Abstract:
A semiconductor integrated circuit includes a phase-locked loop (PLL) circuit configured to generate an oscillation output signal synchronized with a reference clock and a plurality of clock and data recovery (CDR) circuits configured to adjust a phase of the oscillation output signal and a phase of serial data. The PLL circuit converts a voltage output from a loop filter, the voltage functioning to control an oscillation frequency of an oscillator, into a current and delivers the converted current to the plurality of CDR circuits. Therefore, in a case where clock signals used in a plurality of serial transmission channels are synchronized with one another, limitations on layout of clock wiring from the PLL circuit to the CDR circuits and the occurrence of jitter are reduced.

Description:
BACKGROUND  
       [0001]     The present invention relates to semiconductor integrated circuits, and in particular, to a semiconductor integrated circuit that incorporates a phase-locked loop (PLL) circuit and a plurality of clock and data recovery (CDR) circuits for synchronization of clocks used in a plurality of serial transmission channels.  
         [0002]     In the network field, a transmission technology based on serial transmission has been developed. Currently, serial transmission in the network field mainly uses a technique of embedding clock information (encoded data) in a series of data, transmitting the data at the transmitting side and decoding a clock from the transmitted data at the receiving side for synchronization between transceivers.  
         [0003]     Network devices usually include PLL circuits and CDR circuits in order to synchronize clocks between remote places connected over a network.  
         [0004]      FIG. 7  is a block diagram of a known conventional PLL circuit. A PLL circuit  70  includes a phase-frequency detector (PFD)  71 , a charge pump (CP)  72 , a loop filter (LF)  73 , a voltage-to-current converter (V-I)  74 , an oscillator (OSC)  75 , and a frequency divider (DIV)  76 . When a reference clock REFCLK is externally input to the PLL circuit  70 , the phase frequency detector  71  compares the reference clock REFCLK to a divided signal DIVCLK into which the frequency of an oscillation output signal GCLK from the oscillator  75  is divided to detect the phase and frequency differences therebetween. The phase frequency detector  71  outputs an up pulse signal UP or a down pulse signal DOWN to the charge pump  72 , depending on the phase difference and frequency difference. The charge pump  72  charges or discharges the loop filter  73  on the basis of the up pulse signal UP or the down pulse signal DOWN. A control voltage VTUNE determined by an electric charge stored in a capacitance of the loop filter  73  is converted into a frequency control current ICTRL in the voltage-to-current converter  74 . The frequency control current ICTRL is used to control an oscillation frequency of the oscillator  75 .  
         [0005]     In this way, the phase difference and the frequency difference between the reference clock REFCLK and the oscillation output signal GCLK from the oscillator  75  are detected, and in accordance with the detected values, the oscillation frequency of the oscillator  75  is repeatedly changed, so that the phase and frequency of the reference clock REFCLK and the phase and frequency of the oscillation output signal GCLK are synchronized with each other.  
         [0006]      FIG. 8  is a block diagram of a conventional serial-to-parallel converter (S/P) for converting serial data into parallel data by using a CDR circuit. As shown in  FIG. 8 , a serial-to-parallel converter  80  includes a deserializer  84  and a CDR circuit  81  having a phase detector (PD)  82  and a phase control block  83 . The phase detector  82  detects the leading edge and the trailing edge of serial data signals RXP and RXN (which are differential data), and transmits to the phase control block  83  phase differences between the serial data signals RXP and RXN and clocks GCLKI, GCLKQ, GCLKIB, and GCLKQB. The clocks GCLKI, GCLKQ, GCLKIB, and GCLKQB are generated by a PLL circuit and  90  are degrees out of phase with the adjacent clock. The phase control block  83  shifts the phases of the clocks GCLKI, GCLKQ, GCLKIB, and GCLKQB such that the phases of these clocks are synchronized with the phases of the serial data signals RXP and RXN. Clocks FCLKI and FCLKIB, whose phases have been adjusted by the phase control block  83 , are transmitted to the deserializer  84 . The deserializer  84  converts the serial data into parallel data in synchronism with the clocks transmitted from the phase control block  83 .  
