Abstract:
A system including a plurality of transmission lines, a transmitter outputting respective signals to each of the plurality of transmission lines, a receiver receiving each of the plurality of signals via respective transmission lines, the receiver including a connection path connected to a termination voltage, a plurality of termination circuits distributed along the connection path, each termination circuit receiving a unique termination voltage from the connection path, receiving a respective signal and outputting a terminated input signal, a reference voltage generator including multiple reference voltage generator units connected to a common voltage, each reference voltage generator unit uniquely receiving at least one unique termination voltage and outputting a reference voltage, and a plurality of data input buffers receiving respective signals and an appropriate reference voltage of the multiple reference voltages output from the reference voltage generator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a continuation application based on application Ser. No. 11/790,014, filed Apr. 23, 2007 now U.S. Pat. No. 7,403,040, the entire contents of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to reference voltage generators and methods for generating reference voltages. More particularly, one or more aspects of the invention relate to reference voltage generators, semiconductor systems employing such reference voltage generators and method for generating reference voltages that are configured to reduce and/or eliminate for termination mismatches. 
   2. Description of the Related Art 
   When transmitting signals between semiconductor devices, impedance matching may be performed to reduce signal reflection. Signal reflection, i.e., signal degradation, may occur if the impedance of a transmission line does not match that of a load being driven. 
   Signal reflections may cause distortion in the form of, e.g., ringing and/or stair-stepping, which may, in turn, lead to, e.g., false triggering in clock lines, erroneous bits in data, address and control lines, increased clock and signal jitter, etc. 
   Termination circuits such as termination resistors may be used to improve signal integrity, e.g., reduce signal reflection. Termination resistors may be provided internally and/or externally to a semiconductor device. Internally provided termination resistors may be referred to as on-chip termination resistors or on-die termination resistors. 
   Conventional termination circuits may include a receiver including a conventional reference voltage generator that provides a reference voltage based on a ground voltage and a power supply voltage of the receiver. In such devices, any variation in a ground voltage and/or a power supply voltage of a transmitter is not factored into the determination of the reference voltage. Thus, a data error rate of a logic level determination of data input signal may increase, and performance of the transmitter and receiver interface may be degraded. Termination circuits employing and/or generating more accurate reference voltage values are desired. 
   SUMMARY OF THE INVENTION 
   One or more aspects of the invention is therefore directed to reference voltage generators and methods for generating reference voltages, which may be employable by semiconductor systems and which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art. 
   It is therefore a feature of an embodiment of the present invention to provide reference voltage generators capable of generating more accurate reference voltages based on at least one voltage signal from each of a transmitter unit and a receiver unit. 
   It is therefore a separate feature of embodiments of the present invention to provide a semiconductor device system employing a reference voltage generator capable of reducing and/or eliminating signal mismatch by providing more accurate reference voltages relative to conventional systems. 
   At least one of the above and other features and advantages of the present invention may be realized by providing a system including a plurality of transmission lines, a transmitter outputting respective signals to each of the plurality of transmission lines, a receiver receiving each of the plurality of signals via respective transmission lines, the receiver including a connection path connected to a termination voltage, a plurality of termination circuits distributed along the connection path, each termination circuit receiving a unique termination voltage from the connection path, receiving a respective signal and outputting a terminated input signal, a reference voltage generator including multiple reference voltage generator units connected to a common voltage, each reference voltage generator unit uniquely receiving at least one unique termination voltage and outputting a reference voltage, and a plurality of data input buffers receiving respective signals and an appropriate reference voltage of the multiple reference voltages output from the reference voltage generator. 
   The common voltage may be a first voltage signal based on a transmitter voltage. The system may include a first voltage transmission line, and the transmitter may include a first voltage driver outputting the first voltage signal to the first voltage transmission line. The first voltage signal may be supplied to each reference voltage generator through a common resistor. The multiple reference voltage generator units may be a plurality of reference voltage generator units and each reference voltage generator unit may receive a corresponding unique termination voltage. 
