Abstract:
A phase comparing circuit includes a first storage circuit for reading an external clock signal based on a control clock signal; first and second inverters for inverting a signal from the first storage circuit based respectively on first and second threshold levels; third and fourth inverters for inverting respective signals output from the first and second inverters; a delay circuit for delaying the control clock signal by a specific time; a coincidence control circuit for setting the delayed control clock signal to be active when the signals from the third and fourth inverters coincide with each other, and setting it to be inactive when the signals from the third and fourth inverters do not coincide with each other; and a second storage circuit for reading a signal output form the first storage circuit when the delayed control clock signal is active, and outputting the read signal as the control signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a digital phase comparing circuit, and in particular, relates to a phase comparing circuit for preventing an erroneous operation of a next circuit connected thereto when a metastable phenomenon occurs. 
   Priority is claimed on Japanese Patent Application No. 2007-059091, filed Mar. 8, 2007, the contents of which are incorporated herein by reference. 
   2. Description of the Related Art 
     FIG. 4  is a general circuit diagram of a DLL (delay locked loop) used in a DDR SDRAM (double data rate synchronous dynamic RAM) or the like, and  FIG. 5  shows the corresponding operation waveforms. 
   An external clock (signal) CLK, input into an input circuit  1 , is transformed into an output control clock CLK 0  via a delay circuit  2  and a buffer  3 . Based on the clock CLK 0 , data DQ and a data control clock DQS are respectively output from a DQ output circuit  4  and a DQS output circuit  5 . In this process, the delay time with respect to the delay circuit  2  is controlled by a phase comparing circuit  7  and a delay control circuit  8  so that the rising edge of a control clock signal RCLK, output from a dummy circuit  9 , coincides with the rising edge of the input clock CLK. Accordingly, the timings of the output data DQ and the clock DQS coincide with the timing of the input clock CLK. 
   The phase comparing circuit  7  outputs a signal UP, which is (i) L (low) when the control clock signal RCLK rises after the rise of the external clock CLK (see the first half (on the left side) of  FIG. 5 ), and (ii) H (high) when the control clock signal RCLK rises before the rise of the external clock CLK (see the second half (on the right side) of  FIG. 5 ). The delay control circuit  8  receives the UP signal (H or L), and increases or decreases the delay time in accordance with the rising edge of a timing signal CCLK. In  FIG. 5 , the increase/decrease of the delay time is exaggerated in convenience of the explanations. 
     FIG. 6  shows the structure of the conventional phase comparing circuit  7 , and  FIG. 7  shows corresponding operation waveforms. 
   An edge-trigger D-FF (delay flip-flop)  11  transmits data at the data terminal D to the output terminal Q at the rising edge of a signal input into the clock terminal CK. That is, in the first D-FF  11  on the left side of  FIG. 6 , an external clock signal CLK is connected to data terminal D, and the control clock signal RCLK is connected to the clock terminal CK, wherein the external clock signal CLK is latched at the rising edge of the control clock signal RCLK. Generally, in the D-FF, when the transition timing at the data terminal D is sufficiently close to the rising edge of the signal at the clock terminal CK, a metastable phenomenon in which the signal output from the output terminal Q oscillates for a specific period may occur (see the second half of  FIG. 7 ). When the bandwidth of the D-FF is relatively narrow, the signal output from the output terminal Q may not oscillate, but a metastable phenomenon, in which the signal output from the output terminal Q becomes unstable while keeping the center value, may occur. The following explanation is applicable to either case. In particular, with respect to the phase comparing circuit of a DLL, the DLL operates so that the rising edge of the control clock signal RCLK becomes closer to the rising edge of the external clock signal CLK, thereby increasing the possibility of an occurrence of the metastable phenomenon. 
