Patent Publication Number: US-10325671-B2

Title: Memory system having impedance calibration circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119(a) to Korean patent application number 10-2017-0116037, filed on Sep. 11, 2017, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Various exemplary embodiments of the present disclosure generally relate to a memory system. Particularly, the embodiments relate to an impedance calibration circuit of a memory system. 
     2. Description of the Related Art 
     In general, a memory system includes a nonvolatile memory device, a buffer memory device, and a memory controller. The memory controller exchanges data with the buffer memory device at high speed. When the memory controller receives data from the buffer memory device, a data signal output from an output buffer of the buffer memory device is input to an input buffer of the memory controller. At this time, a signal reflection inevitably occurs. The signal reflection refers to a phenomenon in which a portion of a data signal input to the input buffer of the memory controller is reflected to a data transmission line due to mismatching between input impedance of input/output terminal in the input buffer of the memory controller and impedance of the data transmission line. 
     The signal reflection is not problematic when the speed of the output data signal is not high. However, if the speed of the output data signal goes beyond a certain speed, a problem may arise in which the memory controller may not stably receive the data signal due to the signal reflection. That is, a data signal that is output from the buffer memory device may be interfered by a reflected signal of a data signal previously output from the buffer memory device. The signal reflection occurs similarly even when data is output. In order to prevent the signal reflection, an impedance calibration circuit such as an on die termination calibration circuit is provided at a data input/output terminal of the memory controller, so that the impedance of the data input/output terminal of the memory controller is matched to the impedance of the data transmission line. 
     SUMMARY 
     Embodiments provide a memory system including an impedance calibration circuit configured to reduce the area thereof. 
     According to an aspect of the present disclosure, there is provided a memory system including: a buffer memory device including a reference voltage pad; a memory controller including a controller ZQ pad; and a controller calibration resistor, wherein the reference voltage pad, the controller ZQ pad, and the controller calibration resistor are coupled to each other. 
     According to an aspect of the present disclosure, there is provided a memory system including: a buffer memory device; a memory controller including a first pad; and a controller calibration resistor coupled to the memory controller through the first pad, wherein the memory controller performs an impedance calibration operation, based on the resistance of the controller calibration resistor, wherein the buffer memory device determines whether a data signal is logic ‘high’ or logic ‘low’ in a data input operation, based on a voltage output from the memory controller through the first pad. 
     According to an aspect of the present disclosure, there is provided a transmission system including: first and second devices; a transmission line suitable for transferring a signal between the first and second devices, wherein the first device matches impedances between the first device and the transmission line with reference to a first calibration resistance, and wherein the second device matches impedances between the second device and the transmission line with reference to a second calibration resistance and the voltage of the first calibration resistance when the first device completes the impedance matching. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will now be described in more detail hereinafter with reference to the accompanying drawings; however, they may 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 example embodiments to those skilled in the art. 
       In the drawings, dimensions may be exaggerated for clarity. It will be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout. 
         FIG. 1  is a diagram illustrating a memory system according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram illustrating a memory system according to another embodiment of the present disclosure. 
         FIG. 3  is a diagram illustrating a dynamic random access memory (DRAM) of  FIG. 2 . 
         FIG. 4  is a diagram illustrating a nonvolatile memory device of FIG. 
         FIG. 5  is a diagram illustrating a memory controller of  FIG. 1 . 
         FIG. 6  is a diagram illustrating a memory controller according an embodiment of the present disclosure. 
         FIG. 7  is a diagram illustrating a memory system including the memory controller of  FIG. 6 . 
         FIG. 8  is a diagram illustrating a memory controller according another embodiment of the present disclosure. 
         FIG. 9  is a diagram illustrating a memory system including the memory controller of  FIG. 8 . 
         FIG. 10  is a circuit diagram illustrating in detail a reference voltage generating unit. 
         FIG. 11  is a circuit diagram illustrating in detail a pull-up resistor unit. 
         FIG. 12  is a diagram illustrating an impedance calibration operation according to the present disclosure. 
         FIG. 13  is a diagram illustrating another embodiment of the memory system including the memory controller shown in  FIG. 1 . 
         FIG. 14  is a diagram illustrating another embodiment of the memory system including the memory controller shown in  FIG. 1 . 