   Each reference voltage generator unit may include a first resistor between a reference node and the unique termination voltage, and a second resistor between the common voltage and the reference node. The second resistor may be common to all reference voltage generator units. The second resistor may be separate for each reference voltage generator unit. The common voltage may be a first voltage signal based on a transmitter voltage. The common voltage may include a first voltage signal and a second voltage signal. The first voltage signal may be based on a driver ground voltage of the transmitter and the second voltage signal is based on a driver power supply voltage of the transmitter. 
   The system may include a first voltage transmission line and a second voltage transmission line, and wherein the transmitter may include a first voltage driver outputting the first voltage signal to the first voltage transmission line and a second voltage driver outputting the second voltage signal to the second voltage transmission line. Each reference voltage generator unit may generate a reference voltage based on an average of a low signal and a high signal. 
   The low signal may be an average of the first voltage signal and a first termination voltage and the high signal is an average of the second voltage signal and a second termination voltage. The first and second termination voltages may be equal. The first and second termination voltages may be from adjacent termination circuits. 
   At least one of the above and other features and advantages of the present invention may be separately realized by providing a method of generating multiple reference voltages in a system having a plurality of transmission lines, a transmitter outputting respective signals to each of the plurality of transmission lines, and a receiver receiving each of the plurality of signals via respective transmission lines, the method including distributing a termination voltage along a connection path, generating a plurality of terminated input signals based on each of the plurality of signals and a corresponding unique termination voltage received from the connection path, and generating multiple reference voltages from unique termination voltages and a common voltage, and outputting a reference voltage for each of the plurality of terminated input signals. 
   Generating multiple reference voltages from the unique termination voltages and the common voltage may include using a plurality of resistors arranged between the each of the termination voltages received from the connection path and the common voltage. The method may include receiving the common voltage from the transmitter, wherein the common voltage may correspond to a voltage of the transmitter. 
   At least one of the above and other features and advantages of the present invention may be separately realized by providing a machine-readable medium that provides executable instructions, which, when executed by a processor, cause the processor to perform a method of generating multiple reference voltages in a system having a plurality of transmission lines, a transmitter outputting respective signals to each of the plurality of transmission lines, and a receiver receiving each of the plurality of signals via respective transmission lines, the method including distributing a termination voltage along a connection path, generating a plurality of terminated input signals based on each of the plurality of signals and a corresponding termination voltage received from the connection path, and generating multiple reference voltages from unique termination voltages and a common voltage, and outputting a reference voltage for each of the plurality of terminated input signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which: 
       FIG. 1  illustrates a first exemplary embodiment of a semiconductor device system employing one or more aspects of the invention; 
       FIG. 2  illustrates an exemplary data signal transmission path employable by the semiconductor device system shown in  FIG. 1 ; 
       FIG. 3  illustrates an exemplary embodiment of a reference voltage transmission path employable by the semiconductor device system shown in  FIG. 1 ; 
       FIG. 4  illustrates an exemplary embodiment of a receiver employing one or more aspects of the invention; 
       FIG. 5  illustrates a first exemplary embodiment of a reference voltage generator employable by the receiver shown in  FIG. 4 ; 
       FIG. 6  illustrates a second exemplary embodiment of a reference voltage generator employable by the receiver shown in  FIG. 4 ; 
       FIG. 7  illustrates a second exemplary embodiment of a semiconductor device system employing one or more aspects of the invention; 
       FIG. 8  illustrates a first exemplary embodiment of a receiver employable by the semiconductor device system shown in  FIG. 7 ; 
       FIG. 9  illustrates an exemplary embodiment of a reference voltage generator employable by the receiver shown in  FIG. 8 ; 
       FIG. 10  illustrates a second exemplary embodiment of a receiver employable by the semiconductor device system shown in  FIG. 7 ; and 
       FIG. 11  illustrates an exemplary embodiment of a reference voltage generator employable by the receiver shown in  FIG. 10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Korean Patent Application No. 2006-0041798, filed on May 10, 2006, in the Korean Intellectual Property Office, is incorporated by reference herein in its entirety. 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout the specification. 
   In the following description, it will be understood that when elements are described as being connected to each other, they may be directly connected or one or they may be connected via one or more intervening elements. If elements are described as being directly connected, then the elements are directly connected to each other and there are no intervening elements therebetween. 
