Abstract:
A clock synchronization circuit is provided for synchronizing an external clock signal with an internal clock signal. The circuit is connected to a clock buffer adapted to output the internal clock signal. The circuit includes a first loop adapted to receive the external clock signal and output a plurality of reference clock signals having a predetermined phase difference therebetween. A second loop is adapted to delay the plurality of reference clock signals; select a signal from among the plurality of delayed reference clock signals; provide the selected signal to the clock buffer; detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal; generate a plurality of control voltages to reduce the detected phase difference, and control a delay amount of each of the plurality of reference clock signals in response to the plurality of control voltages; so as to synchronize the internal clock signal with the external clock signal.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to semiconductor devices and, in particular, to a clock synchronization circuit and a semiconductor device having the same.  
         BACKGROUND DESCRIPTION  
         [0002]    The operating speed of a central processing unit (CPU), which is a signal processing unit, has been radically improved over the last several years. However, the operating speed of dynamic random access memory (DRAM) semiconductor devices, which correspond to the main memory of a CPU, has not been greatly improved. Rather, it has been identified as a main bottle-neck factor of computer systems. To reduce the difference in operating speed between a CPU and a DRAM semiconductor device, new DRAM semiconductor devices are being developed such as synchronous DRAM (SDRAM) semiconductor devices, Rambus DRAM semiconductor devices, synclink DRAM semiconductor devices, and so forth. These DRAM semiconductor devices have a feature such that data received from an external source or output to the outside is processed in synchronization with an internal clock. The internal clock is generated from an external clock signal which is received from an external source. A circuit which synchronizes the internal clock signal with the external clock signal is referred to as a clock synchronization circuit. A phase locked loop and a delay locked loop are included in the clock synchronization circuit. Among them, the delay locked loop is usually used in the DRAM semiconductor devices.  
           [0003]    [0003]FIG. 1 is a block diagram of a clock synchronization circuit  101  according to the prior art. Referring to FIG. 1, the clock synchronization circuit  101  has a dual loop structure having a core delay locked loop  111  and a peripheral delay locked loop  113 . The core delay locked loop  111  receives an external clock signal inCLK and generates 6 sub clock signals CK 1  through CK 6  (hereinafter collectively referred to as “CK 1 -CK 6 ”). The sub clock signals CK 1 -CK 6  have a predetermined phase difference. The peripheral delay locked loop  113  receives the sub clock signals CK 1 -CK 6 , generates a clock signal Q, and synchronizes the clock signal Q with an external clock signal inCLK using a phase interpolation technique. The peripheral delay locked loop  113  includes a phase selector  121 , a selection phase transformer  131 , a phase interpolator  141 , a phase detector  151  and a controller  161 . The phase interpolator  141  interpolates the phases of signals Φ′ and Ψ′ output from the selective phase transformer  131  to generate the clock signal Q. The phase interpolator  141  receives 16 bits of signals output from the controller  161  in order to determine the degree of interpolation of the phase of the clock signal Q.  
           [0004]    The phase interpolation technique used by the conventional clock synchronization circuit  101  can achieve its effects when the slew rate of an external clock signal inCLK is small, or when a smaller phase boundary can be provided by increasing the number of sub-clock signals CK 1 -CK 6  generated by the core delay locked loop  111 . However, in the former case, the dynamic noise sensitivity of the clock synchronization circuit  101  is increased, so that jitter performance is degraded. In the latter case, a burden on the clock synchronization circuit  101  is increased.  
           [0005]    Accordingly, it would be desirable and highly advantageous to have a clock synchronization circuit having improved jitter performance.  
         SUMMARY OF THE INVENTION  
         [0006]    The problems stated above, as well as other related problems of the prior art, are solved by the present invention, a clock synchronization circuit and a semiconductor device having the same. Both the clock synchronization circuit and semiconductor device have improved jitter performance with respect to prior art devices.  
