BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clock recovery circuit in which clock signals are extracted from an inputted data signal, and more particularly, to an over-sampling type clock recovery circuit which performs sampling with a plurality of clock signals having different phases. 
     2. Description of the Related Art 
     In recent years, a high-speed protocol is proposed, such as Gbit Ethernet and Fiber Channel for data transmission. For this purpose, high speed processing is requested to a clock recovery circuit which extracts a clock signal from a data signal in a high speed transmission and a PLL circuit which establishes frequency synchronization between an internal clock signal and the transmitted clock signal. In order to respond to such a request, as disclosed in 1996 IEEE International Solid-State Circuits Conference, an over-sampling-type clock recovery circuit is proposed in which a transmitted data signal is sampled with a plurality of clock signals with different phases generated by an internal circuit. 
     FIG. 1 shows a block diagram of a clock recovery circuit which is disclosed in the conventional example. A data signal is supplied to eight phase comparators TIPD 0  to TIPD 7 . Each phase comparator TIPD is supplied with every three clock signals of 24 clock signals having fixed delays outputted from a fixed delay circuit FD and detects a phase state between the data signal supplied and three clock signals. 
     When the data signal is delayed more in relation to the clock signals, the phase comparator TIPD detects the leading of the clock signal in phase to set a dn signal as an output of the circuit to an enable state and an up signal as another output of the circuit to a disable state. Similarly, when detecting the delay of the clock signals in phase, the phase comparator TIPD sets the up signal to the enable state and the dn signal to the disable state. FIGS. 2A to  2 F show a locking state. In this case, there is no delay or leading the clock signals in phase as shown in FIGS. 2A to  2 D. Therefore, both of the up signal and the dn signal are set to the disable state, as shown in FIGS. 2E and 2F. On the other hand, FIGS. 3A to  3 D shows the delay state of the clock signals. At this time, the up signal is set to the enable state, as shown in FIG.  3 E and the dn signal is set to the disable state, as shown in FIG.  3 F. In case of the leading state of the clock signals, the up signal is set to the disable state, and the dn signal is set to the enable state. 
     Each of charge pumps CP 0  to CP 7  increases the output voltages when the up signal is set to the enable state and decreases the output voltage when the dn signal is set to the enable state. A low pass filter LPF inputs the output voltages from the charge pumps CP 0  to CP 7  and integrates the change of these voltages. A variable delay circuit VD is supplied with the output voltage of the low pass filter LPF and a reference clock signal which is the output of a voltage controlled oscillator VCO. The variable delay circuit VD delays the reference clock signal in accordance with the output voltage from the low pass filter LPF. The fixed delay circuit FD is supplied with the output of the variable delay circuit FD and generates the 24 clock signals having fixed delays from the supplied clock signal. 
     In this way, in the conventional clock recovery circuit, each of the eight phase comparators TIPD 0  to TIPD 7  detects the leading or delay state of the clock signals in phase to set the up signal or dn signal is set to the enable state. As a result, the voltage outputted from the corresponding charge pump CP increases or decreases. Therefore, the variable delay circuit VD delays the reference clock signal based on the output voltage from the low pass filter LPF. The fixed delay circuit FD generates the 24 clock signals from the delayed reference signal. As a result, the phase leading or delay state of the clock signals in each of the phase comparators TIPD 0  to TIPD 7  is set to an adequate state so that the appropriate sampling of the data signal can be realized. 
     However, in the conventional clock recovery circuit, there is a problem. That is, the data sampling cannot be correctly performed when the phase differences are generated between the 24 clock signals due to the influence of a layout of wiring patterns in the clock recovery circuit. Especially, the data sampling cannot be correctly performed when the phase differences are generated between three clock signals supplied to the phase comparator TIPD. For example, when the a phase shift (delay) of the clock signal is generated in one of the three clock signals as shown in FIG. 3D, the phase comparator TIPD detects a clock delay state so that the up signal is set to the enable state. For this reason, the charge pump CP receives the up signal of the enable state and the fixed delay circuit FD delays the 24 clock signals. As a result, a correct data sampling cannot be performed in the whole clock recovery circuit including other phase comparators TIPD. 