     FIG. 1  illustrates a first exemplary embodiment of a semiconductor device system  500  employing one or more aspects of the invention. The semiconductor device  500  may include a receiver  220 , a transmitter  320 , a first voltage signal generator  130 , and a plurality of transmission lines  30   a  to  30   n  connected between the transmitter  320  and the receiver  220 . 
   The first voltage signal generator  130  may include a first voltage transmission line  35  connected between the transmitter  320  and the receiver  220 . The first voltage transmission line  35  may be connected to a first voltage driver pin  75  of the transmitter  320  and a first voltage input pin  70  of the receiver  220 . 
   The transmitter  320  may include a plurality of data output drivers  50   a  to  50   n , a plurality of data output pins  40   a  to  40   n , and a first voltage driver  60 . The first voltage driver  60  may include a n-type transistor NT, e.g., an NMOS transistor, having a first terminal connected to the first voltage driver pin  75  and a second terminal connected to a driver ground voltage VSSQ. Each of the data output pins  40   a  to  40   n  of the transmitter  320  may be connected to a respective one of the data output drivers  50   a  to  50   n . Each of the data output drivers  50   a  to  50   n  may be connected to a driver power supply voltage VDDQ and the driver ground voltage VSSQ. Each of the data output drivers  50   a  to  50   n  may supply a terminated data input signal IS 1  to ISn to a respective one of the data input pins  10   a  to  10   n  of the receiver  220 . 
   The receiver  220  may be supplied with a receiver power supply voltage VDD and may include a plurality of data input pins  10   a  to  10   n . The receiver  220  may include a reference voltage generator  120 . The reference voltage generator  120  may include a plurality of input terminals connected to the receiver power supply voltage VDD via the power supply line  90  for receiving the termination voltage VDD. As a result of characteristics, e.g., resistance, of, e.g., the power supply line  90 , each of the input terminals may receive a respective termination voltage VDD 1  to VDDn based on the receiver power supply voltage VDD. That is, in embodiments of the invention, each of the termination voltages VDD 1  to VDDn may be different from one another. 
   Each of the data input pins  10   a  to  10   n  of the receiver  220  may be connected to the power supply line  90  via a respective one of a plurality of termination resistors RT. That is, e.g., each input terminal of the reference voltage generator  120  may be connected to the respective one of the data input pins  10   a  to  10   n  via the respective one of the plurality of the termination resistors RT connected therebetween. The reference voltage generator  120  may include a plurality of output terminals for outputting a respective one of the reference voltages VREF 1  to VREFn. 
   Referring to  FIG. 1 , the reference voltage generator  120  may also be connected to the first voltage input pin  70 , and more particularly, to the first voltage signal generator  130 . Thus, in embodiments of the invention, the reference voltage generator  120  may receive a voltage signal corresponding to, e.g., the ground supply voltage VSSQ of the transmitter  320 , via the first voltage signal generator  130 . Accordingly, the voltage signal from the transmitter  320  may be employed by the reference voltage generator  120  to generate the respective reference voltages VREF 1  to VREFn. Thus, in embodiments of the invention, more accurate reference voltages may be generated. 
   The receiver  220  may include a plurality of data input buffers  20   a  to  20   n . Each of the data input buffers  20   a  to  20   n  may receive the respective one of the reference voltages VREF 1  to VREFn from the reference voltage generator  120 . The data input buffers  20   a  to  20   n  may also receive the respective one of the terminated data input signals IS 1  to ISn, and may compare the respective one of the terminated data input signals IS 1  to ISn to the received respective one of the reference voltages VREF 1  to VREFn. Based on the comparison of the respective one of the terminated data input signals IS 1  to ISn to the respective one of the reference voltages VREF 1  to VREFn, the respective one of the data input buffers  20   a  to  20   n  may output a respective input signal CS 1  to CSn. 