           [0007]    According to a first aspect of the invention, there is provided a clock synchronization circuit for synchronizing an external clock signal with an internal clock signal. The circuit is connected to a clock buffer adapted to output the internal clock signal. The circuit includes: a first loop adapted to receive the external clock signal and output a plurality of reference clock signals having a predetermined phase difference therebetween; and a second loop adapted to delay the plurality of reference clock signals, select a signal from among the plurality of delayed reference clock signals, provide the selected signal to the clock buffer, detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal, generate a plurality of control voltages to reduce the detected phase difference, and control a delay amount of each of the plurality of reference clock signals in response to the plurality of control voltages, so as to synchronize the internal clock signal with the external clock signal.  
           [0008]    According to a second aspect of the invention, there is provided a clock synchronization circuit for synchronizing an external clock signal with an internal clock signal. The circuit is connected to a clock buffer. The circuit includes: a first loop adapted to receive the external clock signal and output first through fourth reference clock signals, consecutive pairs of the first through fourth reference clock signals having a 90° phase difference therebetween; and a second loop having first through fourth voltage control delay units adapted to delay the first through fourth reference clock signals, the second loop adapted to select a reference clock signal from among the first through fourth delayed reference clock signals, provide the selected reference clock signal to the clock buffer for conversion to the internal clock signal, detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal, generate a plurality of control voltages having different levels according to the detected phase difference to reduce the detected phase difference, provide the plurality of control voltages to the first through fourth voltage control delay units, and control the delay amount of the selected reference clock signal in response to a control voltage from among the plurality of control voltages, so as to synchronize the internal clock signal with the external clock signal.  
           [0009]    According to a third aspect of the invention, a level of the control voltage applied to a voltage control delay unit among the first through fourth voltage control delay units that outputs the selected reference clock signal is different from levels of other control voltages from among the plurality of control voltages applied to other voltage control delay units among the first through fourth voltage control delay units that generate unselected reference clock signals.  
           [0010]    According to a fourth aspect of the invention, delay amounts applied to the first through fourth reference clock signals are always detected, the selected reference clock signal is switched to an unselected one of the first through fourth delayed reference clock signals having a phase that lags a phase of the selected reference clock signal by 90° when the delay amount of the selected reference clock signal approaches a maximum value, and the selected reference clock signal is switched to an unselected one of the first through fourth delayed reference clock signals having a phase that leads the phase of the selected reference clock signal by 90° when the delay amount of the selected reference clock signal approaches a minimum value.  
           [0011]    According to a fifth aspect of the invention, there is provided a clock synchronization circuit for synchronizing an external clock signal with an internal clock signal. The circuit is connected to a clock buffer. The circuit includes: a first loop adapted to receive the external clock signal and output a first and a second reference clock signal, the reference clock signals being differential signals having a 90° phase difference therebetween; and a second loop having a first voltage control delay unit adapted to delay the first reference clock signal to output a first and a second differential clock signal, and a second voltage control delay unit adapted to delay the second reference clock signal to output a third and a fourth differential clock signal, wherein each of the first and second voltage control delay units is controlled by one of a reference voltage and a control voltage, the second loop adapted to select a differential clock signal among the first through fourth differential clock signals output from the first and second voltage control delay units, provide the selected differential clock signal to the clock buffer, detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal, and provide the control voltage to one of the first and the second voltage control delay units according to the detected phase difference to reduce the detected phase difference, so that a delay amount of the selected differential clock signal is controlled, so as to synchronize the internal clock signal with the external clock signal.  
           [0012]    According to a sixth aspect of the invention, a level of the reference voltage is different from a level of the control voltage.  
