Patent Publication Number: US-2010123492-A1

Title: Control apparatus and clock synchronizing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-296635, filed on Nov. 20, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a control apparatus and a clock synchronizing method for controlling clock synchronization. 
     BACKGROUND 
     A communication apparatus such as a wireless communication base station generates, by using, for example, a digital phase locked loop (DPLL) circuit, a system clock signal (e.g., 3.84 MHz) synchronized with a clock signal extracted from a highway (HWY) line which is a digital synchronous network connected to an upper station. 
       FIG. 10  illustrates a structure example of a DPLL circuit. The DPLL circuit generates an output clock signal whose phase is synchronized with an input clock signal, as with the case of an analog phase locked loop (PLL) circuit (e.g., see Japanese Laid-open Patent Publication No. 2002-152736). In the DPLL circuit, a phase comparator, a loop filter, and an oscillator that are the components of the PLL circuit are configured to process digital signals. 
     Referring to  FIG. 10 , the DPLL circuit includes a HWY interface  5 , a phase comparator  6 , a loop filter  7 , an oscillator  8 , and a frequency divider  9 . 
     In the example illustrated in  FIG. 10 , a digital phase detector (DPD) is used as the phase comparator  6 ; a digital loop filter (DLF) is used as the loop filter  7 ; and a numerically controlled oscillator (NCO) is used as the oscillator  8 . 
     A clock signal extracted from a HWY line is input via the HWY interface  5  to the phase comparator  6  in the DPLL circuit. 
     The phase comparator  6  counts the phase difference between the input clock signal from the HWY interface  5  and an output clock signal that is from the oscillator  8  and that is frequency-divided by the frequency divider  9  (the frequency is converted by multiplying it by 1/N), and outputs the counted result as phase difference information in terms of a digital value. 
     A signal including the phase difference information output from the phase comparator  6  is input via the loop filter  7  to the oscillator  8 . 
     The oscillator  8  changes the output frequency on the basis of the input phase difference information, thereby generating an output clock signal synchronized with the input clock signal from the HWY interface  5  to the phase comparator  6 . 
     The output clock signal from the oscillator  8  is output as a system clock signal from the DPLL circuit. Also, as feedback control, the output clock signal from the oscillator  8  is frequency-divided by the frequency divider  9  and input to the phase comparator  6 . 
     Accordingly, the DPLL circuit generates an output clock signal synchronized with an input clock signal from the HWY line. 
     Next, the process of generating phase difference information in the phase comparator  6  illustrated in  FIG. 10  will be described in detail. 
     A method of detecting the phase difference between multiple input clock signals in the phase comparator  6  will now be described with reference to  FIG. 11 . 
     Referring to  FIG. 11 , a reference (REF) input clock signal indicates an input clock signal extracted from the HWY line and input to the phase comparator  6 . A feedback (FB) input clock signal indicates an output clock signal that is input, so as to perform feedback control, from the oscillator  8  to the phase comparator  6 . 
     A count clock signal is a reference clock signal that is sufficiently fast compared with the REF input clock signal and the FB input clock signal, that is, a high-frequency reference clock signal. 
     The phase comparator  6  detects the rising edge of the REF input clock signal and the FB input clock signal. The phase comparator  6  counts, using the count clock signal, the phase difference between detection of the rising edge of the REF input clock signal and detection of the rising edge of the FB input clock signal. 
       FIG. 12  illustrates a structure example of the phase comparator  6 . 
     Referring to  FIG. 12 , the phase comparator  6  includes an edge detector  301  that detects the rising edge of the REF input clock signal, an edge detector  302  that detects the rising edge of the FB input clock signal, a phase difference counter  303 , a history register  304 , and an adder circuit  305 . 
     The edge detector  301  and the edge detector  302  detect the rising edge of the REF input clock signal and the FB input clock signal, respectively, and send detection information to the phase difference counter  303 . 
     The phase difference counter  303  counts, using the count clock signal, the phase difference between the rising edge of the REF input clock signal and the rising edge of the FB input clock signal on the basis of the detection information obtained from the edge detector  301  and the edge detector  302 , and stores the counted value (hereinafter referred to as a count value) in the history register  304 . 
