perform a high-speed latch operation by increasing discharge speed. 
     Conventionally, a sequential circuit may influence a clock frequency of a system due to a setup time and a clock to output (CQ) delay. Accordingly, when a relatively high-performance sequential circuit, in which the setup time and the CQ delay are relatively small (e.g., very little) is used, increasing a clock frequency may relatively easy. The sequential circuit  100  according to at least one example embodiment of inventive concepts may reduce a setup time and/or the CQ delay, which may increase clock frequency. 
       FIG. 2  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 1 . Referring to  FIG. 2 , a sequential circuit  100 ′ may include a first stage  110 , a second stage  120 ′, a combinational logic  130 , and a latch circuit  140 . Compared with the sequential circuit  100  of  FIG. 1 , the second PMOS transistor P 22 ′ of the sequential circuit  100 ′ of  FIG. 2  may be turned on or off in response to a logic signal LS. 
     In the sequential circuit  100  of  FIG. 1 , when the first node NET 1  is discharged, the second node NET 2  may be charged via the second PMOS transistor P 22 . Contrastively, in the sequential circuit  100 ′ of  FIG. 2 , when the logic signal LS transitions to a low level, the second node NET 2  may be charged via the second PMOS transistor P 22 ′. Alternatively, when the logic signal LS is maintained at a low level, the second node NET 2  may maintain a state of charge. That is, in the sequential circuit of  FIG. 2 , the logic signal LS may prevent the second node NET 2  from discharging via the first NMOS transistor N 21  and charge the second node NET 2  via the second PMOS transistor P 22 ′. 
     The value of the logic signal LS output from the combinational logic  130  may depend on the state of the first node NET 1 . Accordingly, for the sequential circuit  100 ′ of  FIG. 2 , the state of the second node NET 2  (i.e., state of charge and discharge) may be similar to that of the second node NET 2  of the sequential circuit  100  of  FIG. 1 . For example, in the sequential circuit  100  of  FIG. 1  and the sequential circuit  100 ′ of  FIG. 2 , when the clock CLK is at a high level, one of the first and second nodes NET 1  and NET 2  may be discharged according to the state of the data D. In addition, a discharged one of the first and second nodes NET 1  and NET 2  may prevent the other node from discharging and charge the other node. 
     An area of the sequential circuit  100 ′ of  FIG. 2  may be reduced compared to the area of the sequential circuit  100  of  FIG. 1 . In the sequential circuit  100 ′ of  FIG. 2 , the inputs to the first and second PMOS transistors P 21  and P 22 ′ are the clock CLK and the logic signal LS, and the inputs to the first and second NMOS transistors N 21  and N 22  are also the clock CLK and the logic signal LS. Accordingly, the sequential circuit  100 ′ of  FIG. 2  may be implemented as a CMOS composed of NAND gate, and the area of the sequential circuit  100 ′ may be reduced. 
     For the sequential circuit  100  of  FIG. 1 , when the clock CLK is at a high level and the first node NET 1  is discharged, there may be an interval in which the second node NET 2  is floating before the second node NET 2  is fully charged. Contrastively, for the sequential circuit  100 ′ of  FIG. 2 , the logic signal LS may turn the second PMOS transistor P 22 ′ on in advance before the clock CLK transitions to a high level. Accordingly, the stability of the sequential circuit  100 ′ may be improved. 
       FIG. 3  is a circuit diagram illustrating an example embodiment of the combinational logic  130  illustrated in  FIG. 1 . 
     Referring to  FIG. 3 , the combinational logic  130  may include an inverter  131  and an AND-OR-Inverter (AOI) gate  132 . 
     The inverter  131  may receive and invert a voltage of the first node NET 1 . The AOI gate  132  may be implemented to generate the logic signal LS by executing an AOI gate operation with respect to a voltage of the second node NET 2 , the data D, and an output value of the inverter  131 . As illustrated in  FIG. 3 , an output terminal of the AOI gate  132  may be connected to the first connection node CN 1 . 
     Discharge paths of the first and second nodes, which are formed according to the logic signal LS outputted from the combinational logic  130 , and a discharge operation will be described with respect to  FIGS. 4 and 5 . 
       FIG. 4  is a circuit diagram illustrating an example discharge path of the first node NET 1  illustrated in  FIG. 1 . For descriptive convenience, the AOI gate  132  may include a discharge path connected to the first connection node CN 1 . In at least one example embodiment, the discharge path may be implemented with NMOS transistors N 12  and N 13 , which are connected serially. The NMOS transistor N 12  may be turned on in response to the data D, and the NMOS transistor N 13  may be turned on in response to a voltage of the second node NET 2 . Here, an order of the NMOS transistors N 12  and N 13 , which are connected serially as illustrated in  FIG. 4 , is an example. However, the scope and spirit of inventive concepts should not be limited thereto. For example, an NMOS transistor associated with a voltage of the second node NET 2  may be connected to the first connection node CN 1 , and an NMOS transistor associated with the data D may be connected to the ground terminal GND. Meanwhile, the discharge path of the first node NET 1  illustrated in  FIG. 4  is an example. However, the scope and spirit of inventive concepts should not be limited thereto. 
