Patent Publication Number: US-10771233-B1

Title: Clock data recovery circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Patent Application No. 108105499, filed in Taiwan on Feb. 19, 2019; the entirety of which is incorporated herein by reference for all purposes. 
     BACKGROUND 
     The disclosure generally relates to a clock data recovery circuit and, more particularly, to a clock data recovery circuit capable of accelerating the loop response speed and does not easily result in frequency drift. 
     A clock data recovery circuit is widely employed in many applications, but the conventional clock data recovery circuit typically uses the analog loop filter. Therefore, in the situation of that the input terminal of the clock data recovery circuit does not receive any data signal over a certain period of time, it easily causes frequency drift in the resulting sampling clock signal. 
     The frequency drift problem may be reduced by replacing the analog loop filter with more complex digital loop filter, but it would easily result in insufficient loop response speed, thereby causing the conventional clock data recovery circuit to be unable to support applications having higher data signal frequency. 
     SUMMARY 
     An example embodiment of a clock data recovery circuit is disclosed, comprising: a first detecting circuit, arranged to operably detect phases of a data signal and a sampling clock signal to generate a corresponding first detection signal; a first loop filter, coupled with the first detecting circuit, and arranged to operably process the first detection signal to generate a control signal; a second detecting circuit, arranged to operably detect phases of a reference signal and a feedback signal to generate a corresponding second detection signal; a control voltage generating circuit, coupled with the second detecting circuit, and arranged to operably generate a corresponding control voltage according to the second detection signal; a voltage-controlled oscillator, coupled with the control voltage generating circuit, and arranged to operably generate the sampling clock signal according to the control voltage; a phase adjustment circuit, coupled with the first loop filter and the voltage-controlled oscillator, and arranged to operably adjust a phase of the sampling clock signal according to the control signal, so as to generate a phase-adjusted signal; and a frequency divider circuit, coupled with the second detecting circuit and the phase adjustment circuit, and arranged to operably conduct a frequency division operation on the phase-adjusted signal to generate the feedback signal. 
     Both the foregoing general description and the following detailed description are examples and explanatory only, and are not restrictive of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified functional block diagram of a clock data recovery circuit according to one embodiment of the present disclosure. 
         FIGS. 2-4  show simplified schematic diagrams of an adjustment circuit in the clock data recovery circuit according to different embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is made in detail to embodiments of the invention, which are illustrated in the accompanying drawings. The same reference numbers may be used throughout the drawings to refer to the same or like parts, components, or operations. 
       FIG. 1  shows a simplified functional block diagram of a clock data recovery circuit  100  according to one embodiment of the present disclosure. The clock data recovery circuit  100  is utilized to receive a data signal DA_IN and to generate a sampling clock signal SCLK, which can be utilized to sample the data signal DA_IN, according to the data signal DA_IN. As shown in  FIG. 1 , the clock data recovery circuit  100  comprises a first detecting circuit  110 , a first loop filter  120 , a second detecting circuit  130 , a control voltage generating circuit  140 , a voltage-controlled oscillator  150 , a phase adjustment circuit  160 , a frequency divider circuit  170 , and an adjustment circuit  180 . 
     The first detecting circuit  110  is arranged to operably detect phases of the data signal DA_IN and the sampling clock signal SCLK to generate a corresponding first detection signal DEC 1 . The first detecting circuit  110  may be realized with various suitable phase detector circuits, frequency detector circuits, or phase and frequency detector circuits. The first detection signal DEC 1  may be realized with any signal formats capable of representing the relative timing relationship between the phase of the data signal DA_IN and the phase of the sampling clock signal SCLK. 
     The first loop filter  120  is coupled with the first detecting circuit  110 , and arranged to operably process the first detection signal DEC 1  to generate a control signal CTL. The first loop filter  120  may be realized with various suitable digital filters, so as to increase the related loop response speed. 
     The second detecting circuit  130  is arranged to operably detect phases of a reference signal REF and a feedback signal FB to generate a corresponding second detection signal DEC 2 . The aforementioned reference signal REF is designed to have a frequency similar to or identical to the frequency of the data signal DA_IN received by the clock data recovery circuit  100 . In practice, the reference signal REF may be generated by other suitable circuits (not shown in  FIG. 1 ) located outside the clock data recovery circuit  100 . The second detecting circuit  130  may be realized with various suitable phase detector circuits, frequency detector circuits, or phase and frequency detector circuits. The second detection signal DEC 2  may be realized with any signal format capable of representing the relative timing relationship between the phase of the reference signal REF and the phase of the feedback signal FB. 
