Abstract:
A delay locked loop circuit includes a delay unit for receiving an input clock signal, generating an output clock signal whose phase lags that of the input clock signal, and generating multiple delay signals having differently delayed phases in response to the input clock signal; a harmonic lock preventing unit for receiving the input clock signal and the multiple delay signals, outputting a 1 st  signal and a 2 nd  signal, comparing the phases of the input clock signal and the multiple delay signals, and generating the 1 st  signal or the 2 nd  signal depending on the comparison result; an electric charge pump for receiving the 1 st  signal and the 2 nd  signal, generating a phase control signal, and making the voltage of the phase control signal higher or lower than a pre-defined voltage in response to the 1 st  signal and the 2 nd  signal; a filter for eliminating AC included in the phase control signal and transmitting the filtered signal to the delay unit; and start-up circuits for feeding a 1 st  voltage to the electric charge pump in the initial state before the output clock signal is generated and making the electric charge pump provide a 2 nd  voltage to the delay unit; wherein the delay unit provides a delay locked circuit that adjusts the phases of the output clock signal and the multiple delay signals in response to the phase control voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Delay Locked Loop (DLL), and more particularly, to a DLL with multi-phases. 
     2. Description of the Related Art 
     A semiconductor device, which processes signals at high speed with a built-in memory, uses a Phase Locked Loop (PLL) or a Delay Locked Loop (DLL) in order to synchronize input/output data with a system clock. The DLL features less jitter and is simpler and more reliable than the PLL. However, the DLL is disadvantageous in that it has limited phases when compared to the PLL. Recently, various methods to overcome the disadvantage of the DLL have been studied. Since the DLL operates reliably even in the digital block where power noise is serious, it has had a wide range of applications. 
     FIG. 1 is a block diagram of an existing DLL. Referring to FIG. 1, the existing DLL  101  includes a phase comparator  111 , an electric charge pump  121 , a filter  131  and a delay element  141 . The phase comparator  111  compares the phases of an input clock signal (CLKIN) and an output clock signal (CLKOUT). Then, the phase comparator outputs the phase difference as the 1 st  signal (UP) or the 2 nd  signal (DN). The electric charge pump  121  increases or decreases the output voltage (V 1 ) depending on the 1 st  signal (UP) or the 2 nd  signal (DN). The filter  131  eliminates AC included in the output voltage (V 1 ). The delay element  141  reduces or extends the delay time of the output clock signal (CLKOUT) depending on the output voltage of the filter  131  to be synchronized with or frequency-adjusted with the input clock signal (CLKIN). 
     In the existing DLL  101 , the limited phase capture range may cause harmonic lock. 
     FIG. 2A shows the output clock signal of the DLL  101  shown in FIG. 1 synchronized with the input clock signal (CLKIN). The rising edge (r 1 ) of the output clock signal (CLKOUT) is synchronized with the rising edge (r 2 ) of the input clock signal (CLKIN) after N clock periods (TN). 
     FIG. 2B shows the output clock signal (CLKOUT) shown in FIG. 1 in a harmonic lock state. The rising edge (r 3 ) of the output clock signal (CLKOUT) is synchronized with the rising edge (r 4 ) of the input clock signal (CLKIN) after N clock periods (TN). Even though the rising edge (r 3 ) of the output clock signal (CLKOUT) is synchronized with the rising edge of the input clock signal (CLKIN) after N clock periods, the output clock signal is in a frequency-varied harmonic lock state which is an abnormal state. However, the phase comparator  111  judges the state as normal. This is a problem of the existing DLL  101 . 
     IEEE J. Solid-state Circuits, vol 32, pp.1683-1692, Nov 1997 specifies a method of starting the delay line with the minimum delay in the initial operation of the DLL, as a solution to harmonic lock. However, the method specified in the publication can only address harmonic lock in part, and is not a fundamental solution to harmonic lock. 
     U.S. Pat. No. 5,663,665 discloses another method to resolve harmonic lock. The phase discriminator of the &#39;665pPatent detects whether the rising edge of the signal of the last tab of the delay element is the same as that of one signal of the middle tabs. If the phase discriminator detects that case, it judges that harmonic lock is present, and prevents harmonic lock by adjusting the delay speed. However, if none of the rising edges of the signals of the middle tabs is the same as the rising edge of the signal of the last tab, the phase discriminator defined in the &#39;665 patent cannot detect the phase difference and harmonic lock cannot be prevented. 
