Patent Publication Number: US-11658656-B2

Title: Low power clock gating cell and an integrated circuit including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2020-0161569, filed on Nov. 26, 2020, Korean Patent Application No. 10-2020-0166964, filed on Dec. 2, 2020, Korean Patent Application No. 10-2021-0048027, filed on Apr. 13, 2021, and Korean Patent Application No. 10-2021-0106184, filed on Aug. 11, 2021 in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
     TECHNICAL FIELD 
     The inventive concept relates to a clock gating cell, and more particularly, to a clock gating cell with low power consumption and an integrated circuit including the same. 
     DISCUSSION OF RELATED ART 
     In a clock synchronization system including a clock network, a clock gating cell may be used as a digital circuit for supplying a clock signal. Because a clock gating cell selectively outputs a clock signal in response to a control signal, the system&#39;s power efficiency may be increased through an operation of the clock gating cell. 
     However, a clock gating cell still consumes power due to a clock signal even when no clock signal is output. 
     SUMMARY 
     The inventive concept provides a clock gating cell that includes a pair of transistors and receives a clock signal for reduced power consumption and an integrated circuit including the same. 
     According to an example embodiment of the inventive concept, there is provided a clock gating cell including: a first circuit configured to receive an enable signal and an inverted output clock signal and generate a first signal through a first node; a second circuit configured to receive the first signal and generate an inverted first signal; a third circuit configured to receive the first signal, the inverted first signal, and an input clock signal, generate the first signal by being connected to the first circuit through the first node, and generate the inverted output clock signal through a second node; and a fourth circuit configured to receive the first signal, generate the inverted output clock signal by being connected to the third circuit through the second node, and generate the output clock signal, wherein the third circuit includes a pair of transistors receiving the input clock signal. 
     According to an example embodiment of the inventive concept, there is provided an integrated circuit including a clock gating cell that receives at least one control signal and an input clock signal and generates an output clock signal, wherein the clock gating cell includes: a first circuit including a first function circuit configured to generate a first signal through a first node based on the at least one control signal and a first keeper circuit configured to maintain a voltage level of the first signal; a second circuit configured to receive the first signal and generate an inverted first signal; a third circuit including a second function circuit configured to generate the first signal through the first node based on the at least one control signal, a second keeper circuit configured to maintain a voltage level of the first signal, a first charge circuit configured to generate an inverted output clock signal by charging a second node based on the first signal, and a discharge circuit configured to discharge the second node; and a fourth circuit including a second charge circuit configured to generate the inverted output clock signal by charging the second node based on the first signal, and the third circuit includes a pair of transistors receiving the input clock signal. 
     According to an example embodiment of the inventive concept, there is provided a latch circuit including: a first circuit configured to receive an inverted output clock signal and a data signal and generate a first signal through a first node; a second circuit configured to receive the data signal, an input clock signal, and an output signal, generate the first signal by being connected to the first circuit through the first node, and generate the inverted output clock signal through a second node; a third circuit configured to generate the inverted output clock signal by being connected to the second circuit through the second node; and a fourth circuit configured to receive the first signal and generate an output signal and an inverted output signal, wherein the second circuit includes a pair of transistors receiving the input clock signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the inventive concept will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG.  1    is a block diagram showing an integrated circuit (IC) according to an example embodiment of the inventive concept; 
         FIG.  2    is a logic diagram of a clock gating cell according to an example embodiment of the inventive concept; 
         FIGS.  3 A and  3 B  are circuit diagrams of clock gating cells according to example embodiments of the inventive concept; 
         FIG.  4    is a circuit diagram of a clock gating cell according to an example embodiment of the inventive concept; 
         FIG.  5    is a diagram showing an operation of a clock gating cell according to an example embodiment of the inventive concept; 
         FIG.  6    is a timing diagram for describing an operation of a clock gating cell according to an example embodiment of the inventive concept; 
         FIGS.  7 ,  8 ,  9     10 ,  11 A, and  11 B are circuit diagrams of clock gating cells according to example embodiments of the inventive concept; 
         FIG.  12    is a diagram showing a logic symbol of a latch circuit according to an example embodiment of the inventive concept; 
         FIG.  13    is a block diagram showing a latch circuit according to an embodiment of the inventive concept; 
         FIGS.  14  and  15    are circuit diagrams of latch circuits according to example embodiments of the inventive concept; 
         FIGS.  16 A and  16 B  are diagrams schematically showing partial layouts of a clock gating cell and a latch circuit according to example embodiments of the inventive concept. 
         FIG.  17    is a diagram showing an IC including a clock gating cell according to an embodiment of the inventive concept; and 
         FIG.  18    is a flowchart of a method of manufacturing an IC, according to an example embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG.  1    is a diagram showing an integrated circuit (IC) according to an example embodiment of the inventive concept. 
     Referring to  FIG.  1   , a clock gating cell  10  may be included in an IC  1  manufactured through a semiconductor process. The clock gating cell  10  may also be referred to as a clock gating circuit or an integrated clock gating (ICG) cell. In the present specification, the term “clock gating” may refer to suspending or resuming an operation of a digital circuit by selectively providing an output clock signal to a functional block. 
     The clock gating cell  10  may receive an input clock signal CLK and an enable signal E and generate an output clock signal ECK. The clock gating cell  10  may be set to an enable state or a disable state according to the enable signal E. For example, the clock gating cell  10  may be set to the disable state when the enable signal E is at a first logic level (e.g., a logic low level). The clock gating cell  10  in the disable state may generate the output clock signal ECK of a constant level. 
     For example, the clock gating cell  10  may be set to the enable state when the enable signal E is at a second logic level (e.g., a logic high level). The clock gating cell  10  in the enable state may generate the output clock signal ECK that is toggled according to the input clock signal CLK. 
     The clock gating cell  10  may generate the output clock signal ECK and supply the output clock signal ECK to a functional block  15 . The functional block  15  may refer to a digital circuit that operates in response to a negative edge or a positive edge of the output clock signal ECK. For example, the functional block  15  may be a positive edge triggered flipflop or a negative edge triggered flipflop. 
     The clock gating cell  10  may include a circuit element for directly receiving the input clock signal CLK. Although  FIG.  1    shows one circuit element, e.g., A, a plurality of circuit elements may be provided. As the number of circuit elements for receiving an input clock signal increases, the capacitance due to the input clock signal CLK increases, and thus the self-power consumption of the clock gating cell  10  may increase. 
     The clock gating cell  10  according to the present embodiment may reduce the capacitance due to the input clock signal CLX by reducing the number of circuit elements that directly receive the input clock signal CLK. Therefore, the power consumption of the clock gating cell  10  caused by the input clock signal CLK may be reduced, and thus, a low power clock gating cell may be implemented. 
       FIG.  2    is a logic diagram of a clock gating cell according to an example embodiment of the inventive concept. 
     Referring to  FIG.  2   , the clock gating cell  10  may include a first circuit  11 , a first inverter  12 , and a second circuit  13 . The logic diagram of  FIG.  2    may be implemented in various circuit configurations. 
     The first circuit  11  may receive an enable signal E, an input clock signal CLK, a first signal SIG 1 , an inverted first signal SIG 1 N, and an inverted output clock signal ECKN and may be connected to an intermediate node N 11 . For example, the first signal SIG 1  may be generated from the intermediate node N 11 . In other words, the first signal SIG 1  may be a signal at the intermediate node N 11 . 
     For example, the first circuit  11  may include a first OR gate  111  that receives the input clock signal CLK and the first signal SIG 1  and generates a first result signal R 1 , a first AND gate  112  that receives the inverted first signal SIG 1 N and the first result signal R 1  and generates a second result signal R 2 , a second inverter  113  that generates an inverted enable signal EN by inverting the enable signal E, a second OR gate  114  that receives the inverted enable signal EN of the second inverter  113  and the second result signal R 2  and generates a third result signal R 3 , and a first NAND gate  115  that receives the third result signal R 3  and the inverted output clock signal ECKN and generates the first signal SIG 1 . The first inverter  12  may generate the inverted first signal SIG 1 N by inverting the first signal SIG 1 . 
     The second circuit  13  may receive the inverted enable signal EN, the inverted first signal SIG 1 N, the input clock signal CLK, and the first signal SIG 1  and generate the output clock signal ECK. 