     In addition to the above conventional example, a digital signal receiving apparatus is disclosed in Japanese Laid Open Patent application (JP-A-Showa 61- 145945).  In this reference, the digital signal receiving apparatus is composed of a reproducing section and a majority determining section and a conversion section. The reproducing section reproduces clock signals having a basic clock signal frequency fr and a frequency n (n is a positive integer equal to or larger then 3) times of the basic clock signal frequency fr locked to a digital reproduction signal in phase. The majority determining section extracts n samples values during one bit of the digital reproduction signal based on nfr clock signals, and determines binary values of the n sample values on the majority side as a value during the bit. The converting section converts the determined value to have 1/fr width. Thus, the digital reproduction signal is shaped in units of basic clocks fr of the digital reproduction signal. 
     Also, a data sampling converting circuit is disclosed in Japanese Laid Open Patent application (JP-A-Showa 61- 214842).  In this reference, the data sampling converting circuit is composed of a clock reproducing circuit, a frequency dividing circuit and a determining circuit. The clock reproducing circuit reproduces a clock pulse from a character multiplexed signal. The frequency dividing circuit divides the reproduced clock signal in frequency to 1 to n-th, and generates n sampling pulses with different phases. The determining circuit performs sampling of the character multiplexed signal with the n sampling pulses and determines based on majority determination of m continuous sampling results whether a digital data is in a high level or a low level. 
     Also, a demodulation data identifying and determining apparatus is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 3-69238). In this reference, the demodulation data identifying and determining apparatus is composed of a detecting and demodulating circuit, a comparator, a clock reproducing circuit, a timing determining section, a latch circuit. The detecting and demodulating circuit demodulate an input signal to output a base band signal. The comparator converts the base band signal into a binary signal. The clock reproducing circuit reproduces a reproduction clock signal having the same frequency as a bit rate of a transmission data, and generates a clock signal faster than the reproduction clock signal. The timing determining section performs sampling of the binary signal using the clock signal and performs majority determination to a plurality of values corresponding to a plurality of sampling points to output the result of the majority determination. The latch circuit latches the output from the timing determining section in accordance with the reproduction clock signal to output as a reproduced digital data. 
     Also, a digital signal reproducing circuit is disclosed in Japanese Laid Open Patent application (JP-A-Heisei 4-11431). In this reference, the digital signal reproducing circuit is composed of a demodulating section, a sampling section and a majority determining section. The demodulating section demodulates a digital modulated signal. The sampling section performs sampling of the demodulated digital signal in accordance with clock signals from a clock source. The majority determining section performs majority determination to a plurality of sampling values supplied from the sampling section. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised to solve the above problems. Therefore, an object of the present invention is to provide an over-sampling type clock recovery circuit which can correct phases of clock signals. 
     Another object of the present invention is to provide an over-sampling type clock recovery circuit in which correct sampling of a data signal can be realized irrespective of a layout of clock signals. 
     In order to achieve a first aspect of this present invention, an over-sampling type clock recovery circuit includes a phase difference determining section and a phase adjusting section. The phase difference determining section determines a phase difference between a data signal and a plurality of clock signals using majority determination, to produce a phase adjustment signal. The phase adjusting section adjusts phases of the plurality of clock signals based on the phase adjustment signal. 
     The phase difference determining section includes N (N is an integer equal to or more than 1) phase comparators, a majority determining circuit and an adjustment signal generating section. The N phase comparators are provided for respective bits of data of the data signal. Each of the N phase comparators compares a corresponding one of the bits and a set of clock signals in phase to produce a comparison phase difference signal. The majority determining circuit determines majority of the N comparison phase difference signals and changing minority of the N comparison phase difference signals to match to the majority of the N comparison phase difference signals to generate N majority phase difference signals. The adjustment signal generating section generates the phase adjustment signal from the N majority phase difference signals. 
     In this case, each of the phase comparators generates as the comparison phase difference signals, a first signal when the data signal leads the clock signal set, and a second signal when the data signal is delayed than the clock signal set. The majority determining circuit determines the first signals to change each of the second signals into the first signal when the first signals are more than the second signals, and determines the second signals to change each of the first signals into the second signal when the second signals are more than the first signals. Also, the phase adjusting section includes an oscillator for generating a reference clock signal, a delay unit for delaying the reference clock signal based on the phase adjustment signal, and a clock signal generating section for generating the plurality of clock signals from the delayed reference signal such that each of the plurality of clock signals has a predetermined delay. 