   In a circuit for solving this problem, as shown in  FIG. 6 , the signal output from the D-FF  11  is applied via an inverter  12  to the data terminal of a D-FF  13 , and the control clock signal RCLK is delayed by a time d 1  by means of a delay circuit  14 , and then applied as a signal UPCLK to the clock terminal CK of the D-FF  13 , so that the signal output from the first D-FF  11  is latched by the second D-FF  13  after the delay time d 1  has elapsed. That is, positive feedback is applied so as to prevent the signal from being affected by the above-described signal oscillation. The metastable phenomenon is a probability event, and the occurrence probability decreases in inverse proportion to an exponential function with respect to the duration time tMET (see  FIG. 7 ) of the metastable phenomenon. That is, the occurrence probability is decreased by providing a sufficient amount of delay time d 1  (between the two D-FFs  11  and  13 ) to a low value which can actually be disregarded (see, for example, Japanese Unexamined Patent Application, First Publication No. 2005-228426). 
   However, in recent years, the operation frequency of DRAMs has increased rapidly, and thus it is difficult to provide a sufficient amount of delay time d 1 . That is, if the delay time d 1  is reduced in inverse proportion to the operation frequency so as to provide a sufficient operation margin, the occurrence probability of the metastable phenomenon rapidly increases. That is, as shown in the second half of  FIG. 7 , if the delay time d 1  becomes shorter than the metastable duration time tMET, the relevant oscillation cannot be sufficiently reduced by using the second D-FF  13 , which may cause an erroneous operation of the delay control circuit  8  (see  FIG. 4 ) 
   SUMMARY OF THE INVENTION 
   In light of the above circumstances, an object of the present invention is to provide a phase comparing circuit for reducing the above-described delay time d 1  while reliably preventing an erroneous operation of a next circuit connected to the phase comparing circuit. 
   Therefore, the present invention provides a phase comparing circuit for comparing phases between an external clock signal and a control clock signal, and outputting a control signal based on a result of the comparison, the phase comparing circuit comprising: 
   a first storage circuit for reading the external clock signal in accordance with the control clock signal; 
   a first inverter circuit for inverting a signal output from the first storage circuit based on a first threshold level; 
   a second inverter circuit for inverting a signal output from the first storage circuit based on a second threshold level; 
   a third inverter circuit for inverting a signal output from the first inverter circuit; 
   a fourth inverter circuit for inverting a signal output from the second inverter circuit; 
   a delay circuit for delaying the control clock signal by a specific time and outputting the delayed control clock signal; 
   a coincidence control circuit for setting the delayed control clock signal to be active when the signals output from the third and fourth inverter circuits coincide with each other, and setting it to be inactive when the signals output from the third and fourth inverter circuits do not coincide with each other; and 
   a second storage circuit for reading a signal output form the first storage circuit when the delayed control clock signal is active, and outputting the read signal as the control signal. 
   In a typical example, the third and fourth inverter circuits each have a hysteresis characteristic. 
   In a preferable example, the coincidence control circuit includes: 
   a comparison circuit for comparing the signals output from the third and fourth inverter circuits, determining whether the signals coincide with each other, and outputting a determined result; 
   a latch circuit for reading a signal output from the comparison circuit, and outputting the read signal in accordance with the delayed control clock signal; and 
   a gate circuit which is open or closed in accordance with a signal output from the latch circuit. 
   In another typical example, the first and second inverter circuits are each formed by a current mirror amplifier which inverts an input signal based on a reference voltage generated by a resistive division. 
   In accordance with the present invention, even if a metastable phenomenon occurs in the phase comparing circuit, an erroneous operation of a next circuit connected to the phase comparing circuit can be prevented. Therefore, it is possible to considerably reduce the delay time which is a known measure to handle the metastable phenomenon. Accordingly, the relevant DLL can be operated at a high frequency while providing a sufficient operation margin. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the structure of a phase comparing circuit as a first embodiment of the present invention. 
       FIG. 2  is a waveform diagram used for explaining the operation in the first embodiment. 
       FIG. 3  is a block diagram showing the structure of a phase comparing circuit as a second embodiment of the present invention. 
       FIG. 4  is a general circuit diagram of a DLL. 
       FIG. 5  is a waveform diagram used for explaining the operation of the DLL in  FIG. 4 . 
       FIG. 6  is a block diagram showing the structure of a conventional example of the phase comparing circuit. 
       FIG. 7  is a waveform diagram used for explaining the operation of the conventional phase comparing circuit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, embodiments of the present invention will be described with reference to the appended figures. 