     Referring to  FIGS. 1 to 4 , a discharge operation about the first node NET 1  is described below. When the data D has a high level and the voltage of the second node NET 2  has a high level, a voltage of the first node NET 1  may be discharged in response to the clock CLK having a high level. At this time, the combinational logic  130  illustrated in  FIG. 3  may generate the logic signal LS having a low level. Because a voltage of the first connection node CM is a low level, the logic signal LS may be maintained at a low level while the voltage of the first node NET 1  is discharged. Accordingly, the voltage of the second node NET 2  may not be fundamentally discharged. 
       FIG. 5  is a circuit diagram illustrating an example discharge path of a second node NET 2  illustrated in  FIG. 1 . 
     Referring to  FIG. 5 , a discharge path of the second node NET 2  may be implemented with NMOS transistors N 21  and N 22 , which are connected serially. 
     Referring to  FIGS. 1 to 5 , an example discharge operation about the second node NET  2  is described below. When a voltage of the first node NET 1  is a high level, a voltage of the second node NET 2  is a low level, and data D has a low level, the AOI gate  132  of the combinational logic  130  may generate the logic signal LS having a high level. The voltage of the second node NET 2  may be discharged in response to the logic signal LS having a high level and the clock CLK having a high level with reference to the sequential circuit  100  illustrated in  FIG. 1 . At the same or substantially the same time (e.g., simultaneously and/or concurrently), the first node NET 1  may be charged with the power supply terminal VDD in response to the voltage of the second node NET 2  of a low level and may keep or maintain a voltage at a high level. While the second node NET 2  is discharged, the discharging of the first node NET 1  may be suppressed, prevented and/or blocked (e.g., fundamentally blocked). 
     As described with reference to  FIGS. 4 and 5 , when a discharge operation is performed with respect to one of the first node NET 1  and the second node NET 2 , the discharge operation about the other node thereof may be suppressed, prevented and/or blocked (e.g., fundamentally blocked) based on the logic signal LS provided from the combinational logic  130 . 
       FIG. 6  is a circuit diagram illustrating an example embodiment of the latch circuit  140  illustrated in  FIG. 1 . 
     Referring to  FIGS. 1 to 6 , the latch circuit  140  may include a PMOS transistor P 41 , NMOS transistors N 41  and N 42 , and a buffer BUF. The PMOS transistor P 41  may connect the power supply terminal VDD to an output node OUT in response to a voltage of the second node NET 2 . The NMOS transistor N 41  may be connected to the output node OUT and may operate in response to the clock CLK. The NMOS transistor N 42  may connect a drain terminal of the NMOS transistor N 41  to the ground terminal GND in response to the voltage of the second node NET 2 . The buffer BUF may buffer a voltage of the output node OUT. In at least one example embodiment, the buffer BUF may be activated in response to the clock CLK or may be activated in response to an inverted version of the voltage of the second node NET 2 . 
     An order of the NMOS transistors N 41  and N 42 , which are connected serially as illustrated in  FIG. 6 , is an example. However, the scope and spirit of inventive concepts should not be limited thereto. For example, positions of the NMOS transistors N 41  and N 42 , which are serially connected, may be interchanged. 
     A sequential circuit according to at least one example embodiment of inventive concepts may be implemented to receive a scan signal. The sequential circuit may be used as a portion of a scan chain for a scan test operation. 
       FIG. 7  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts. 
     Referring to  FIG. 7 , a sequential circuit  200  may include a first stage  210 , a second stage  220 , a combinational logic  230 , and a latch circuit  240 . Compared with the first stage  110  illustrated in  FIG. 1 , the first stage  210  of the sequential circuit  200  may further include PMOS transistors P 13  and P 14  and NMOS transistors N 12  and N 13 , which are connected to receive a scan input SI and a scan enable signal SE associated with a scan signal, and an NMOS transistor N 14 , which is connected to receive a voltage of the second node NET 2 . A configuration of the first stage  210  is an example, and the scope and spirit of inventive concepts should not be limited thereto. For example, positions of the first PMOS transistor P 11  and the third and fourth PMOS transistors P 13  and P 14  may be interchanged. 
     Moreover, compared with the combinational logic  130  illustrated in  FIGS. 1 and 3 , the combinational logic  230  of the sequential circuit  200  may include an AOI gate  232 , which additionally receives an inverted version (e.g., /SE) of the scan enable signal SE. 
     The second stage  220  may be the same or substantially the same as the second stage  120  shown in  FIG. 1 . 
     The latch circuit  240  may be similar to the latch circuit  140  shown in  FIG. 1 , but may further include an inverter  244  coupled to an output of a latch  242 . The latch  242  may be the same or substantially the same as the latch circuit  140  in  FIG. 1 . The inverter  244  may invert the output of the latch  242 . 