           [0013]    According to a seventh aspect of the invention, delay amounts of the first through fourth differential clock signals are always detected, the selected differential clock signal is switched to an unselected one of the first through fourth differential clock signals having a phase that lags the phase of the selected clock signal by 90° when the delay amount of the selected differential clock signal approaches a maximum value, and the selected differential clock signal is switched to an unselected one of the first through fourth differential clock signals having a phase that leads the phase of the selected clock signal by 90° when the delay amount of the selected differential clock signal approaches a minimum value.  
           [0014]    According to an eighth aspect of the invention, there is provided a semiconductor device including: a clock buffer adapted to output an internal clock signal suitable for internal use by the semiconductor device; a first loop adapted to receive the external clock signal and output a plurality of reference clock signals having a predetermined phase difference therebetween; and a second loop adapted to delay the plurality of reference clock signals, select a signal from among the plurality of delayed reference clock signals, provide the selected signal to the clock buffer for conversion to the internal clock signal, detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal, generate a plurality of control voltages to reduce the detected phase difference, and control a delay amount of each of the plurality of reference clock signals in response to the plurality of control voltages, so as to synchronize the internal clock signal with the external clock signal.  
           [0015]    According to a ninth aspect of the invention, the semiconductor device is a synchronous dynamic random access memory (SDRAM) semiconductor device.  
           [0016]    According to a tenth aspect of the invention, there is provided a semiconductor device including: a clock buffer adapted to output an internal clock signal suitable for internal use by the semiconductor device; a first loop adapted to receive an external clock signal and output first through fourth reference clock signals having a 90° phase difference therebetween; and a second loop having first through fourth voltage control delay units adapted to delay the first through fourth reference clock signals, the second loop adapted to select a reference clock signal among the first through fourth reference clock signals output from the first through fourth voltage control delay units and provide the selected reference clock signal to the clock buffer for conversion to the internal clock signal, detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal, generate a plurality of control voltages having different levels according to the detected phase difference to reduce the detected phase difference, provide the plurality of control voltages to the first through fourth voltage control delay units, and control delay amounts of the first through fourth reference clock signals in response to the plurality of control voltages, so as to synchronize the internal clock signal with the external clock signal.  
           [0017]    According to an eleventh aspect of the invention, there is provided a semiconductor device including: a clock buffer adapted to output an internal clock signal suitable for internal use by the semiconductor device; a first loop adapted to receive an external clock signal and output first and second reference clock signals of differential types having a 90° phase difference therebetween; and a second loop having a first voltage control delay unit adapted to delay the first reference clock signal to output a first and a second differential clock signal, and a second voltage control delay unit adapted to delay the second reference clock signal to output a third and a fourth differential clock signal, wherein each of the first and second voltage control delay units is controlled by one of a reference voltage and a control voltage, the second loop adapted to select a signal among the first through fourth differential clock signals output from the first and second voltage control delay units, provide the selected signal to the clock buffer for conversion to the internal clock signal, detect a phase difference between the internal clock signal output from the clock buffer and the external clock signal, and provide the control voltage to one of the first and the second voltage control delay units according to the detected phase difference to reduce the detected phase difference, so that a delay amount of the selected signal is controlled, so as to synchronize the internal clock signal with the external clock signal.  