     FIG. 2  illustrates an exemplary data signal transmission path of an open-drain type data driver employable by the semiconductor device system  500  shown in  FIG. 1 . Referring to  FIGS. 1 and 2 , each of the transmission lines  30   a  to  30   n  may have one end connected to a respective one of the data output pins  40   a  to  40   n  of the transmitter  320  and another end connected to a respective one of the data input pins  10   a  to  10   n  of the receiver  220 . The end of the respective transmission line  30   a  to  30   n  connected to the respective data input pin  10   a  to  10   n  may be connected to a first terminal of a respective termination resistor RT. Another terminal of the respective termination resistor may be connected to a termination voltage VTT, e.g., VDD 1 . In some embodiments of the invention, a low voltage VOL, a high voltage VOH and a corresponding reference voltage VREF of each of the terminated data input signals IS may be defined by the following three equations, respectively.
   VOL=VTT×RON /( RON+RT )  (1) VOH=VTT  (2)   VREF =( VOL+VOH )/2 =VTT ×( RON+RT/ 2))/( RON+RT )  (3) 
   In Equation (1), VOL corresponds to a low voltage of the respective terminated data input signal IS, VTT corresponds to a termination voltage, RON corresponds to a resistance from the driver ground voltage VSSQ to the respective data input pin  10   a  to  10   n  of the receiver  220 , and RT corresponds to a resistance of the respective termination resistor. In Equation (2), VOH corresponds to a high voltage of the respective terminated data input signal IS. 
     FIG. 3  illustrates an exemplary embodiment of a reference voltage transmission path of an open-drain type voltage driver employable by the semiconductor device system  500  shown in  FIG. 1 . Referring to  FIGS. 1 and 3 , the first voltage transmission line  35  may have one end connected to the first voltage input pin  70  of the receiver  220  and another end connected to the first voltage driver pin  75  of the transmitter  320 , i.e., the first voltage driver  60  of the transmitter  320 . As shown in  FIG. 3 , the reference voltage transmission path may include a pull-up resistor RU and a pull down resistor RD. The pull-up resistor RU may be connected between the respective termination voltage VTT and a reference node NR. The pull-down resistor RD may be connected between the reference node NR and a voltage node NL connected to the first voltage input pin  70  for receiving a respective voltage signal VL. In some embodiments of the invention, a reference voltage of the reference voltage transmission path may be defined by Equation (4).
   VREF =( VOL+VOH )/ 2   =VTT ×( RON+RU )/( RON+RD+RU )  (4) 
   In equation (4), VOL and VOH respectively correspond to low voltage and a high voltage of the voltage signal VL, RON corresponds to a resistance from the driver ground voltage VSSQ to the voltage node NL, RU corresponds to a resistance of the pull-up resistor, RD corresponds to a resistance of the pull-down resistor, and VTT corresponds to a termination voltage supplied to a respective terminal of the pull-up resistor RU. 
     FIG. 4  illustrates additional features of the exemplary embodiment of the receiver  220  shown in  FIG. 1 . As discussed above, the receiver  220  may include the reference voltage generator  120 . In some embodiments of the invention, the reference voltage generator  120  may include a plurality of reference voltage generating units  120   a  to  120   n.    
     FIG. 5  illustrates a first exemplary embodiment of a reference voltage generator  121  employable as the reference voltage generator  120  of the receiver  220  shown in  FIG. 4 . Referring to  FIG. 5 , the reference voltage generator  121  may include a plurality of reference voltage generating units  121   a  to  121   n . As shown in  FIG. 5 , each of the reference voltage generating units  121   a  to  121   n  may include a pull-up resistor RU and a pull-down resistor RD. For each reference voltage generating unit  121   a  to  121   n , the pull-up resistor RU may be connected to the pull-down resistor RD at a respective reference node NR 1  to NRn. As shown in  FIG. 5 , the respective reference voltage VREF 1  to VREFn may correspond to a voltage at the respective reference node NR 1  to NRn. 
   More particularly, e.g., each pull-up resistor RU may be connected between the respective termination voltage VDD 1  to VDDn and the respective reference node NR 1  to NR 2 , and each pull-down resistor RD may be connected between the respective reference node NR 1  to NR 2  and the voltage node NL. As discussed above, a voltage of the voltage node NL may correspond to the voltage signal VL from the first voltage signal generator  130 . In embodiments of the invention, the pull-up resistors RU and/or the pull down resistors RD may be variable resistors having a resistance in accordance with a resistance control circuit (not shown). 