     For example, the second circuit  13  may include a third OR gate  131  that receives the inverted enable signal EN and the inverted output clock signal ECKN and generates a fourth result signal R 4 , a second AND gate  132  that receives the inverted first signal SIG 1 N and the fourth result signal R 4  and generates a fifth result signal R 5 , a fourth OR gate  133  that receives the input clock signal CLK and the fifth result signal R 5  and generates a sixth result signal R 6 , a second NAND gate  134  that receives the first signal SIG 1  and the sixth result signal R 6  and generates the inverted output clock signal ECKN, and a third inverter  135  that generates the output clock signal ECK by inverting the inverted output clock signal ECKN. 
     In  FIG.  2   , gates that directly receive the input clock signal CLK in the clock gating cell  10  are the first OR gate  111  of the first circuit  11  and the fourth OR gate  133  of the second circuit  13 . The logic diagram of  FIG.  2    is not limited to any circuit diagram, but, in the case of a configuration in which the input clock signal CLK is input to two gates, power consumption of the clock gating cell  10  may increase due to the input clock signal CLK. 
     According to the present embodiment to be described in more detail below, a plurality of gates for receiving the input clock signal CLK may be merged to implement the clock gating cell  10  including only a pair of transistors (in other words, two transistors) directly receiving the input clock signal CLK. Therefore, power consumption of the clock gating cell  10  may be reduced, and, as the clock gating cell  10  of low power is implemented, power consumption of the IC  1  including the clock gating cell  10  may also be reduced. 
       FIGS.  3 A and  3 B  are circuit diagrams of clock gating cells according to example embodiments of the inventive concept. 
       FIG.  3 A  is a circuit diagram of the clock gating cell  10  of  FIG.  2    according to an example embodiment. The clock gating cell  10  may include first, second, third and fourth blocks C 1 , C 2 , C 3  and C 4 . 
     The first block C 1  may include a first keeper circuit C 11  and a first function circuit C 12 , the second block C 2  may include a first charge circuit C 21 , a second function circuit C 22 , a second keeper circuit C 23 , and a discharge circuit C 24 , and the third block C 3  may include a second charge circuit C 31 . 
     In the present specification, a keeper circuit may refer to a circuit that maintains a voltage level of a particular node at a certain value. A charge circuit may refer to a circuit that charges a voltage level of a particular node to a logic high level, and a discharge circuit may refer to a circuit that discharges a voltage level of a particular node to a logic low level. A function circuit may refer to a circuit that performs a logic operation based on an input signal and operates according to a result of the operation. 
     In addition, the first and second keeper circuits C 11  and C 23 , the first and second function circuits C 12  and C 22 , the first and second charge circuits C 21  and C 31 , and the discharge circuit C 24  may each be implemented with one transistor as described below with reference to  FIG.  4   , but the inventive concept is not limited thereto. 
     The first block C 1  may receive the enable signal E and the inverted output clock signal ECKN and may be connected to a power voltage node VDD, a first node N 1 , and a ground node. As shown in  FIG.  3 A , the first signal SIG 1  may be generated by the first node N 1 . In other words, the first signal SIG 1  may be generated through the first node N 1 . 
     For example, the first block C 1  may include the first keeper circuit C 11 , the first function circuit C 12 , and a first transistor N 11 . 
     The first keeper circuit C 11  may receive the inverted output clock signal ECKN and may be connected to the power voltage node VDD and the first node N 1 . For example, the first keeper circuit C 11  may maintain the first signal SIG 1  of the first node N 1  at a particular logic level according to the inverted output clock signal ECKN. In other words, the first keeper circuit C 11  may maintain the first signal SIG 1  of the first node N 1  at a particular logic level in response to the inverted output clock signal ECKN. 
     The first function circuit C 12  may receive the enable signal E and may be connected to the first node N 1  and a source of the first transistor N 11 . For example, the first function circuit C 12  may generate the inverted enable signal EN by inverting the received enable signal E and control the logic level of the first signal SIG 1  of the first node N 1  according to the inverted enable signal EN. 
     Although  FIG.  2    shows that the first function circuit C 12  receives one enable signal E, the inventive concept is not limited thereto, and the first function circuit C 12  may be connected to a plurality of signal lines and receive a plurality of other signals. For example, the first function circuit C 12  may receive the enable signal E and a scan enable signal (e.g., SE of  FIG.  10   ) instructing a test operating mode of the clock gating cell  10 . 
     The first transistor N 11  may receive the inverted output clock signal ECKN and may be connected to the first function circuit C 12  and the ground node. For example, the first transistor N 11  may be connected between the first function circuit C 12  and the ground node and may be controlled by the inverted output clock signal ECKN applied to its gate. 
     The second block C 2  may receive the inverted enable signal EN, the inverted first signal SIG 1 N, and the input clock signal CLK and may be connected to the power voltage node VDD, the first node N 1 , a second node N 2 , and the ground node. For example, the inverted output clock signal ECKN may be generated by the second node N 2 . 
     For example, the second block C 2  may include the first charge circuit C 21 , the second function circuit C 22 , the second keeper circuit C 23 , second and third transistors P 21  and N 21 , and the discharge circuit C 24 . 
     The first charge circuit C 21  may receive the first signal SIG 1  and may be connected to the power voltage node VDD and a third node N 3 . For example, the first charge circuit C 21  may charge the second node N 2  by turning ON/OFF the second transistor P 21  connected through the third node N 3 . For example, the first charge circuit C 21  may charge the first node N 1  according to a result of an operation of the second function circuit C 22 . Therefore, for example, the logic level of the first signal SIG 1  generated by the first node N 1  may be a logic high level. In  FIG.  3 A , the circle shown at the first charge circuit C 21  may denote an inverter. Therefore, the first charge circuit C 21  may use the inverted first signal SIG 1 N. 
     The second function circuit C 22  may receive the enable signal E and may be connected to the first node N 1  and the third node N 3 . In  FIG.  3 A , the circle shown at the second function circuit C 22  may denote an inverter. Therefore, the second function circuit C 22  may use the inverted enable signal EN. 
     Although  FIG.  3 A  shows that the second function circuit C 22  receives one inverted enable signal EN, the inventive concept is not limited thereto, and the second function circuit C 22  may receive a plurality of other signals. For example, the second function circuit C 22  may receive the inverted enable signal EN and an inverted scan enable signal. In addition, as shown in  FIG.  2   , polarities of respective signals for controlling the first function circuit C 12  and the second function circuit C 22  may be opposite to each other. 
     The second keeper circuit C 23  may receive the inverted first signal SIG 1 N and may be connected to the first node N 1  and a fourth node N 4 . For example, the second keeper circuit C 23  may maintain the first signal SIG 1  of the first node N 1  at a particular logic level according to the inverted first signal SIG 1 N. 
     The second transistor P 21  may receive the input clock signal CLK and may be connected to the second node N 2  and the third node N 3 . For example, the second transistor P 21  may be connected between the second node N 2  and the third node N 3  and may be controlled by the input clock signal CLK applied to its gate. 
     The third transistor N 21  may receive the input clock signal CLK and may be connected to the fourth node N 4  and the ground node. For example, the third transistor N 21  may be connected between the fourth node N 4  and the ground node and may be controlled by the input clock signal CLK applied to its gate. 
     The discharge circuit C 24  may receive the first signal SIG 1  and may be connected to the second node N 2  and the fourth node N 4 . For example, the discharge circuit C 24  may discharge the second node N 2  by turning ON/OFF the third transistor N 21  connected through the fourth node N 4 . Therefore, for example, the logic level of the inverted output clock signal ECKN generated by the second node N 2  may be a logic low level. 
     The third block C 3  may receive the first signal SIG 1  and the inverted output clock signal ECKN, may generate the output clock signal ECK, and may be connected to the second node N 2  and the power voltage node VDD. According to an example embodiment of the inventive concept, the third block C 3  may include the second charge circuit C 31  and an inverter C 32 . 
     The second charge circuit C 31  may receive the first signal SIG 1  and may be connected to the power voltage node VDD and the second node N 2 . For example, the second charge circuit C 31  may charge the second node N 2 . Therefore, for example, the logic level of the inverted output clock signal ECKN generated by the second node N 2  may be a logic high level. 