     The history register  304  includes multiple registers. In the multiple registers, count values  1  to m counted by the phase difference counter  303  are sequentially stored in cycles of, for example, the reciprocal of the frequency of the REF input clock signal (which may be called comparison frequency). 
     The adder circuit  305  adds the count values  1  to m stored in the history register  304  and computes the total value. The adder circuit  305  outputs the computed total value as phase difference information. 
     As a specific example, when the comparison frequency is 8 kHz and the frequency of the count clock signal is 77.76 MHz, the phase difference detection resolution ΔP of the phase comparator  6  will be computed by the following equation 1: 
     
       
         
           
             
               
                 
                   
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     When the phase difference between the input clock signals (REF input clock signal and FB input clock signal) input to the phase comparator  6  is 180 degrees in the case where the DPLL circuit is locked, that is, in the case where it is determined that the input clock signal and the output clock signal of the DPLL circuit are synchronized with each other and converge, the count value counted using the count clock signal in the locked state is obtained using the following equation 2: 
       (77.76 MHz/8 kHz)×(180 deg/360 deg)=4860count  [2] 
     In the foregoing example, the phase difference count value in the locked state changes by 4860. A fluctuation of the count value is normally ±1 to 10 counts in accordance with the degree of stableness of the oscillator  8  (e.g., about 0.1 m to 1 m Unit Interval (UI) in terms of the phase difference). Therefore, each count value stored in the history register  304  will be 4860±1 to 10. For example, as illustrated in  FIG. 13 , when eight count values are stored in the history register  304 , the total value computed by the adder circuit  305  will be 38880±8 to 80. 
     The input clock signal input to the DPLL circuit may have instantaneous phase jump due to disturbance caused by, for example, noise or cross talk that occurs in the circuit or apparatus. Even when the input clock signal has phase jump, the DPLL circuit generates an output clock signal (which may be called system clock signal) synchronized with the input clock signal. Therefore, the communication apparatus causes a system error. 
     For example, as illustrated in  FIG. 14 , when 0.1 UI instantaneous phase jump occurs, the count value obtained by the phase difference counter  303  is increased or decreased by 972 counts compared with the count value obtained in the locked state. 
     The count value based on the phase jump, which is computed by the phase difference counter  303 , is input via the loop filter  7  at a subsequent stage to the oscillator  8 , and the phase jump affects the output clock signal from the oscillator  8 . That is, the output clock signal also has phase jump in accordance with the input clock signal of the DPLL circuit. 
     In order to overcome the foregoing problem, for example, measures have been taken to change the characteristics of the loop filter  7  provided in the DPLL circuit, that is, to reduce the cut-off frequency of the loop filter  7 . 
     By reducing the cut-off frequency, although the above-described effects of the phase jump may be alleviated, the response characteristics of the DPLL circuit may become slower, or an expensive oscillator with lower phase noise characteristics may become necessary, resulting in an increase in the manufacturing cost. Accordingly, new problems are caused. 
     Japanese Laid-open Patent Publication No. 2002-152736 discloses the technique of inhibiting, in a PLL circuit, a phase comparator from performing phase comparison when it is determined that phase jump has occurred. However, when the phase comparator is inhibited from performing phase comparison, a voltage controlled crystal oscillator (VCXO) may operate by itself on the basis of a fixed value that is not in accordance with the phase difference between the input and output clock signals. Accordingly, the output clock signal may have a frequency fluctuation in accordance with the accuracy of the VCXO. 
     SUMMARY 
     According to an aspect of the invention, a control apparatus that performs control to synchronize an output clock signal with an input clock signal includes a counting part that counts a phase difference between the input clock signal and the output clock signal, an extracting part that extracts a count value indicating, of phase differences counted by the counting part, a phase difference within a certain range, and a clock generating part that generates the output clock signal, the clock generating part being capable of controlling a frequency of the output clock signal on the basis of the extracted count value indicating the phase difference within the certain range. 