     In addition, in some cases, the DLL user requires a randomly delayed clock signal. However, the existing DLL delays the output clock signal only for a pre-defined time. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is a first object of the present invention to provide a Delay Locked Loop (DLL) that completely prevents harmonic lock. 
     It is a second object of the present invention to provide a DLL that can selectively output randomly delayed clock signals. 
     In accordance with the present invention, there is provided a delay locked loop comprising: a delay unit for receiving an input clock signal, generating an output clock signal whose phase lags that of the input clock signal, and generating multiple delay signals having different phase delays in response to the input clock signal; a harmonic lock preventing unit for receiving the input clock signal and the multiple delay signals, comparing the phases of the input clock signal and the delay signals, generating a 1 st  signal or a 2 nd  signal depending on the comparison result, and outputting the 1 st  signal or the 2 nd  signal; an electric charge pump for receiving the 1 st  signal and the 2 nd  signal, generating a phase control signal, and making the voltage of the phase control signal higher or lower than a pre-defined voltage in response to the 1 st  signal and the 2 nd  signal; a filter for eliminating AC included in the phase control signal and transmitting the filtered signal to the delay unit; and start-up circuits for feeding a 1 st  voltage to the electric charge pump in the initial state before the output clock signal is generated and making the electric charge pump provide a 2 nd  voltage to the delay unit, wherein the delay unit provides a delay locked circuit that adjusts the phases of the output clock signal and the multiple delay signals in response to the phase control voltage. 
     Preferably, the delay unit includes: multiple delay elements connected in series for delaying the input clock signal, and outputting the output clock signal and the multiple delay signals; and a bias unit for controlling the delay amount of the multiple delay elements depending on the voltage of the output signal of the filter. Preferably, the multiple delay signals are output from specific elements of the multiple delay elements. 
     Preferably, the harmonic lock preventing unit includes: multiple phase detectors for comparing two adjacent signals out of the input clock signal and multiple delay signals, activating the 1 st  signal if the phase is found to lag and activating the 2 nd  signal if the phase is found to lead, and receiving two adjacent signals out of the input clock signal and the multiple delay signals; a 1 st  NOR gate and a 2 nd  NOR gate for each receiving the outputs of half of the multiple phase detectors; an OR gate for receiving the outputs of the 1 st  NOR gate and the 2 nd  NOR gate and outputting the 1 st  signal; and an AND gate for receiving the outputs of the 1 st  NOR gate and the 2 nd  NOR gate and outputting the 2 nd  signal. Preferably, the phase detectors include: a 1 st  delay flip-flop for receiving the power voltage and the input clock signal or the 1 st  delay signal and generating the output of the phase detector; a 2 nd  delay flip-flop for receiving the power voltage and the 2 nd  delay signal which has a phase closest to that of the 1 st  delay signal; and a 2 nd  NAND gate for receiving the outputs of the 1 st  delay flip-flop and the 2 nd  delay flip-flop, and providing the outputs to the 1 st  delay flip-flop and the 2 nd  delay flip-flop. In addition, preferably, the electric charge pump makes the voltage of the phase control signal higher than the pre-defined voltage if the 1 st  signal is activated, and makes the voltage of the phase control signal lower than the pre-defined voltage if the 2 nd  signal is activated. The electric charge pump includes a 1 st  transistor which outputs a power voltage and is gated by the output of the start-up circuit. 
     Preferably, if the phase control voltage becomes higher than the pre-defined voltage, the phases of the output clock signal and multiple delay signals are advanced. If the phase control voltage becomes lower than the pre-defined voltage, the phases of the output clock signal and multiple delay signals are delayed. Preferably, the start-up circuit includes: a 1 st  delay flip-flop for receiving the input clock signal and the output clock signal; and a 2 nd  delay flip-flop for receiving the outputs of the 1 st  delay flip-flop and the output clock signal and generating the output of the start-up circuit. Preferably, if the output clock signal is logic low in the initial state before the output clock signal is generated, the start-up circuit outputs the logic low, the 1 st  transistor is turned on and the power voltage is provided to the filter so that the output clock signal can be quickly synchronized with the input clock signal. 