     The inverter C 32  may generate the output clock signal ECK by inverting the inverted output clock signal ECKN. Here, the inverter C 32  of the third block C 3  may be omitted. In this case, the clock gating cell  10  may generate the inverted output clock signal ECKN instead of the output clock signal ECK. 
     The fourth block C 4  may generate the inverted first signal SIG 1 N by inverting the first signal SIG 1 . The fourth block C 4  may be implemented by an inverting circuit. 
     The first charge circuit C 21  and the second charge circuit C 31  may determine the logic level of the first signal SIG 1  and the logic level of the inverted output clock signal ECKN under the control of the second transistor P 21 . 
     For example, when the logic level of the input clock signal CLK is a logic low level, the second transistor P 21  may be turned on by the input clock signal CLK and the first charge circuit C 21  may determine the logic level of the inverted output clock signal ECKN of the second node N 2 . 
     For example, when the logic level of the input clock signal CLK is a logic high level, the second transistor P 21  may be turned off by the input clock signal CLK and the first charge circuit C 21  is unable to determine the logic level of the inverted output clock signal ECKN of the second node N 2 . In this case, the second charge circuit C 31  may determine the logic level of the inverted output clock signal ECKN of the second node N 2 . 
     For example, when the logic level of the input clock signal CLK is a logic low level, the second transistor P 21  may be turned on by the input clock signal CLK. In this case, based on the ON/OFF state of the second function circuit C 22 , the second charge circuit C 31  may determine the logic level of the first signal SIG 1  of the first node N 1 . 
     In the clock gating cell  10  according to the present embodiment, the first signal SIG 1  may be generated by latching the enable signal E according to the input clock signal CLK. For example, the first signal SIG 1  may be generated from the enable signal E when the logic level of the input clock signal CLK is a logic low level and may maintain a previous output value when the logic level of the input clock signal CLK is a logic high level. 
     In addition, the output clock signal ECK may maintain a logic low level when the logic level of the first signal SIG 1  is a logic low level and may be generated from the input clock signal CLK when the logic level of the first signal SIG 1  is a logic high level. 
     A clock gating cell according to a comparative example may include a latch circuit and a logic gate and, since the latch circuit and the logic gate each receive the input clock signal CLK directly, power consumption due to the input clock signal CLK is large. On the contrary, according to the present embodiment, the clock gating cell  10  may include only a pair of transistors that receive the input clock signal CLK by merging the input clock signal CLK input to a latch circuit and the input clock signal CLK input to a logic gate. Therefore, parasitic capacitance due to the input clock signal CLK may decrease, and power consumption of the clock gating cell  10  may be reduced. 
       FIG.  3 B  may be an alternative embodiment of  FIG.  2   , wherein a clock gating cell  15  may include first, second, third and fourth blocks C 15 , C 25 , C 35  and C 45 . 
     The first block C 15  may include a first keeper circuit C 115  and a first function circuit C 125 , the second block C 25  may include a first charge circuit C 215 , a second function circuit C 225 , a second keeper circuit C 235 , and a discharge circuit C 245 , and the third block C 35  may include a second charge circuit C 315 . 
     Unlike in  FIG.  3 A , signals for controlling the first keeper circuit C 115 , the second keeper circuit C 235 , and the discharge circuit C 245  may not each be limited to any one signal. 
     In addition, a plurality of control signals A 1 , A 2 , and A 3  may be used to control the first function circuit C 125  and the second function circuit C 225 , and any one of the control signals A 1 , A 2 , and A 3  may be the enable signal E. Although  FIG.  3 B  shows three control signals, the number of control signals is not limited thereto. 
     In detail, the first block CIS may include the first function circuit C 125  configured to generate the first signal SIG 1  through the first node N 1  based on at least one of the control signals A 1 , A 2 , and A 3  and the first keeper circuit C 115  configured to maintain the voltage level of the first signal SIG 1 . 
     The second block C 25  may include the second function circuit C 225  configured to generate the first signal SIG 1  through the first node N 1  based on at least one of the control signals A 1 , A 2 , and A 3 , the second keeper circuit C 235  configured to maintain the voltage level of the first signal SIG 1 , the first charge circuit C 215  configured to generate the inverted output clock signal ECKN by charging the second node N 2  based on the first signal SIG 1 , and the discharge circuit ( 245  configured to discharge the second node N 2 . 
     The third block C 35  may include the second charge circuit C 315  configured to generate the inverted output clock signal ECKN by charging the second node N 2  based on the first signal SIG 1 . 
     The fourth block C 45  may be configured to receive the first signal SIG 1  and generate the inverted first signal SIG 1 N. 
       FIG.  4    is a circuit diagram of a clock gating cell according to an example embodiment of the inventive concept. 
     Referring to  FIG.  4   , a clock gating cell  10   a  may include first, second, third and fourth blocks C 1   a , C 2   a , C 3   a  and C 4   a .  FIG.  4    may correspond to an embodiment of  FIG.  3 A , and descriptions identical to those of  FIG.  2    may be omitted below. In addition, in this specification, transistors may have structures that provide complementary transistors (e.g., an n-channel transistor and a p-channel transistor), and, as non-limiting examples, may be implemented as planar field effect transistors (FETs), fin field effect transistors (FinFETs), gate all around field effect transistors (GAAFETs), vertical field effect transistors (VFETs), etc. 
     The first block C 1   a  may include an inverter I 11   a , a first PFET transistor P 11   a , and first and second NFET transistors N 11   a  and N 12   a.    
     The inverter I 11   a  may generate the inverted enable signal EN by inverting the enable signal E. The first PFET transistor P 11   a  may be connected between the power voltage node VDD and a first node N 1   a  and may be controlled by the inverted output clock signal ECKN applied to its gate. The first and second NFET transistors N 11   a  and N 12   a  may be connected in series between the first node N 1   a  and the ground node and may be controlled by the inverted enable signal EN and the inverted output clock signal ECKN, respectively. For example, the inverted enable signal EN may be applied to the gate of the first NFET transistor N 11   a , and the inverted output clock signal ECKN may be applied to the gate of the second NFET transistor N 12   a.    
     The second block C 2   a  may include third, fourth and fifth PFET transistors P 21   a , P 22   a , and P 23   a  and third, fourth and fifth NFET transistors N 21   a , N 22   a , and N 23   a.    
     The third PFET transistor P 21   a  may be connected between the power voltage node VDD and a third node N 3   a  and may be controlled by the inverted first signal SIG 1 N applied to its gate. The fourth PFET transistor P 22   a  may be connected between the third node N 3   a  and the first node N 1   a  and may be controlled by the inverted enable signal EN applied to its gate. The fifth PFET transistor P 23   a  may be connected between the third node N 3   a  and a second node N 2   a  and may be controlled by the input clock signal CLK applied to its gate. 
     The third NFET transistor N 21   a  may be connected between the first node N 1   a  and a fourth node N 4   a  and may be controlled by the inverted first signal SIG 1 N applied to its gate. The fourth NFET transistor N 22   a  may be connected between the second node N 2   a  and the fourth node N 4   a  and may be controlled by the first signal SIG 1  applied to its gate. The fifth NFET transistor N 23   a  may be connected between the fourth node N 4   a  and the ground node and may be controlled by the input clock signal CLK applied to its gate. 
     The third block C 3   a  may include a sixth PFET transistor P 31   a  and an inverter I 31   a . The sixth PFET transistor P 31   a  may be connected between the power voltage node VDD and the second node N 2   a  and may be controlled by the first signal SIG 1  applied to its gate. The inverter I 31   a  may generate the output clock signal ECK by inverting the inverted output clock signal ECKN. 
     The fourth block C 4   a  may include an inverter I 41   a . The inverter I 41   a  may generate the inverted first signal SIG 1 N by inverting the first signal SIG 1 . 
     Since the clock gating cell  10   a  according to the present embodiment includes only one pair of transistors directly receiving the input clock signal CLK, the clock gating cell  10   a  may have a simplified layout. In other words, connections to clock pins through gate lines may be minimized. 
     For example, the layout of the clock gating cell  10   a  may include only one pair of gate lines to which the input clock signal CLK is applied, and signals other than the input clock signal CLK may be input to gate lines of transistors adjacent to the fifth PFET transistor P 23   a  and the fifth NFET transistor N 23   a.    