         [0007]      FIG. 9  is a block diagram of another conventional serial-to-parallel converter (S/P). A serial-to-parallel converter  90  includes, instead of the phase control block  83  shown in  FIG. 8 , a coarse loop  92 , which is similar to a PLL circuit, and a fine loop  91  for performing control from a phase detector, and a deserializer  93 . The coarse loop  92  operates in the same manner as the PLL circuit described above, i.e., compares a low-frequency reference clock REFCLK and a divided signal DUVCLK into which the frequency of an oscillation output signal generated in an oscillator  915  is divided to detect a phase difference therebetween and controls an oscillation frequency of the oscillator  915 . If the frequency of the reference clock REFCLK is equal to the frequency of serial data signals RXP and RXN, the fine loop  91  performs an adjustment such that the phases of high-speed clocks GCLKI, GCLKQ, GCLKIB, and GCLKQB which are generated in the oscillator  915 , are coincident with the phases of the serial data signals. The high-speed clocks GCLKI and GCLKIB, whose phases have been adjusted in the fine loop  91 , are transmitted to the deserializer  93 . The deserializer  93  then converts the serial data into parallel data in synchronism with the high-speed clocks GCLKI and GCLKIB from the fine loop  91 .  
         [0008]     With increases in the amount of traffic on networks in recent years, a technique of binding a plurality of serial transmission channels is used. In this case, it is necessary to perform data communications while the frequencies of clocks in the plurality of serial transmission channels are coincident with each other. The technique is broadly classified into two methods described below.  
         [0009]     A first method is a method of supplying a clock from a single PLL circuit to each of the plurality of channels.  FIG. 10  is a schematic diagram illustrating this method. Serial-to-parallel converters  80  in  FIG. 10  are similar to the serial-to-parallel converter  80  shown in  FIG. 8 . This method has the advantage of being capable of entirely synchronizing the frequencies of clocks in the channels.  
         [0010]     However, as the line becomes finer in semiconductor process technology, line widths and spaces between lines are reduced, and therefore, line resistance and capacitance between lines tend to increase. As a result, as the clock frequency increases its speed from several gigabytes to several tens of gigabytes, deterioration of signal quality, such as a decrease in clock amplitude, an occurrence of jitter, and the like, tends to occur in channels that are remote from each other in layout. To address this problem, a buffer inserted into a path of clock wiring or a shielding of wire is required. This imposes limitations on layout of clock wiring on a semiconductor chip.  
         [0011]     A second method is a method of arranging a serial-to-parallel converter such as the one shown in  FIG. 9  in each channel, and providing commonality among reference clocks of the serial-to-parallel converters.  FIG. 11  is a schematic diagram illustrating this method. This second method can reduce clock deterioration dependent on the position on layout more than the first method because high-speed clocks are generated in each channel.  
         [0012]     However, since a reference clock is shared in the second method, the influence of driven load capacitance is not avoided. Therefore, limitations on layout of clock wiring are still present, although not as much as in the high-speed clock described above. Additionally, both the layout area and power consumption are increased because a coarse loop and a fine loop are disposed in each channel. Moreover, since each coarse loop uses a low-frequency reference clock for synchronization, it is necessary to increase a frequency gain of an oscillator, which results in the occurrence of jitter.  
         [0013]     As the second method, various techniques are discussed. For example, Japanese Unexamined Patent Application Publication No. 2000-243939 discloses a technique that allows layout of wiring for a reference clock merely by embedding a reference-clock transmission block in each channel and arranging the channels in a different sequence.  
         [0014]     Japanese Unexamined Patent Application Publication No. 2004-015032 discloses a technique that suppresses noise within a chip by arranging a line for a reference clock in a dedicated area on an outermost section of the chip and also reduces the length of the line for the reference clock.  
         [0015]     Japanese Unexamined Patent Application Publication No. 11-205133 discloses a technique that reduces variations in the frequency of an oscillator by providing a PLL circuit in a receiver with two loops, one loop functioning to compare a reference clock and a feedback clock and the other functioning to compare serial data and the feedback clock, switching between the two loops at start-up and at which data synchronization is performed, and suppressing a current in a tuning-current generating circuit in an oscillator when synchronization with serial data is performed.  