     In accordance with a second aspect of the present invention, there is provided a delay locked loop comprising: a delay unit for receiving an input clock signal, generating an output clock signal whose phase lags the input clock signal, and generating 1 st  multiple delay signals and 2 nd  multiple delay signals having different phase delays in response to the input clock signal; a harmonic lock preventing unit for receiving the input clock signal and the 1 st  multiple delay signals, outputting a 1 st  signal and a 2 nd  signal, comparing the phases of the input clock signal and the 1 st  multiple delay signals, and generating the 1 st  signal or the 2 nd  signal depending on the comparison result; an electric charge pump for receiving the 1 st  signal and the 2 nd  signal, generating a phase control signal, and making the voltage of the phase control signal higher or lower than a pre-defined voltage in response to the 1 st  signal and the 2 nd  signal; a filter for eliminating AC included in the phase control signal and transmitting the filtered signal to the delay unit; and a decoder for decoding a received address signal and providing the decoded signal to the delay unit, wherein the delay unit selectively outputs some of the 2 nd  multiple delay signals in response to the decoded signal and provides a delay locked circuit that adjusts the phases of the output clock signal and the 1 st  multiple delay signals in response to the output signal of the filter. 
     Preferably, the delay unit includes: multiple delay elements connected to the input clock signal in series; a bias unit for controlling the delay amount of the multiple delay elements depending on the voltage of the output signal of the filter; and multiple switching elements, connected respectively to the outputs of the delay elements and switched by the decoded signal, for outputting the 2 nd  multiple delay signals, wherein the 1 st  multiple delay signals are output by specific elements of the delay elements. It is preferable that the switching elements are transmission gates. 
     Preferably, the number of 2 nd  multiple delay signals is larger than the number of 1 st  multiple delay signals. 
     In accordance with another aspect of the present invention, there is provided a delay locked loop comprising: a delay unit for receiving an input clock signal, generating an output clock signal whose phase lags the input clock signal, and generating 1 st  and 2 nd  multiple delay signals having different phase delays in response to the input clock signal; a harmonic lock preventing unit for receiving the input clock signal and the 1 st  multiple delay signals, outputting a 1 st  signal and a 2 nd  signal, comparing the phases of the input clock signal and the 1 st  multiple delay signals and generating the 1 st  signal or the 2 nd  signal depending on the comparison result; an electric charge pump for receiving the 1 st  signal and the 2 nd  signal, generating a phase control signal, and making the voltage of the phase control signal higher or lower than a pre-defined voltage in response to the 1 st  signal and the 2 nd  signal; a filter for eliminating AC included in the phase control signal and transmitting the filtered signal to the delay unit; start-up circuits for feeding a 1 st  voltage to the electric charge pump in the initial state before the output clock signal is generated and making the electric charge pump provide a 2 nd  voltage to the delay unit; and a decoder for decoding a received address signal and providing the decoded signal to the delay unit, wherein the delay unit selectively outputs 2 nd  multiple delay signals in response to the decoded signal and adjusts the phases of the output clock signal and 1 st  multiple delay signals in response to the output signal of the filter. The present invention can prevent harmonic lock. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a block diagram of an existing Delay Locked Loop (DLL). 
     FIGS. 2A and 2B show the output clock of the DLL shown in FIG. 1, synchronized with the input clock, and in a harmonic lock state, respectively. 
     FIG. 3 is a block diagram of the DLL according to a preferred embodiment of the present invention. 
     FIG. 4 is a circuit diagram showing a start-up circuit, an electric charge pump and a filter shown in FIG.  3 . 
     FIG. 5 shows in detail a delay unit shown in FIG.  3 . 
     FIG. 6 is a circuit diagram of a harmonic lock preventing unit shown in FIG.  3 . 
     FIG. 7 is a circuit diagram of one of phase detectors shown in FIG.  6 . 
     FIG.  8 A through FIG. 8E show waveforms when delay signals of the unit delay shown in FIG. 5 are found to be unlocked. 