     At least one example embodiment of inventive concepts is exemplified as the scan input SI and the scan enable signal SE are inputted to the first stage  210  in the sequential circuit  200  illustrated in  FIG. 7 . However, the scope and spirit of inventive concepts should not be limited thereto. For example, a sequential circuit according to at least one example embodiment of inventive concepts may be implemented such that the scan input SI and the scan enable signal SE are inputted to the combinational logic. 
       FIG. 8  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 7 , Referring to  FIG. 8 , a sequential circuit  200 ′ may include a first stage  210 , a second stage  220 ′, a combinational logic  230 , and a latch circuit  240 . Compared with the sequential circuit  200  of  FIG. 7 , the second PMOS transistor P 22 ′ of the sequential circuit  200 ′ of  FIG. 8  may be turned on or off in response to a logic signal LS. 
       FIG. 9  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts. 
     Referring to  FIG. 9 , the sequential circuit  300  may include a first stage  310 , a second stage  320 , a combinational logic  330  and a latch circuit  340 . 
     Compared with the sequential circuit  200  illustrated in  FIG. 7 , the sequential circuit  300  may include a combinational logic  330 , which is configured to be different from the sequential circuit  200  so as to additionally receive the scan input SI and the scan enable signal SE and to generate the logic signal LS. 
     The combinational logic  330  may include an inverter  331 , an AOI gate  332 , and a multiplexer  333 . The multiplexer  333  may be implemented to select one of the data D and the scan input SI as an input of the AOI gate  332  in response to the scan enable signal SE. A configuration of the combinational logic  330  illustrated in  FIG. 9  is an example, and the scope and spirit of inventive concepts should not be limited thereto. 
     The first stage  310  and the second stage  320  may be the same or substantially the same as the first stage  110  and the second stage  120 , respectively. The latch circuit  340  may include a latch  342  and an inverter  344 . The latch  342  and the inverter  344  may be the same or substantially the same as the latch  242  and the inverter  244 , respectively. 
     At least some example embodiments of inventive concepts are described with regard to a data path and a scan path implemented so as to have a shared structure. However, the scope and spirit of inventive concepts should not be limited thereto. For example, sequential circuits according to example embodiments of inventive concepts may be implemented with a structure in which a data path and a scan path are divided. 
       FIG. 10  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 9 . Referring to  FIG. 10 , a sequential circuit  300 ′ may include a first stage  310 , a second stage  320 ′, a combinational logic  330 , and a latch circuit  340 . Compared with the sequential circuit  300  of  FIG. 9 , the second PMOS transistor P 22 ′ of the sequential circuit  300 ′ of  FIG. 10  may be turned on or off in response to a logic signal LS. 
       FIG. 11  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts. 
     Referring to  FIG. 11 , a sequential circuit  400  may include a first stage  410 , a second stage  420 , a combinational logic  430  and a latch circuit  440 . 
     The first stage  410  receives the scan input SI through a configuration different from the sequential circuit  300  of  FIG. 9 , and the combinational logic  430  receives the data D through a configuration different from the sequential circuit  300  of  FIG. 9 . 
     The first stage  410  may include: a first PMOS transistor P 11 , which connects the first node NET 1  to a common node of both a third PMOS transistor P 13  and a fourth PMOS transistor P 14  in response to the clock CLK; a second PMOS transistor P 12 , which connects the first node NET 1 , to the power supply terminal VDD in response to a voltage of the second node NET 2 ; the third PMOS transistor P 13 , which connects one end of the first PMOS transistor P 11  to the power supply terminal VDD in response to the scan input SI; the fourth PMOS transistor P 14 , which connects the one end of the first PMOS transistor P 11  to the power supply terminal VDD in response to the scan enable signal SE; a first NMOS transistor N 11 , which connects the first connection node CN 1  to the first node NET 1  in response to the clock CLK; a second NMOS transistor N 12 , which connects one end of the third NMOS transistor N 13  to the first node NET 1  in response to the scan input SI; and a third NMOS transistor N 13 , which connects the second connection node CN 2  to the one end of the second NMOS transistor N 12  in response to the scan enable signal SE. A configuration of the first stage  410  is an example, and the scope and spirit of inventive concepts should not be limited thereto. For example, positions of the second NMOS transistor N 12  and the third NMOS transistor N 13 , which are connected serially, may be interchanged. 