         [0016]     The techniques described in Japanese Unexamined Patent Application Publication Nos. 2000-243939 and 2004-015032 can reduce limitations on layout and suppress the influence of noise caused by a circuit adjacent to a clock line, but do not solve a problem of the increase in the layout area and power consumption because a clock itself is supplied to each channel. Additionally, the problem of the occurrence of jitter remains unsolved because it is necessary to increase the gain of a voltage-controlled oscillator.  
         [0017]     The technique described in Japanese Unexamined Patent Application Publication No. 11-205133 suppresses variations in the frequency of the oscillator and thus reduces the occurrence of jitter by not using a reference clock and reducing a tuning current when synchronization with serial data is performed. However, in a state where no serial data is input over a fixed period of time, the first loop for the reference clock is temporarily switched on and, upon receipt of serial data, the second loop is switched on again. Therefore, it is necessary to use a reference clock whose jitter is low. As a result, the problem of the limitations on layout of wiring for the reference clock, and the increase in power consumption caused by a buffer arranged in a path of distribution of the reference clock remains unsolved.  
       SUMMARY  
       [0018]     It is an object of the present invention to provide a semiconductor integrated circuit that solves the above-described problems in the known art, avoids limitations on layout of clock wiring from a PLL circuit to CDR circuits when clocks used in a plurality of serial transmission channels are synchronized with each other, and reduces the occurrence of jitter.  
         [0019]     According to an exemplary aspect of the present invention, a semiconductor integrated circuit includes a phase-locked loop (PLL) circuit and a plurality of clock and data recovery (CDR) circuits. The PLL circuit is configured to generate an oscillation output signal synchronized with a reference clock and includes a phase-frequency detector, a loop filter, an oscillator, and a voltage-to-current converter configured to convert a control voltage output from the loop filter and used for controlling an oscillation frequency of the oscillator into a current. The CDR circuits are configured to adjust a phase of the oscillation output signal with respect to a phase of serial data. The semiconductor integrated circuit also includes a path for distributing the converted current to the plurality of CDR circuits.  
         [0020]     According to various exemplary embodiments, each of the CDR circuits may include an oscillator whose oscillation frequency is controlled on the basis of the distributed current.  
         [0021]     According to various exemplary embodiments, each of the CDR circuits may include a current-to-voltage converter configured to convert the distributed current into a voltage, and a voltage-controlled oscillator whose oscillation frequency is controlled on the basis of the converted voltage.  
         [0022]     As described above, a semiconductor integrated circuit according to an exemplary aspect of the present invention distributes to each channel a current for controlling an oscillation frequency (center frequency) of an oscillator in a PLL circuit, instead of distributing a clock. Therefore, no consideration is required for layout of clock wiring, which is to be considered in the known art. In other words, since a control current is not subjected to the influence of noise on wiring, such as crosstalk, no signal deterioration is present in even a CDR circuit that is remote from the PLL circuit. As a result, a line for shielding and a buffer in a path of wiring are not required. Power consumption can be reduced accordingly. Furthermore, since an oscillator in each CDR circuit may control its oscillation frequency (may adjust the phase of the oscillation frequency with respect to that of serial data) by using a mirrored current from a PLL circuit, a circuit with a reduced frequency gain can be designed, and therefore, the occurrence of jitter can be reduced. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a schematic diagram of a semiconductor integrated circuit according to an exemplary embodiment of the present invention;  
         [0024]      FIG. 2  is a schematic diagram of a PLL circuit used in the semiconductor integrated circuit according to an exemplary embodiment of the present invention;  
         [0025]      FIG. 3  is a schematic diagram of a voltage-to-current converter shown in  FIG. 2  according to an exemplary embodiment;  
         [0026]      FIG. 4  is a schematic diagram of another voltage-to-current converter according to an exemplary embodiment;  
         [0027]      FIG. 5  is a schematic diagram of a serial-to-parallel converter included in the semiconductor integrated circuit according to an exemplary embodiment;  
         [0028]      FIG. 6  is a schematic diagram of another serial-to-parallel converter included in the semiconductor integrated circuit according to an exemplary embodiment;  
         [0029]      FIG. 7  is a schematic diagram of a conventional typical PLL circuit;  
         [0030]      FIG. 8  is a schematic diagram of a conventional typical serial-to-parallel converter;  
         [0031]      FIG. 9  is a schematic diagram of another conventional typical serial-to-parallel converter;  
         [0032]      FIG. 10  is a diagram illustrating a conventional method of supplying clocks to a plurality of channels; and  
         [0033]      FIG. 11  is a diagram illustrating a conventional method of providing commonality among reference clocks in serial links. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0034]     A semiconductor integrated circuit according to various exemplary embodiments of the present invention is described below with reference to the attached drawings.  