         FIG. 15  is a diagram illustrating another embodiment of the memory system including the memory controller shown in  FIG. 1 . 
         FIG. 16  is a diagram illustrating another embodiment of the memory system including the memory controller shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present invention are described below in more detail with reference to the accompanying drawings. We note, however, that the present invention may be embodied in different forms and variations, and should not be construed as being limited to the embodiments set forth herein. Rather, the described embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the present invention to those skilled in the art to which this invention pertains. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. 
     In the entire specification, when an element is referred to as being “connected” or ° “coupled” to another element, it can be directly connected or coupled to the another element or be indirectly connected or coupled to the another element with one or more intervening elements interposed therebetween. In addition, when an element is referred to as “including” a component, this indicates that the element may further include another component instead of excluding another component unless there is different disclosure. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. 
     As used herein, singular forms may include the plural forms as well, unless the context clearly indicates otherwise. 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well-known process structures and/or processes have not been described in detail in order not to unnecessarily obscure the present invention. 
     Hereinafter, the various embodiments of the present invention will be described in detail with reference to the attached drawings. 
       FIG. 1  is a diagram illustrating a memory system  1000  according to an embodiment of the present disclosure. 
     Referring to  FIG. 1 , the memory system  1000  may include a nonvolatile memory device  1100  that retains stored data even when power is cut off, a buffer memory device  1300  for temporarily storing data, and a memory controller  1200  that controls the nonvolatile memory device  1100  and the buffer memory device  1300  under the control of a host  2000 . 
     The host  2000  may communicate with the memory system  1000 , using at least one of various communication methods such as a universal serial bus (USB), a serial AT attachment (SATA), a high speed interchip (HSIC), a small computer system interface (SCSI), Firewire, a peripheral component interconnection (PCI), a PCI express (PCIe), a nonvolatile memory express (NVMe), a universal flash storage (UFS), a secure digital (SD), a multimedia card (MMC), an embedded MMC (eMMC), a dual in-line memory module (DIMM), a registered DIMM (RDIMM), a load reduced DIMM (LRDIMM), and the like. 
     The memory controller  1200  may control overall operations of the memory system  1000 , and control data exchange between the host  2000  and the nonvolatile memory device  1100 . For example, the memory controller  1200  may program or read data by controlling the nonvolatile memory device  1100  in response to a request of the host  2000 . Also, the memory controller  1200  may store information of main memory blocks and sub-memory blocks, which are included in the nonvolatile memory device  1100 , and select the nonvolatile memory device  1100  to perform a program operation on a main memory block or a sub-memory block according to the amount of data loaded for the program operation. In some embodiments, the nonvolatile memory device  1100  may include a flash memory. 
     The memory controller  1200  may control data exchange between the host  2000  and the buffer memory device  1300 , or temporarily store system data for controlling the nonvolatile memory device  1100  in the buffer memory device  1300 . The buffer memory device  1300  may be used as a working memory, a cache memory, or a buffer memory for the memory controller  1200 . The buffer memory device  1300  may store codes and commands, which are executed by the memory controller  1200 . Also, the buffer memory device  1300  may store data processed by the memory controller  1200 . 
     The memory controller  1200  may temporarily store data input from the host  2000  in the buffer memory device  1300  and then transmit the data temporarily stored in the buffer memory device  1300  to the nonvolatile memory device  1100  to be stored in the nonvolatile memory device  1100 . Also, the memory controller  1200  may receive data and a logical address, which are input from the host  2000 , and transform the logical address to a physical address indicating a region in which data is to be actually stored in the nonvolatile memory device  1100 . Also, the memory controller  1200  may store, in the buffer memory device  1300 , a logical-to-physical address mapping table that establishes a mapping relationship between the logical address and the physical address. 
     In some embodiments, the buffer memory device  1300  may include a double data rate synchronous dynamic random access memory (DDR SDRAM), a low power double data rate fourth-generation (LPDDR4) SDRAM, a graphics double data rate (GDDR) SRAM, a low power DDR (LPDDR), a rambus dynamic random access memory (RDRAM), etc. 
       FIG. 2  is a diagram illustrating a memory system  1000  according to another embodiment of the present disclosure. 