           [0018]    These and other aspects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments, which is to be read in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a block diagram of a clock synchronization circuit according to the prior art;  
         [0020]    [0020]FIG. 2 is a block diagram illustrating a clock synchronization circuit according to an illustrative embodiment of the invention together with a clock buffer;  
         [0021]    [0021]FIG. 3 is a timing diagram illustrating clock switching of the second loop of FIG. 2 according to an illustrative embodiment of the invention;  
         [0022]    [0022]FIG. 4, which is a block diagram illustrating the second loop of FIG. 2 in further detail according to an illustrative embodiment of the invention;  
         [0023]    [0023]FIG. 5 is a block diagram illustrating the operations of the first and second charge pumps of FIG. 4 according to an illustrative embodiment of the invention;  
         [0024]    [0024]FIGS. 6A and 6B, which are phase diagram illustrating the delay control methods performed by the first through fourth voltage control delay units of FIG. 2 according to an illustrative embodiment of the invention;  
         [0025]    [0025]FIG. 7 is a block diagram illustrating the window finder of FIG. 4 in further detail according to an illustrative embodiment of the invention;  
         [0026]    [0026]FIG. 8 is a block diagram of a clock synchronization circuit according to the second illustrative embodiment of the invention together with a clock buffer;  
         [0027]    [0027]FIG. 9 is a block diagram illustrating the second loop of FIG. 8 in further detail according to an illustrative embodiment of the invention; and  
         [0028]    [0028]FIG. 10 is a phase diagram illustrating a delay control method performed by the second loop of FIG. 8 according to an illustrative embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]    It is to be understood that the present invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or a combination thereof. Preferably, the present invention is implemented as a combination of both hardware and software, the software being an application program tangibly embodied on a program storage device. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (CPU), a random access memory (RAM), and input/output (I/O) interface(s). The computer platform also includes an operating system and microinstruction code. The various processes and functions described herein may either be part of the microinstruction code or part of the application program (or a combination thereof) which is executed via the operating system. In addition, various other peripheral devices may be connected to the computer platform such as an additional data storage device.  
         [0030]    It is to be further understood that, because some of the constituent system components depicted in the accompanying Figures may be implemented in software, the actual connections between the system components may differ depending upon the manner in which the present invention is programmed. Given the teachings herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention.  
         [0031]    A general description of the present invention will now be provided to introduce the reader to the concepts of the invention. Subsequently, more detailed descriptions of various aspects of the invention will be provided with respect to FIGS. 2 through 10.  
         [0032]    Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention must not be interpreted as being restricted to the embodiments. The embodiments are provided to more completely explain the scope of the present invention to those skilled in the art. In the entire specification, like reference numerals denote the same members. Furthermore, each embodiment disclosed and described in this specification includes a conductive-type embodiment that is complementary to each embodiment.  
         [0033]    [0033]FIG. 2 is a block diagram illustrating a clock synchronization circuit  201  according to an illustrative embodiment of the invention together with a clock buffer  221 . The clock synchronization circuit  201  includes first and second loops  211  and  213 . The first loop  211  receives an external clock signal inCLK and generates first through fourth reference clock signals RefCLK 1  through RefCLK 4  (hereinafter collectively referred to as “REFCLK 1 -REFCLK 4 ”). Each consecutive pair of the first through fourth reference clock signals RefCLK 1 -RefCLK 4  have a phase difference of 90 degrees. That is, the phase of the second reference clock signal RefCLK 2  lags the phase of the first clock signal RefCLK 1  by 90 degrees, the phase of the third reference clock signal RefCLK 3  lags the phase of the second reference clock signal RefCLK 2  by 90 degrees, and the phase of the fourth reference clock signal RefCLK 4  lags the phase of the third reference clock signal RefCLK 3  by 90 degrees. If the number of the clock reference clock signal RefCLK 1  through RefCLK 4  is increased, the phase difference between the reference clock signals RefCLK 1  through RefCLK 4  is reduced.  
         [0034]    The second loop  213  receives the first through fourth reference clock signals RefCLK 1  through RefCLK 4  and outputs a clock signal iCLK. That is, the second loop  213  controls the delay amounts of the first through fourth reference clock signals RefCLK 1  through RefCLK 4  using an analog control voltage, and synchronizes an internal clock signal fCLK with an external clock signal inCLK. The second loop  213  selects one among the first through fourth clock signals CLK 1 -CLK 4  obtained by controlling the delay amounts of the first through fourth reference clock signals RefCLK 1 -RefCLK 4 , and provides the selected clock signal as a clock signal iCLK to the clock buffer  221 .  