         [0035]      FIG. 1  is a schematic diagram showing an exemplary embodiment of a semiconductor integrated circuit. According to various exemplary embodiments, a semiconductor integrated circuit  10  shown in  FIG. 1  includes a PLL circuit (PLL)  11  and a plurality of serial-to-parallel converters (S/Ps)  12  to which CDR circuits are applied.  
         [0036]     According to various exemplary embodiments, the PLL circuit  11  shown in  FIG. 1  receives a reference clock REFCLK and outputs a plurality of frequency control currents ICTRL_PLL having the same current value. The serial-to-parallel converters  12  (S/P&lt; 0 &gt;, . . . , S/P&lt;N&gt;) receive the frequency control currents ICTRL_PLL from the PLL circuit  11  and serial data signals RXP (RXP&lt; 0 &gt;, . . . , RXP&lt;N&gt;) and RXN (RXN&lt; 0 &gt;, . . . , RXN&lt;N&gt;), respectively. According to various exemplary embodiments and as described later, each of the frequency control currents ICTRL_PLL is a mirrored current of a frequency control current ICTRL_PLL into which an output voltage of a loop filter in the PLL circuit  11  is converted by a voltage-current converter, the frequency of control current being used for controlling an oscillation frequency of an oscillator. The number of the mirrored frequency control currents may be the same as that of the serial-to-parallel converters  12 .  
         [0037]      FIG. 2  is a schematic diagram of a PLL circuit used in the semiconductor integrated circuit according to an exemplary embodiment of the present invention. According to various exemplary embodiments, a PLL circuit  20  (the PLL circuit  11  in  FIG. 1 ) includes a phase-frequency detector (PFD)  21 , a charge pump (CP)  22 , a loop filter (LF)  23 , a voltage-to-current converter (V-I)  24 , an oscillator (OSC)  25 , and a frequency divider (DIV)  26 . The oscillator  25  is a current-controlled oscillator whose oscillation frequency is controlled on the basis of the frequency control current ICTRL_PLL.  
         [0038]     According to various exemplary embodiments, the phase-frequency detector  21  receives a reference clock REFCLK from the outside and a divided signal DIVCLK into which the frequency of an oscillation output signal of the oscillator  25  is divided by the frequency divider  26 . The phase-frequency detector  21  may then compare the reference clock REFCLK and the divided signal DIVCLK to detect the difference in phase and frequency therebetween, and may output an up pulse signal UP or a down pulse signal DOWN on the basis of the phase and frequency differences. According to various exemplary embodiments, the charge pump  22  charges or discharges a capacitance (not illustrated) in the loop filter  23  on the basis of the up pulse signal UP or the down pulse signal DOWN. Also, the loop filter  23  may generate a control voltage VTUNE for controlling the oscillator  25  from the electric charge stored in the capacitance, and the control voltage VTUNE is converted into the frequency control current ICTRL_PLL by the voltage-to-current converter  24 . The oscillation frequency of the oscillator  25  is controlled on the basis of the frequency control current ICTRL_PLL.  
         [0039]      FIG. 3  is a schematic diagram showing an exemplary embodiment of the voltage-to-current converter shown in  FIG. 2 . According to various exemplary embodiments, a voltage-to-current converter  30  includes a comparator  31 , a counter  32 , and a current distribution circuit  33 . The current distribution circuit  33  may include a plurality of constant-current power supplies  34 . As shown in  FIG. 3 , each of the constant-current power supplies  34  has a plurality of current elements  35  that may flow relative amounts of currents equal to 1, ½, ¼, ⅛, 1/16, 1/32, and 1/64, and switches  36  for switching on and off the current elements  35 .  