     Referring to  FIG. 2 , the buffer memory device  1300  may be provided in the form of a module including one or more dynamic random access memories (DRAMs)  1305 . The DRAMs  1305  may exchange data signals with the memory controller  1200  by sharing the same data transmission line. Here, the DRAM may be a double data rate fourth-generation synchronous dynamic random access memory (DDR4 SDRAM). 
     When a data signal output from the DRAM  1305  is input to an input buffer of the memory controller  1200 , signal reflection may occur. The signal reflection refers to a phenomenon in which a portion of a data signal input to the input buffer of the memory controller is reflected to the data transmission line due to the mismatching between input impedance of input/output terminal in the input buffer of the memory controller and impedance of the data transmission line. An impedance calibration operation may be required to prevent data signal distortion caused by the signal reflection. 
       FIG. 3  is a diagram illustrating the one or more DRAMs  1305  of  FIG. 2 . 
     Referring to  FIG. 3 , the DRAMs  1305  may include a memory cell array  510 , a row decoder  520 , a sense amplifier circuit  530 , a column decoder  540 , a control logic  550 , a command decoder  560 , a mode register set (MRS) circuit  570 , an address buffer  580 , a data input/output circuit  590 , and a refresh circuit  500 . 
     The memory cell array  510  is a data storage in which a lot of memory cells are arranged in row and column directions. The memory cell array  510  includes a plurality of DRAM memory cells, and data stored in the DRAM memory cell may disappear when power is cut off. The sense amplifier circuit  530  may read data stored in the memory cell array  510  by sensing and amplifying a voltage difference between a bit line pair, based on the distribution of charges stored in a selected memory cell. 
     Data DQ input through the data input/output circuit  590  is written in the memory cell array  510 , based on an address signal ADD. The data DQ read from the memory cell array  510  based on the address signal ADD is output to the outside through the data input/output circuit  590 . The address signal ADD is input to the address buffer  580  to designate a memory in or from which data is to be written or read. The address buffer  580  temporarily stores the address signal ADD input from the outside. 
     The data input/output circuit  590  may receive a reference voltage input from an external device through a memory reference voltage pad VREF_M. The reference voltage may be a voltage that becomes a reference for determining whether a data signal is logic ‘high’ or logic ‘low’ when data is input. 
     The row decoder  520  decodes a row address in the address signal ADD output from the address buffer  580 , to designate a word line coupled to a memory cell to or from which data is to be input or output. That is, the row decoder  120  enables a corresponding word line by decoding the row address output from the address buffer  580  in a data write or read mode. 
     The column decoder  540  decodes a column address in the address signal ADD output from the address buffer  580 , to designate a bit line coupled to a memory cell to or from which data is to be input or output. 
     The command decoder  560  receives command signals CMD such as, e.g., a signal including /CBR, /CKE, etc., applied externally, and decodes the signal, thereby internally generating the decoded command signal. The MRS circuit  570  sets an internal mode register in response to the address signal ADD and an MRS command for designating an operation mode of the DRAM  1305 . The control logic  550  may control an operation of the DRAM  1305  in response to a command output from the command decoder  560 . 
     The refresh circuit  500  may control a refresh operation of reading data stored and then re-writing the read data in preparation for a case where charges stored in a capacitor of each of the DRAM memory cells included in the memory cell array  510  disappear. 
     In addition, although not shown in  FIG. 3 , the DRAM  1305  may further include a clock circuit for generating a clock signal, a power circuit for generating or dividing an internal voltage by receiving a power voltage applied externally, and the like. 
       FIG. 4  is a diagram illustrating the nonvolatile memory device  1100  of  FIG. 1 . 
     Referring to  FIG. 4 , the nonvolatile memory device  1100  may include a memory cell array  100  that stores data. The nonvolatile memory device  1100  may include peripheral circuits  200  configured to perform a program operation for storing data in the memory cell array  100 , a read operation for outputting the stored data, and an erase operation for erasing the stored data. The nonvolatile memory device  1100  may include a control logic  300  that controls the peripheral circuits  200  under the control of the memory controller  1200  (shown in  FIG. 1 ). The memory cell array  100  may include a plurality of NAND flash memory cells, and data stored in the NAND flash memory cell may not disappear when power is cut off. 