     FIG.  9 A through FIG. 9E show that the unlocked delay signals as shown in FIG. 8 are synchronized after passing through the harmonic lock preventing unit shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 is a block diagram of a Delay Locked Loop (DLL) according to the preferred embodiment of the present invention. Referring to FIG. 3, the DLL  301  includes a harmonic lock preventing unit  311 , an electric charge pump  321 , a filter  331 , a delay unit  341 , a start-up circuit  351  and a decoder  361 . 
     The delay unit  341  delays the input clock signal (CLKIN) for a pre-defined time and outputs the output clock signal (CLKOUT). In addition, the delay unit  341  outputs 1 st  multiple delay signals (d 1 ˜dm) and 2 nd  multiple delay signals (dl 1 ˜din) whose phase lags that of the input clock signal (CLKIN) and leads that of the output clock signal (CLKOUT). The 1 st  multiple delay signals (d 1 ˜dm) are transmitted to the harmonic lock preventing unit  311 . The delay unit  341  is connected to the decoder  361 , and outputs at least one delay signal out of the 2 nd  multiple delay signals (dl 1 ˜din) depending on the output signals (ADDOUT) of the decoder  361 . Since the delay unit  341  includes a Voltage Controlled Delay Line (VCDL), the delay time of the output clock signal (CLKOUT), the 1 st  multiple delay signals (d 1 ˜dm) and the 2 nd  multiple delay signals (dl 1 ˜din) are adjusted by a signal (V 4 ) output from the filter  331 . 
     The decoder  361  decodes the received address signal (ADDIN) and outputs the decoded address signal (ADDOUT). 
     Since the delay unit  341  outputs the 2 nd  multiple delay signals (dl 1 ˜din) having different phases depending on the decoded address signal (ADDOUT), the DLL user can choose to use dl 1 ˜din, as well as the output clock signal (CLKOUT) to then output the chosen signal as a clock signal RCLK for another purpose of use. 
     The start-up circuit  351  receives the input clock signal (CLKIN) and the output clock signal (CLKOUT) and sets the initial voltage of signal (V 4 ) which is provided by the filter  331  to the delay unit  341  before the input clock signal (CLKIN) input to the delay unit  341  is output from the delay unit  341 . The input clock signal (CLKIN) has a large phase difference from the output clock signal (CLKOUT) output in the initial state before the input clock signal (CLKIN) input to the delay unit  341  in a coarse mode during power-on is output as the output clock signal (CLKOUT) from the delay unit  341 . In the initial power-on state in which supply of the power voltage to a device having the DLL  301  starts, the output clock signal (CLKOUT) is logic low, and the output clock signal (CLKOUT) of the initial logic low level resets the start-up circuit  351 , thereby making the output of the flipflops  411  and  412  go low. Accordingly, the PMOS transistor  431  is turned on and the voltage of the signal (V 4 ) increases fast to be a predetermined voltage level. Thus, in the initial power-on state, the output clock signal (CLKOUT) is quickly synchronized with the input clock signal (CLKIN). 
     After the coarse mode ends, the DLL  301  operates in a fine mode to accurately match phase. The DLL of the present invention  301  prevents harmonic lock in fine mode and matches phase accurately. 
     The harmonic lock preventing unit  311  receives the 1 st  multiple delay signals (d 1 ˜dm) output from the delay unit  341  and outputs 1 st  signals (UP) and 2 nd  signals (DN). The harmonic lock preventing unit  311  compares the phases of adjacent delay signals out of the 1 st  multiple delay signals (d 1 ˜dm). If the phase difference of the delay signals exceeds a pre-defined value, the harmonic lock preventing unit activates the 1 st  signal (UP) or the 2nd signal (DN). The harmonic lock preventing unit  311  operates as follows. In a phase detector ( 611  shown in FIG.  7 A), when the input clock signal (CLKIN) is activated to logic high, the output (P 1 ) goes logic high. Then the input clock signal (CLKIN) has a predetermined delay time at the delay unit  341  so that a signal (d 1 ) of a high level is produced. The output (P 1 ) of the phase detector  611  is transitioned into logic low by the signal (d 1 ). The phase detector  611  is driven by outputs (d 1 ˜dm) of the delay unit  341 , as shown in FIG. 6, to thus prevent a harmonic lock, which is a problem occurring to a delay lock loop  301 . This is possible because the harmonic lock preventing unit  311  shown in FIG. 6 which features the present invention, generates the 1 st  or 2 nd  signal (UP or DN) to be fed back within one period when the outputs (d 1 ˜dm) of the delay unit  341  is larger or smaller than the period of the input clock signal (CLKIN) as a reference signal. 