     In addition, in  FIG.  4   , a source and a drain of the fifth PFET transistor P 23   a  receiving the input clock signal CLK may be connected to a node inside a circuit instead of the power voltage node VDD or the ground node. A source of the fifth NFET transistor N 23   a  may also be connected to a node inside a circuit. 
       FIG.  5    is a diagram showing an operation of a clock gating cell according to an example embodiment of the inventive concept. 
     Referring to  FIG.  5   , the fifth PFET transistor P 23   a  and the fifth NFET transistor N 23   a  that receive the input clock signal CLK may determine logic levels of the first signal SIG 1  generated by the first node N 1   a  and the inverted output clock signal ECKN generated by the second node N 2   a . For example, the fifth PFET transistor P 23   a  may control a charging path of the first node N 1   a  and the second node N 2   a  based on the input clock signal CLK, and the fifth NFET transistor N 23   a  may control a discharging path of the first node N 1   a  and the second node N 2   a  based on the input clock signal CLK. 
     The fifth PFET transistor P 23   a  may determine the logic level of the first signal SIG 1  generated by the first node N 1   a  through a first route R 1 . For example, the first route R 1  may represent a charging path of the first signal SIG. 
     In addition, the fifth PFET transistor P 23   a  may determine the logic level of the inverted output clock signal ECKN generated by the second node N 2   a.    
     The fifth NFET transistor N 23   a  may determine the logic level of the first signal SIG 1  generated by the first node N 1   a  through a second route R 2 . For example, the second route R 2  may represent a discharging path of the first signal SIG 1 . 
     In addition, the fifth PFET transistor P 23   a  may determine the logic level of the inverted output clock signal ECKN generated by the second node N 2   a  through a third route R 3 . For example, the third route R 3  may represent a discharging path of the inverted output clock signal ECKN. 
     As described above, since one transistor controls the charging or discharging of two signals, the number of circuit elements directly receiving an input clock signal may be reduced. In other words, the number of clock pins and the clock pin capacitance may be reduced. 
       FIG.  6    is a timing diagram for describing an operation of a clock gating cell according to an example embodiment of the inventive concept. 
     Referring to  FIG.  6   , signals according to the lapse of time in the clock gating cell  10   a  of  FIG.  4    are shown. In the timing diagram of  FIG.  6   , a propagation delay may be ignored, and the input clock signal CLK may oscillate according to a period T. 
     At a time point t 1 , the input clock signal CLK may transit from a logic low level to a logic high level. The enable signal E may maintain a logic low level, and thus the first signal SIG 1  may maintain a logic low level, the inverted output clock signal ECKN may maintain a logic high level, and the output clock signal ECK may maintain a logic low level. 
     At a time point t 2 , the enable signal E may transit from the logic low level to a logic high level. The input clock signal CLK may maintain the logic high level, and thus the first signal SIG 1  may maintain the logic low level, the inverted output clock signal ECKN may maintain the logic high level, and the output clock signal ECK may maintain the logic low level. 
     At a time point t 3 , the input clock signal CLK may transit from the logic high level to the logic low level. The enable signal E may maintain the logic high level, and the first signal SIG 1  may transit from the logic low level to a logic high level according to the enable signal E. The inverted output clock signal ECKN may maintain the logic high level, and the output clock signal ECK may maintain the logic low level. 
     At a time point t 4 , the input clock signal CLK may transit from the logic low level to the logic high level. The enable signal E may maintain the logic high level, and the first signal SIG 1  may also maintain the logic high level. The inverted output clock signal ECKN may transit from the logic high level to a logic low level, and thus the output clock signal ECK may transit from the logic low level to a logic high level. 
     At a time point t 5 , the input clock signal CLK may maintain the logic high level, and the enable signal E may transit from the logic high level to the logic low level. In addition, the first signal SIG 1  may maintain the logic high level, the inverted output clock signal ECKN may maintain the logic low level and the output clock signal ECK may maintain the logic high level. 
     At a time point t 6 , when the input clock signal CLK transits from the logic high level to the logic low level, the first signal SIG 1  may transit from the logic high level to the logic low level. In addition, the inverted output clock signal ECKN may transit from the logic low level to the logic high level, and thus the output clock signal ECK may transit from the logic high level to the logic low level. 
     At a time point t 7  after one period T, the input clock signal CLK may transit from the logic high level to the logic low level. The enable signal E may maintain a logic low level, and thus the first signal SIG 1  may maintain a logic low level, the inverted output clock signal ECKN may maintain a logic high level, and the output clock signal ECK may maintain a logic low level. 
     At a time point t 8 , the input clock signal CLK may maintain the logic low level, and the enable signal E may transit from the logic low level to the logic high level. Therefore, the first signal SIG 1  may transit from the logic low level to the logic high level, the inverted output clock signal ECKN may maintain the logic high level, and the output clock signal ECK may maintain the logic low level. 
     At a time point t 9 , the input clock signal CLK may transit from the logic low level to the logic high level. The enable signal E may maintain the logic high level, and the first signal SIG 1  may maintain the logic high level. The inverted output clock signal ECKN may transit from the logic high level to a logic low level, and thus the output clock signal ECK may transit from the logic low level to a logic high level. 
     Similar to the clock gating cell according to the comparative example, in the clock gating cells  10 ,  10   a  according to the present embodiment, the first signal SIG 1 , which is generated by latching the enable signal E according to the input clock signal CLK, may be generated, and an operation for generating the output clock signal ECK may be performed based on the first signal SIG 1 . 
     While the clock gating cell according to the comparative example includes at least two pairs of transistors for receiving the input clock signal CLK to perform the above-stated operations, the clock gating cells  10 ,  10   a ,  10   b ,  10   c ,  10   d , and  10   e  according to example embodiments of the inventive concept include only one pair of transistors for receiving the input clock signal CLK each, and thus input capacitance and power consumption due to the input clock signal CLK may be reduced. 
       FIGS.  7  to  10 ,  11 A, and  11 B  are circuit diagrams of clock gating cells according to example embodiments of the inventive concept. 
     Each of clock gating cells  10   b ,  10   c ,  10   d ,  10   e ,  10   f , and  10   g  of  FIGS.  7  to  10 ,  11 A, and  11 B  is an embodiment of the clock gating cell  10  of  FIG.  2    and may correspond to a modified embodiment of the clock gating cell  10   a  of  FIG.  4   . Therefore, descriptions identical to those already given above may be omitted below. 
     In addition, signals according to the lapse of time in clock gating cells  10   b ,  10   c , and  10   d  of  FIGS.  7  to  9    may be similar to those described above with reference to the timing diagram of  FIG.  6   . In addition, in  FIGS.  7  to  11 B , elements which correspond to those shown in  FIG.  4    will be denoted by “b”, “c”, “d”, “e”, “f” and “g”, respectively. For example, the first PFET transistor P 11   b  of  FIG.  7    may correspond to the first PFET transistor P 11   a  of  FIG.  4   . 
     Referring to  FIG.  7   , first, second, third and fourth blocks C 1   b , C 2   b , C 3   b  and C 4   b  may be similar to the first to fourth blocks C 1   a  to C 4   a  of  FIG.  4   , respectively. In an example embodiment of the inventive concept, some of the transistors receiving the first signal SIG 1  may receive delayed first signals SIG 1 D through a buffer, instead of the first signal SIG 1 . 
     In an example embodiment of the inventive concept, the fourth block C 4   b  may include first and second inverters I 41   b  and I 42   b  to generate the delayed first signals SIG 1 D. The first inverter I 41   b  may generate the inverted first signal SIG 1 N by inverting the first signal SIG 1 , and the second inverter I 42   b  may generate the delayed first signal SIG 1 D by inverting the inverted first signal SIG 1 N. 
     At least one of a fourth NFET transistor N 22   b  of the second block C 2   b  and a sixth PFET transistor P 31   b  of the third block C 3   b  may receive the delayed first signal SIG ID instead of the first signal SIG 1 . 
     Although  FIG.  7    shows that both the fourth NFET transistor N 22   b  and the sixth PFET transistor P 31   b  receive the delayed first signal SIG 1 D, the inventive concept is not limited thereto, and the fourth NFET transistor N 22   b  may receive the first signal SIG 1  and the sixth PFET transistor P 31   b  may receive the delayed first signal SIG 1 D. 