     The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of a DPLL circuit; 
         FIG. 2  illustrates an example of a phase comparator; 
         FIG. 3  illustrates an operation example of the phase comparator which generates phase difference information in the case where no phase jump occurs; 
         FIG. 4  illustrates an operation example of a phase-jump determining circuit when phase jump occurs; 
         FIG. 5  illustrates an operation example of a phase-jump correcting circuit when phase jump occurs; 
         FIG. 6  illustrates an operation example of a selector circuit when phase jump occurs; 
         FIG. 7  illustrates an operation example of the phase-jump determining circuit in the case where the occurrence of phase jump is instantaneous; 
         FIG. 8  illustrates an operation example of the selector circuit in the case where the occurrence of phase jump is instantaneous; 
         FIG. 9  illustrates an operation example of the phase comparator which generates phase difference information in the case where the occurrence of phase jump is not instantaneous; 
         FIG. 10  illustrates an example of a DPLL circuit in the related art; 
         FIG. 11  illustrates a method of detecting a phase difference between multiple input clock signals in a phase comparator; 
         FIG. 12  illustrates an example of a phase comparator in the related art; 
         FIG. 13  illustrates an operation example of the phase comparator in the related art, which generates phase difference information (in the case where no phase jump occurs); and 
         FIG. 14  illustrates an operation example of the phase comparator in the related art, which generates phase difference information in the case where phase jump occurs. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will be described with reference to the drawings. 
     (1) DPLL Circuit 
       FIG. 1  illustrates an example of a DPLL circuit used in the present embodiment. 
     Referring to  FIG. 1 , the DPLL circuit includes a phase comparator  1 , a loop filter  2 , an oscillator  3 , and a frequency divider  4 . 
     For example, a DPD is used as the phase comparator  1 ; a DLF is used as the loop filter  2 ; and an NCO is used as the oscillator  3 . The loop filter  2  is, for example, an infinite impulse response (IIR) filter, and a digital signal processor (DSP), for example, is used as the loop filter  2 . 
     A clock signal extracted from an external line (e.g., a digital synchronous network connected to an upper station) is input to the phase comparator  1  in the DPLL circuit. 
     The phase comparator  1  counts the phase difference between the input clock signal extracted from the external line and an output clock signal that is from the oscillator  3  and that is frequency-divided by the frequency divider  4  (the frequency is converted by multiplying it by 1/N), and outputs the counted result as phase difference information in terms of a digital value. 
     A signal including the phase difference information output from the phase comparator  1  is filtered by the loop filter  2 , and the filtered signal is input to the oscillator  3 . 
     The oscillator  3  generates an output clock signal synchronized with the input clock signal, which is input from the external line to the phase comparator  1 , by changing the output frequency on the basis of the input phase difference information. 
     The output clock signal generated by the oscillator  3  is output as a system clock signal from the DPLL circuit. Also, as feedback control, the output clock signal from the oscillator  3  is frequency-divided by the frequency divider  4  and input to the phase comparator  1 . 
     Accordingly, the DPLL circuit of the present embodiment generates an output clock signal synchronized with an input clock signal from the external line. 
     Hereinafter, a clock signal extracted from the external line and input to the phase comparator  1  will be referred to as a REF input clock signal, and a clock signal output from the oscillator  3 , frequency-divided, and then input to the phase comparator  1  will be referred to as a FB input clock signal. 
     (2) Phase Comparator 
       FIG. 2  illustrates an example of the phase comparator used in the present embodiment. 
     Referring to  FIG. 2 , the phase comparator  1  includes an edge detector  101  that detects the rising edge of the REF input clock signal, an edge detector  102  that detects the rising edge of the FB input clock signal, a phase difference counter  103 , a history register  104 , an adder circuit  105 , a phase-jump determining circuit  106 , a phase-jump correcting circuit  107 , and a selector circuit  108 . 
     The edge detector  101  detects the rising edge of the REF input clock signal, and sends detection information to the phase difference counter  103 . Similarly, the edge detector  102  detects the rising edge of the FB input clock signal, and sends detection information to the phase difference counter  103 . 
     The phase difference counter  103  counts, using a count clock signal, the phase difference between the rising edge of the REF input clock signal and the rising edge of the FB input clock signal on the basis of the detection information obtained from the edge detector  101  and the edge detector  102  by using, for example, the method described with reference to  FIG. 11 , and stores the counted value (hereinafter referred to as a count value) in the history register  104 . 