   First Embodiment 
     FIG. 1  is a block diagram showing the structure of a phase comparing circuit as a first embodiment of the present invention. This phase comparing circuit is used in a DLL circuit (refer to  FIG. 4 ). 
   In  FIG. 1 , reference numeral  21  indicates a first D-FF used for reading the external clock signal CLK at the rising edge of the control clock signal RCLK output from the dummy output circuit  9  (see  FIG. 4 ) of the DLL circuit. Reference numeral  22  indicates an inverter used for inverting and outputting the signal output from the D-FF  21 . Reference numeral  23  indicates a second D-FF used for reading the signal output from the inverter  22  at the rising edge of a signal UPCLK. 
   Reference symbols INVA 1  and INVB 1  each indicate an inverter used for inverting and outputting the signal output from the inverter  22 , wherein the inverter INVA 1  has a threshold level VthA which is higher than the threshold level VthB of the inverter INVB 1  (see  FIG. 2 ). Such a difference in the threshold level can be provided by an asymmetric sizing. That is, the input threshold level VthA can be higher than the intermediate value (between the relevant high and low levels) by reducing the channel width of a relevant NMOS transistor with respect to a corresponding PMOS transistor so that the NMOS has a relatively small channel width in comparison with general inverters. Also for the inverter INVB 1 , an asymmetric sizing is performed in the opposite direction. 
   Reference symbols COA and COB respectively indicate capacitors connected to the output terminals of the inverters INVA 1  and INVB 1 . Reference symbols INVA 2  and INVB 2  are inverters which respectively invert and output signals NA 1  and NB 1  output from the inverters INVA 1  and INVB 1 . The inverters INVA 2  and INVB 2  each have a hysteresis characteristic. That is, the threshold level when the signal NA 1  (or NB 1 ) is changed from “H” (high) to “L” (low) is lower than the intermediate value of the power source voltage, and the threshold level when the signal NA 1  (or NB 1 ) is changed from “L” to “H” is higher than the intermediate value of the power source voltage. Such an inverter operation is conventionally known, and can be implemented by adding an inverter used for feeding back the relevant signal from the output side to the input side to an ordinary inverter. 
   In the present embodiment, it is assumed that the capacitors COA and COB have the same capacitance, and the inverters INVA 2  and INVB 2  have the same hysteresis characteristic. 
   Reference numeral  30  indicates an EX-NOR circuit used for outputting an exclusive NOR result between a signal NA 2  output from the inverter INVA 2  and a signal NB 2  output from the inverter INVB 2 . The signal output from the EX-NOR circuit  30  is input into the data terminal D of a flow-through D latch  31  used for preventing the occurrence of a hazard. In the D latch  31 , when an “L” signal is applied to the G terminal, data at the data terminal D is output from the output terminal Q, and when the signal at the G terminal rises up to “H”, the data at the data terminal D is latched and output from the output terminal Q. A signal N 2  output from the output terminal Q is applied to the first input terminal of an AND gate  32 . 
   Reference numeral  33  indicates a delay circuit used for outputting the control clock signal RCLK after delaying it by a specific time d 1 , and the signal output from the delay circuit  33  is input into the terminal G of the D latch  31  and the second input terminal of the AND gate  32 . When the signal N 2  is “H”, the AND gate  32  outputs the signal, which is output from the delay circuit  33 , as the signal UPCLK to the clock terminal CK of the D-FF  23  and the input terminal of a delay circuit  34 . The delay circuit  34  delays the signal UPCLK by a time d 2 , and then outputs the delayed signal as a signal CCLK to the delay control circuit  8  (see  FIG. 4 ) of the DLL circuit. 
   Below, the operation of the phase comparing circuit will be explained with reference to a waveform diagram of  FIG. 2 . 