     The combinational logic  430  may include: an inverter  431 , which inverts a voltage of the first node NET 1 ; a PMOS transistor P 31 , which connects a common node of PMOS transistors P 31 , P 32 , and P 33  to the first connection node CN 1  in response to an inverted voltage/NET 1  of the first node NET 1 ; the PMOS transistor P 32 , which connects one end of the PMOS transistor P 31  to the power supply terminal VDD in response to the data D; the PMOS transistor P 33 , which connects the one end of the PMOS transistor P 31  to the power supply terminal VDD in response to an inverted scan enable signal /SE; a PMOS transistor P 34 , which connects the one end of the PMOS transistor P 31  to the power supply terminal VDD in response to the voltage of the second node NET 2 ; an NMOS transistor N 31 , which connects the ground terminal GND to the first connection node CN 1  in response to the inverted voltage/NET 1  of the first node NET 1 ; an NMOS transistor N 32 , which connects one end of an NMOS transistor N 33  to the first connection node CN 1  in response to the data; the NMOS transistor N 33 , which connects the second connection node CN 2  to the NMOS transistor N 32  in response to the inverted scan enable signal /SE; and an NMOS transistor N 34 , which connects the ground terminal GND to the second connection node CN 2  in response to the voltage of the second node NET 2 . The configuration of the combinational logic  430  is an example, and the scope and spirit of inventive concepts should not be limited thereto. For example, positions of the PMOS transistor P 31  and the PMOS transistors P 32 , P 33  and P 34 , which are connected serially, may be interchanged. Moreover, positions of the NMOS transistors N 32 , N 33  and N 34 , may be variously interchanged. 
     The second stage  420  may be the same or substantially the same as the second stage  120 . The latch circuit  440  may include a latch  442  and an inverter  444 . The latch  442  and the inverter  444  may be the same or substantially the same as the latch  242  and the inverter  244 , respectively. 
     Unlike the sequential circuit  300  illustrated in  FIG. 9 , as described with reference to  FIG. 11 , the sequential circuit  400  may be implemented with a structure in which a scan path and a data path are divided. 
       FIG. 12  is a diagram illustrating an example scan path in the sequential circuit  400  of  FIG. 11 . 
     Referring to  FIGS. 11 and 12 , when the scan enable signal SE is at a high level, a scan path may be formed. Even though PMOS transistors P 14  and P 33  and NMOS transistors N 11 , N 13 , N 32 , N 33 , which are meaningless or do not operate, from among components of the sequential circuit  400  of  FIG. 11  are removed, a scan path, which is the same or substantially the same as illustrated in  FIG. 12 , may be formed according to a scan enable signal SE of a high level. The scan path according to an example embodiment of inventive concepts may include an inverter  434  composed of transistors P 31 , P 32 , P 34 , and N 31  of  FIG. 11 . The inverter  434  of the scan path may not appear on a data path. Accordingly, the sequential circuit  400  may be designed such that an operation of the scan path is slower in speed than that of the data path. 
       FIG. 13  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 11 . Referring to  FIG. 13 , a sequential circuit  400 ′ may include a first stage  410 , a second stage  420 ′, a combinational logic  430 , and a latch circuit  440 . Compared with the sequential circuit  400  of  FIG. 11 , the second PMOS transistor P 22 ′ of the sequential circuit  400 ′ of  FIG. 13  may be turned on or off in response to a logic signal LS. 
       FIG. 14  is a diagram illustrating an example scan path in the sequential circuit of  FIG. 13 . Referring to  FIGS. 13 and 14 , when the scan enable signal SE is at a high level, a scan path may be formed. Since the scan path of  FIG. 14  is similar to the scan path of  FIG. 12 , its detailed descriptions will be omitted herein. 
     A sequential circuit according to at least one example embodiment of inventive concepts may be implemented to receive a reset signal. 
       FIG. 15  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts. 
     Referring to  FIG. 15 , a sequential circuit  500  may include a first stage  510 , a second stage  520 , a combinational logic  530 , and a latch circuit  550 . 
     The first stage  510  and the combinational logic  530  may be the same or substantially the same as the first stage  410  and the combinational logic  430 , respectively. 
     The second stage  520  is configured to be different from that of the sequential circuit  400  illustrated in  FIG. 11  so as to receive a reset signal R. 
     The second stage  520  may include a PMOS transistor P 23  connecting the power supply terminal VDD and a common node of both PMOS transistors P 21  and P 22  in response to the reset signal R and an NMOS transistor N 23  connecting the ground terminal GND with the second node NET 2  in response to the reset signal R. 
     The latch circuit  550  may include a latch  542  and an inverter  544 . The latch  542  and the inverter  544  may be the same or substantially the same as the latch  242  and the inverter  244 , respectively. 
       FIG. 16  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 15 . Referring to  FIG. 16 , a sequential circuit  500 ′ may include a first stage  510 , a second stage  520 ′, a combinational logic  530 , and a latch circuit  540 . Compared with the sequential circuit  500  of  FIG. 15 , the second PMOS transistor P 22 ′ of the sequential circuit  500 ′ of  FIG. 16  may be turned on or off in response to a logic signal LS. 
     A sequential circuit according to at least one example embodiment of inventive concepts may be implemented with a structure in which an AOI gate illustrated in  FIGS. 3 to 16  is transformed into various forms. 