         [0035]    As shown in FIG. 3, which is a timing diagram illustrating clock switching of the second loop  213  of FIG. 2 according to an illustrative embodiment of the invention, when the delay amount of the selected clock signal iCLK increases and approaches a maximum value or when the delay amount thereof decreases and approaches a minimum value, the selected clock signal is switched to another clock signal. For example, if the delay amount of the first clock signal CLK 1  approaches the maximum value in a state where the first clock signal CLK 1  has been selected, the first clock signal CLK 1  is switched to the second clock signal CLK 2 , while the first, third and fourth clock signals CLK 1 , CLK 3  and CLK 4  return to the original phase relationship provided from the first loop  211 . If the delay amount of the first clock signal CLK 1  approaches the minimum value, the first clock signal CLK 1  is switched to the fourth clock signal CLK 4 . As described above, the selected clock signal iCLK is switched to another clock signal before it reaches the limit of the maximum value or minimum value of its delay amount, whereby the total phase region can be covered.  
         [0036]    Referring back to FIG. 2, the second loop  213  includes first through fourth voltage control delay units  231  through  234 , a multiplexer  241 , a phase detector  251  and a controller  261 .  
         [0037]    The first through fourth voltage control delay units  231  through  234  delay the first through fourth reference clock signals RefCLK 1 -RefCLK 4 , respectively, output from the first loop  211 . Each of the first through fourth voltage control delay units  231  through  234  consists of four delay elements, as shown in FIG. 4, which is a block diagram illustrating the second loop  213  of FIG. 2 in further detail according to an illustrative embodiment of the invention. Among the first through fourth voltage control delay units  231  through  234 , a voltage control delay unit for outputting a selected clock signal iCLK is controlled by a control voltage which is different from control voltages by which voltage control delay units for outputting non-selected clock signals are controlled. For example, if the first clock signal CLK 1  is selected, a control voltage VC 1  supplied from the controller  261  to the first voltage control delay unit  231  is different from control voltages VC 2  supplied to the second through fourth voltage control delay units  232  through  234 . The control voltages VC 2 , which are supplied to the second through fourth voltage control delays  232  through  234  for outputting non-selected clock signals, are the same.  
         [0038]    In the first through fourth voltage control delay units  231  through  234 , the delay time for each of the first through fourth clock signals CLK 1 -CLK 4  varies with the number of delay elements included in each of the delay units. That is, when a large number of delay elements are provided, the delay time for each of the first through fourth clock signals CLK 1  through CLK 4  becomes long (i.e., increasing the number of delay units increases the delay time). On the other hand, when a small number of delay elements are provided, the delay time for each of the first through fourth clock signals CLK 1  through CLK 4  becomes short (i.e., decreasing the number of delay units decreases the delay time). Also, the delay amount of each of the first through fourth clock signals CLK 1  through CLK 4  varies with the sizes of the control voltages VC 1  and VC 2  applied to the first through fourth voltage control delay units.  
         [0039]    A delay control method of the first through fourth voltage control delay units  231  through  234  will now be described with reference to FIGS. 6A and 6B, which are phase diagram illustrating the delay control methods performed by the first through fourth voltage control delay units  231  through  234  of FIG. 2 according to an illustrative embodiment of the invention. FIG. 6A refers to the case where the delay amount of the selected clock signal iCLK increases, and FIG. 6B refers to the case where the delay amount of the selected clock signal iCLK decreases. The selected clock signal iCLK is delayed within the delay control range of the first through fourth voltage control delay units  231  through  234 . The selected clock signal iCLK must be switched to another clock signal before its delay amount reaches the maximum or minimum value, and must be able to be continuously switched in any direction. Here, as a delay range to be covered by the first through fourth voltage control delays  231  through  234  becomes narrower, the number of delay elements included in each of the first through fourth voltage control delays  231  through  234  decreases. Thus, power consumption is reduced, and the jitter performance is improved.  