     In order to achieve another aspect of the present invention, a method of n over-sampling type clock recovery circuit includes: 
     determining a phase difference between a data signal and a plurality of clock signals using majority determination, to produce a phase adjustment signal; and 
     adjusting phases of the plurality of clock signals based on the phase adjustment signal. 
     In order to achieve still another aspect of the present invention, an over-sampling type clock recovery circuit includes an actual phase difference determining section for determining a phase difference between a data signal and a plurality of clock signals in units of bits of data of the data signals, to produce comparison phase difference signals, an adjustment signal generating section for determining majority of the comparison phase difference signals and generating a phase adjustment signal based on the determined majority, and a phase adjusting section for adjusting phases of the plurality of clock signals based on the phase adjustment signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional example of a clock recovery circuit; 
     FIGS. 2A to  2 F are timing charts to explain a lock state when there is no delay or leading of clock signals to a data signal in phase; 
     FIGS. 3A to  3 F are timing charts to explain a delayed state of the clock signals than a data signal in phase; 
     FIG. 4 is a block circuit diagram illustrating the structure of a clock recovery circuit according to an embodiment of the present invention; 
     FIGS. 5A to  5 Y are timing charts illustrating clock signals and a data signal; 
     FIGS. 6A to  6 F are timing charts to explain a locking state when there is no delay or leading of the clock signals than or to the data signal in phase; 
     FIGS. 7A to  7 F are timing charts to explain a delaying state of the clock signals than the data signal in phase; 
     FIGS. 8A to  8 F are timing charts to explain a leading state of the clock signals to the data signal in phase; 
     FIGS. 9A to  9 P are timing charts to explain a part of the operation of a majority determining circuit; 
     FIGS. 10A to  10 P are timing charts to explain another part of the operation of the majority determining circuit; and 
     FIG. 11 is a diagram showing the input-output characteristic of a variable delay circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, an over-sampling type clock recovery circuit of the present invention will be described below in detail with reference to the accompanied drawings. 
     FIG. 4 is a block circuit diagram illustrating the structure of the over-sampling type clock recovery circuit according to an embodiment of the present invention. The over-sampling type clock recovery circuit is composed of a plurality of phase comparators TIPD 0  to TIPD 7 , a majority determining circuit DEC, a plurality of charge pumps CP 0  to CP 7 , a low pass filter LPF, a voltage controlled oscillator VCO, a variable delay circuit VD, and a fixed delay circuit FD. It should be noted that the same reference symbols are allocated to the same components as those of the conventional clock recovery circuit. 
     In the embodiment, each of eight phase comparators TIPD 0  to TIPD 7  is provided to perform sampling of a data signal of 8 bits with three clock signals. That is, each of the phase comparators TIPD 0  to TIPD 7  inputs the 3 clock signals having fixed delays and the data signal transmitted thereto and detects the phase state between the data signal and the clock signals. When the data signal delays than the clock signal, the phase comparator detect the leading of the clock signal to the data signal to set a corresponding one of up signals up 0  to up 7  to a disable state and to set a corresponding one of dn signals dn 0  to dn 7  to an enable state. In the same way, when detecting the delay of the clock signals than the data signal, the phase comparator sets the corresponding up signal to the enable state and the corresponding dn signal to the disable state. 
     The majority determining circuit DEC is connected with the output terminals of the respective phase comparators TIPD 0  to TIPD 7 . The majority determining circuit DEC determines the majority of the up or dn signals of the enable or disable states from the phase comparators TIPD 0  to TIPD 7  to determine whether the clock signals leads in phase than the data signal as the whole circuit. As the result of the majority determination, the states of the up signals and dn signals on the minority side are corrected to match to those of the up signals and dn signals on the majority side. Then, the majority determining circuit DEC outputs the up signals and dn signals on the majority side and the corrected up signals and dn signals on the minority side to the charge pumps CP 0  to CP 7  as upd signals and dnd signals, respectively. 
