Abstract:
A clock data recovery circuit may include: a phase comparison unit suitable for comparing input data with a phase of a multi-phase clock, and for generating an up/down signal corresponding to the comparison result; a filtering unit suitable for counting the up/down signal based on an upper threshold value and a lower threshold value, for setting, when an overflow occurs, the lower threshold value to an initial value for the count of the up/down signal, or when a underflow occurs, the upper threshold value to the initial value for the count of the up/down signal, and for generating a control code corresponding to one of the underflow and the overflow; and a phase rotating unit suitable for adjusting the phase of the multi-phase clock in response to the control code outputted from the filtering unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2016-0017027, filed on Feb. 15, 2016, in the Korean Intellectual Property Office, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Exemplary embodiments relate to a semiconductor design technology and, more particularly, to a clock data recovery (CDR) circuit. 
     DISCUSSION OF THE RELATED ART 
     Generally, a system performing serial data communication through a small number of data buses uses a clock data recovery method. The clock data recovery method generates a clock signal to be a reference from serial data, and uses the generated clock signal as a strobe signal for receiving data. Therefore, generally a transmitter may transmit data having information related to the clock signal, and a receiver may include therein a clock data recovery (CDR) circuit for generating a clock signal from the data and receive, in synchronization with the generated clock signal, data transmitted from the transmitter. 
     Typically, for minimizing signal distortion due to a noise or jitter and increasing a valid window for the data, the CDR circuit of the receiver compares the phase of the clock signal generated from the inputted clock signal data with a transition time of the inputted data and adjusts the phase of the clock signal. 
       FIG. 1  is a block diagram illustrating a conventional CDR circuit  10 . 
     Referring to  FIG. 1 , the CDR circuit  10  includes a sampler  12 , a phase detector  14 , a digital loop filter (DLF)  16  and a phase rotator  18 . 
     The sampler  12  samples input data DIN using a multi-phase clock R_CLK&lt;0:15&gt; provided from the phase rotator  18 , and outputs phase shift information. Furthermore, the sampler  12  samples the input data DIN according to the multi-phase clock R_CLK&lt;0:15&gt; and generates output data DOUT. The phase detector  14  receives the phase shift information outputted from the sampler  12 , and outputs an up/down signal UP/DN corresponding to a period in which data transition occurs. The DLF  16  may be implemented with a filtering circuit, and receives the up/down signal UP/DN outputted from the phase detector  14 , and outputs a control code CTRL&lt;0:m&gt; for controlling the operation of the phase rotator  18 . The phase rotator  18  receives a clock CLK&lt;0:15&gt; from a clock generator (not shown) such as a phase lock loop (PLL), generates the multi-phase clock R_CLK&lt;0:15&gt;, and adjusts the phase of the multi-phase clock R_CLK&lt;0:15&gt; in response to the control code CTRL&lt;0:m&gt; outputted from the DLF  16 . 
     As described above, the CDR circuit  10  receives the input data DIN and outputs the output data DOUT using the multi-phase clock R_CLK&lt;0:15&gt; having a phase adjusted depending on the phase of the input data DIN. 
     In the CDR circuit  10 , a delay time of several cycles required for the DLF  16  to perform digital filtering is called loop latency. Due to such loop latency, a limit-cycle phenomenon (i.e., a bang-bang error) is exacerbated, and the jitter of the CDR circuit  10  is accordingly increased. In the DLF  16 , to reduce the limit-cycle phenomenon, only when several up/down signals UP/DN are collected and the number thereof becomes a predetermined number or more, the control code CTRL&lt;0:m&gt; is provided to the phase rotator  18 . In other words, the DLF  16  counts the up/down signal UP/DN and is able to provide the control code CTRL&lt;0:m&gt; only when an underflow or overflow occurs. Thereafter, the DLF  16  initializes a counter provided therein, and then counts a next up/down signal UP/DN. 
     However, in a state in which the counter provided in the DLF  16  has been initialized, when the phase of the multi-phase clock R_CLK&lt;0:15&gt; of the CDR circuit  10  fluctuates due to a noise generated from the input data DIN or a random noise generated from the phase rotator  18  itself, many cycles are consumed to recover the phase, and a lot of operating time is thus required. 
     SUMMARY 
     Various embodiments are directed to a clock data recovery (CDR) circuit capable of being rapidly recovered, when a noise occurs, to its original state by initializing, using a lower threshold value and an upper threshold value, a counter provided in a digital loop filter, and an integrated circuit including the same, and a CDR method. 
     Also, various embodiments are directed to a CDR circuit capable of realizing the same CDR bandwidth (BW) as that of a conventional circuit despite using a counter having a depth of ½ of that of the conventional circuit, and an integrated circuit including the same, and a CDR method. 