         [0040]    When a condition occurs in which the delay variation speed of the selected clock signal iCLK is three times as fast as the delay variation speeds of three unselected clock signals, switching between the clock signals CLK 1  through CLK 4  can be continuously conducted in every direction. In FIG. 6A, the initial phases of the first through fourth clock signals CLK 1  through CLK 4  exist at 0 degree, 90 degrees, 180 degrees and 270 degrees, respectively, and the first clock signal CLK 1  at 0 degree is initially selected. When the first clock signal CLK 1  is rotated counterclockwise due to an increase in its delay amount, the second through fourth clock signals CLK 2  through CLK 4  rotate clockwise, during which the first and second clock signals CLK 1  and CLK 2  meet at +67.5 degrees. Then, the second clock signal CLK 2  is selected as the clock signal iCLK, and the first, third and fourth clock signals CLK 1 , CLK 3  and CLK 4  are moved from the original position to a position departing by 7.5 degrees. FIG. 6B shows the case where the first clock signal CLK 1  is switched to the second clock signal CLK 2 , and then the second clock signal CLK 2  is rotated clockwise due to a decrease in its delay amount. When the second clock signal CLK 2  is rotated clockwise, the first, third and fourth clock signals CLK 1 , CLK 3  and CLK 4  are rotated counterclockwise. During that time, when the first and second clock signals CLK 1  and CLK 2  meet each other, the first clock signal CLK 1  is re-selected as the clock signal iCLK, while the second through fourth clock signals CLK 2  through CLK 4  are moved from the original positions to positions departing by 7.5 degrees. In this method, a delay range to be covered by a voltage control delay unit is −67.5° to +67.5°.  
         [0041]    The multiplexer  241  is connected to the first through fourth voltage control delay units  231  through  234 , and receives the first through fourth clock signals CLK 1 -CLK 4  and selects one among the first through fourth clock signals CLK 1 -CLK 4  in response to a control signal MC output from the controller  261 .  
         [0042]    The clock buffer  221  converts the voltage level of the clock signal iCLK output from the multiplexer  241  and outputs the internal clock signal fCLK. The clock buffer  221  is widely used in semiconductor devices, in particular, in SDRAM semiconductor devices. In this case, the clock buffer  221  converts the voltage level of the received clock signal iCLK into a voltage level suitable to the inside of SDRAM semiconductor devices to generate the internal clock signal fCLK.  
         [0043]    The phase detector  251  receives the internal clock signal fCLK and the external clock signal inCLK, compares the phases of the two signals to each other to generate phase information signals up and dn, and provides the phase information signals up and dn to the controller  261 . The phase detector  251  can be implemented by a typical phase detector.  
         [0044]    The controller  261  receives the phase information signals up and dn, and outputs the control voltages VC 1  and VC 2  for controlling the delay amounts of the first through fourth voltage control delay units  231  through  234  on the basis of phase information included in the phase information signals up and dn. The controller  261  also receives the first through fourth clock signals CLK 1 -CLK 4  output from the first through fourth voltage control delay units  231  through  234  and detects a phase window wherein the first through fourth clock signals CLK 1 -CLK 4  exist, to determine which clock signal is to be selected by the multiplexer  241 .  
         [0045]    A detailed block diagram of the controller  261  is shown in FIG. 4 which, as noted above, is a block diagram illustrating the second loop  213  of FIG. 2 in further detail according to an illustrative embodiment of the invention. Referring to FIG. 4, the controller  261  includes a window finder  411 , a state decoder  421 , and first and second charge pumps  431  and  432 .  