     The electric charge pump  321  receives the 1 st  signal (UP) and the 2 nd  signal (DN), and generates a phase control signal (V 2 ). If the 1 st  signal (UP) is activated, the voltage of the phase control signal (V 2 ) rises. If the 2 nd  output signal (DN) is activated, the voltage of the phase control signal (V 2 ) falls. 
     The filter  331  removes AC signals included in the phase control signal (V 2 ) and sends the signal (V 4 ) having pure DC signals to the delay unit  341 . 
     As described above, the harmonic lock preventing unit  311  detects the phase differences between the 1 st  multiple delay signals (d 1 ˜dm) output from the delay unit  341 . The harmonic lock preventing unit  311  detects whether the phase differences exceed a pre-defined range and accurately synchronizes the output clock signal (CLKOUT) with the input clock signal (CLKIN). Therefore, the output clock signal (CLKOUT) is prevented from entering the harmonic lock state. 
     FIG. 4 shows embodiments of a start-up circuit  351 , an electric charge pump circuit  321  and a filter circuit  331 . 
     Referring to FIG. 4, the start-up circuit  351  includes a 1 st  delay flip-flop  411  and a 2 nd  delay flip-flop  412 . 
     The 1 st  delay flip-flop  411  receives an input clock signal (CLKIN) and the output clock signal (CLKOUT), and generates a clock signal (P 1 ) whenever the level of the input clock signal (CLKIN) increases from logic low to logic high. The 1 st  delay flip-flop  411  outputs the clock signal (P 1 ) generated as a result of the division by two of the input clock signal (CLKIN). A 2 nd  delay flip-flop  412  outputs the signal (S 1 ) generated as a result of the division by four of the clock signal (P 1 ) output by the 1 st  delay flip-flop  411 . Reset terminals of the 1 st  delay flip-flop  411  and the 2 nd  delay flip-flop  412  receive the output clock signal (CLKOUT). If the output clock signal (CLKOUT) is logic high, the two flip-flops  411  &amp;  412  operate normally. If the output clock signal (CLKOUT) is logic low, the flip-flops  411  &amp;  412  are reset. If the 2 nd  delay flip-flop  412  is reset, the start-up circuit  351  outputs the signal (S 1 ) as a logic low signal irrespective of the input. That is, if the output clock signal (CLKOUT) is logic low in the initial state of the delay unit  341 , the start-up circuit  351  outputs the signal (S 1 ) at a logic low level. If the delay lock loop  301  operates normally, that is, if the periods of the input clock signal (CLKIN) and the output clock signal (CLKOUT) are equal to each other, the output clock signal (CLKOUT) is always synchronized with the input clock signal (CLKIN) of logic high, to thus reset the signal (S 1 ), so that the PMOS transistor  431  will never be driven. Thus, after the normal operation, the start-up function is stopped. 
     The electric charge pump  321  includes a 1 st  PMOS transistor  431  and a 2 nd  PMOS transistor  432 , a 1 st  current source  421 , and a 2 nd  current source  422 , and outputs the phase control signal (V 2 ). The 1 st  NMOS transistor  433  is turned on and outputs the constant current generated from the 1 st  current source  421  as the phase control signal (V 2 ), when the output of the start-up circuit  351  is logic low. The 2 nd  PMOS transistor  432  outputs the constant current and increases the voltage of the phase control signal (V 2 ) when the 1 st  signal (UP) is logic low. The 1 st  NMOS transistor  433  reduces the voltage of the phase control signal (V 2 ) through a constant current source  422  when the 2 nd  signal (DN) is logic high. 
     As described above, the electric charge pump  321  makes the voltage of the phase control signal (v 2 ) higher than a pre-defined voltage when the 1 st  signal (up) is activated to logic low and make the voltage of the phase control signal (V 2 ) lower than a predetermined voltage when the 2 nd  signal (UP) is activated to logic high. 