     The control voltage generating circuit  140  is coupled with the second detecting circuit  130 , and arranged to operably generate a corresponding control voltage VC according to the second detection signal DEC 2 . In this embodiment, when the second detection signal DEC 2  indicates that the phase of the reference signal REF is ahead the phase of the feedback signal FB, the control voltage generating circuit  140  may increase the magnitude of the control voltage VC. On the contrary, when the second detection signal DEC 2  indicates that the phase of the reference signal REF is behind the phase of the feedback signal FB, the control voltage generating circuit  140  may decrease the magnitude of the control voltage VC. 
     The voltage-controlled oscillator  150  is coupled with the control voltage generating circuit  140 , and arranged to operably generate the aforementioned sampling clock signal SCLK according to the control voltage VC. The voltage-controlled oscillator  150  may increase the frequency of the outputted sampling clock signal SCLK when the control voltage generating circuit  140  increases the magnitude of the control voltage VC; and may decrease the frequency of the sampling clock signal SCLK when the control voltage generating circuit  140  decreases the magnitude of the control voltage VC. 
     The phase adjustment circuit  160  is coupled with the first loop filter  120  and the voltage-controlled oscillator  150 , and arranged to operably adjust the phase of the sampling clock signal SCLK according to the control signal CTL to generate a phase-adjusted signal ACLK. In practice, the phase adjustment circuit  160  may be realized with various phase interpolation circuits, programmable delay chains, or other suitable circuits. 
     The frequency divider circuit  170  is coupled with the second detecting circuit  130  and the phase adjustment circuit  160 , and arranged to operably conduct a frequency division operation on the phase-adjusted signal ACLK to generate the aforementioned feedback signal FB, so that the frequency of the feedback signal FB is lower than the frequency of the phase-adjusted signal ACLK. In practice, the frequency divider circuit  170  may be realized with various suitable integer frequency dividers or fractional frequency dividers. 
     The adjustment circuit  180  is coupled with the first detecting circuit  110  and the voltage-controlled oscillator  150 , and arranged to operably control the voltage-controlled oscillator  150  according to the first detection signal DEC 1  to change the phase of the sampling clock signal SCLK. In other words, the aforementioned voltage-controlled oscillator  150  not only adjusts the frequency of the sampling clock signal SCLK according to the control voltage VC generated by the control voltage generating circuit  140 , but also adjusts the phase of the sampling clock signal SCLK under control of the adjustment circuit  180 . 
     During the operation of the clock data recovery circuit  100 , the aforementioned first detecting circuit  110 , adjustment circuit  180 , and voltage-controlled oscillator  150  collectively form a first loop. The second detecting circuit  130 , the control voltage generating circuit  140 , the voltage-controlled oscillator  150 , the phase adjustment circuit  160 , and the frequency divider circuit  170  collectively form a second loop. 
     Since the aforementioned phase adjustment circuit  160  adjusts the phase of the sampling clock signal SCLK under control of the first loop filter  120  to generate the phase-adjusted signal ACLK, the aforementioned first loop and second loop are enabled to reach respective steady states at the same time. As a result, the first loop and the second loop can operate simultaneously. That is, the first loop and the second loop in the clock data recovery circuit  100  can share the sampling clock signal SCLK generated by the voltage-controlled oscillator  150  at the same time. 
     Therefore, there is no need to arrange additional switch circuits in the clock data recovery circuit  100  to switch the voltage-controlled oscillator  150  between the aforementioned first loop and second loop. As a result, the probability of that frequency drift occurs in the sampling clock signal SCLK outputted from the voltage-controlled oscillator  150  can be effectively reduced. 
     In practice, the control voltage generating circuit  140  may be realized with a cooperation of a charge pump and a loop filter. In the embodiment of  FIG. 2 , for example, the control voltage generating circuit  140  comprises a charge pump  242  and a second loop filter  244 . 
     The charge pump  242  is coupled with the second detecting circuit  130 , and arranged to operably generate a corresponding output voltage according to the second detection signal DEC 2 . The second loop filter  244  is coupled with the charge pump  242  and the voltage-controlled oscillator  150 , and arranged to operably compensate and filter the output voltage of the charge pump  242 , so as to generate the aforementioned control voltage VC. In practice, the aforementioned charge pump  242  and second loop filter  244  may be realized with various suitable circuit structures. 