         [0046]    The window finder  411  receives the first through fourth clock signals CLK 1 - CLK 4  and a selection code signal sel, and finds a phase window where the selected clock signal iCLK exists, thereby determining whether the current selected clock signal iCLK is to be switched. FIG. 7 is a block diagram illustrating the window finder  411  of FIG. 4 in further detail according to an illustrative embodiment of the invention. Referring to FIG. 7, the window finder  411  includes inverters  711  through  714 , NAND gates  721  through  724 , D flip-flops  731  through  734 , and multiplexers  741  and  742 . The inverters  711  through  714  and the NAND gates  721  through  724  receive the first through fourth clock signals CLK 1 -CLK 4  and make a window between the clock edges of each of the first through fourth clock signals CLK 1 -CLK 4 . The window is sampled by the rising edge of the selected clock signal iCLK, to find a phase window where the current selected clock signal iCLK exists. The window information is output as signals up_sel and dn_sel while passing through the multiplexers  741  and  742  which are controlled by a selection code signal. If the selection code signal is ‘00’ and the second clock signal CLK 2  must be selected since the first clock signal has been selected, the signal up_sel is output to increase the current selection code signal. The window finder  411  is an important block to determine jitter upon switching between clock signals. Therefore, the structure of the window finder  411  must be designed to be as symmetrical as possible, to reduce a path mismatch between the paths of the first through fourth clock signals CLK 1  through CLK 4  and the path of a selected clock signal iCLK.  
         [0047]    Referring back to FIG. 4, the state decoder  421  receives the output signals up_sel and dn_sel of the window finder  411 , determines the next selection code from a current selection code, and provides a signal MC depending on the determined selection code to the multiplexer  241 . Also, the state decoder  421  provides the selection code signal sel to the window finder  411 .  
         [0048]    The first and second charge pumps  431  and  432  receive the output signals up and dn of the phase detector  251  and generate differential control voltages VC 1  and VC 2 . Here, when the first charge pump  431  is provided with the output signal up of the phase detector  251 , then the second charge pump  432  is provided with the output signal dn of the phase detector  251 . When the first charge pump  431  is provided with the output signal dn of the phase detector  251 , then the second charge pump  432  is provided with the output signal up of the phase detector  251 .  
         [0049]    [0049]FIG. 5 is a block diagram illustrating the operations of the first and second charge pumps  431  and  432  of FIG. 4 according to an illustrative embodiment of the invention. Referring to FIGS. 4 and 5, the first charge pump  431  provides the control voltage VC 1  to a voltage control delay unit for generating a selected clock signal iCLK, and the second charge pump  432  provides the control voltage VC 2  to voltage control delay units for generating non-selected clock signals. The first and second charge pumps  431  and  432  provide the same current. The capacitors  511  through  514  are connected to the first through fourth voltage control delay units  231  through  234 , respectively. When the capacities of the capacitors  511  through  514  are all the same, a selected clock signal iCLK is delayed at a speed that is three times as fast as the delay speeds of unselected clock signals. When the selected clock signal iCLK is switched to another clock signal, the first and second charge pumps  431  and  432  are switched to corresponding capacitors, so as to provide the control voltages VC 1  and VC 2  to corresponding voltage control delay units  231  through  234 , and charge re-distribution occurs in each capacitor, so that the delay control range of −67.5° to +67.5° shown in FIGS. 6A and 6B is established.  
         [0050]    [0050]FIG. 8 is a block diagram of a clock synchronization circuit  801  according to the second illustrative embodiment of the invention together with a clock buffer  221 . The same reference numerals of FIG. 8 as those of FIG. 2 denote the same elements. Referring to FIG. 8, a clock synchronization circuit  801  includes first and second loops  811  and  813 . The first loop  811  generates first and second reference clock signals RefCLK 1  and RefCLK 2  having a phase difference of  90  degrees. The second loop  813  includes first and second voltage control delay units  831  and  832 , a multiplexer  241 , a phase detector  251  and a controller  861 . The clock synchronization circuit  801  can perform the same function as the function of the clock synchronization circuit  201  shown in FIG. 2, since the first and second reference clock signals RefCLK 1  and RefCLK 2  applied to the first and second voltage control delay units  831  and  832  are differential signals and clock signals output from the first and second voltage control delay units  831  and  832  are also differential signals.  