     The memory cell array  100  may include a plurality of memory blocks MB 1  to MBk (where k is a positive integer)  110  (hereinafter, referred to as “memory blocks  110 ”). Local lines LL and bit lines BL 1  to BLn (where n is a positive integer) may be coupled to the memory blocks  110 . For example, the local lines LL may include a first select line, a second select line, and a plurality of word lines arranged between the first and second select lines. Also, the local lines LL may further include dummy lines arranged between the first select line and the word lines and between the second select line and the word lines. Here, the first select line may be a source select line, and the second select line may be a drain select line. For example, the local lines LL may include word lines, drain and source select lines, and source lines. For example, the local lines LL may further include dummy lines. For example, the local lines LL may further include pipe lines. The local lines LL may be coupled to the memory blocks  110 , respectively, and the bit lines BL 1  to BLn may be commonly coupled to the memory blocks  110 . The memory blocks  110  may be implemented in a two-dimensional or three-dimensional structure. For example, memory cells may be arranged in a direction parallel to a substrate in memory blocks  110  having a two-dimensional structure. For example, memory cells may be arranged in a direction vertical to a substrate in memory blocks  110  having a three-dimensional structure. 
     The peripheral circuits  200  may perform program, read, and erase operations of a selected memory block  110  under the control of the control logic  300 . For example, the peripheral circuits  200 , under the control of the control logic  300 , may supply verify and pass voltages to the first select line, the second select line, and the word lines, selectively discharge the first select line, the second select line, and the word lines, and verify memory cells coupled a selected word line among the word lines. For example, the peripheral circuits  200  may include a voltage generating circuit  210 , a row decoder  220 , a page buffer group  230 , a column decoder  240 , an input/output circuit  250 , and a sensing circuit  260 . 
     The voltage generating circuit  210  may generate various operating voltages Vop used for program, read, and erase operations in response to an operation signal OP_CMD. Also, the voltage generating circuit  210  may selectively discharge the local lines LL in response to the operation signal OP_CMD. For example, the voltage generating circuit  210  may generate a program voltage, a verify voltage, pass voltages, a turn-on voltage, a read voltage, an erase voltage, a source line voltage, and the like under the control of the control logic  300 . 
     The row decoder  220  may transfer the operating voltages Vop to local lines LL coupled to a selected memory block  110  in response to a row address RADD. 
     The page buffer group  230  may include a plurality of page buffers PB 1  to PBn  231  coupled to the bit lines BL 1  to BLn. The page buffers PB 1  to PBn  231  may operate in response to page buffer control signals PBSIGNALS. For example, the page buffers PB 1  to PBn  231  may temporarily store data received through the bit lines BL 1  to BLn, or sense voltages or current of the bit lines BL 1  to BLn in a read or verify operation. 
     The column decoder  240  may transfer data between the input/output circuit  250  and the page buffer group  230  in response to a column address CADD. For example, the column decoder  240  may exchange data with the page buffers  231  through data lines DL, or exchange data with the input/output circuit  250  through column lines CL. 
     The input/output circuit  250  may transfer a command CMD and address ADD, which are received from the memory controller  1200  (shown in  FIG. 1 ), to the control logic  300 , or exchange data DATA with the column decoder  240 . 
     The sensing circuit  260 , in a read operation and a verify operation, may generate a reference current in response to a permission bit VRY_BIT&lt;#&gt;, and output a pass signal PASS or a fail signal FAIL by comparing a sensing voltage VPB received from the page buffer group  230  with a reference voltage generated by the reference current. 
     The control logic  300  may control the peripheral circuits  200  by outputting the operation signal OP_CMD, the row address RADD, the page buffer control signals PBSIGNALS, and the permission bit VRY_BIT&lt;#&gt; in response to the command CMD and the address ADD. Also, the control logic  300  may determine whether the verify operation has passed or failed in response to the pass or fail signal PASS or FAIL received from the sensing circuit  260 . 
       FIG. 5  is a diagram illustrating the memory controller  1200  of  FIG. 1 . 
     Referring to  FIG. 5 , the memory controller  1200  may include a processor  710 , an error correction code (ECC) circuit  720 , a host interface  730 , a nonvolatile memory device interface  740 , a buffer memory device interface  750 , and a bus  760 . 
     The bus  760  may provide channels between components of the memory controller  1200 . 