     In this embodiment, when the second detecting circuit  130  has detected that the phase of the reference signal REF is ahead the phase of the feedback signal FB, the second detecting circuit  130  may utilize the second detection signal DEC 2  to control the charge pump  242  to increase the output voltage, to thereby increase the control voltage VC. When the second detecting circuit  130  has detected that the phase of the reference signal REF is behind the phase of the feedback signal FB, the second detecting circuit  130  may utilize the second detection signal DEC 2  to control the charge pump  242  to decrease the output voltage, to thereby decrease the control voltage VC. 
     In the embodiment of  FIG. 2 , the aforementioned adjustment circuit  180  may be realized with a combination of a current source circuit  282  and a current sink circuit  284 . The current source circuit  282  is arranged to operably increase the current passing through the voltage-controlled oscillator  150  according to the first detection signal DEC 1 . The current sink circuit  284  is arranged to operably decrease the current passing through the voltage-controlled oscillator  150  according to the first detection signal DEC 1 . 
     When the first detecting circuit  110  has detected that the phase of the data signal DA_IN is ahead the phase of the sampling clock signal SCLK, the first detecting circuit  110  may utilize the first detection signal DEC 1  to control the adjustment circuit  180  to increase the current passing through the voltage-controlled oscillator  150 , so that the voltage-controlled oscillator  150  would advance the phase of the sampling clock signal SCLK. On the other hand, when the first detecting circuit  110  has detected that the phase of the data signal DA_IN is behind the phase of the sampling clock signal SCLK, the first detecting circuit  110  may utilize the first detection signal DEC 1  to control the adjustment circuit  180  to decrease the current passing through the voltage-controlled oscillator  150 , so that the voltage-controlled oscillator  150  would delay the phase of the sampling clock signal SCLK. 
     By utilizing the adjustment circuit  180  to directly adjust the current passing through the voltage-controlled oscillator  150  according to the detecting result of the first detecting circuit  110 , the phase of the sampling clock signal SCLK outputted from the voltage-controlled oscillator  150  can be changed, thereby significantly increasing the loop response speed of the aforementioned first loop. Even if the clock data recovery circuit  100  does not receive the data signal DA_IN for a longer period of time, because the second detecting circuit  130  still works for every reference period and keep adjusting the voltage-controlled oscillator  150 , the frequency of the sampling clock signal SCLK outputted from the voltage-controlled oscillator  150  can still remain stable. In this way, it effectively reduces the possibility of that the frequency drift occurs in the sampling clock signal SCLK outputted from the voltage-controlled oscillator  150 . 
     Please note that the aforementioned circuit structures are merely exemplary embodiments rather than a restriction to the practical implementations. As shown in  FIG. 3 , the current sink circuit  284  in the aforementioned adjustment circuit  180  may be omitted in some embodiments. 
     As shown in  FIG. 4 , for another example, the current source circuit  282  in the adjustment circuit  180  may be omitted in some embodiments. 
     In some embodiments, the aforementioned adjustment circuit  180  may be instead realized with a variable capacitor circuit coupled with the input terminal of the voltage-controlled oscillator  150 . In this embodiment, the first detecting circuit  110  may utilize the first detection signal DEC 1  to change the capacitance of the adjustment circuit  180 , so as to change the input voltage of the voltage-controlled oscillator  150 , thereby adjusting the phase of the sampling clock signal SCLK. 
     In another embodiment, the adjustment circuit  180  may be instead realized with various voltage adjustment circuits capable of changing the input voltage of the voltage-controlled oscillator  150  according to the first detection signal DEC 1 . 
     Certain terms are used throughout the description and the claims to refer to particular components. One skilled in the art appreciates that a component may be referred to as different names. This disclosure does not intend to distinguish between components that differ in name but not in function. In the description and in the claims, the term “comprise” is used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to.” The term “couple” is intended to compass any indirect or direct connection. Accordingly, if this disclosure mentioned that a first device is coupled with a second device, it means that the first device may be directly or indirectly connected to the second device through electrical connections, wireless communications, optical communications, or other signal connections with/without other intermediate devices or connection means. 
     The term “and/or” may comprise any and all combinations of one or more of the associated listed items. In addition, the singular forms “a,” “an,” and “the” herein are intended to comprise the plural forms as well, unless the context clearly indicates otherwise. 
     The term “voltage signal” used throughout the description and the claims may be expressed in the format of a current in implementations, and the term “current signal” used throughout the description and the claims may be expressed in the format of a voltage in implementations. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention indicated by the following claims.