   More particularly, in the following description, resistors may be identified as variable and/or may be simply referred to as a resistor, however, any of the resistors may be a resistor having a predetermined value and/or a variable resistor. That is, embodiments of the invention, are not limited by the exemplary embodiments shown in the accompanying drawing figures. 
   As shown in  FIG. 5 , the reference voltage generator  120  may receive the voltage signal, e.g., VL. Therefore, the reference voltage generator  120  may generate respective reference voltages VREF 1  to VREFn based on the voltage signal, e.g., VL, of the transmitter  320  as well as respective termination voltages VDD 1  to VDDn of the receiver  220 . Thus, embodiments of the invention may enable more accurate reference voltage(s) to be generated by the reference voltage generator  120 . Therefore, embodiments of the invention may also enable a data error rate of logic level determination of input data signals to be reduced and performance of the semiconductor device system  500  to be maintained and/or improved. 
     FIG. 6  illustrates a second exemplary embodiment of a reference voltage generator  122  employable as the reference voltage generator  120  of the receiver shown in  FIG. 4 . In general, only differences between the first exemplary reference voltage generator  121  shown in  FIG. 5  and the second exemplary reference voltage generator  122  shown in  FIG. 6  will be described below. 
   As shown in  FIG. 6 , in some embodiments of the invention, the reference voltage generator  122  may include a plurality of reference voltage generating units  122   a  to  122   n . Each of the reference voltage generating units  122   a  may include a pull-up resistor RU connected between a respective one of the termination voltages VDD 1  to VDDn and a respective one of the reference nodes NR 1  to NRn. In some embodiments of the invention, as shown in  FIG. 6 , a common pull-down resistor RDC may be connected between all of the reference nodes NR 1  and the voltage node NL. 
     FIG. 7  illustrates a second exemplary embodiment of a semiconductor device system  600  employing one or more aspects of the invention. In general, only differences between the first exemplary semiconductor device system  500  shown in  FIG. 1  and the second exemplary semiconductor device system  600  shown in  FIG. 7  will be described below. In some embodiments of the invention, the semiconductor device system  600  may include a transmitter  340 , a receiver  240 , the plurality of transmission lines  30   a  to  30   n , and a first and second voltage generator  150 . 
   Similar to the transmitter  320  of the first exemplary semiconductor device system  500 , the transmitter  340  of the second exemplary semiconductor device system  600  may include the plurality of data output drivers  50   a  to  50   n  and the plurality of data out put pins  40   a  to  40   n . The transmitter  340  may also include a plurality of voltage driver pins  75   a ,  75   b  instead of the voltage driver pin  75  of the transmitter  320 . Each of the data output drivers  50   a  to  50   n  may be connected to the driver power supply voltage VDDQ and the driver ground voltage VSSQ. 
   Similar to the receiver  220  of the first exemplary semiconductor device system  500 , the receiver  240  of the second exemplary semiconductor device system  600  may include the plurality of data input pins  10   a  to  10   n  and a plurality of data input buffers  20   a  to  20   n . The receiver  240  may also include a reference voltage generator  140  and a plurality of voltage input pins  70   a ,  70   b , instead of the voltage generator  120  and the voltage input pin  70 , respectively, of the transmitter  220 . 
   The first and second voltage signal generator  150  may include a plurality of voltage transmission lines  35   a ,  35   b . Each of the transmission lines  35   a ,  35   b  of the first and second voltage generator  150  may be connected between a respective one of the voltage driver pins  75   a ,  75   b  and a respective one of the voltage input pins  70   a ,  70   b . More particularly, each voltage driver  60   a ,  60   b  of the transmitter  340  may be connected between a driver supply voltage, e.g., VDDQ, VSSQ, of the transmitter  340  and a respective one of the voltage driver pins  75   a ,  75   b.    