     In an embodiment, a clock data recovery circuit may include: a phase comparison unit suitable for comparing input data with a phase of a mufti-phase clock, and for generating an up/down signal corresponding to the comparison result; a filtering unit suitable for counting the up/down signal based on an upper threshold value and a lower threshold value, for setting, when an overflow occurs, the lower threshold value to an initial value for the count of the up/down signal, or when a underflow occurs, the upper threshold value to the initial value for the count of the up/down signal and for generating a control code corresponding to one of the underflow and the overflow; and a phase rotating unit suitable for adjusting the phase of the multi-phase clock in response to the control code outputted from the filtering unit. 
     In an embodiment, an integrated circuit may include: an up/down signal generation unit suitable for comparing a reference signal and a feedback signal and for generating an up/down signal including phase shift information; an underflow/overflow prediction unit suitable for predicting, based on the up/down signal and a sum signal, an underflow/overflow and for generating a control code, and for outputting a select signal for selecting a lower threshold value when the overflow occurs, and for selecting an upper threshold value when the underflow occurs; a counting unit suitable for counting, based on the upper threshold value and the lower threshold value, the up/down signal and for outputting the sum signal, and for setting, when the underflow or overflow occurs, an initial value of the sum signal in response to the select signal; and a feedback unit suitable for generating the feedback signal in response to the control code. 
     In an embodiment, a clock data recovery method may include: comparing input data and a phase of a multi-phase clock and generating an up/down signal corresponding to the comparison result; counting the up/down signal based on an upper threshold value and a lower threshold value and outputting a sum signal; predicting, based on the up/down signal and the sum signal, an underflow/overflow and generating a control code; and adjusting the multi-phase clock in response to the control code, wherein the lower threshold value is set to an initial value of the sum signal when the overflow occurs, and the upper threshold value is set to an initial value of the sum signal when the underflow occurs. 
     When the up/down signal indicating an up state is inputted in a state in which the sum signal has reached the upper threshold value, the control code indicating occurrence of the overflow may be generated, and the lower threshold value may be set to the initial value of the sum signal. When the up/down signal indicating a down state is inputted in a state in which the sum signal has reached the upper threshold value, the control code indicating occurrence of the underflow may be generated, and the upper threshold value may be set to the initial value of the sum signal. The comparing of the input data and the phase of the multi-phase clock and the generating of the up/down signal corresponding to the phase shift information may comprise: sampling the input data using the multi-phase clock and outputting the phase shift information; and receiving the phase shift information and generating the up/down signal corresponding to a period in which data transition occurs. 
     In an embodiment, a clock data recovery circuit may include: a data transition detection unit suitable for receiving input data, detecting transition of the input data based on a multi-phase clock, and generating transition information indicating the transition of the input data; a digital unit suitable for counting the transition information, and generating one of overflow information and underflow information, the overflow information indicating an overflow corresponding that the transition information is counting by an upper threshold value and the overflow information indicating an underflow corresponding that the transition information is counting by a lower threshold value; and a phase adjusting unit suitable for adjusting the phase of the multi-phase clock for controlling timing of output data corresponding to the input data based on the one of the overflow information and the underflow information, wherein the digital unit is configured to set the lower threshold value to an initial value for the counting when the overflow occurs, and set the upper threshold value to the initial value of the counting when an underflow occurs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a general clock data recovery (CDR) circuit. 
         FIG. 2  is a circuit diagram illustrating a digital loop filter (DLF), according to an embodiment of the present invention. 
         FIG. 3  is a flowchart illustrating an algorithm of the digital loop filter (DLF) shown in  FIG. 2 . 
         FIG. 4  is a block diagram illustrating a phase rotating unit, according to an embodiment of the present invention. 
         FIGS. 5A and 5B  are waveform diagrams illustrating the operation of the conventional digital loop filter (DLF) and of an inventive DLF, according to an embodiment of the present invention, respectively, under a first condition. 
         FIGS. 6A and 6B  are waveform diagrams illustrating the operation of the conventional DLF and of the inventive DLF, respectively, under a second condition. 
         FIGS. 7A and 7B  are waveform diagrams illustrating the operation of the conventional DLF and of the inventive DLF, respectively, under a third condition. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present disclosure. 
       FIG. 2  illustrates a digital loop filter DLF  100 , according to an embodiment of the present invention. 