         [0040]     According to various exemplary embodiments, when the comparator  31  receives predetermined reference voltages VTHH and VTHL and a control voltage VTUNE, the comparator  31  then compares the reference voltages to the control voltage. If the control voltage VTUNE is larger than the reference voltage VTHH, the comparator  31  may output a count up signal. COUNT_UP; if the control voltage VTUNE is smaller than the reference voltage-VTHL, the comparator  31  may output a count down signal COUNT_DOWN.  
         [0041]     The counter  32  may output control signals  1 ON,  1 / 2 ON, . . . ,  1 / 64 ON for switching the current elements  35  included in the constant-current power supplies  34  on or off on the basis of the count up signal COUNT_UP or the count down signal COUNT_DOWN from the comparator  31 .  
         [0042]     According to various exemplary embodiments, the current elements  35  generate currents by being switched on by the corresponding switches  36  receiving the control signals  1 ON,  1 / 2 ON, . . . ,  1 / 64 ON from the counter  32 , and the currents generated from the current elements  35  are then summed. The total current is output as the frequency control currents ICTRL_PLL for controlling the oscillator  25  from the constant-current power supplies  34 . The number of constant-current power supplies  34  may be the same as that of CDR circuits. All the constant-current power supplies  34  may be controlled by the single counter  32 . The current distribution circuit  33  delivers the frequency control currents ICTRL_PLL, which are controlled by the counter  32  and have the same current value, to the CDR circuits.  
         [0043]      FIG. 4  is a schematic diagram showing another exemplary embodiment of the voltage-to-current converter. According to various exemplary embodiments, a voltage-to-current converter  40  shown in  FIG. 4  includes a differential amplifier  41  and a current mirror  42  for delivering the currents to the CDR circuits. The differential amplifier  41  may generate voltages VH and VL on the basis of the difference between a predetermined reference voltage VTH and a control voltage VTUNE. For example, if the control voltage VTUNE is larger than the reference voltage VTH, a current that passes through a p-channel metal-oxide semiconductor transistor (hereinafter referred to as PMOS)  412 , an n-channel metal-oxide semiconductor transistor (hereinafter referred to as NMOS)  417 , and an NMOS  419  is larger than a current that passes through a PMOS  414 , an NMOS  418 , and the NMOS  419 , and therefore, the levels of the voltages VH and VL increase. In contrast to this, if the control voltage VTUNE is smaller than the reference voltage VTH, the levels of the voltages VH and VL decrease. In  FIG. 4 , VB 1 , VB 2 , and VB 3  represent bias voltages.  
         [0044]     According to various exemplary embodiments, the current mirror  42  includes a plurality of constant-current power supplies having the same shape. Each of the constant-current power supplies includes a PMOS  421 , a PMOS  422 , an NMOS  423 , and an NMOS  424  connected in series between a power supply VDD and a ground GND. The voltage VH may be input to a gate of the PMOS  421 , and the voltage VL may be input to a gate of the NMOS  424 . In accordance with the levels of the voltages VH and VL, a frequency control current ICTRL_PLL may be output from an output terminal in which a drain of the PMOS  422  and a drain of the NMOS  423  are connected to each other. As described above, in the current mirror  42 , the frequency control current may be controlled on the basis of the voltages VH and VL input from the differential amplifier  41 , and the frequency control currents ICTRL_PLL, which have the same current value as each other and are of the same number as the CDR circuits, are generated and delivered.  
         [0045]     In the exemplary embodiments described above, the oscillator (OSC) in the PLL circuit is a known current-controlled oscillator (ICO). However, the oscillator in the PLL circuit may be a voltage-controlled oscillator (VCO) that is controlled on the basis of the control voltage VTUNE. In the case of the voltage-controlled oscillator, the control voltage VTUNE shown in  FIG. 2  may be input directly to the voltage-controlled oscillator and also input to the voltage-to-current converter. The voltage-to-current converter may generate the frequency control currents ICTRL_PLL, which have the same current value as each other and are of the same number as the CDR circuits.  