     Referring to  FIG.  8   , first, second, third and fourth blocks C 1   c , C 2   c , C 3   c  and C 4   c  may be similar to the first to fourth blocks C 1   a  to C 4   a  of  FIG.  4   , respectively. In an example embodiment of the inventive concept, at least one of the inverter I 11   a  of the first block C 1   a  and the inverter I 31   a  of the third block C 3   a  of  FIG.  4    may be omitted. 
     In an example embodiment of the inventive concept, the first block C 1   c  may directly receive the inverted enable signal EN from the outside instead of the enable signal E. In an example embodiment of the inventive concept, the third block C 3   c  may immediately output the inverted output clock signal ECKN generated through a second node N 2   c.    
     Referring to  FIG.  9   , first, second, third and fourth blocks C 1   d , C 2   d , C 3   d , and C 4   d  may be similar to the first to fourth blocks C 1   a  to C 4   a  of  FIG.  4   , respectively. In an example embodiment of the inventive concept, the first block C 1   d  may be connected to the second block C 2   d  instead of the power voltage node VDD. For example, a first PFET transistor P 11   d  may be connected between a third node N 3   d  of the second block C 2   d  and a first node N 1   d  and may be controlled by the inverted output clock signal ECKN. In other words, because the first PFET transistor P 11   d  is connected to an internal node of a clock gating cell  10   d  instead of the power voltage node VDD, the load on a routing path on a layout may be reduced. 
     Referring to  FIG.  10   , first, second, third and fourth blocks C 1   e , C 2   e , C 3   e  and C 4   e  may be similar to the first to fourth blocks C 1   a  to C 4   a  of  FIG.  4   , respectively. In an example embodiment of the inventive concept, a clock gating cell  10   e  may operate in a plurality of operation modes. To accomplish this, the clock gating cell  10   e  may further receive a scan enable signal SE. 
     In an example embodiment of the inventive concept, the clock gating cell  10   e  may operate in a normal operation mode in which data transmission is performed according to the scan enable signal SE or a test mode in which a test operation is performed. For example, the normal operation mode may be indicated when the scan enable signal SE is at a first logic level (e.g., a logic low level) and a scan test mode may be indicated when the scan enable signal SE is at a second logic level (e.g., a logic high level). 
     Unlike the first block C 1   a  of  FIG.  4   , the first block C 1   e  of the clock gating cell  10   e  may further include a NOR gate G 11   e  instead of the inverter I 11   a . The NOR gate G 11   e  may receive the enable signal E and the scan enable signal SE and output a second signal SIG 2 . 
     The first NFET transistor N 11   e  of the first block C 1   e  may be connected between a first node N 1   e  and a source of a second NFET transistor N 12   e  and may be controlled by the second signal SIG 2  applied to its gate. In addition, a fourth PFET transistor P 22   e  of the second block C 2   e  may be controlled by the second signal SIG 2  instead of the inverted enable signal EN. 
     Although the embodiments of  FIG.  7  to  10    have been described in order, at least two of the embodiments may be duplicated and combined with each other. In an example embodiment of the inventive concept, as the embodiments of  FIGS.  8  and  9    are combined, the first block C 1   d  of the clock gating cell  10   d  may be connected to the second block C 2   d  instead of the power voltage node VDD, and an inverter I 11   d  of the first block C 1   d  and an inverter I 31   d  of a third block C 3   d  may be omitted. 
     In an example embodiment of the inventive concept, the embodiments of  FIGS.  7  and  10    may be combined. Referring to  FIG.  11 A , a first block C 1   f  of a clock gating cell  10   f  may include a NOR gate G 11   f , thereby operating in the normal operation mode or the scan test mode. In addition, a fourth block C 4   f  may include a first inverter I 41   f  that generates the inverted first signal SIG 1 N by inverting the first signal SIG 1  and a second inverter I 42   f  may generate the delayed first signal SIG 1 D by inverting the inverted first signal SIG 1 N. Therefore, a sixth PFET transistor P 31   f  of a third block C 3   f  may receive the delayed first signal SIG 1 D through a buffer. 
     In an example embodiment of the inventive concept, referring to  FIG.  11 B , a third block C 3   g  of a clock gating cell  10   g  may further include a seventh PFET transistor P 32   g  as compared to the third block C 3   f  of the clock gating cell  10   f . The seventh PFET transistor P 32   g  may be connected between a second node N 2   g  and the power voltage node VDD and may be controlled by the clock input CLK applied to its gate. In an example embodiment of the inventive concept, the number of the seventh PFET transistor P 32   g  may be one or greater, wherein the number of the seventh PFET transistor P 32   g  may correspond to the number of fourth NFET transistors N 22   g  and the number of fifth NFET transistors N 23   g . In addition, the seventh PFET transistor P 32   g  may also be added to the third block C 3   b  of  FIG.  7   , the third block C 3   d  of  FIG.  9   , and the third block C 3   e  of  FIG.  10   . 
     The clock gating cell  10   g  may drive a load cell by outputting the clock output ECK to the load cell. In this case, when the parasitic capacitance of the load cell is large, the numbers of the fourth NFET transistors N 22   g , the fifth NFET transistors N 23   g , and inverters I 31   g  connected to the second node N 2   g  may be increased, thereby increasing the intensity of the inverted clock output ECKN and reducing the rising time of the inverted clock output ECKN. 
     According to an example embodiment of the inventive concept, since the seventh PFET transistor P 32   g  is added, the inverted clock output ECKN may be balanced in correspondence to the number of the fourth NFET transistors N 22   g  and the number of the fifth NFET transistors N 23   g . Therefore, the overall power consumption and/or a sacrifice in the operating speed may be minimized and the clock output ECK may be supplied stably.  FIG.  12    is a diagram showing a logic symbol of a latch circuit according to an example embodiment of the inventive concept, and  FIG.  13    is a block diagram of a latch circuit according to an example embodiment of the inventive concept. 
       FIG.  12    is a diagram showing a logic symbol of a latch circuit according to an example embodiment of the inventive concept, and  FIG.  13    is a block diagram of a latch circuit according to an example embodiment of the inventive concept. 
     Referring to  FIG.  12   , a latch circuit  20  may generate an output signal Q based on a data signal D in response to the input clock signal CLK. For example, the latch circuit  20  may be triggered in response to a negative edge or a positive edge of the input clock signal CLK. 
       FIG.  13    may be a block diagram of the latch circuit  20  as an embodiment of  FIG.  12   . 
     The latch circuit  20  may include fifth, sixth, seventh and eighth blocks C 5 , C 6 , C 7  and C 8 . The fifth to eighth blocks C 5  to C 8  may be similar to the first to fourth blocks C 1  to C 4  of  FIG.  2   , respectively. Therefore, descriptions identical to those already given above will be omitted below. As described later with reference to  FIGS.  14  and  15   , first and second keeper circuits CSI and C 63 , first and second function circuits C 52  and C 62 , first and second charge circuits C 61  and C 71 , a discharge circuit C 64 , and an output circuit C 8  may each include at least one transistor. 
     The fifth block C 5  may receive the inverted output clock signal ECKN and the data signal D and may be connected to the power voltage node VDD, a fifth node N 5 , and the ground node. The fifth block C 5  may include the first keeper circuit C 51 , the first function circuit C 52 , and a fourth transistor N 51 . 
     The first keeper circuit C 51  may be similar to the first keeper circuit C 1  of  FIG.  3 A , and the fourth transistor N 51  may be similar to the first transistor N 11  of  FIG.  3 A . 
     The first function circuit C 52  may receive the data signal D and may be connected to the fifth node N 5  and a source of the fourth transistor N 51 . A third signal SIG 3  may be generated by the fifth node N 5 . 
     The sixth block C 6  may receive the data signal D, the output signal Q, the input clock signal CLK, and an inverted output signal QN and may be connected to the power voltage node VDD, the fifth node N 5 , a sixth node N 6 , and the ground node. The sixth block C 6  may include the second function circuit C 62 , the second keeper circuit C 63 , the first charge circuit C 61 , fifth and sixth transistors P 61  and N 61 , and the discharge circuit C 64 . 