   In the operation in which no metastable phenomenon occurs (see the first half of  FIG. 2 ), when the external clock signal CLK (=“H”) is latched, the state of the signal N 1  output from the inverter  22  is defined as “L”. Therefore, the signals NA 1  and NB 1 , which are respectively output from the inverters INVA 1  and INVB 1 , are stably “H”, and thus the signals NA 2  and NB 2 , which are respectively output from the inverters INVA 2  and INVB 2 , are both “L”. That is, they coincide with each other. As a result, the signal output from the EX-NOR  30  becomes “H”, and the signal N 2  output from the D latch  31  becomes “H”, so that the delayed signal (output from the delay circuit  33 ) of the control clock signal RCLK passes through the AND gate  32 , and is supplied as the signal UPCLK to the D-FF  23  and the delay circuit  34 . Accordingly, the signal “L” output from the inverter  22  is read by the D-FF  23 , so that the D-FF  23  outputs the state signal UP which is “L”, and the signal CCLK is output from the delay circuit  34 . These signals UP and CCLK are applied to the delay control circuit  8  (see  FIG. 4 ), thereby activating the delay control circuit  8 . 
   In contrast, in the operation in which a metastable phenomenon occurs (see the second half of  FIG. 2 ), as the inverter INVA 1  has the relatively high threshold level VthA, and the operational bandwidth is restricted by the capacitor COA, the charging time of the signal NA 1  output from the inverter INVA 1  is longer than the discharging time thereof. Therefore, oscillation having a relatively small amplitude occurs on the “H” side, while the signal level of the signal NA 1  gradually decreases. In addition, as the inverter INVA 2  has a hysteresis and a relatively low threshold level, the signal NA 2  keeps “L” for a specific period. 
   On the other hand, as the inverter INVB 1  has the relatively low threshold level VthB, the output signal NB 1  rapidly decreases its level while oscillating. Therefore, the signal NB 2  output from the inverter INVB 2  becomes “H” relatively early. That is, after the signal NB 2  becomes “H”, the signal NA 2  keeps “L” for a specific time during which the signal output from the EX-NOR  30  is “L”. When the delayed signal of the control clock signal RCLK is output from the delay circuit  33  during the above specific time, an “L” signal is latched at the D latch  31 , so that the AND gate  32  is closed, and no signal UPCLK is output. Therefore, when the metastable phenomenon continues during the delay time d 1 , the delay control circuit  8  (see  FIG. 4 ) is not activated, thereby preventing an erroneous operation due to the metastable phenomenon. 
   Additionally, as the input threshold level of the inverter  22  is set at the center, that is, between the thresholds VthA and VthB, it can be regarded that the input logic value of the D-FF  23  is defined before the signal NA 2  rises. Therefore, although the signals UPCLK and CCLK are generated when the signal NA 2  rises before the delay time d 1  has elapsed, no metastable phenomenon occurs because the signal N 1  output from the inverter  22  is defined prior to the rise of the signal NA 2 . 
   In addition, the reason that the inverters INVA 2  and INVB 2  each have a hysteresis is to prevent the signals NA 1  and NB 1  from being inverted when they oscillate with a relatively small amplitude. 
   Additionally, the operational bandwidth can be reduced by another method in which the size of the inverter INVA 2  is larger than that of the inverter INVA 1 . In this case, the relevant capacitor can be omitted. 
   Furthermore, similar effects can be also obtained when the threshold levels of the inverters INVA 1  and INVB 1  are the same, and the threshold levels of the inverters INVA 2  and INVB 2  are different from each other. 
   Second Embodiment 
   A second embodiment of the present invention will be shown in  FIG. 3 . In comparison with the first embodiment, the present embodiment has a distinctive feature in which in place of the inverters INVA 1  and INVB 1  which respectively have the thresholds VthA and VthB, current mirror amplifiers AMPA and AMPB are provided. The reference voltages of the amplifiers AMPA and AMPB are respectively voltages VrefA and VrefB (VrefA&gt;VrefB), which are generated by a resistive division using resistors  41  to  43 . The basic operation of the present embodiment is almost identical to that of the first embodiment (refer to  FIG. 2 ). In the present embodiment, logic thresholds can be precisely controlled, and thus be set to optimum values. 
   While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary embodiments of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. 
   INDUSTRIAL APPLICABILITY 
   The present invention is preferably applied to a DLL circuit used in a DDR SDRAM, or the like.