     The processor  710  may control overall operations of the memory controller  1200  and perform a logical operation. The processor  710  may communicate with the external host  2000  through the host interface  730  and communicate with the nonvolatile memory device  1100  through the nonvolatile memory device interface  740 . Also, the processor  710  may communicate with the memory buffer device  1300  through the buffer memory device interface  750 . 
     The ECC circuit  720  may perform an ECC operation. The ECC circuit  720  may perform ECC encoding on data to be written in the nonvolatile memory device  1100  through the nonvolatile memory device interface  740 . The ECC encoded data may be transferred to the nonvolatile memory device  1100  through the nonvolatile memory device interface  740 . The ECC circuit  720  may perform ECC decoding on data received from the nonvolatile memory device  1100  through the nonvolatile memory device interface  740 . In an embodiment, the ECC circuit  720  may be included in the nonvolatile memory device interface  740  as a component of the nonvolatile memory device interface  740 . 
     The host interface  730  may communicate with the external host  2000  (shown in  FIG. 1 ) under the control of the processor  710 . The host interface  730  may communicate with the host  2000 , using at least one of various communication methods, such as a universal serial bus (USB), a serial AT attachment (SATA), a high speed intership (HSIC), a small computer system interface (SCSI), Firewire, a peripheral component interconnection (PCI), a PCI express (PCIe), a nonvolatile memory express (NVMe), a universal flash storage (UFS), a secure digital (SD), a multimedia card (MMC), an embedded MMC (eMMC), a dual in-line memory module (DIMM), a registered DIMM (RDIMM), and a load reduced DIMM (LRDIMM). 
     The nonvolatile memory device interface  740  may communicate with the nonvolatile memory device  1100  under the control of the processor  710 . The nonvolatile memory device interface  740  may communicate a command, an address, and data with the nonvolatile memory device  1100  through a channel. 
     In an embodiment, the processor  710  may control an operation of the memory controller  1200  using codes. The processor  710  may load codes from a read only memory (ROM) provided inside the memory controller  1200 . In another embodiment, the processor  710  may load codes from the nonvolatile memory device  1100  through the nonvolatile memory device interface  740 . 
     In an embodiment, the bus  760  of the memory controller  1200  may be divided into a control bus and a data bus. The data bus may transmit data in the memory controller  1200 , and the control bus may transmit control information such as a command and an address in the memory controller  1200 . When the bus  760  is divided in to the control bus and the data bus, the data bus and the control bus are separated from each other, and may not interfere or influence with each other. The data bus may be coupled to the host interface  730 , the ECC circuit  720 , the nonvolatile memory device interface  740 , and the buffer memory device interface  750 . The control bus may be coupled to the host interface  730 , the processor  710 , the nonvolatile memory device interface  740 , and the buffer memory device interface  750 . 
     The buffer memory device interface  750  may communicate with the buffer memory device  1300  under the control of the processor  710 . The buffer memory device interface  750  may communicate a command, an address, and data with the buffer memory device  1300  through a channel. 
       FIG. 6  is a diagram illustrating the memory controller  1200  according an embodiment of the present disclosure. 
     Referring to  FIG. 6 , the memory controller  1200  may include an impedance calibration circuit  1210  and a reference voltage generating unit  1220  that generates a reference voltage Vref. The impedance calibration circuit  1210  may include a pull-up counter  1211  that generates a pull-up code pu&lt;n:1&gt; in response to the reference voltage Vref generated by the reference voltage generating unit  1220 , a pull-up resistor unit  1212  that varies the resistance of a pull-up resistor in response to the pull-up code pu&lt;n:1&gt;, and a comparator  1213 . 
     The reference voltage generating unit  1220  may generate a reference voltage Vref for a ZQ calibration operation. In an embodiment, the reference voltage Vref may be a half of an input/output power voltage VDDQ, i.e., 0.5×VDDQ. In addition, the memory controller  1200  may always output the reference voltage Vref in a state in which the input/output power voltage VDDQ is applied. In general, since the memory controller  1200  is to output the reference voltage Vref to the outside with a large current driving capability, the memory controller  1200  may consume a large current to generate the reference voltage Vref. An external device, e.g., the buffer memory device  1300  may use the reference voltage Vref output by the memory controller  1200  in a data input operation. In other words, the buffer memory device  1300  may use the reference voltage Vref as a voltage that becomes a reference for determining whether a data signal is logic ‘high’ or logic ‘low’ in the data input operation. The buffer memory device  1300  may be configured in the form of a module including a plurality of DRAMs  1305  (shown in  FIG. 2 ). Therefore, the memory controller  1200  may be required to generate and output the reference voltage Vref with a large current driving capability. 