   For example, in some embodiments of the invention, as shown in  FIG. 7 , one of the voltage drivers, e.g.,  60   b , may be connected between the driver power supply voltage VDDQ and the respective voltage driver pin  75   b , and another of the voltage drivers, e.g.,  60   a , may be connected between the driver ground voltage VSSQ and the respective voltage driver pin  75   a . The voltage transmission line  35   a  connected to the voltage driver  60   a , which may be connected to the driver ground voltage VSSQ, may provide a first voltage signal VL to the receiver  240 . The voltage transmission line  35   b  connected to the voltage driver  60   b , which may be connected to the driver power supply voltage VDDQ, may provide a second voltage signal VH to the receiver  240 . The first voltage signal VL may correspond to a low voltage signal and the second voltage signal VH may correspond to a high voltage signal of the transmitter  340 . 
   Accordingly, as shown in  FIG. 7 , the second exemplary reference voltage generator  140  may receive a plurality of voltage signals, e.g., VH and VL. Therefore, the reference voltage generator  140  may generate respective reference voltages VREF 1  to VREFn based on a plurality of voltage signals, e.g., VH and VL, of the transmitter  340  as well as respective termination voltages VDD 1  to VDDn of the receiver  240 . Thus, embodiments of the invention may enable more accurate reference voltage(s) to be generated by the reference voltage generator  240 . Therefore, embodiments of the invention may also enable a data error rate of logic level determination of input data signals to be reduced and performance of the semiconductor device system  600  to be maintained and/or improved. 
     FIG. 8  illustrates additional features of the exemplary receiver  240  employable by the semiconductor device system shown in  FIG. 7 . More particularly,  FIG. 8  illustrates additional features of the voltage generator  140  of the receiver  240 . As shown in  FIG. 8 , the voltage generator  140  may include a plurality of reference voltage generating units  140   a  to  140   n . Each of the reference voltage generating units  140   a  to  140   n  may receive each of the plurality of voltage signals, e.g., VH and VL, as well as the respective one of the termination voltages VDD 1  to VDDn. Thus, as discussed above, in some embodiments of the invention, the reference voltage generator  140  may generate respective reference voltages VREF 1  to VREFn using one or more of the voltage signals VH and VL corresponding to, e.g., drive supply and ground voltages VDDQ and VSSQ of the transmitter  340 . 
     FIG. 9  illustrates an exemplary embodiment of the reference voltage generator  140  employable by the receiver  240  shown in  FIG. 8 . As shown in  FIG. 9 , the reference voltage generator  140  may include n reference voltage generating units  140   a  to  140   n . Each of the reference voltage generating units  140   a  to  140   n  may include a plurality of resistors, and more particularly, e.g., a pull-up variable resistor RU, a pull-down variable resistor RD, a first resistor R 1  and a second resistor R 2 . Resistances of the pull-up variable resistor RU and the pull-down variable resistor RD may be set by a resistance control circuit (not shown), which may be included in the receiver  240 . As discussed above, embodiments of the invention are not limited to the types of resistors, e.g., variable resistor or resistor having a predetermined resistance, shown in the accompanying Figures. 
   More particularly, as shown, e.g., in  FIG. 9 , for each voltage generating unit  140   a  to  140   n , the pull-down resistor RD may be connected between the respective termination voltage VDD 1  to VDDn and the first voltage signal VL, which may be supplied via a first voltage node NL of the reference voltage generator  140 , and the pull-up resistor RU may be connected between the respective termination voltage VDD 1  to VDDn and the second voltage signal VH, which may be supplied via a second voltage node NH of the reference voltage generator  140 . The first voltage node NL of the reference voltage generator  140  may be connected to a respective pull-down node ND of the voltage generating unit  140   a  to  140   n , and the second voltage node NH of the voltage generator  140  may be connected to a respective pull-up node NU of the voltage generating unit  140   a  to  140   n . The first resistor R 1  may be connected between the respective pull-up node NU and a respective reference node NR 1  to NRn of the voltage generating unit  140   a  to  140   n , and the second resistor R 2  may be connected between the respective pull-down node ND and the respective reference node NR 1  to NRn of the respective voltage generating unit  140   a  to  140   n . The respective reference node NR 1  to NRn may have a voltage corresponding to the respective reference voltage VREF 1  to VREFn generated by the reference voltage generator  150 . 