     The history register  104  includes multiple registers. In the multiple registers, count values  1  to m counted by the phase difference counter  103  are sequentially stored. When the count values are updated, that is, when a new count value is stored as a count value  8  in the history register  104 , count values  2  to  8  before being updated are shifted to become count values  1  to  7 , and the count value  1  before being updated is deleted from the history register  104 . The cycle of updating the count values may be the reciprocal of the frequency of the REF input clock signal (comparison frequency). 
     The adder circuit  105  extracts the count values  1  to m stored in the history register  104  and computes the total value by adding the extracted count values  1  to m. The adder circuit  105  generates phase difference information by using the computed total value, and outputs the phase difference information. 
     The phase-jump determining circuit  106  extracts the count values stored in the history register  104  and, on the basis of comparison between the extracted count values with a preset count value range, determines whether phase jump has occurred in the REF input clock signal. That is, the phase-jump determining circuit  106  determines that no phase jump has occurred when the count values stored in the history register  104  each indicate a phase difference within a certain range, and determines that phase jump has occurred when any of the count values stored in the history register  104  indicates a phase difference that is not within the certain range. 
     A count value indicating a phase difference within the certain range, which serves as the foregoing determination criterion in the phase-jump determining circuit  106 , may be appropriately set. For example, in the case of a base station apparatus, it is preferable that the count value correspond to a count value in a locked state±phase difference 0.1 mUI to 1 mUI. 
     When the phase-jump determining circuit  106  determines, on the basis of the count values stored in the history register  104 , that no phase jump has occurred, the phase-jump determining circuit  106  sends a signal indicating that no phase jump has occurred to the selector circuit  108 . When the phase-jump determining circuit  106  determines, on the basis of any of the count values stored in the history register  104 , that phase jump has occurred, the phase-jump determining circuit  106  sends a signal indicating that phase jump has occurred to the phase-jump correcting circuit  107  and the selector circuit  108 . Alternatively, the phase-jump determining circuit  106  may not necessarily send a signal indicating that no phase jump has occurred to the selector circuit  108  when the phase-jump determining circuit  106  determines that no phase jump has occurred. 
     Upon receipt of the signal from the phase-jump determining circuit  106  indicating that phase jump has occurred, the phase-jump correcting circuit  107  extracts the count values stored in the history register  104  other than the count value based on which the occurrence of phase jump has been determined. Using values obtained by applying an operation described later to the extracted count values, the phase-jumping correcting circuit  107  generates phase difference information, and outputs the phase difference information. 
     The items of phase difference information output from the adder circuit and the phase-jump correcting circuit  107  are input to the selector circuit  108 . 
     On the basis of the signal indicating that no phase jump has occurred or the signal indicating that phase jump has occurred, which is received from the phase-jump determining circuit  106 , the selector circuit  108  selects the phase difference information to output. That is, when the signal indicating that no phase jump has occurred is received or when no signal indicating that phase jump has occurred is received, the selector circuit  108  outputs the phase difference information from the adder circuit  105 . When the signal indicating that phase jump has occurred is received, the selector circuit  108  outputs the phase difference information from the phase-jump correcting circuit  107 . 
     The phase difference information output from the selector circuit  108  is input to the loop filter  2  (which is not illustrated in  FIG. 2 ). 
     (3) Details of Phase Difference Information Generating Operation 
     Referring to  FIGS. 3 to 8 , the operation of generating phase difference information using the phase comparator illustrated in  FIG. 2  will be specifically described. In  FIGS. 3 to 8 , parts common to those in  FIG. 2  are given the same reference numerals. 
     In the following description, for example, it is assumed that the frequency of the REF input clock signal (which may be called comparison frequency) is 8 kHz, the frequency of the count clock signal is 77.76 MHz, and the number of registers in the history register  104  is 8 (m=8). 
     In the above example, when the phase difference relationship in a locked state is 180 degrees, equation 2 gives that the count value obtained by using the count clock signal, which indicates the phase difference between the REF input clock signal and the FB input clock signal in a locked state, is 4860. Thus, for example, it is set that a count value indicating a phase difference within a certain range set in the phase-jump determining circuit  106  is 4860±1 to 10. 
       FIG. 3  illustrates the case in which all the count values  1  to  8  stored in the history register  104  are within the range of 4860±1 to 10. 