     Each of the charge pumps CP inputs corresponding ones of the upd signals and dnd signals from the majority determining circuit DEC and changes the output voltage in accordance with the input signals to outputs to the low pass filter LPF. The low pass filter LPF integrates the changes of the output voltages from the charge pumps CP 0  to CP 7  to output to the variable delay circuit VD. The variable delay circuit VD inputs a reference clock signal having a predetermined frequency supplied from the voltage controlled oscillator VCO and the output of the low pass filter LPF. The variable delay circuit VD delays the reference clock signal in accordance with the output voltage from the low pass filter LPF. Moreover, the fixed delay circuit FD inputs the output of the variable delay circuit VD, and generates 24 clock signals having fixed delays from the inputted clock signal. The 24 clock signals are supplied to the phase comparators TIPD 0  to TIPD 7  every three, respectively. 
     The operation of the clock recovery circuit having the above-mentioned structure will be described. FIGS. 5A to  5 Y are time charts to explain the data signal supplied to the eight phase comparators TIPD 0  to TIPD 7  and the 24 clock signals used to perform sampling of the data signal. Also, FIGS. 6A to  6 F,  7 A to  7 F, and  8 A to  8 F are timing charts to explain the operation of the each of the phase comparators TIPD 0  to TIPD 7 . As described in the conventional example, each phase comparator detects the phase state between the data signal and corresponding set of three clk n−1 , clk n  and clk n+1  of the 24 clock signals. 
     FIGS. 6A to  6 F show a lock state in which the phases between the data signal and the clock signals are coincident with each other. Since there is no need to change the phases of the clock signals in the lock state, both of the up signal and the dn signal are set to the disable state, as shown in FIGS. 6E and 6F. FIGS. 7A to  7 F show the leading state of the data signal to the clock signals in phase. When the clock signals delay than the data signal, the up signal is set to the enable state as shown in FIG.  7 E and the dn signal is set to the disable state as shown in  7 F. As a result, the phases of the clock signal are led. FIGS. 8A to  8 F show the delay state of the data signal than the clock signals. When the clock signals lead than the data signal, the up signal is set to the disable state as shown in FIG.  8 E and the dn signal is set to the enable state as shown in FIG.  8 F. As a result, the phases of the clock signals are delayed. 
     Then, the up signal and dn signal of each phase comparator are supplied to the majority determining circuit DEC. The majority determining circuit DEC inputs the up signals and dn signals supplied from the respective phase comparators TIPD 0  to TIPD 7 . The majority determining circuit DEC determines the majority of the detecting results of the respective phase comparators TIPD 0  to TIPD 7 , that is, the phase difference data based on the supplied up signals and dn signals. 
     As described above, in the over-sampling type clock recovery circuit, sampling of the data signal with the clock signals having the fixed phase differences. Therefore, if the data signal is not to be high or low continuously, the phase difference data detected in each phase comparator becomes the same value even though there is any delay due to the phase difference of the clock signals. When the data signal is high or low continuously, there is no rising edge in the data signal. Therefore, the phase comparator determines that the data signal is in the locking state and sets the up signal and dn signal to the disable state. However, because there is a limitation of the number of data bits continuously having the same value depending upon a transmission system, the phase of the data signal can be necessarily detected in a certain bit unit. Therefore, in this embodiment, the data in units of 8 bits is used. 
     From the result of the majority determination of the number of up signals or dn signals set to the enable state or disable state, it is determined whether the clock signals in the circuit leads to or delays than the data signal as the whole clock recovery circuit. 
     FIGS. 9A to  9 P show the data signal and the clock signals clk 00  to clk 08 , the up signals up 01  to up 02  outputted from the phase comparators TIPD 0  to TIPD 2  supplied, and the upd signals upd 00  to upd 02  corrected by the majority determining circuit DEC. In the state shown in FIGS. 9A to  9 P, if there is not a phase shift in each clock signal, the clock signals are detected by all the phase comparators TIPD 0  to TIPD 7  to be in the phase delay state. As a result, the up signals are set to the enable state, and the dn signals are set to the disable state. 
     However, because there is a phase shift, i.e., the phase leading (proceeding) in the clock signal clk 08  of FIG. 9J, the phase comparator TIPD 2  supplied with the clock signals clk 06  to clk 08  determines to be in the locking state. Therefore, the phase comparator TIPD 2  sets the up signal to the disable state, as shown in FIG. 9M, and the dn signal to the disable state. However, when the up signals are inputted from all the phase comparators TIPD 0  to TIPD 7  shown in FIG. 4, the majority determining circuit DEC determines the majority of the up signals and the dn signals. Therefore, the majority determining circuit DEC determines that the clock recovery circuit is in the phase delay state of the clock signals as the whole circuit. Thus, the upd 02  signal as the output signal of the majority determining circuit DEC corresponding to the up 02  signal is set to the enable state. Therefore, in the operation since then, an influence due to the phase shift of the clock signal clk 08  is canceled. 