     A node of the reference voltage Vref and a ZQ calibration node ZQ_cal may be coupled to two inputs of the comparator  1213 . The comparator  1213  may generate a division voltage Vdiv by comparing the reference voltage Vref and a voltage of the ZQ calibration node ZQ_cal. 
     The division voltage Vdiv and a clock signal CLK may be input to the pull-up counter  1211 . The pull-up counter  1211  may change a value of the pull-up code pu&lt;n:1&gt;, based on the division voltage Vdiv. Also, the pull-up counter  1211  may change the value of the pull-up code pu&lt;n:1&gt; in synchronization with the clock signal CLK. 
     The pull-up resistor unit  1212  may change the resistance of the pull-up resistor, based on the value of the pull-up code pu&lt;n:1&gt; output from the pull-up counter  1211 . If the resistance of the pull-up resistor is changed, the voltage of the ZQ calibration node ZQ_cal may be changed. 
     In an embodiment, when the voltage of the ZQ calibration node ZQ_cal is higher than the reference voltage Vref, the pull-up counter  1211  may change the value of the pull-up code pu&lt;n:1&gt; in synchronization with the clock signal CLK such that the resistance of the pull-up resistor of the pull-up resistor unit  1212  increases. The pull-up resistor unit  1212  may increase the resistance of the pull-up resistor in response to the value of the pull-up code pu&lt;n:1&gt;. As a result, the voltage of the ZQ calibration node ZQ_cal may decrease. 
     In an embodiment, when the voltage of the ZQ calibration node ZQ_cal is lower than the reference voltage Vref, the pull-up counter  1211  may change the value of the pull-up code pu&lt;n:1&gt; in synchronization with the clock signal CLK such that the resistance of the pull-up resistor of the pull-up resistor unit  1212  decreases. The pull-up resistor unit  1212  may decrease the resistance of the pull-up resistor in response to the value of the pull-up code pu&lt;n:1&gt;. As a result, the voltage of the ZQ calibration node ZQ_cal may increase. 
     According to the impedance calibration operation described above, the voltage of the ZQ calibration node ZQ_cal may be set equal to or substantially equal to the reference voltage Vref. The impedance calibration operation may be performed in a power-up operation or be performed in response to an external request. Also, the impedance calibration operation may be performed in real time in a state in which the input/output power voltage VDDQ is applied. When the impedance calibration operation is performed in real time, there is an advantage in that the memory controller can respond to a change in input/output power voltage VDDQ, temperature, or the like in real time. 
     The reference voltage Vref may be output to the external device e.g., the buffer memory device  1300  through a controller reference voltage pad VREF_C  1202 . In addition, the ZQ calibration node ZQ_cal may be coupled to the external device, e.g., the buffer memory device  1300  through a ZQ pad ZQ_C  1201 . The reference voltage Vref may be used as a voltage that becomes a reference for determining whether a data signal is logic ‘high’ or logic ‘low’ when data is input/output between the memory controller  1200  and the buffer memory device  1300 . 
       FIG. 7  is a diagram illustrating a memory system including the memory controller  1200  of  FIG. 6 . 
     Referring to  FIG. 7 , the memory controller  1200  may output the reference voltage Vref generated by the reference voltage generating unit  1220  (shown in  FIG. 6 ) through the controller reference voltage pad VREF_C  1202 . The output reference voltage Vref may be input to the buffer memory device  1300  through a memory reference voltage pad VREF_M  1302  of the buffer memory device  1300 . The reference voltage Vref may be used as a voltage that becomes a reference for determining whether a data signal is logic ‘high’ or logic ‘low’ when data is input/output between the memory controller  1200  and the buffer memory device  1300 . 