     According to the embodiment of  FIG. 2  the DLF  10  may include a counting unit  120 , an underflow/overflow prediction unit  140 , and a control code generation unit  160 . 
     The underflow/overflow prediction unit  140  predicts, based on an up/down signal UP/DN and a sum signal SUM, an underflow/overflow and generates a loop output signal DLF OUT&lt;0:1&gt; indicating occurrence of an underflow or overflow. Furthermore, the underflow/overflow prediction unit  140  outputs a select signal SEL for selecting one of a lower threshold value MIN and an upper threshold value MAX. For example, when an overflow occurs, the underflow/overflow prediction unit  140  outputs the select signal SEL for selecting the lower threshold value MIN. Also, for example, when an underflow occurs, the underflow/overflow prediction unit  140  outputs the select signal SEL for selecting the upper threshold value MAX. In more detail, the underflow/overflow prediction unit  140  may generate, when an up/down signal UP/DN indicating an up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow and output a select signal SEL for selecting the lower threshold value MIN. In addition, the underflow/overflow prediction unit  140  may generate, when an up/down signal UP/DN indicating a down state is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an underflow and output a select signal SEL for selecting the upper threshold value MAX. 
     The counting unit  120  counts the up/down signal UP/DN based on the upper threshold value MAX and the lower threshold value MIN, and output the sum signal SUM. Furthermore, the counting unit  120  sets an initial value of the sum signal SUM in response to the select signal SEL when an underflow or overflow occurs. For reference, the up/down signal UP/DN may be a signal that is outputted from a phase detector (e.g., the phase detector  14  of  FIG. 1 ) and inputted to the DLF  100 . The up/down signal UP/DN may include phase shift information. 
     In more detail, according to the embodiment of  FIG. 2 , the counting unit  120  may include an adder  122 , a multiplexer  124 , and a plurality of flip-flops  126 _ 1  to  126 _N. 
     The adder  122  adds the up/down signal UP/DN and the sum signal SUM and outputs a preliminary sum signal P_SUM. The multiplexer  124  then selects and outputs, in response to the select signal SEL outputted from the underflow/overflow prediction unit  140 , any one of the preliminary sum signal P_SUM, the lower threshold value MIN and the upper threshold value MAX. The plurality of flip-flops  126 _ 1  to  126 _N latch an output signal of the multiplexer  124  in synchronization with a digital operating clock CLK_DIG and output the sum signal SUM. In the embodiment of  FIG. 2 , it is illustrated, as an example, the case where each of the preliminary sum signal P_SUM and the sum signal SUM is a signal constructed by eight bits (i.e., N=8) and, accordingly, the plurality of flip-flops  126 _ 1  to  126 _N are eight flip-flops  126 _ 1  to  126 _ 8 . However, the invention is not limited in this way. 
     As described above, the counting unit  120  adds, when the up/down signal UP/ON is inputted, the up/down signal UP/DN and the preliminary sum signal P_SUM that has been previously calculated, and outputs the sum signal SUM. When a select signal SEL indicating occurrence of an overflow is inputted from the underflow/overflow prediction unit  140 , the lower threshold value MIN is outputted as the initial value of the sum signal SUM. When a select signal SEL indicating occurrence of an underflow is inputted from the underflow/overflow prediction unit  140 , the upper threshold value MAX is outputted as the initial value of the sum signal SUM. 
     The control code generation unit  160 , according to the embodiment of  FIG. 2 , outputs a control code CTRL&lt;0:m&gt; based on the loop output signal DLF_OUT&lt;0:1&gt;. 
     The control code generation unit  160  may include an accumulator  162 , a decoder  164 , and an output unit  166 . 
     The accumulator  162  accumulates and adds loop output signals DLF_OUT&lt;0:1&gt; in synchronization with the digital operating clock CLK_DIG. The accumulator  162  then outputs an accumulation signal ACCM&lt;0:k&gt;. In an embodiment, the accumulator  162  may add, in synchronization with the digital operating clock CLK_DIG, a previous accumulation value of a two-bit loop output signal DLF_OUT&lt;0:1&gt; and a current value of the two-bit loop output signal DLF_OUT&lt;0:1&gt; and generate a five-bit accumulation signal ACCM&lt;0:5&gt; (that is, k=5). 
     The decoder  164  decodes the accumulation signal ACCM&lt;0:k&gt; and generates a decoded signal DEC&lt;0:m&gt;. The output unit  166  synchronizes the decoded signal DEC&lt;0:m&gt; with the digital operating clock CLK_DIG and outputs the control code CTRL&lt;0:m&gt;. 
       FIG. 3  is a flowchart illustrating an algorithm of the digital loop filter (DLF)  100  shown in  FIG. 2 . 
     Referring to  FIG. 3  first, initial values of the respective signals are set at step S 100 . For example, the preliminary sum signal P_SUM may be set to ‘0’, the sum signal SUM may be set to ‘0’, the upper threshold value MAX may be set to ‘+2’, and the lower threshold value MIN may be set to ‘−2’. The upper threshold value MAX and the lower threshold value MIN may be set depending on design options. 
     For reference, the up/down signal UP/DN may be constructed by a two-bit signal and be set as follows. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 DN 
                 UP 
                 OUTPUT VALUE 
                 STATE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 0 
                 NOP 
               