     The second function circuit C 62  may receive the data signal D and may be connected to the fifth node N 5  and a seventh node N 7 . In addition, in  FIG.  13   , the circle shown at the second function circuit C 62  may denote an inverter. Therefore, the second function circuit C 62  may use an inverted data signal DN. In addition, polarities of respective signals for controlling the first function circuit C 52  and the second function circuit C 62  may be opposite to each other. 
     The second keeper circuit C 63  may receive the output signal Q and may be connected to the fifth node N 5  and an eighth node N 8 . 
     The first charge circuit C 61  may receive the inverted output signal QN and may be connected to the power voltage node VDD and the seventh node N 7 . The circle shown at the first charge circuit C 61  may denote an inverter. Therefore, the first charge circuit C 61  may use the output signal Q. 
     The fifth transistor P 61  may receive the input clock signal CLK and may be connected to the sixth node N 6  and the seventh node N 7 . For example, the fifth transistor P 61  may be connected between the sixth node N 6  and the seventh node N 7  and may be controlled by the input clock signal CLK applied to its gate. The inverted output clock signal ECKN may be generated by the sixth node N 6 . 
     The sixth transistor N 61  may receive the input clock signal CLK and may be connected to the eighth node N 8  and the ground node. For example, the sixth transistor N 61  may be connected between the eighth node N 8  and the ground node and may be controlled by the input clock signal CLK. 
     The discharge circuit C 64  may receive the third signal SIG 3  and may be connected to the sixth node N 6  and the eighth node N 8 . 
     The seventh block C 7  may include the second charge circuit C 71 . The second charge circuit C 71  may receive the inverted output signal QN from the first charge circuit C 61  and may be connected to the power voltage node VDD and the sixth node N 6 . 
     The eighth block C 8  may receive the third signal SIG 3  and may generate the output signal Q and/or the inverted output signal QN. 
     Similar to that described above, the first charge circuit C 61  and the second charge circuit (C 71  may determine the logic level of the third signal SIG 3  and the logic level of the inverted output clock signal ECKN under the control of the fifth transistor P 61 . 
     For example, when the logic level of the input clock signal CLK is a logic low level, the fifth transistor P 61  may be turned on by the input clock signal CLK and the first charge circuit C 61  may determine the logic level of the inverted output clock signal ECKN of the sixth node N 6 . 
     For example, when the logic level of the input clock signal CLK is a logic high level, the fifth transistor P 61  may be turned off by the input clock signal CLK and the first charge circuit C 61  is unable to determine the logic level of the inverted output clock signal ECKN of the sixth node N 6 . In this case, the second charge circuit C 71  may determine the logic level of the inverted output clock signal ECKN of the sixth node N 6 . 
     For example, when the logic level of the input clock signal CLK is a logic low level, the fifth transistor P 61  may be turned on by the input clock signal CLK. In this case, based on the ON/OFF state of the second function circuit C 62 , the second charge circuit C 71  may determine the logic level of the third signal SIG 3  of the fifth node N 5 . 
     Because the latch circuit  20  according to the present embodiment includes only one pair of transistors receiving the input clock signal CLK, a low power latch circuit of which power consumption due to the input clock signal CLK is reduced may be implemented. 
       FIGS.  14  and  15    are circuit diagrams of latch circuits according to example embodiments of the inventive concept. 
     Referring to  FIG.  14   , as an embodiment of  FIG.  13   , a latch circuit  20   a  may operate in response to a negative edge of the input clock signal CLK input thereto. In other words, the latch circuit  20   a  may be a negative edge-triggered latch circuit. 
     A fifth block C 5   a  may receive the data signal D and the inverted output clock signal ECKN and may be connected to a fifth node N 5   a . The third signal SIG 3  may be generated by the fifth node N 5   a.    
     The fifth block C 5   a  may include a seventh PFET transistor P 51   a  and sixth and seventh NFET transistors N 5   a  and N 52   a.    
     The seventh PFET transistor P 51   a  may be connected between the power voltage node VDD and the fifth node N 5   a  and may be controlled by the inverted output clock signal ECKN. The sixth and seventh NFET transistors N 51   a  and N 52   a  may be connected in series between the fifth node N 5   a  and the ground node and may be controlled by the data signal D and the inverted output clock signal ECKN, respectively. 
     A sixth block C 6   a  may receive the output signal Q, the data signal D, the third signal SIG 3 , and the input clock signal CLK and may be connected to a sixth node N 6   a . The inverted output clock signal ECKN may be generated by the sixth node N 6   a.    
     The sixth block C 6   a  may include eighth, ninth and tenth PFET transistors P 61   a , P 62   a , and P 63   a  and eighth, ninth and tenth NFET transistors N 61   a , N 62   a , and N 63   a.    
     The eighth PFET transistor P 61   a  may be connected between the power voltage node VDD and a seventh node N 7   a  and may be controlled by the output signal Q. The ninth PFET transistor P 62   a  may be connected between the seventh node N 7   a  and the fifth node N 5   a  and may be controlled by the data signal D. The tenth PFET transistor P 63   a  may be connected between the seventh node N 7   a  and the sixth node N 6   a  and may be controlled by the input clock signal CLK. 
     The eighth NFET transistor N 61   a  may be connected between the fifth node N 5   a  and an eighth node N 8   a  and may be controlled by the output signal Q. The ninth NFET transistor N 62   a  may be connected between the sixth node N 6   a  and the eighth node N 8   a  and may be controlled by the third signal SIG 3 . The tenth NFET transistor N 63   a  may be connected between the eighth node N 8   a  and the ground node and may be controlled by the input clock signal CLK. 
     A seventh block C 7   a  may be connected to the power voltage node VDD and the sixth node N 6   a . In an example embodiment of the inventive concept, the seventh block C 7   a  may include an eleventh PFET transistor P 71   a  that is connected between the power voltage node VDD and the sixth node N 6   a  and is controlled by the inverted output signal QN. 
     In an example embodiment of the inventive concept, an eighth block C 8   a  may include an inverter I 81   a  that generates the output signal Q by inverting the third signal SIG 3  and an inverter I 82   a  that generates the inverted output signal QN by inverting the output signal Q. 
     Referring to  FIG.  15   , as an embodiment of  FIG.  13   , a latch circuit  20   b  may operate in response to a positive edge of the input clock signal CLK input thereto. In other words, the latch circuit  20   b  may be a positive edge-triggered latch circuit. 
     Because fifth, sixth, seventh and eighth blocks C 5   b , C 6   b , C 7   b  and C 8   b  of the latch circuit  20   b  may be similar to the fifth to eighth blocks C 5   a  to C 8   a  of  FIG.  13   , respectively, descriptions identical to those already given above may be omitted below. In  FIG.  15   , elements which correspond to those shown in  FIG.  14    will be denoted by “b”. 
     The fifth block C 5   b  may include twelfth and thirteenth PFET transistors P 51   b  and P 52   b  and an eleventh NFET transistor N 51   b . The twelfth and thirteenth PFET transistors P 51   b  and P 52   b  may be connected in series between the power voltage node VDD and a fifth node N 5   b  and may be controlled by the inverted output clock signal ECKN and the data signal D, respectively. The eleventh NFET transistor N 51   b  may be connected between the fifth node N 5   b  and the ground node and may be controlled by the inverted output clock signal ECKN. 
     The sixth block C 6   b  may include fourteenth, fifteenth and sixteenth PFET transistors P 61   b , P 62   b , and P 63   b  and twelfth, thirteenth and fourteenth NFET transistors N 61   b , N 62   b , and N 63   b.    
     The fourteenth PFET transistor P 61   b  may be connected between the power voltage node VDD and a seventh node N 7   b  and may be controlled by the input clock signal CLK. The fifteenth PFET transistor P 62   b  may be connected between the fifth node N 5   b  and the seventh node N 7   b  and may be controlled by the output signal Q. The sixteenth PFET transistor P 63   b  may be connected between a sixth node N 6   b  and the seventh node N 7   b  and may be controlled by the third signal SIG 3 . 
     The twelfth NFET transistor N 61   b  may be connected between the fifth node N 5   b  and an eighth node N 8   b  and may be controlled by the data signal D. The thirteenth NFET transistor N 62   b  may be connected between the sixth node N 6   b  and the eighth node N 8   b  and may be controlled by the input clock signal CLK. The fourteenth NFET transistor N 63   b  may be connected between the eighth node N 8   b  and the ground node and may be controlled by the output signal Q. 