     FIG. 10  illustrates a second exemplary embodiment of a receiver  240 ′ employable by the semiconductor device system shown in  FIG. 7 . In general, only differences between the first exemplary receiver  240  shown in  FIG. 8  and the second exemplary receiver  240 ′ shown in  FIG. 10  will be described below. As shown in  FIG. 10 , the second exemplary receiver  240 ′ may include a reference voltage generator  160  instead of the reference voltage generator  140 . As shown in  FIG. 10 , the reference voltage generator  160  may include a fewer number of reference voltage generating units  160   a  to  160   c  than a number of terminated data input signals IS 1  to ISn. That is, e.g., the reference voltage generator  160 , for receiving four terminated data input signals IS 1  to IS 4 , may include two, i.e., less than four, voltage generating units  160   a  and  160   c . More particularly, in some embodiments of the invention, each voltage generating unit  160   a ,  160   c  may be shared by a plurality, e.g., two, of the terminated data input signals IS 1  to ISn. 
   Accordingly, as shown in  FIG. 10 , each generated reference voltage, e.g., VREF 1 , VREF 3 , may be supplied to a plurality of the data input buffers, e.g.,  20   a ,  20   b ,  20   c ,  20   d.    
     FIG. 11  illustrates an exemplary embodiment of the reference voltage generator  160  employable by the receiver  240 ′ shown in  FIG. 10 . In general, only differences between the exemplary reference voltage generator  160  shown in  FIG. 11  and the exemplary reference voltage generator  140  shown in  FIG. 9  will be described below. Each of the reference voltage generating units  160   a ,  160   b  may include a plurality of resistors, and more particularly, e.g., a pull-up variable resistor RU, a pull-down variable resistor RD, a first resistor R 1  and a second resistor R 2 . Resistances of the pull-up variable resistor RU and the pull-down variable resistor RD may be set by a resistance control circuit (not shown), which may be included in the receiver  240 ′. 
   More particularly, as shown, e.g., in  FIG. 11 , one of the voltage generating units  160   a ,  160   c  may be associated with two of the termination voltages, e.g., VDD 1 , VDD 2 , VDD 3 , VDD 4 , and thus, in some embodiments of the invention, there may be half as many reference voltage generating units, e.g.,  160   a ,  160   c , as termination voltages, e.g., VDD 1  to VDD 4  and/or terminated data input signals, e.g., IS 1  to IS 4 . For each voltage generating unit  160   a ,  160   c , the pull-up resistor RU, the first resistor R 1 , the second resistor R 2  and the pull-down resistor RD may be connected between one of the respective termination voltages, e.g., VDD 1 , and another of the respective termination voltages, e.g., VDD 2 . That is, e.g., the pull-up resistor RU, the first resistor R 1 , the second resistor R 2  and the pull-down resistor RD of the first reference voltage generating unit  160   a  may be connected in series between the first termination voltage VDD 1  and the second termination voltage VDD 2 , and the pull-up resistor RU, the first resistor R 1 , the second resistor R 2  and the pull-down resistor Rd of the second reference voltage generating unit  160   c  may be connected in series between the third termination voltage VDD 3  and the fourth termination voltage VDD 3 . For each of the voltage generating units  160   a ,  160   c , e.g., a respective pull-up node NU may correspond to a node between the pull-up resistor RU and the first resistor R 1 , and the pull-up node NU may be connected to the second voltage node NH, and a respective pull-down node ND may correspond to a node between pull-down resistor RD and the second resistor R 2 , and the pull-down node ND may be connected to the first voltage node NL. A respective reference node NR 1 , NR 3  may correspond to a node between the respective first and second resistors R 1 , R 2 . 
   Exemplary embodiments of the present invention have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. While embodiments of the present invention have been described relative to a hardware implementation, the processing of present invention may be implemented in software, e.g., by an article of manufacture having a machine-accessible or readable medium including data that, when accessed by a machine, e.g., a processor, cause the machine to perform a method, according to one or more aspects of the invention, for generating a plurality of reference voltages. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.