               
                 0 
                 1 
                 +1 
                 UP 
               
               
                 1 
                 0 
                 −1 
                 DOWN 
               
               
                 1 
                 1 
                 X 
                 — 
               
               
                   
               
             
          
         
       
     
     Like rise, the two-bit loop output signal DLF_OUT&lt;0:1&gt; may be set as follows. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 DLF_OUT&lt;1&gt; 
                 DLF_OUT&lt;0&gt; 
                 OUTPUT VALUE 
                 STATE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 0 
                 NO FLOW 
               
               
                 0 
                 1 
                 +1 
                 OVERFLOW 
               
               
                 1 
                 0 
                 −1 
                 UNDERFLOW 
               
               
                 1 
                 1 
                 X 
                 — 
               
               
                   
               
             
          
         
       
     
     The counting unit  120  adds the up/down signal UP/DN and the sum signal SUM and outputs the preliminary sum signal P_SUM at step S 110 . 
     The underflow/overflow prediction unit  140  predicts, based on the up/down signal UP/DN and the sum signal SUM, an underflow/overflow and generates a loop output signal DLF OUT&lt;0:1&gt; indicating an underflow or an overflow. 
     In this regard when an up/down signal UP/DN indicating an up state (i.e., ‘+1’) is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX at step S 120 , the underflow/overflow prediction unit  140  generates a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow (i.e., ‘+1’), and outputs a select signal SEL for selecting the lower threshold value MIN, and the counting unit  120  enables the sum signal SUM to be initialized to the lower threshold value MIN in response to the select signal SEL at step S 130 . 
     On the other hand, when an up/down signal UP/DN indicating a down state (i.e., ‘−1’) is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN at step S 140 , the underflow/overflow prediction unit  140  generates a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an underflow (i.e., ‘−1’), and outputs a select signal SEL for selecting the upper threshold value MAX, and the counting unit  120  enables the sum signal SUM to be initialized to the upper threshold value MAX in response to the select signal SEL at step S 150 . 
     If the sum signal SUM is not in a state in which it has reached the upper threshold value MAX or the lower threshold value MIN, the underflow/overflow prediction unit  140  generates a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of no-flow (i.e. ‘0’), and outputs a select signal SEL for selecting a preliminary sum signal P_SUM, and the counting unit  120  enables the preliminary sum signal P_SUM to be outputted as the sum signal SUM in response to the select signal SEL at step S 160 . 
     The above-mentioned operations S 110  to S 160  may be continuously repeated. 
     The control code generation unit  160  may output the control code CTRL&lt;0:m&gt; based on the loop output signal DLF_OUT&lt;0:1&gt; outputted from the underflow/overflow prediction unit  140 . 
     The conventional DLF counts the up/down signal UP/DN based on the upper threshold value MAX and the lower threshold value MIN and outputs the sum signal SUM, but initialize, when an underflow/overflow occurs, the sum signal SUM to a center value (I.e., a value of ‘0’). On the other hand, the inventive DLF  100  according to the embodiment of  FIG. 2 , counts the up/down signal UP/DN based on the upper threshold value MAX and the lower threshold value MIN and outputs the sum signal SUM, but also initializes, when an overflow occurs, the sum signal SUM to the lower threshold value MIN and initializes, when an underflow occurs, the sum signal SUM to the upper threshold value MAX. Therefore, through improvement in the underflow/overflow processing algorithm of an internal counter (i.e., counting unit  120 ) of the DLF  100 , the operating speed is improved, and jitter and a bit error rate (BER) can be reduced. Furthermore, despite using a counter having a depth of ½ of that of the conventional circuit, it is possible to realize the same CDR bandwidth (BW) as that of the conventional circuit, whereby the entire area of the CDR circuit can be reduced. 
       FIG. 4  is a block diagram illustrating a phase rotating unit  200  according to an embodiment of the present invention. 