     When the count value counted by the phase difference counter  103  is stored as the count value  8  in the history register  104 , the phase-jump determining circuit  106  determines that no phase jump has occurred in the count value  8 . Thus, the selector circuit  108  outputs the phase difference information generated by the adder circuit  105 . The phase difference information generated by the adder circuit  105  is information indicating a total value 38880±8 to 80, which is obtained by adding all the count values  1  to  8 . 
       FIGS. 4 to 6  illustrate the case in which phase jump has occurred in the REF input clock signal, and the count value  8  becomes 5832, which exceeds the range of 4860±1 to 10. It is assumed that the count values  1  to  7  already stored in the history register  104  are within the range of 4860±1 to 10. 
     Referring to  FIG. 4 , the phase-jump determining circuit  106  determines that phase jump has occurred in the count value  8 , and sends a signal indicating that phase jump has occurred to the phase-jump correcting circuit  107  and the selector circuit  108  (which is not illustrated in  FIG. 4 ). 
     Referring to  FIG. 5 , when the phase-jump correcting circuit  107  receives the signal indicating that phase jump has occurred in the count value  8 , the phase-jump correcting circuit  107  calculates an operation using the count values  1  to  7  excluding the count value  8 . Specifically, the value 34020±7 to 70, which is the sum of count values of the count values  1  to  7 , is multiplied by 8/7, which is “the number of count values stored in the history register  104 ” divided by “the number of count values indicating a phase difference within the certain range”. The phase-jump correcting circuit  107  outputs phase difference information indicating the value 38880±8 to 80, which is the value obtained by calculating the operation. 
       FIG. 6  illustrates the operation of the selector circuit  108  when phase jump has occurred. 
     When the selector circuit  108  receives a signal indicating that phase jump has occurred from the phase-jump determining circuit  106 , the selector circuit  108  performs control to select and output, of the phase difference information generated by the adder circuit  105  and the phase difference information generated by the phase-jump correcting circuit  107 , the phase difference information generated by the phase-jump correcting circuit  107 . Therefore, the selector circuit  108  outputs the phase difference information indicating the value 38880±8 to 80, which is the value computed by the phase-jump correcting circuit  107  in  FIG. 5 . 
     Accordingly, phase jump occurred in the count value  8  may be prevented from affecting the control operation performed in the oscillator  3 . That is, even when instantaneous phase jump occurs in an input clock signal input to the phase comparator  1  in the DPLL circuit, an output clock signal generated by the oscillator  3  may be prevented from suddenly fluctuating due to the occurrence of phase jump. 
     (4) Modification 
     In the above-described embodiment, when instantaneous phase jump occurs in an input clock signal input to the phase comparator  1  in the DPLL circuit in a locked state, an output clock signal generated by the oscillator  3  may be prevented from suddenly fluctuating due to the occurrence of phase jump. 
     In contrast, in this modification, when phase jump in an input clock signal input to a phase comparator is instantaneous, the same operation as the above-described embodiment is performed. Furthermore, a function is added to control an output clock signal output from an oscillator to fluctuate in accordance with phase jump in an input clock signal when this phase jump is not instantaneous, that is, when the input clock signal and the output clock signal consecutively produce phase differences that are not within a certain range. 
     That is, in this modification, for example, a phase-jump determining circuit determines whether phase jump that occurs in an input clock signal input to a phase comparator is instantaneous, on the basis of count values stored in a history register. 
     The case in which the input clock signal and the output clock signal consecutively produce phase differences that are not within a certain range is the case in which normal pulling in occurs. 
     Referring now to  FIGS. 7 to 9 , a specific operation in this modification will be described. 
       FIGS. 7 to 9  illustrate a phase difference counter  203 , a history register  204 , an adder circuit  205 , a phase-jump determining circuit  206 , a phase-jump correcting circuit  207 , and a selector circuit  208 . Parts that are common in  FIGS. 7 to 9  are given the same reference numerals. 
     As with the description of the above-described embodiment, it is assumed that the frequency of a REF input clock signal (which may be called comparison frequency) is 8 kHz, the frequency of a count clock signal is 77.76 MHz, the number of registers in the history register  204  is 8 (m=8), and a count value indicating a phase difference within a certain range set in the phase-jump determining circuit  206  is 4860±1 to 10. 