       FIG. 17  is a circuit diagram illustrating an example embodiment of a sequential circuit in which an AOI gate of the sequential circuit  400  illustrated in  FIG. 11  is transformed. 
     Referring to  FIG. 17 , a sequential circuit  600  may include a PMOS transistor P 34  having a different connection from the sequential circuit  400  illustrated in  FIG. 11 . The PMOS transistor P 34  may connect the power supply terminal VDD to the first connection node CN 1  in response to a voltage of the second node NET 2 . 
       FIG. 18  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 17 . Compared with the sequential circuit  600  of  FIG. 17 , the second PMOS transistor P 22 ′ of the sequential circuit  600 ′ of  FIG. 18  may be turned on or off in response to a logic signal LS. 
       FIG. 19  is a circuit diagram illustrating another example embodiment of a sequential circuit in which an AOI gate of the sequential circuit  400  illustrated in  FIG. 11  is transformed. 
     Referring to  FIG. 19 , a sequential circuit  700  may include an NMOS transistor N 31  having a different connection from the sequential circuit  400  illustrated in  FIG. 11 . The NMOS transistor N 31  may connect the first connection node CN 1  to the second connection node CN 2  in response to an inverted voltage/NET 1  of the first node NET 1 . 
       FIG. 20  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 19 . Compared with the sequential circuit  700  of  FIG. 19 , the second PMOS transistor P 22 ′ of the sequential circuit  700 ′ of  FIG. 20  may be turned on or off in response to a logic signal LS. 
       FIG. 21  is a circuit diagram illustrating another example embodiment of a sequential circuit in will an AOI gate of the sequential circuit  400  illustrated in  FIG. 11  is transformed. 
     Referring to  FIG. 21 , the sequential circuit  800  may include a PMOS transistor P 34  and an NMOS transistor N 31  having a different connection from the sequential circuit  400  illustrated in  FIG. 11 . The PMOS transistor P 34  may connect the power supply terminal VDD to the first connection node CN 1  in response to a voltage of the second node NET 2 . The NMOS transistor N 31  may connect the first connection node CN 1  to the second connection node CN 2  in response to an inverted voltage/NET 1  of the first node NET 1 . 
       FIG. 22  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 21 . Compared with the sequential circuit  800  of  FIG. 21 , the second PMOS transistor P 22 ′ of the sequential circuit  800 ′ of  FIG. 22  may be turned on or off in response to a logic signal LS. 
     Example embodiments of inventive concepts are described with regard to a discharge path of the first stage in sequential circuits illustrated in  FIGS. 1 to 22  using a portion of combinational logic. However, the scope and spirit of inventive concepts should not be limited thereto. For example, the sequential circuit according to at least some example embodiments of inventive concepts may include the discharge path in the first stage regardless of the combinational logic. 
       FIG. 23  is a diagram illustrating a sequential circuit according to another example embodiment of inventive concepts. 
     Referring to  FIG. 23 , the sequential circuit  900  may include a first stage  910 , a second stage  920 , a combinational logic  930  and a latch circuit  940 . The second stage  920 , the combinational logic  930  and the latch circuit  940  may be the same or substantially the same as the second stage  120 , the combinational logic  130  and the latch circuit  140 , respectively. 
     The first stage  910  of the sequential circuit  900  may be similar to the first stage  110  in  FIG. 1 , but may further include a discharge circuit  192  connected between the first connection node CN 1  and the ground terminal GND. A discharge operation about the first node NET 1  of a first stage  910  may be performed through the discharge circuit  192 , which is not associated with a combinational logic  930 . 
     As illustrated in  FIG. 23 , the discharge circuit  192  may include: an NMOS transistor N 15 , which connects one end of an NMOS transistor N 17  to the first connection node CN 1  in response to the data an NMOS transistor N 16  connected to the NMOS transistor N 15  in parallel; and the NMOS transistor N 17  connecting the ground terminal GND to a common node of both the NMOS transistors N 15  and N 16  in response to a voltage of the second node NET 2 . In at least one example embodiment, a gate terminal of the NMOS transistor N 16  may be connected to an output terminal of the combinational logic  930 . The discharge circuit illustrated in  FIG. 23  is an example, and the scope and spirit of inventive concepts should not be limited thereto. 
       FIG. 24  is a circuit diagram illustrating another example embodiment of sequential circuit of  FIG. 23 . Referring to  FIG. 24 , a sequential circuit  900 ′ may include a first stage  910 , a second stage  920 ′, a combinational logic  930 , and a latch circuit  940 . Compared with the sequential circuit  900  of  FIG. 23 , the second PMOS transistor P 22 ′ of the sequential circuit  900 ′ of  FIG. 24  may be turned on or off in response to a logic signal LS. 
       FIG. 25  is a flowchart illustrating an operating method of a sequential circuit according to an example embodiment of inventive concepts. Below, an operating method of a sequential circuit will be described with reference to  FIGS. 1 to 25 . 