     Referring to  FIG. 4 , the phase rotating unit  200  receives a clock CLK&lt;0:15&gt; and generates a multi-phase clock R_CLK&lt;0:15&gt;. The phase rotating unit  200  also controls the phase of the multi-phase clock R_CLK&lt;0:15&gt; in response to a control code CTRL&lt;0:m&gt; so that the multi-phase clock R_CLK&lt;0:15&gt; can be shifted to an optimum sampling position for the input data DIN. 
     According to the illustrated embodiment of  FIG. 4 , the phase rotating unit  200  may include a clock selector  710  and a phase interpolator  730 . 
     The clock selector  710  selects, in response to some of the bits CTRL&lt;0:k&gt; of the control code CTRL&lt;0:m&gt; (k is an integer greater than 0 and less than m), two clocks among a plurality of input clocks CLK 0  to CLK 15  and outputs them as first and second selected clocks SEL_CLK 0  and SEL_CLK 1 . The clock selector  710  may include a first multiplexer (MUX)  712  and a second MUX  714 . The first MUX  712  selects, in response to the bits CTRL&lt;0:k&gt;, one clock among a plurality of input clocks CLK 0  to CLK 7  and outputs the selected clock as the first selected clock SEL_CLK 0 . The second MUX  714  selects, in response to the bits CTRL&lt;0:k&gt;, one clock among a plurality of input clocks CLK 8  to CLK 15  and outputs the selected clock as the second selected clock SEL_CLK 1 . 
     The phase interpolator  730  mixes the first and second selected clocks SEL_CLK 0  and SEL_CLK 1  and generates a multi-phase clock R_CLK&lt;0:15&gt;. The phase interpolator  730  may mix the first and second selected clocks SEL_CLK 0  and SEL_CLK 1  at a mixing ratio determined by the other bits CTRL&lt;k+1:m&gt; of the control code CTRL&lt;0:m&gt;. The multi-phase clock R_CLK&lt;0:15&gt; may have a phase between the first select clock SEL_CLK 0  and the second selected clock SEL_CLK 1 , and this phase may be determined depending on the mixing ratio. 
     Hereinafter, with reference to  FIGS. 5A to 7B , an operation of the DLF  100  described with reference to  FIGS. 1 to 4  will be explained. 
       FIGS. 5A and 5B  are waveform diagrams illustrating the operation of the conventional DLF and the inventive DLF  100 , respectively, under a first condition. For example, the first condition may refer to a condition in which no noise is introduced while loop latency is “0”. In each drawing, the term ‘CDR PHASE’ means a degree with which a phase (hereinafter, referred to as a ‘CDR phase’) of a multi-phase clock R_CLK&lt;0:15&gt; of a CDR circuit  10  is displaced from a target locking point, i.e., a phase of an input data DIN). 
     First, referring to  FIG. 5A , there is illustrated the operation of the conventional DLF under the first condition. In the case where the CDR phase follows the target locking point, an up/down signal UP/DN indicating an up state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 1 )}, if the up/down signal UP/DN indicating the up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow is generated. Accordingly, the sum signal SUM is initialized to ‘0’, and the CDR phase is adjusted. 
     In the case where the CDR phase precedes the target locking point, an up/down signal UP/DN indicating a down state is inputted, it is counted and a sum signal SUM is generated. At time Z, if the up/down signal UP/DN indicating the down state is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an underflow is generated. Accordingly, the sum signal SUM is initialized to ‘0’, and the CDR phase is adjusted. 
     Referring to  FIG. 5B , there is illustrated an operation of the DLF  100 , according to the embodiment of  FIG. 2 , under the first condition. First, in the case where the CDR phase follows the target locking point, an up/down signal UP/DN indicating an up state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 3 )}, if the up/down signal UP/DN indicating the up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow (i.e., DLF_OUT&lt;1&gt;) is generated. Accordingly, the sum signal SUM is initialized to the lower threshold value MIN, and the CDR phase is adjusted. Thereafter, at time {circle around ( 4 )}, if the up/down signal UP/DN indicating the down state is inputted in a state in which the sum signal SUM has been initialized to the lower threshold value MIN, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an underflow (i.e., DLF_OUT&lt;0&gt;) is generated. Accordingly, the sum signal SUM is initialized to the upper threshold value MAX, and the CDR phase is adjusted. 