       FIGS. 7 and 8  illustrate the operation of the phase-jump determining circuit  206  in the case where phase jump that occurs in the REF input clock signal is instantaneous. Specifically,  FIGS. 7 and 8  illustrate the case in which, by updating count values stored in the history register  204 , a count value indicating a phase difference within the certain range (count value  8  in  FIG. 7 ) is stored after a count value indicating a phase difference that is not within the certain range (count value  7  in  FIG. 7 ). 
     In the foregoing case, the phase-jump determining circuit  206  determines that phase jump has occurred in the count value  7  indicating the count value 5832 (=4860+972) stored in the history register  204 . Also, the phase-jump determining circuit  206  determines that no phase jump has occurred in the count value  6  which is before the count value  7  or in the count value  8  which is after the count value  7  since the count values  6  and  8  are each a count value indicating a phase difference that is within the certain range, and determines that the phase jump in the count value  7  is instantaneous. 
     Therefore, on the basis of the determination result, the phase-jump determining circuit  206  sends a signal indicating that phase jump has occurred to the phase-jump correcting circuit  207  and the selector circuit  208  (which is not illustrated in  FIG. 7 ). 
     Referring to  FIG. 8 , the adder circuit  205  extracts the count values  1  to  8  stored in the history register  204  illustrated in  FIG. 7 , and computes the total value by adding the extracted count values  1  to  8 . In this case, phase difference information generated by the adder circuit  205  is information indicating the total value 39852±7 to 70, which is obtained by adding all the count values  1  to  8 . Using the computed total value, the adder circuit  205  generates phase difference information, and outputs the phase difference information. 
     When the phase-jump correcting circuit  207  receives from the phase-jump determining circuit  206  the signal indicating that phase jump has occurred in the count value  7 , the phase-jump correcting circuit  207  calculates an operation similar to the case illustrated in  FIG. 5  by using the count values  1  to  6  and  8  excluding the count value  7 . Specifically, the sum 34020±7 to 70, which is obtained by adding count values of the count values  1  to  6  and  8 , is multiplied by 8/7. The phase-jump correcting circuit  207  outputs phase difference information indicating the value 38880±8 to 80, which is obtained by calculating the operation. 
     The selector circuit  208  selects phase difference information to output, on the basis of a signal indicating that no phase jump has occurred or a signal indicating that phase jump has occurred, and a signal indicating that phase differences that are not within the certain range have consecutively occurred in the input clock signal, which are received from the phase-jump determining circuit  206 . That is, when no signal indicating that phase jump has occurred is received, when a signal indicating that no phase jump has occurred is received, or when a signal indicating that phase differences that are not within the certain range have consecutively occurred in the input clock signal is received, the selector circuit  208  outputs the phase difference information from the adder circuit  205 . In contrast, when a signal indicating that phase jump has occurred is received, the selector circuit  208  outputs the phase difference information from the phase-jump correcting circuit  207 . 
     When the selector circuit  208  receives a signal indicating that phase jump has occurred from the phase-jump determining circuit  206 , the selector circuit  208  performs control to select and output, of the phase difference information generated by the adder circuit  205  and the phase difference information generated by the phase-jump correcting circuit  207 , the phase difference information generated by the phase-jump correcting circuit  207 . Therefore, the selector circuit  208  outputs the phase difference information indicating the value 38880±8 to 80, which is the value computed by the phase-jump correcting circuit  207 . 
     Next,  FIG. 9  illustrates the operation of the phase-jump determining circuit  206  in the case where phase jump in the REF input clock signal is not instantaneous, that is, phase differences that are not within the certain range consecutively occur. Specifically,  FIG. 9  illustrates the case in which, by updating count values stored in the history register  204 , a count value indicating a phase difference that is not within the certain range (count value  8  in  FIG. 9 ) is stored after a count value indicating a phase difference that is not within the certain range (count value  7  in  FIG. 9 ). 
     When the count value counted by the phase difference counter  203  is stored as the count value  8  in the history register  204 , the count values in the history register  204  are updated. 
     The adder circuit  205  extracts the count values  1  to  8  stored in the history register  204 , and computes the total value by adding the extracted count values  1  to  8 . In this case, phase difference information generated by the adder circuit  205  is information indicating the total value 40824±6 to 60, which is the value obtained by adding all the count values  1  to  8 . Using the computed total value, the adder circuit  205  generates phase difference information, and outputs the phase difference information. 