     A seventh block C 7   b  may be connected to the ground node and the sixth node N 6   b . In an example embodiment of the inventive concept, the seventh block C 7   b  may further include a fifteenth NFET transistor N 71   b  that is connected between the ground node and the sixth node N 6   b  and is controlled by the inverted output signal QN. 
     An eighth block C 8   b  may be similar to the eighth block C 8   a  of  FIG.  14   . 
     The fifth to seventh blocks C 5   a  to C 7   a  of  FIG.  14    may be divided into a pull-up unit and a pull-down unit based on the fifth node N 5   a  and the sixth node N 6   a . In the same regard, the fifth to seventh blocks C 5   b  to C 7   b  of  FIG.  15    may be divided into a pull-up unit and a pull-down unit based on the fifth node N 5   b  and the sixth node N 6   b.    
     For example, referring to  FIG.  14   , the seventh PFET transistor P 51   a  of the fifth block C 5   a  may be classified as a pull-up unit, whereas the sixth and seventh NFET transistors N 51   a  and N 52   a  may be classified as a pull-down unit. In the sixth block C 6   a , the eighth to tenth PFET transistors P 61   a , P 62   a , and P 63   a  may be classified as a pull-up unit, whereas the eighth to tenth NFET transistors N 61   a , N 62   a , and N 63   a  may be classified as a pull-down unit. The eleventh PFET transistor P 71   a  of the seventh block C 7   a  may be classified as a pull-up unit. 
     Referring to  FIG.  15   , the twelfth and thirteenth PFET transistors P 51   b  and P 52   b  of the fifth block C 5   b  may be classified as a pull-up unit, whereas the eleventh NFET transistor N 51   b  may be classified as a pull-down unit. In the sixth block C 6   b , the fourteenth to sixteenth PFET transistors P 61   b , P 62   b , and P 63   b  may be classified as a pull-up unit, whereas the twelfth to fourteenth NFET transistors N 61   b , N 62   b , and N 63   b  may be classified as a pull-down unit. The fifteenth NFET transistor N 71   b  of the seventh block C 7   b  may be classified as a pull-down unit. 
     By being divided into a pull-up unit and a pull-down unit, the negative edge-triggered latch circuit  20   a  and the positive edge-triggered latch circuit  20   b  may have structures symmetrical around the fifth nodes N 5   a  and N 5   b  and the sixth nodes N 6   a  and N 6   b . In addition, pull-up units of the latch circuits  20   a  and  20   b  may be implemented by PFET transistors, whereas pull-down units may be implemented by NFET transistors. 
       FIGS.  16 A and  16 B  are diagrams schematically showing partial layouts of a clock gating cell and a latch circuit according to example embodiments of the inventive concept. For example,  FIG.  16 A  may be a diagram schematically showing a layout corresponding to the second block C 2   a  of the clock gating cell  10   a  of  FIG.  4   , and more particularly, the fifth PFET transistor P 23   a  included in the second block C 2   a  and a surrounding region thereof. For example,  FIG.  16 B  may be a diagram schematically showing a layout corresponding to the sixth block C 6   b  of the clock gating cell  20   b  of  FIG.  15   , and more particularly, the thirteenth NFET transistor N 62   b  included in the sixth block C 6   b  and a surrounding region thereof. However, the inventive concept is not limited thereto, and  FIG.  16 B  may correspond to second blocks C 2   b , C 2   c , C 2   d , C 2   e , and C 2   f  of clock gating cells  10   b ,  10   c ,  10   d ,  10   e , and  10   f  of  FIGS.  7  to  11 B  and the sixth block C 6   a  of the latch circuit  20   a  of  FIG.  14   . A first layout L 1  may be a schematic partial representation of clock gating cells  10   b  to  10   f  or the latch circuit  20   a  on a plane including an X-axis and a Y-axis. 
     In  FIGS.  16 A and  16 B , only some layers and some patterns may be shown for convenience of illustration, and vias may be shown even though they are located under a pattern of a wiring layer to indicate connections between the pattern of the wiring layer and patterns therebelow. Further, a pattern consisting of a conductive material like a pattern of a wiring layer may be referred to as a conductive pattern or may be simply referred to as a pattern. 
     As shown in  FIGS.  16 A and  16 B , at least one gate electrode may extend in the Y-axis direction. In addition, at least one active region may extend in the X-axis direction and may constitute a transistor together with a gate electrode. In some embodiments of the inventive concept, at least one fin may extend in the X-axis direction in active regions and constitute a finFET together with a gate electrode. 
     Descriptions will be given below with reference to  FIGS.  4  and  16 A  together, and, for convenience of explanation, it will be assumed, but is not limited to, that the first layout L 1  corresponds to the fifth PFET transistor P 23   a  included in the second block C 2   a  of the clock gating cell  10   a  ( FIG.  4   ) and the surrounding region thereof. 
     A first gate electrode G 1  may be connected to a clock pattern P_CLK, and the input clock signal CLK may be applied to the clock pattern P_CLK. In a first region R 1  of a PFET active region, a first pattern A, a second pattern B, and the first gate electrode G 1  may form a PFET transistor. The PFET transistor may correspond to the fifth PFET transistor P 23   a  of the clock gating cell  10   a.    
     The first pattern A may be connected to a source or a drain of the fifth PFET transistor P 23   a . For example, the first pattern A may correspond to the second node N 2   a  and the inverted output clock signal ECKN may be applied thereto. The first pattern A may be connected to an output inverter. The second pattern B may be connected to the source or the drain of the fifth PFET transistor P 23   a  and, for example, may correspond to the third node N 3   a.    
     According to example embodiments of the inventive concept, because the clock gating cell  10   a  includes only one PFET transistor and one NFET transistor for receiving the input clock signal CLK, signals other than the input clock signal CLK may be applied to a third pattern C connected to a second gate electrode G 2 . For example, the inverted first signal SIG 1 N provided to a gate of the third PFET transistor P 21   a  or the inverted enable signal EN provided to a gate of the fourth PFET transistor P 22   a  may be applied to the third pattern C. 
     Transistors other than transistors receiving the input clock signal CLK, e.g., the third and fourth PFET transistors P 21   a  and P 22   a  adjacent to the fifth PFET transistor P 23   a , may not be electrically connected to the clock pattern P_CLK. 
     In addition, the first gate electrode G 1  may form an NFET transistor in an NFET active region. Therefore, the first layout L 1  may include two transistors receiving the input clock signal CLK due to the first gate electrode G 1 . Here, a source and a drain of at least one of the two transistors may be connected to nodes other than the power voltage node VDD and the ground node. For example, a source of a PFET transistor formed in the first region R 1  may be connected to the third node N 3   a  and a drain of the PFET transistor may be connected to the second node N 2   a.    
     Referring to  FIG.  16 B , a second layout L 2  may be a schematic layout corresponding to the sixth block C 6   b  of the clock gating cell  20   b  of  FIG.  15   , and more particularly, the thirteenth NFET transistor N 62   b  included in the sixth block C 6   b  and a surrounding region thereof. The second layout L 2  may be a schematic representation of a portion of the latch circuit  20   b  on a plane including an X-axis and a Y-axis. 
     Descriptions will be given below with reference to  FIGS.  15  and  16 B  together, and, for convenience of explanation, it will be assumed, but is not limited to, that the second layout L 2  corresponds to the thirteenth NFET transistor N 62   b  included in the sixth block C 6   b  of the latch circuit  20   b  ( FIG.  15   ) and the surrounding region thereof. In addition, descriptions identical to those already given above with reference to  FIG.  16 A  may be omitted below. 
     A first gate electrode G 1  may be connected to a clock pattern P_CLK, and the input clock signal CLK may be applied to the clock pattern P_CLK. In a second region R 2  of an NFET active region, the first gate electrode G 1  may form an NFET transistor. The NFET transistor may correspond to the thirteenth NFET transistor N 62   b  of the latch circuit  20   b . A fourth pattern D may be connected to a source or a drain of the thirteenth NFET transistor N 62   b . For example, the fourth pattern D may correspond to the sixth node N 6   b  and the inverted output clock signal ECKN may be applied thereto. A fifth pattern E may be connected to the source and the drain of the thirteenth NFET transistor N 62   b  and, for example, may correspond to the eighth node N 8   b.    