     As described above, in the case of the DLF  100  shown in  FIG. 5B , the repetition period of the up/down signal UP/DN is short compared to that of the conventional DLF shown in  FIG. 5A . Therefore, even when the CDR phase is displaced from the target locking point, it can be rapidly recovered to its original state. Furthermore, in the case of the conventional DLF shown in  FIG. 5A , when an overflow or an underflow occurs, the sum signal SUM is initialized to ‘0’, whereby tracking is restarted. Therefore, to embody a counter having a CDR bandwidth (BW) of 4, the upper threshold value MAX and the lower threshold value MIN should be respectively set to +4/−4. However, in the case of the DLF  100  shown in  FIG. 5B , when an overflow or an underflow occurs, the sum signal SUM is respectively initialized to the lower threshold value MIN or the upper threshold value MAX. Therefore, to embody a counter having the same CDR bandwidth (BW) of 4, the upper threshold value MAX and the lower threshold value MIN may be respectively set to +2/−2. As a result, the DLF  100  is capable of realizing, despite using the counter having a depth of ½ of that of the conventional circuit, the same CDR bandwidth BW as that of the conventional circuit. 
       FIGS. 6A and 6B  are waveform diagrams illustrating the operation of the conventional DLF and the operation of the inventive DLF  100 , respectively, under a second condition. For reference, the second condition may refer to a condition in which no noise is introduced while the loop latency is ‘1’. In  FIGS. 6A and 6B , there is illustrated the case where the CDR phase is adjusted after one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’. 
     First, referring to  FIG. 6A , there is illustrated the operation of the conventional DLF under the second condition. In the case where the CDR phase follows the target locking point, an up/down signal UP/DN indicating an up state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 1 )}, if the up/down signal UP/DN indicating the up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow is generated, whereby the sum signal SUM is initialized to ‘0’. After one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, the CDR phase is adjusted. 
     Subsequently, in the case where the CDR phase precedes the target locking point, an up/down signal UP/DN indicating a down state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 2 )}, if the up/down signal UP/DN indicating the down state is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an underflow is generated, whereby the sum signal SUM is initialized to ‘0’. After one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, the CDR phase is adjusted. 
     Referring to  FIG. 6B , there is illustrated an operation of the inventive DLF  100 , according to the embodiment of  FIG. 2 , under the second condition. First, in the case where the CDR phase follows the target locking point, an up/down signal UP/DN indicating an up state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 3 )}, if the up/down signal UP/DN indicating the up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow is generated, whereby the sum signal SUM is initialized to the lower threshold value MIN. After one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, the CDR phase is adjusted. At time {circle around ( 4 )}, if the up/down signal UP/DN indicating the down state is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an underflow is generated, whereby the sum signal SUM is initialized to the upper threshold value MAX. After one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, the CDR phase is adjusted. 
     As described above, in the case of the DLF  100  shown in  FIG. 6B , the repetition period of the up/down signal UP/DN is short, despite being under the condition in which the loop latency is present, compared to that of the conventional DLF shown in  FIG. 6A . Therefore, even when the CDR phase is displaced from the target locking point, it can be rapidly recovered to its original state. Furthermore, compared to the conventional DLF shown in  FIG. 6A , the DLF  100  shown in  FIG. 6B  is capable of realizing, despite being under the condition in which the loop latency is present and despite using the counter having a depth of ½ of that of the conventional DLF, the same CDR bandwidth BW as that of the conventional circuit. 
       FIGS. 7A and 7B  are waveform diagrams illustrating the operation of the conventional DLF and the operation of the inventive DLF  100 , respectively, under a third condition. For example, the third condition may refer to a condition in which a noise is introduced while the loop latency is ‘1’. In  FIGS. 7A and 7B , there is illustrated the case where the CDR phase is adjusted after one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, and where the CDR phase is further displaced from the target locking point by the noise compared to the typical case. 
     First, referring to  FIG. 7A , there is illustrated the operation of the conventional DLF under the third condition. In the case where the CDR phase follows the target locking point, an up/down signal UP/DN indicating an up state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 1 )}, if the up/down signal UP/DN indicating the up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow is generated, whereby the sum signal SUM is initialized to ‘0’. After one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, the CDR phase is adjusted. 