         [0046]     Each of the phase-frequency detector, the charge pump, the loop filter, and the frequency divider may be a known device. The voltage-to-current converter may be a combination of the exemplary embodiments described above, and may be a combination of a known device and current mirrors which are of the same number as the CDR circuits.  
         [0047]      FIG. 5  is a schematic diagram showing an exemplary embodiment of a serial-to-parallel converter included in the semiconductor integrated circuit. According to various exemplary embodiments, a serial-to-parallel converter (S/P)  50  includes a deserializer  57  and a CDR circuit  51  having a phase detector (PD)  52 , a charge pump (CP)  53 , a loop filter (LF)  54 , a voltage-to-current converter (V-I)  55 , and an oscillator (OSC)  56 .  
         [0048]     The phase detector  52  may compare the phases of clocks GCLKI, GCLKQ, GCLKIB, and GCLKQB with the phases of serial data signals RXP and RXN. If the phases of the clock signals are earlier, the phase detector  52  outputs an early signal EARLY; if the phases of the clock signals are later, the phase detector  52  outputs a late signal LATE. The charge pump  53  outputs a control voltage VTUNE_FINE on the basis of the early signal EARLY or the late signal LATE. The control voltage VTUNE_FINE may be smoothed in the loop filter  54  and then converted into a frequency control current ICTRL_FINE in the voltage-to-current converter  55 . The oscillator  56  may then receive a total current of the frequency control current ICTRL_FINE and the frequency control current ICTRL_PLL, which is supplied from the PLL circuit as a mirrored current. The high-speed clocks GCLKI and GCLKIB, whose phases have been adjusted in this way, may be transmitted to the deserializer  57 . The deserializer  57  may convert serial data into parallel data in synchronism with the high-speed clocks GCLKI and GCLKIB.  
         [0049]     Whereas a known CDR circuit includes a coarse loop and a fine loop, the CDR circuit in various exemplary embodiments requires no circuit for such a coarse loop. Therefore, both the layout area and power consumption can be reduced. Additionally, the frequency gain of the oscillator  56  can be smaller than that in the oscillator used in the PLL circuit. In this case, the occurrence of jitter can be reduced.  
         [0050]      FIG. 6  is a schematic diagram showing another exemplary embodiment of the serial-to-parallel converter included in the semiconductor integrated circuit. According to various exemplary embodiments, a serial-to-parallel converter (S/P)  60  shown in  FIG. 6  includes a deserializer  65  and a CDR circuit  61  having an oscillator (OSC)  64 , a phase detector (PD)  62 , and a phase control block  63 . Since the oscillator  64  receives the frequency control current ICTRL_PLL from the PLL circuit, the oscillator  64  oscillates with the same frequency with which the oscillator in the PLL circuit oscillates. The phase control block  63  may perform an adjustment on the basis of an early signal EARLY and a late signal LATE input from the phase detector  62  such that the phases of clocks GCLKI, GCLKQ, GCLKIB, and GCLKQB become coincident with the phases of serial data signals RXP and RXN. High-speed clocks FCLKI and FCLKIB, whose phases have been adjusted in this way, may be transmitted to the deserializer  65 . The deserializer  65  may convert serial data into parallel data in synchronism with the high-speed clock signals FCLKI and FCLKIB.  
         [0051]     The oscillator  64  may be a general current-controlled oscillator. Alternatively, the oscillator  64  may be an oscillator whose frequency gain is smaller than that in the oscillator used in the PLL circuit. In this case, the occurrence of jitter can be reduced. Each of the phase control block  63  and the phase detector  62  can be a known device.  
         [0052]     The CDR circuit may be of any type as long as the CDR circuit controls the frequency of the oscillator on the basis of the frequency control current ICTRL_PLL received from the PLL circuit. For example, the CDR circuit may be a combination of the two exemplary embodiments described above or may be a known device. Additionally, the CDR circuit may include a current-to-voltage converter configured to convert the supplied frequency control current ICTRL_PLL into a control voltage, so that the oscillator can be a voltage-controlled circuit.  
         [0053]     While the semiconductor integrated circuit according to various exemplary embodiments of the present invention has been described with reference to these embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It is apparent that changes and modifications may be made without departing from the sprit and scope of the claimed invention.