     In the memory controller  1200 , the ZQ calibration node ZQ_cal (shown in  FIG. 6 ) may be coupled to a controller calibration resistor Rcal_C  1400  through the controller ZQ pad ZQ_C  1201 . The impedance calibration operation described with reference to  FIG. 6 , i.e., an operation of correcting the resistance of the pull-up resistor of the pull-up resistor unit  1212  may be performed based on the controller calibration resistor Rcal_C  1400 . In an embodiment, when the reference voltage Vref is, for example, 0.5×VDDQ, the resistance of the pull-up resistor of the pull-up resistor unit  1212  may be corrected to be substantially equal to that of the controller calibration resistor Rcal_C  1400 . In another example, when the reference voltage Vref is, for example, ⅔×VDDQ, the resistance of the pull-up resistor of the pull-up resistor unit  1212  may be corrected to be approximately ½ of that of the controller calibration resistor Rcal_C  1400 . In another example, when the reference voltage Vref is, for example, ⅓×VDDQ, the resistance of the pull-up resistor of the pull-up resistor unit  1212  may be corrected to be approximately 1.5 times greater than that of the controller calibration resistor Rcal_C  1400 . 
     The buffer memory device  1300  may be coupled to a memory calibration resistor Rcal_M  1401  through a memory ZQ pad ZQ_M  1301 . The buffer memory device  1300  may also include an impedance calibration circuit having a form similar to that of the impedance calibration circuit of the memory controller  1200 , and may autonomously perform an impedance calibration operation, based on the memory calibration resistor Rcal_M  1401 . The controller calibration resistor Rcal_C  1400  and the memory calibration resistor Rcal_M  1401  may be resistors having the same magnitude. In addition, when the controller calibration resistor Rcal_C  1400  is terminated to a ground voltage VSS, the memory calibration resistor Rcal_M  1401  may also be terminated to the ground voltage VSS. When the controller calibration resistor Rcal_C  1400  is terminated to the input/output power voltage VDDQ, the memory calibration resistor Rcal_M  1401  may also be terminated to the input/output power voltage VDDQ. 
       FIG. 8  is a diagram illustrating the memory controller  1200  according another embodiment of the present disclosure. 
     Referring to  FIG. 8 , unlike the memory controller  1200  described with reference to  FIG. 6 , the memory controller  1200  described in  FIG. 8  may not include the controller reference voltage pad VREF_C  1202  for outputting the reference voltage Vref. If the impedance calibration operation is completed as described with reference to  FIG. 6 , the voltage of the ZQ calibration node ZQ_cal may be set equal to or substantially equal to the reference voltage Vref. Therefore, the voltage of the ZQ calibration node ZQ_cal may be output instead of the reference voltage Vref. 
     As described above, when the memory controller  1200  does not includes the controller reference voltage pad VREF_C  1202  for outputting the reference voltage Vref, the number of pads can be decreased, improving the overall productivity. 
     The memory controller  1200  may always output the reference voltage Vref in the state in which the input/output power voltage VDDQ is applied as described in  FIG. 6 . In this case, when the controller reference voltage pad VREF_C  1202  is removed as shown in  FIG. 8 , and the voltage of the ZQ calibration node ZQ_cal is output instead of the reference voltage Vref, the impedance calibration operation may be performed in real time in the state in which the input/output power voltage VDDQ is applied. In other words, the impedance calibration operation is not performed in a power-up operation or by an external request, but is always performed in the state in which the input/output power voltage VDDQ is applied. Thus, the impedance calibration operation can be performed in real time. According to the impedance calibration operation performed in real time, it is possible to efficiently counteract a change in power voltage or temperature of the memory system  100 . 
       FIG. 9  is a diagram illustrating a memory system including the memory controller  1200  of  FIG. 8 . 
     Referring to  FIG. 9 , the memory controller  1200  may not include the controller reference voltage pad VREF_C  1202  for outputting the reference voltage Vref generated by the reference voltage generating unit  1220 . In addition, if the impedance calibration operation is completed as described with reference to  FIG. 8 , the voltage of the ZQ calibration node ZQ_cal may be set equal to or substantially equal to the reference voltage Vref. Therefore, the voltage of the ZQ calibration node ZQ_cal may be output instead of the reference voltage Vref. Consequently, as shown in  FIG. 9 , the controller ZQ pad ZQ_C  1203  of the memory controller  1200  may be coupled to the controller calibration resistor Rcal_C  1400  and the memory reference voltage pad VREF_M  1302 . In other words, the buffer memory device  1300  may receive, from the memory controller  1200 , the voltage of the ZQ calibration node ZQ_cal, which is substantially equal to the reference voltage Vref, instead of the reference voltage Vref, to use the voltage of the ZQ calibration node ZQ_cal as the reference voltage Vref. That is, the buffer memory device  1300  may use the voltage of the ZQ calibration node ZQ_cal of the memory controller  1200  as a reference for determining whether a data signal is logic ‘high’ or logic ‘low’ in the data input operation. 