     The filter  331  includes a capacitor  441 , and eliminates AC included in the phase control signal (v 2 ) to provide the signal (V 4 ) having a pure DC signal to the delay unit  341 . In addition, the capacitor  441  is charged to a certain voltage and the signal (V 4 ) is maintained at a pre-defined voltage. The voltage increase or decrease according to the 1 st  or 2 nd  signal (UP or DN) generated due to a phase difference and the delay time of the delay unit  341  is varied. 
     As described above, the start-up circuit  351  outputs logic low in the initial state before the output clock signal (CLKOUT) is normally output. Then, the 1 st  PMOS transistor  431  is turned on and the power voltage (vdd) is provided to the delay unit  341  through the filter  331  so that the output clock signal (CLKOUT) can be quickly synchronized with the input clock signal (CLKIN). 
     FIG. 5 shows in detail an embodiment of the delay unit  341  shown in FIG.  3 . Referring to FIG. 5, the delay unit  341  includes multiple delay elements (B 1 ˜Bn), multiple switching elements (Q 1 ˜Qn−1) and a bias unit 511. 
     The multiple delay elements (B 1 ˜Bn) connected in series to the input clock signal (CLKIN) delay the input clock signal (CLKIN) for a pre-defined time and output the 1 st  multiple delay signals (d 1 ˜dm), the 2 nd  multiple delay signals (dl 1 ˜din) and the output clock signal (CLKOUT). Each of the 1 st  multiple delay signals (d 1 ˜dm) is output by specific delay elements, that is, every two delay elements, four delay elements or eight delay elements. Each of the 2 nd  multiple delay signals (dl 1 ˜din) is output by one delay element. Therefore, the number of 2 nd  multiple delay signals (dl 1 ˜din) is more than the number of 1 st  multiple delay signals (d 1 ˜dm). However, in some cases, the number of 1 st  multiple delay signals (d 1 ˜dm) can be equal to the number of 2 nd  multiple delay signals (dl 1 ˜din). 
     The multiple switching elements (Q 1 ˜Qn−1) are turned on or off depending on the output signal (ADDOUT) of the decoder 361. For example, if the switching elements (Q 1 ˜Qn−1) are transmission gates and one among the delay signals (dl 1 ˜din) is selected by an N-bit decoder to then be output as a clock signal (RCLK). In other words, the clock signal (RCLK) having an arbitrary phase delay can be produced according to the value of the N-bit decoder. 
     The bias unit changes the voltage level (bias level) of a 3 rd  signal (AP) or a 4 th  signal (AN) depending on the voltage of the signal (V 4 ) of the filter  331 , and the delay time is changed according to the voltage levels. 
     The number of delay elements (B 1 ˜Bn) is selected depending on the purpose of the DLL ( 301 ). 
     FIG. 6 is a circuit diagram of one embodiment of the harmonic lock preventing unit  311  shown in FIG.  3 . Referring to FIG. 6, the harmonic lock preventing unit  311  includes multiple phase detectors ( 611 ˜ 618 ), a 1 st  NOR gate through a 3 rd  NOR gate ( 621 ˜ 623 ) and a 1 st  NAND gate  631 . 
     The phase detectors  611  through  618  each receive two delay signals out of the 1 st  multiple delay signals (d 1 ˜dm) whose phase difference is small, as well as the input clock signal (CLKIN), and detect the phase difference of the received signals. The phase detectors  611  through  618  will be described in detail with reference to FIG.  7 . The 1 st  NOR gate  621  and the 2 nd  NOR gate respectively receive outputs P 1 ˜P 4  and P 5 ˜P 8  of the phase detectors. The 3 rd  NOR gate  623  receives outputs P 9  and P 10  of the 1 st  NOR gate  621  and the 2 nd  NOR gate  622 , and outputs the 1 st  signal (UP). The 1 st  NAND gate  631  receives outputs P 9  and P 10  of the 1 st  NOR gate and the 2 nd  NOR gate, and outputs the 2 nd  signal (DN). 