     The phase-jump determining circuit  206  extracts the count values stored in the history register  204 , and determines that phase jump has occurred in both the count value  7  and the count value  8  that is newly stored by the foregoing update, both of which indicate the count value 5832 (=4860+972). Also, since the phase-jump determining circuit  206  determines that phase jump has occurred in multiple count values that are consecutively stored in the history register  204 , the phase-jump determining circuit  206  determines that the phase jump in the count values  7  and  8  is not instantaneous, that is, phase differences that are not within the certain range have consecutively occurred in the input clock signal. A criterion for determining that phase differences that are not within the certain range have consecutively occurred in the input clock signal, that is, the number of times the phase jump has consecutively occurred in count values, can appropriately be set to 2 or greater. 
     Therefore, on the basis of the determination result, the phase-jump determining circuit  206  sends a signal indicating that phase jump has occurred to the phase-jump correcting circuit  207 , and sends a signal indicating that phase differences that are not within the certain range have consecutively occurred in the input clock signal to the selector circuit  208 . 
     Upon receipt of the signal indicating that phase jump has occurred in the count values  7  and  8 , the phase-jump correcting circuit  207  calculates an operation using the count values  1  to  6  excluding the count values  7  and  8 . Specifically, as in the case of  FIG. 5 , the sum 29160±6 to 60, which is obtained by adding count values of the count values  1  to  6 , is multiplied by 8/6. The phase-jump correcting circuit  207  outputs phase difference information indicating the value 38880±8 to 80, which is obtained by calculating the operation. 
     When the selector circuit  208  receives from the phase-jump determining circuit  206  a signal indicating that phase differences that are not within the certain range have consecutively occurred in the input clock signal, the selector circuit  208  performs control to select and output, of the phase difference information generated by the adder circuit  205  and the phase difference information generated by the phase-jump correcting circuit  207 , the phase difference information generated by the adder circuit  205 . Therefore, the selector circuit  208  outputs the phase difference information indicating the value 40824±6 to 60, which is the value computed by the adder circuit  205 . 
     In the above description, on the basis of the determination result, the phase-jump determining circuit  206  sends a signal indicating that phase jump has occurred to the phase-jump correcting circuit  207 . Alternatively, when it is determined that phase differences that are not within the certain range have consecutively occurred in the input clock signal, the phase-jump determining circuit  206  may not send a signal indicating that phase jump has occurred to the phase-jump correcting circuit  207 . 
     Accordingly, when phase jump in an input clock signal input to the phase comparator is instantaneous, the same operation as the above-described embodiment is performed. Furthermore, when phase jump in an input clock signal is not instantaneous, that is, when phase differences that are not within the certain range consecutively occur, an output clock signal output from the oscillator may be controlled to fluctuate in accordance with the phase jump. 
     Accordingly, when instantaneous phase jump occurs in an input clock signal input to the phase comparator in the DPLL circuit, an output clock signal output from the oscillator may be prevented from suddenly fluctuating due to the occurrence of phase jump. When phase differences that are not within the certain range consecutively occur in the input clock signal, an output clock signal output from the oscillator may be controlled so as to suppress the phase differences. 
     (5) Others 
     In the above-described embodiment and the modification of the embodiment, when the phase-jump determining circuit determines that phase jump has occurred, the phase-jump determining circuit may alternatively send a signal indicating that phase jump has occurred to the adder circuit. The adder circuit may be controlled not to add the count values stored in the history register upon receipt of the signal indicating that phase jump has occurred. 
     In the above-described embodiment and the modification of the embodiment, among the count values stored in the history register, a count value indicating a phase difference that is not within a set certain range may alternatively be replaced with a count value indicating a phase difference within the certain range, or may be deleted. 
     According to an embodiment of the present invention, the occurrence of phase jump in an output clock signal due to the effect of instantaneous phase jump that occurs in an input clock signal in a DPLL circuit may be suppressed. 
     According to an embodiment of the present invention, the occurrence of phase jump in an output clock signal may be suppressed without changing the setting of the cut-off frequency of a loop filter provided in the DPLL circuit. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention(s) has (have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.