     Referring to  FIG. 25 , a voltage of the first node NET 1  and/or a voltage of the second node NET 2  may be charged with a power supply voltage in response to the clock CLK, the data or the logic signal LS (S 110 ). The voltage of the first node NET 1  may be discharged in response to the clock CLK (S 120 ). The voltage of the second node NET 2  may be discharged in response to the data D, the clock CLK, and the logic signal LS. Here, the logic signal LS may be generated from the combinational logic described with reference to  FIGS. 1 to 24 . The combinational logic may generate the logic signal LS such that the voltage of the first node NET 1  is not discharged when the voltage of the second node NET 2  is discharged or such that the voltage of the second node NET 2  is not discharged when the voltage of the first node NET 1  is discharged. 
     Afterwards, data corresponding to whether the voltage of the second node NET 2  is discharged may be latched, and the sequential circuit may output the latched data as a result value (e.g., Q or /Q). In at least one example embodiment, the latched data may be able to be inverted. 
     Moreover, the sequential circuit may reset the voltage of the first node NET 1  or the voltage of the second node NET 2  in response to the reset signal R. 
     Meanwhile, a latch operation of data is described with reference to  FIG. 25 . However, the sequential circuit according to at least one example embodiment of inventive concepts may form a scan path in response to the scan enable signal SE, thereby making it possible to latch the scan input SI inputted thereto. 
     In an operating method of the sequential circuit described with reference to  FIG. 25 , an illustrated order or the number of steps or operations should not limit the scope and spirit of inventive concepts. For example, steps S 120  and S 130  may be performed in order or in parallel. 
       FIG. 26  is a timing diagram illustrating example operating timing of a sequential circuit according to an example embodiment of inventive concepts. Below, an operation of a sequential circuit will be described with reference to  FIGS. 25 and 26 . 
     As shown in  FIG. 26 , the combinational logic may control discharging of a first node at a first stage and a second node at a second stage such that the first node is discharged during a first time interval and the second node is discharged during a second time interval, wherein the first and second time intervals are different and non-overlapping time intervals. 
     Referring to  FIGS. 25 and 26 , during interval T 1  (at S 110 ), a voltage of the second node NET 2  may be charged in response to the clock CLK of a low level, the data D of a low level, and the logic signal LS of a high level. In the case, a voltage of the first node NET 1  may be maintained at a high level. During interval T 2  (at S 130 ), the voltage of the second node NET 2  may be discharged in response to the clock CLK having a high level, the data D having a low level, and the logic signal LS having a high level. During interval T 3  (at S 110 ), when it is assumed that the data D is transitioned, the logic signal LS may be transitioned from a high level to a low level. At this time, the voltage of the second node NET 2  may be charged in response to the clock CLK having a low level, the data D of a low level before a transition, and the logic signal LS of a high level before a transition. Moreover, the voltage of the first node NET 1  may be maintained continuously or substantially continuously at a high level. During interval T 4  (at S 130 ), a transition of the data D may be completed. The voltage of the first node NET 1  may be discharged in response to the clock CLK having a high level, the data having a high level, and the logic signal LS having a low level. At this time, the voltage of the second node NET 2  may be maintained at a high level. In the case, an output terminal Q may output a voltage, which corresponds to the voltage of the second node NET 2 , having a high level. During interval T 5  (at S 110 ), the voltage of the first node NET 1  may be charged in response to the clock CLK having a low level, the data D having a high level, and the logic signal LS having a low level. At this time, the voltage of the second node NET 2  may be maintained at a high level, and the output terminal Q may be maintained at a high level. The timing illustrated in  FIG. 26  is an example, and the scope and spirit of inventive concepts should not be limited thereto. 
     In example embodiments, during the intervals T 3  (at S 110 ) to T 5  (at S 110 ), the charged second node NET 2  voltage may be maintained at a high level in response to the low level of the logic signal LS. 
     Sequential circuits according to at least some example embodiments of inventive concepts may be applied to at least one configuration of a solid state drive (SSD). 
       FIG. 27  is a block diagram illustrating a SSD according to an example embodiment of inventive concepts. 
     Referring to  FIG. 27 , a SSD  1000  may include a plurality of nonvolatile memories  1100  and a SSD controller  1200 . 
     The nonvolatile memories  1100  may be implemented to optionally receive an external high-voltage Vppx. The nonvolatile memories  1100  may include a plurality of nonvolatile memory packages connected to a plurality of channels CH 1  to Chi, respectively. In this example, ‘i’ is an integer of two or more. 
     The SSD controller  1200  may be connected to the nonvolatile memories  1100  through the channels CH 1  to Chi, respectively. The SSD controller  1200  may include at least one processor  1210 , a buffer memory  1220 , an error correction code (FCC) circuit  1230 , a host interface  1240 , and a nonvolatile memory interface  1250 . 