         [0051]    [0051]FIG. 9 is a block diagram illustrating the second loop  813  of FIG. 8 in further detail according to an illustrative embodiment of the invention. Referring to FIG. 9, the controller  861  includes a charge pump  875 , a boundary detector  871  and a state decoder  873 . Each of the first and second voltage control delay units  831  and  832  includes seven delay elements. A voltage control delay unit for outputting a selected clock signal iCLK, and a voltage control delay unit for outputting non-selected clock signals, are controlled by different voltages. If clock signals CLK 3  and CLK 4  output from the second voltage control delay unit  832  are selected, the second voltage control delay unit  832  is provided with a control voltage VC from the controller  861 , while the first voltage control delay unit  831  is provided with a reference voltage Vref. The first voltage control delay unit  831  outputs first and second clock signals CLK 1  and CLK 2  of differential types having a 180° phase difference, and the second voltage control delay unit  832  outputs the third and fourth clock signals CLK 3  and CLK 4  of differential types having 90° phase differences with respect to the first and second clock signals CLK 1  and CLK 2 , respectively.  
         [0052]    The boundary detector  871  determines whether to increase or decrease a current selection code, using the phase relationship between the first through fourth clock signals CLK 1  through CLK 4 . The state decoder  873  receives the output of the boundary detector  871  and determines a next selection code from the current selection code, so as to control the multiplexer  241 . The output signal of the phase detector  251  drives the charge pump  875 . The delay amount of a selected voltage control delay unit is controlled by the control voltage VC generated by the charge pump  875 , and the delay amount of the other unselected voltage control unit is controlled by the reference voltage Vref.  
         [0053]    [0053]FIG. 10 is a phase diagram illustrating a delay control method performed by the second loop  813  of FIG. 8 according to an illustrative embodiment of the invention. In the case where the first clock signal CLK 1  is selected, when the first clock signal CLK 1  is consistent with the third clock signal CLK 3  having a 90° phase due to an increase in its delay amount, it is switched to the third clock signal CLK 3 . In the same case, when the first clock signal CLK 1  is consistent with the fourth clock signal CLK 4  having a 90° phase due to a decrease in its delay amount, it is switched to the fourth clock signal CLK 4 . Here, the first and second clock signals CLK 1  and CLK 2  maintain a 180° phase difference between them since they are differential signals, and the third and fourth clock signals CLK 3  and CLK 4  also maintain a 180° phase difference between them since they are differential signals. As described above, the delay range of the first clock signal CLK 1  is between −90° and +90°, so that the clock synchronization circuit  801  provides a wider delay range than the delay range provided by the clock synchronization circuit  201  of the first embodiment (FIG. 2).  
         [0054]    The clock synchronization circuit  801  can cover the entire phase region since continuous switching can be made at the clock boundary between two signals of the clock signals CLK 1  through CLK 4 , similar to the clock synchronization circuit  201  shown in FIG. 2. In the clock synchronization circuit  801 , each of the first and second voltage control delay units  831  and  832  has a greater number of delay elements than the number of delay elements included in each of the first through fourth voltage control delay units  231  through  234  in the clock synchronization circuit  201  of FIG. 2. However, the total number of delay elements in the clock synchronization circuit  801  is smaller than that of the delay elements in the clock synchronization circuit  201 .  
         [0055]    The clock synchronization circuits  201  and  801  according to the first and second embodiments of the present invention, respectively, can be realized in semiconductor devices, in particular, in SDRAM semiconductor devices.  
         [0056]    As described above, the clock synchronization circuits  201  and  801  according to the present invention each having a dual loop include voltage control delay units  231  through  234  and voltage control delay units  831  and  832 , respectively, such that the influence of dynamic noise is reduced and jitter performance is enhanced.  
         [0057]    Although the illustrative embodiments have been described herein with reference to the accompanying drawings, it is to be understood that the present system and method is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.