     FIGS. 10A to  10 P show timing charts of the data signal, the clock signals clk 00  to clk 08 , the up signals and upd signals upd 00  to upd 02 . Here, a case is shown where a phase shift of the data signal occurs at a certain bit to delay the phase of the data signal. When the phase shift occurs in the bit of the data signal, the majority determining circuit DEC could correct the phase shift, as in the above case where the clock signal is shifted in phase. That is, if there is not a phase shift in the bit of the data signal, the phase delay state of the clock signals is detected by the phase comparators TIPD 0  and TIPD 1 . Therefore, the up signals up 01  and up 02  are set to the enable state. However, in this case, because there is a phase delay shift in the third bit of the data signal, the phase comparator TIPD 2  inputting the third bit determines to be in locking state and sets the up signal to the disable state and the dn signal to the disable state. The majority determining circuit DEC determines the majority of the up signals and the dn signals outputted from all the phase comparators TIPD 0  to TIPD 7  shown in FIG.  4 . Therefore, the majority determining circuit DEC determines that the clock signal delays than the data signal as the whole circuit. As a result, the majority determining circuit DEC sets the upd signal upd 02  corresponding to the up 02  signal outputted from the phase comparator TIPD 2  to the enable state and the dnd signal dnd 02  to the disable state. Therefore, in the operation since then, an influence due to the phase shift of the data signal is canceled. 
     In this way, because the phase shift of the clock signal or data signal is corrected by the majority determining circuit DEC, the respective charge pumps CP 0  to CP 7  supplied with the upd signals upd 0  to upd 7  and dnd signals dnd 0  to dnd 7 , converts the phase difference data obtained from the upd signals and dnd signals into the voltage values. That is, when the upd signal is set to the enable state, the output voltage is increased, and when the dnd signal is set to the enable state, the output is decreased. The low pass filter LPF is supplied with the output voltages of the charge pumps PC 0  to PC 7  and integrates the change of this voltage. The variable delay circuit VD is supplied with the output voltage of the low pass filter LPF and the reference clock signal outputted from the voltage controlled oscillator VCO. The variable delay circuit VD delays and outputs the reference clock signal in accordance with the output voltage of the low pass filter LPF. FIG. 11 shows relation of a delay quantity to the input voltage of the variable delay circuit VD. The reference clock signal delayed by the variable delay circuit VD is supplied to the fixed delay circuit FD. Then, the fixed delay circuit generates the 24 clock signals having the same phase difference between the clock signals from the delayed reference clock signal, to output the clock signals to each of the phase comparator. 
     Therefore, when a phase shift is generated in a part of the clock signals having the fixed phases, or when the phase shift is generated in a part of the data signal, the majority determining circuit DEC corrects the phase difference data, even though the erroneous phase difference data is outputted from a part of the phase comparators TIPD 0  to TIPD 7 . Therefore, it is possible to avoid the generation of inappropriate leading or delay of the clock signals in the fixed delay circuit FD owing to the erroneous phase difference data, resulting in the correct sampling of the data signal. 
     The embodiment shows an example of the present invention. It would not need to say that the number of bits of the data signal, the number of phase comparators and the number of clock signals having the fixed phases can be set suitably in accordance with the required speed. 
     As described above, according to the present invention, a majority determining circuit DEC is provided to input a plurality of phase difference data which are the outputs from a plurality of phase comparators TIPD 0  to TIPD 7 . Also, the majority determining circuit DEC determines the majority of the phase difference data, and to correct and output the phase difference data on the minority side to the phase difference data on the majority side. Therefore, even if phase shifts of the plurality of the clock signals or the phase shift of the data signal are generated due to the influence of the layout, the phase difference data on the minority side generated from this phase difference can be corrected to the phase difference data on the majority side.
An over-sampling type clock recovery circuit includes a phase difference determining section and a phase adjusting section. The phase difference determining section determines a phase difference between a data signal and a plurality of clock signals using majority determination, to produce a phase adjustment signal. The phase adjusting section adjusts phases of the plurality of clock signals based on the phase adjustment signal.