     The processor  1210  may be implemented to process an overall operation of the SSD controller  1200 . The processor  1210  may include a plurality of sequential circuits FF 1  and FF 2 . Here, each of the sequential circuits FF 1  and FF 2  may be implemented with a sequential circuit described with reference to  FIGS. 1 and 26 . An example embodiment of inventive concepts is exemplified as the number of the sequential circuits FF 1  and FF 2  serially connected is two. However, the scope and spirit of inventive concepts should not be limited thereto. 
     The buffer memory  1220  may temporarily store data used in an operation of the SSD controller  1200 . The buffer memory  1220  may include a plurality of memory lines, which stores data or a command. Here, a plurality of memory lines may be mapped to cache lines using various methods. At least some example embodiments of inventive concepts are exemplified as the buffer memory  1220  in  FIG. 27  is arranged in the SSD controller  1200 . However, the scope and spirit of inventive concepts may not be limited thereto. For example, the buffer memory  1220  according to an example embodiment of inventive concepts may be placed outside the SSD controller  1200 . 
     The ECC circuit  1230  may calculate a value of an error correction code of data to be programmed during a write operation, may correct data, which is read during a reading operation, based on the value of the error correction code, and/or may correct an error repaired from the nonvolatile memories  1100 . An example embodiment of inventive concepts is exemplified with regard to the ECC circuit  1230  detecting and correcting an error by a sector. However, the scope and spirit of inventive concepts may not be limited thereto. 
     The ECC circuit  1230  may generate a fail bit in data received from the nonvolatile memories  1100  or an error correction code (ECC) for correcting an error bit. The ECC circuit  1230  may perform an error correction encoding provided to the nonvolatile memories  1100  and may generate data to which a parity bit is added. The parity bit may be stored in the nonvolatile memories  1100 . Moreover, the ECC circuit  1230  may perform error correction decoding with respect to data outputted from the nonvolatile memories  1100 . The ECC circuit  1230  may correct an error using a parity bit. Although not illustrated, a code memory, which stores code data for an operation of the SSD controller  1200 , may be further included in the SSD controller  1200 . The code memory may be implemented with a nonvolatile memory. 
     The host interface  1240  may provide an interface function for interfacing with an external device. The host interface  1240  may be connected to an external host through a parallel advanced technology attachment (PATA) bus, a serial advanced technology attachment (SATA) bus, a small computer system interface (SCSI), a universal serial bus (USB), a Peripheral Component Interconnect Express (PCIe), a secure digital (SD), a serial attached SCSI (SAS), a universal flash storage (UFS), an embedded multimedia card (eMMC), a multimedia card (MMC), a NAND interface, or the like. The nonvolatile memory interface  1250  may provide an interface function between the SSD controller  1200  and the nonvolatile memories  1100 . Although not illustrated, the SSD controller  1200  may install a wireless communication function (e.g., WiFi). 
     As high-speed sequential circuits FF 1  and FF 2  are applied to the processor  1210 , the SSD  1000  according to at least one example embodiment of inventive concepts may include the processor  1210 . Accordingly, the SSD  1000  may operate at a relatively high clock frequency. 
       FIG. 28  is a block diagram illustrating an electronic device  2000  according to an example embodiment of inventive concepts. 
     Referring to  FIG. 28 , an electronic device  2000  may include at least one processor  2100 , a buffer memory  2200 , a code memory  2300 , a nonvolatile memory interface  2600 , a nonvolatile memory device  2700 , and a host interface  2800 . The electronic device  2000  may be one of the following devices or a combination of two or more thereof: a data storage medium (e.g., solid state drive (SSD), a memory stick, a universal flash storage (UFS) device), a memory card (e.g., a secure digital (SD), a multimedia card (MMC), an embedded MMC (eMMC), or the like), a smart card, a mobile device (e.g., a smartphone and Galaxy™ series), a smartphone, a tablet personal computer (PC), a mobile phone, a video phone, an e-book reader, a desktop PC, a laptop PC, a netbook computer, a personal digital assistant (PDA), a portable multimedia player (PMP), a MP3 player, a mobile medical device, an electronic bracelet, an electronic necklace, an electronic appcessory, a camera, a wearable device, an electronic clock, a wrist watch, a home appliance (e.g., a refrigerator, an air conditioner, a vacuum cleaner, an oven, an microwave oven, a washing machine, an air cleaner, or the like), an artificial intelligence robot, a television (TV), a digital video disk (DVD) player, an audio system, kinds of medical devices (e.g., a magnetic resonance angiography (MRA) camera, a magnetic resonance imaging (MRI) camera, a computed tomography (CT) camera, a ultrasonic machine, or the like), a navigation device, a global positioning system (GPS) receiver, an event data recorder (EDR), a flight data recorder (FDR), a set-top box, a TV box (e.g., Samsung HomeSync™, AppleTV™, or googleTV™), an electronic dictionary, a car infotainment device, an electronic equipment for ship (e.g., a navigation system for ship, a gyrocompass, or the like), an avionics system, a security device, electronic clothes, an electronic key, a camcorder, a game console, a head-mounted display (HMD), a flat panel display device, an electronic picture frame, an electronic album, furniture or a portion of building or structure, which is includes a communication function, an electronic board, an electronic signature receiving device, or a projector. 