     The harmonic lock preventing unit  311  generates a phase error signal and activates the 1 st  signal (UP) or the 2 nd  signal (DN) when the delay time (t 1 ) of the 1 st  multiple delay signals (d 1 −dm) exceeds the time defined in the following Formula 1.              t1   =       T   n     ×   4             [     Formula                 1     ]                                
     Here, T is the period of the input clock signal (CLKIN) and n is the number of delay elements (B 1 ˜Bn) shown in FIG.  5 . For example, if the period of the input clock signal (CLKIN) is 20[ns] and the number of the delay elements (B 1 ˜Bn) is 20, the delay time (t 1 ) is 4[ns]. The reason why        (     T   n     )                          
     is multiplied by 4 is that the 1 st  multiple delay signals (d 1 ˜dm) are output by every fourth delay element. If the 1 st  multiple delay signals (d 1 ˜dm) were output by every second delay element,        (     T   n     )                          
     would be multiplied by 2. If the 1 st  multiple delay signals (d 1 ˜dm) were output by every eighth delay element,        (     T   n     )                          
     would be multiplied by 8. 
     Referring to FIG.  7 A and FIG. 7B, the phase detector  611  includes a 1 st  delay flip-flop  711 , a 2 nd  delay flip-flop  712  and a 2 nd  NAND gate  721 . The input clock signal (CLKIN) received by the 1 st  delay flip-flop  711  and the delay signal (d 1 ) received by the 2 nd  delay flip-flop  712  have a phase difference. That is, the delay signal (d 1 ) is delayed for more time than the input clock signal (CLKIN). The phase detector  611  generates a pulse signal (P 1 ) having a pulse width corresponding to the delay time. 
     FIG.  8 A through FIG. 8E show waveforms when the 1 st  multiple delay signals (d 1 ˜dm) shown in FIG. 5 are found to be unlocked. 
     If the respective periods of the 1 st  multiple delay signals (d 1 ˜dm) are greater than the period of input clock signal (CLKIN), the phase detectors  611  through  618  included in the harmonic lock preventing unit  311  detect the respectively delayed phase, generate the signals shown in FIGS. 8A and 8C, and transmit them to the 1 st  NOR gate  621  and the 2 nd  NOR gate  622 . Then, the 1 st  NOR gate  621  generates the signal shown in FIG.  8 B and the 2 nd  NOR gate  622  generates the signal shown in FIG.  8 D. Because the outputs (P 9 , P 10 ) of the 1 st  NOR gate  621  and the 2 nd  NOR gate  622  are combined by an OR gate  623 , the 1 st  signal (UP) has the waveform shown in FIG.  8 E. The electric charge pump  321  increases the voltage of the phase control signal (V 2 ) and accelerates the operation of the delay elements (B 1 ˜Bn) of the delay unit  341 . Then, the delay of the delay signals of the 1 st  multiple delay signals (d 1 ˜dm) and the output clock signal (CLKOUT) is reduced. The output clock signal (CLKOUT) is quickly synchronized with the input clock signal (CLKIN). That is, the output clock signal (CLKOUT) is prevented from entering the harmonic lock state. 
     FIG.  9 A through FIG. 9E show that the unlocked delay signals as shown in FIG. 8 are synchronized after passing through the harmonic lock preventing unit  311  shown in FIG.  6 . As shown in FIG.  9 A through FIG. 9E, if the phases and periods of the input clock signal (CLKIN) and the output clock signal (CLKOUT) are synchronized, the phase detectors  611  through  618  detect no phase error. Then, since the 1 st  signal (UP) is output as logic high, the voltage of the phase control signal (V 2 ) is maintained. 
     As described above, according to the present invention, the delay unit  341  generates the 1 st  multiple delay signals (d 1 ˜dm). The harmonic lock preventing unit  311  detects the phases of the 1 st  multiple delay signals (d 1 ˜dm) and prevents the phase of the output clock signal (CLKOUT) from lagging. Therefore, the output clock signal (CLKOUT) is prevented from entering the harmonic lock state. In addition, the delay unit  341  generates the 2 nd  multiple delay signals (dl 1 ˜din). The decoder  361  selects some of the 2 nd  multiple delay signals (dl 1 ˜din) and the DLL user can selectively use the delay signals having different phases. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.