     According to example embodiments of the inventive concept, because the latch circuit  20   b  includes only one PFET transistor and one NFET transistor for receiving the input clock signal CLK, signals other than the input clock signal CLK may be applied to a sixth pattern F connected to a second gate electrode G 2 . For example, the output signal Q provided to a gate of the fifteenth PFET transistor P 62   b  and a gate of the fourteenth NFET transistor N 63   b  may be applied to the sixth pattern F. 
     Transistors other than transistors receiving the input clock signal CLK, e.g., the fourteenth and fifteenth NFET transistors N 63   b  and N 71   b  adjacent to the thirteenth NFET transistor N 62   b , may not be electrically connected to the clock pattern P_CLK. 
     In addition, the first gate electrode G 1  may form a PFET transistor in a PFET active region. Therefore, the second layout L 2  may include two transistors receiving the input clock signal CLK due to the first gate electrode G 1 . Here, a source and a drain of at least one of the two transistors may be connected to nodes other than the power voltage node VDD and the ground node. For example, a source of an NFET transistor formed in the second region R 2  may be connected to the sixth node N 6   b  and a drain of the NFET transistor may be connected to the eighth node N 8   b.    
       FIG.  17    is a diagram showing an IC including a clock gating cell according to an example embodiment of the inventive concept. 
     Referring to  FIG.  17   , an IC  100  processing digital signals may include a first clock gating cell CGC 1 , a second clock gating cell CGC 2 , a power controller PC, a first combinational logic block CL 1 , a second combinational logic block CL 2 , and a plurality of flip-flops PF 1 , PF 2 , NF 1 , and NF 2 . The IC  100  may include a clock gating cell described above with reference to  FIGS.  1  to  11   . 
     The power controller PC may control the power of the IC  100  and may generate a first enable signal E 1  and a second enable signal E 2 . For example, the power controller PC may generate an inactive first enable signal E 1  to reduce power consumption by a digital circuit including at least one first positive edge triggered flip-flop PF 1 , the first combinational logic block CL 1 , and at least one second positive edge triggered flip-flop PF 2 . In addition, the power controller PC may generate an inactive second enable signal E 2  to reduce power consumption by a digital circuit including at least one first negative edge triggered flip-flop NF 1 , the second combinational logic block CL 2 , and at least one second negative edge triggered flip-flop NF 2 . 
     The first clock gating cell CGC 1  may receive an input clock signal CLK and may stop or resume the supply of a first output clock signal ECK 1  based on the first enable signal E 1 . For example, as described above with reference to  FIG.  1   , the first clock gating cell CGC 1  may generate the first output clock signal ECK 1  that is maintained at logic ‘0’ in a disabled state. Therefore, the first output clock signal ECK 1  may be supplied to a positive edge-triggered flip-flop, for example, at least one first positive edge-triggered flip-flop PF 1  and at least one second positive edge-triggered flip-flop PF 2 . In addition, the second clock gating cell CGC 2  may receive the input clock signal CLK and may stop or resume the supply of a second output clock signal ECK 2  based on the second enable signal E 2 . For example, as described above with reference to  FIG.  1   , the second clock gating cell CGC 2  may generate the second output clock signal ECK 2  that is maintained at logic ‘1’ in the disabled state. Therefore, the second output clock signal ECK 2  may be supplied to a negative edge-triggered flip-flop, for example, at least one first negative edge-triggered flip-flop NF 1  and at least one second negative edge-triggered flip-flop NF 2 . 
       FIG.  18    is a flowchart of a method of manufacturing an IC according to an example embodiment of the inventive concept. 
     Referring to  FIG.  18   , an IC (e.g.,  100  of  FIG.  17   ) including a clock gating cell as described above may be manufactured. 
     In some embodiments of the inventive concept, a clock gating cell may be a standard cell. A standard cell is a unit of a layout included in the integrated circuit IC and may be simply referred to as a cell. The integrated circuit IC may include a plurality of different standard cells, each of which may provide a unique function. The standard cells may have a structure complying with pre-set rules based on a semiconductor process for manufacturing the integrated circuit IC. For example, the standard cells may have a certain length or a multiple of the certain length on a plane perpendicular to a direction in which layers are stacked. 
     A standard cell library (or cell library) D 2  may include information regarding standard cells, e.g., function information, characteristic information, layout information, etc., and may include information regarding a clock gating cell. As described above with reference to the drawings, a clock gating cell defined by the standard cell library D 2  may not only provide high operational reliability, but also provide high efficiency, e.g., reduced area and low power consumption. 
     In operation S 10 , a logical synthesis operation for generating a netlist D 3  from RTL data D 1  may be performed. For example, a semiconductor design tool (e.g., a logical synthesis tool) may perform a logical synthesis with reference to the standard cell library D 2  from the RTL data D 1  composed in Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL) and Verilog, thereby generating the netlist D 3  including a bitstream or a netlist. The standard cell library D 2  may include information regarding good performance of a clock gating cell, and standard cells may be included in the integrated circuit IC with reference to such information during a logical synthesis process. 
     In operation S 20 , a Place &amp; Routing (P&amp;R) operation for generating layout data D 4  from the netlist D 3  may be performed. As shown in  FIG.  18   , the P&amp;R operation S 20  may include a plurality of sub-operations S 21 , S 22 , and S 23 . In sub-operation S 21 , an operation for arranging standard cells may be performed. For example, a semiconductor design tool (e.g., a P&amp;R tool) may arrange a plurality of standard cells with reference to the standard cell library D 2  from the netlist D 3 . For example, the semiconductor design tool may arrange a layout of clock gating cells defined by the netlist D 3  with reference to the standard cell library D 2 . In sub-operation S 22 , an operation for generating interconnections may be performed. The interconnections may electrically connect output pins to input pins of standard cells and may include, for example, at least one via and at least one conductive pattern. In sub-operation S 23 , an operation for generating the layout data D 4  may be performed. The layout data D 4  may have a format, e.g., GDSII, and may include geometric information regarding the standard cells and the interconnections. 
     In operation S 30 , an Optical Proximity Correction (OPC) may be performed. An OPC may refer to an operation for forming a pattern of a desired shape by correcting distortion such as refraction caused by light characteristics in photolithography included in a semiconductor process for manufacturing the integrated circuit IC, and the pattern on a mask may be determined by applying the OPC to the layout data D 4 . In some embodiments of the inventive concept, the layout of the integrated circuit IC may be limitedly modified in operation S 30 , and the limited modification of the integrated circuit IC in operation S 30  is a post-processing for optimizing the structure of the integrated circuit IC and may be referred to as design polishing. 
     In operation S 40 , an operation for manufacturing a mask may be performed. For example, as the OPC is applied to the layout data D 4 , patterns on a mask may be defined to form patterns to be formed on a plurality of layers, and at least one mask (or a photomask) for forming respective patterns of the plurality of layers may be fabricated. 
     In operation S 50 , an operation for manufacturing the integrated circuit IC may be performed. For example, the integrated circuit IC may be manufactured by patterning the plurality of layers by using the at least one mask fabricated in operation S 40 . As shown in  FIG.  18   , operation S 50  may include sub-operations S 51  and S 52 . In sub-operation S 51 , a front-end-of-line (FEOL) process may be performed. The FEOL process may refer to a process of forming individual devices such as transistors, capacitors, and resistors on a substrate during a process of manufacturing an IC. For example, the FEOL process may include planarizing and cleaning a wafer, forming a trench, forming a well, forming a gate line, and forming a source and a drain, etc. In sub-operation S 52 , a back-end-of-line (BEOL) process may be performed. The BEOL process may refer to a process of interconnecting individual devices such as transistors, capacitors, and resistors during a process of manufacturing an IC. For example, the BEOL process may include silicidation of gate, source, and drain regions, adding a dielectric, planarizing, forming a hole, adding a metal layer, forming a via, forming a passivation layer, etc. Next, the IC may be packaged in a semiconductor package and used as a component for various applications. As described above, due to the low power consumption characteristics of a clock gating cell, the IC may exhibit low power consumption and high efficiency, and, as a result, the efficiency of an application including the IC may be improved. 
     While the inventive concept has been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made thereto without departing from the spirit and scope of the inventive concept set forth in the following claims.