     Subsequently, in the case where the CDR phase precedes the target locking point, an up/down signal UP/DN indicating a down state is inputted, it is counted and a sum signal SUM is generated. In this case, if the CDR phase is further displaced from the target locking point by a noise compared to the typical case, the time it takes to recover the CDR phase to the target locking point is increased. That is, at time {circle around ( 2 )}, if the up/down signal UP/DN indicating the down state is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN, an underflow occurs, whereby the sum signal SUM is initialized to ‘0’, and after one cycle, the CDR phase is adjusted. Thereafter, at time {circle around ( 2 )}′, an underflow occurs again, so that the sum signal SUM is initialized to ‘0’, and after one cycle, the CDR phase is adjusted. 
     Referring to  FIG. 7B , there is illustrated an operation of the inventive DLF  100 , according to an embodiment of the present invention, under the third condition. In the case where the CDR phase follows the target locking point, an up/down signal UP/DN indicating an up state is inputted, it is counted and a sum signal SUM is generated. At time {circle around ( 3 )}, if the up/down signal UP/DN indicating the up state is inputted in a state in which the sum signal SUM has reached the upper threshold value MAX, a loop output signal DLF_OUT&lt;0:1&gt; indicating occurrence of an overflow is generated, whereby the sum signal SUM is initialized to the lower threshold value MIN. After one cycle after the loop output signal DLF_OUT&lt;0:1&gt; has been outputted by the loop latency ‘1’, the CDR phase is adjusted. 
     Subsequently, in the case where the CDR phase precedes the target locking point, an up/down signal UP/DN indicating a down state is inputted, it is counted and a sum signal SUM is generated. In this case, if the CDR phase is further displaced from the target locking point by a noise compared to the typical case, the time it takes to recover the CDR phase to the target locking point is increased. That is, at time {circle around ( 4 )}, if the up/down signal UP/DN indicating the down state is inputted in a state in which the sum signal SUM has reached the lower threshold value MIN, an underflow occurs, whereby the sum signal SUM is initialized to ‘0’, and after one cycle, the CDR phase is adjusted. Thereafter, at time {circle around ( 4 )}′, an underflow occurs again, so that the sum signal SUM is initialized to the upper threshold value MAX, and after one cycle, the CDR phase is adjusted. 
     As described above, in the case of the DLF  100  shown in  FIG. 7B , the repetition period of the up/down signal UP/DN is short, despite being under the condition in which a noise is introduced while the loop latency is present, compared to that of the conventional DLF shown in  FIG. 7A . Therefore, even when the CDR phase is displaced from the target locking point, it can be rapidly recovered to its original state. Therefore, the operating speed is improved, and jitter and a bit error rate (BER) can be reduced. 
     Furthermore, compared to the conventional DLF shown in  FIG. 7A , the DLF  100  shown in  FIG. 7B  is capable of realizing, despite being under the condition in which a noise is introduced while the loop latency is present and despite using the counter having a depth of ½ of that of the conventional DLF, the same CDR bandwidth BW as that of the conventional circuit. Therefore, there is an effect of promoting a reduction in the entire area of the CDR circuit. 
     As described above, according to the described embodiments, a CDR circuit is provided having improved operating speed, reduced jitter and bit error rate (BER). The CDR circuit includes, inter alia, an improved DLF having a counter that employs an improved underflow/overflow processing algorithm. 
     Furthermore, the CDR circuit can realize, despite using a counter having a depth that is one half (½) of that of the conventional circuit, the same CDR bandwidth (BW) as that of the conventional circuit, thus promoting a reduction of the entire area of the CDR circuit. 
     We note, that in some instances, as would be apparent to those skilled in the relevant art to which this invention pertains, a feature or element of one described embodiment may be used singly or in combination with other features or elements of another embodiment, unless otherwise specifically indicated. 
     Although various embodiments have been described for illustrative purposes, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the disclosure as defined in the following claims.