     The processor  2100  may be implemented to control an overall operation about the electronic device  2000 . Here, the processor  2100  may be a central processing unit, an application processor, a graphic processor, or the like. In at least one example embodiment, the processor  2100  may include a secure processor or a secure element (SE), and the like. For example, the processor  2100  may have a tamper-resistant function to be protected from a tempering attack such as a micro-probing, a software attack, an eavesdropping, a fault generation, or the like. In at least one example embodiment, the processor  2100  may include at least two sequential circuits FF 1  and FF 2 , which are connected serially. Here, each of the sequential circuits FF 1  and FF 2  may be implemented with the sequential circuit described with reference to  FIGS. 1 and 25 . In at least one example embodiment, each of the sequential circuits FF 1  and FF 2  may be implemented with the same or substantially the same type of sequential circuit. In at least one other example embodiment, each of the sequential circuits FF 1  and FF 2  may be implemented with different types of sequential circuits. 
     The buffer memory  2200  may operate according to control of the processor  2100 . The buffer memory  2200  may temporarily store data which is processed by the processor  2100  or may buffer data, which is transmitted to the nonvolatile memory device  2700 , or data which is transmitted from the nonvolatile memory device  2700 . In at least one example embodiment, the buffer memory  2200  may be a random access memory (RAM), a static random access memory (SRAM), and a phase-change random access memory (PRAM). 
     The code memory  2300  may be implemented to store a code and/or an application for managing or operating the electronic device  2000 . In at least one example embodiment, the code memory  2300  may be a read only memory (ROM) or a PRAM. Data exchange with the nonvolatile memory device  2700  may be performed through the nonvolatile memory interface  2600 . The host interface  2800  may be connected to an external host through a parallel advanced technology attachment (PATA) bus, a serial advanced technology attachment (SATA) bus, a small computer system interface (SCSI), a universal serial bus (USB), a Peripheral Component Interconnect Express (PCIe), SD, a serial attached SCSI (SAS), a UFS, an eMMC, an MMC, a NAND interface, or the like. 
     Although not illustrated, the electronic device  2000  may install a wireless communication function (e.g., Wi-Fi). Meanwhile, the electronic device  2000  may further include a component, which is not illustrated in  FIG. 28 , or may not include at least one (except a cryptographic processing circuit) of the components illustrated in  FIG. 28 . 
       FIG. 29  is a block diagram illustrating a mobile device  3000  according to an example embodiment of inventive concepts. 
     Referring to  FIG. 29 , a mobile device  3000  may include a processor (AP/ModAP)  3100 , a storage device  3200 , a display/touch module  3300 , and a buffer memory  3400 . 
     The processor  3100  may be implemented to control an overall operation of the mobile device  3000  and a wired/wireless communication with an external device. For example, the processor  3100  may be an application processor (AP), an integrated modem application processor (hereinafter referred to as “ModAP”), or the like. The processor  3100  may include at least two sequential circuits FF 1  and FF 2 , which are described with reference to  FIGS. 1 to 24 . 
     In at least one example embodiment, sequential circuits FF 1  and FF 2  of the processor  3100  may be implemented with a sequential circuit described with reference to  FIGS. 1 and 24 . In at least one other example embodiment, the processor  3100  may include the first sequential circuit FF 1  and the second sequential circuit FF 2 , which are serially connected to perform a scan test in response to a scan signal. Here, each of the first sequential circuit FF 1  and the second sequential circuit FF 2  may be implemented with a sequential circuit receiving the scan input SI and the scan enable signal SE which are described with reference to  FIGS. 7 to 22 . 
     The buffer memory  3400  may be implemented to temporarily store data, which is needed when the mobile device  3000  performs a process operation. The display/touch module  3300  may be implemented to display data processed from the processor  3100  or to receive data from a touch panel. The storage device  3200  may be implemented so as to store data of a user. The storage device  3200  may be an eMMC, an SSD, a UFS, or the like. 
     Sequential circuits and/or operating methods thereof according to one or more example embodiments of inventive concepts may suppress and/or minimize influence due to a clock change by complementarily determining whether voltages of nodes are discharged in response to a logic signal and a clock according to voltages of complementary nodes and a state of data. 
     Sequential circuits and/or operating methods thereof according to one or more example embodiments of inventive concepts may increase a discharge speed by being implemented with NMOS transistors of a 2-stack structure. Accordingly, a latch operation may be performed at a relatively high speed. 
     Sequential circuits and/or operating methods thereof according to one or more example embodiments of inventive concepts may enable increases in a lock frequency by reducing a setup time or a CQ delay. 
     Therefore, it should be understood that the example embodiments discussed herein are not limiting, but illustrative. While inventive concepts have been described with reference to example embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of inventive concepts.