     The memory controller  1200  of  FIG. 7  may always output the reference voltage Vref in the state in which the input/output power voltage VDDQ is applied. As described with reference to  FIG. 9 , the memory controller  1200  may output the voltage of the ZQ calibration node ZQ_cal instead of the reference voltage Vref through the controller ZQ pad ZQ_C  1203 , and the buffer memory device  1300  may receive the voltage of the ZQ calibration node ZQ_cal through the memory reference voltage pad VREF_M  1302  to use the voltage of the ZQ calibration node ZQ_cal instead of the reference voltage Vref. When a reference voltage to be used by the buffer memory device  1300  is output through the controller ZQ pad ZQ_C  1203  as shown in  FIG. 9 , the impedance calibration operation of the memory controller  1200  may be performed in real time. In other words, the memory controller  1200  may always output the voltage of the ZQ calibration node ZQ_cal instead of the reference voltage Vref, and hence the impedance calibration operation can also be performed in real time. According to the impedance calibration operation performed in real time, there is an advantage in that the memory controller can efficiently respond to a change in input/output power voltage VDDQ, temperature, or the like in real time. 
       FIG. 10  is a circuit diagram illustrating in detail the reference voltage generating unit  1220 . 
     Referring to  FIG. 10 , the reference voltage generating unit  1220  includes a first resistor R 1  having one end coupled to a node of the input/output power voltage VDDQ, a first pass gate PG 1  that is provided between the first resistor R 1  and a node of the reference voltage Vref and is controlled by a pair of enable signals en and /en, a second resistor R 2  having one end coupled to a node of the ground voltage VSS, a second pass gate PG 2  that is provided between the second resistor R 2  and the node of the reference voltage Vref and is controlled by the pair of enable signals en and /en, a third resistor R 3  provided between the node of the input/output power voltage VDDQ and the node of the reference voltage Vref, a fourth node R 4  provided between the nodes of the ground voltage VSS and the reference voltage Vref, a first capacitor CAP 1  provided between the nodes of the input/output power voltage VDDQ and the reference voltage Vref, and a second capacitor CAP 2  provided between the nodes of the ground voltage VSS and the reference voltage Vref. 
     In an embodiment, when the reference voltage Vref is, for example, 0.5×VDDQ, the resistance ratio of the first resistor R 1  to the second resistor R 2  may be substantially equal to that of the third resistor R 3  to the fourth resistor R 4 , and each resistance ratio may be 1:1. However, the first resistor R 1  and the second resistor R 2  may have resistance values lower than those of the third resistor R 3  and the fourth resistor R 4 . According to the above-described configuration, noise can be reduced when the impedance calibration operation is performed, and current consumption can be reduced when the impedance calibration operation is not performed. 
       FIG. 11  is a circuit diagram illustrating in detail the pull-up resistor unit  1212 . 
     Referring to  FIG. 11 , the pull-up resistor unit  1212  may include first to nth transistors TR 1  to TRn. Bits of a pull-up code pu&lt;n:1&gt; output from the pull-up counter  1211  may be input to gate nodes of the first to nth transistors TR 1  to TRn, respectively, and an input/output power voltage VDDQ is applied to source nodes of the first to nth transistors TR 1  to TRn. A division voltage Vdiv may be commonly coupled to drain nodes of the first to nth transistors TR 1  to TRn. 
     The transistors have different resistance values. Among the transistors, transistors may have resistance values that are two times, four times, eight times, . . . , i.e. 2 n  times the resistance value of a transistor to which the most significant bit pu&lt;n&gt; of the pull-up code pu&lt;n:1&gt; is input. In other words, the transistors may have sizes that are two times, four times, eight times, . . . , i.e. 2 n  times the size of a transistor to which the last significant bit pu 1 &lt; 1 &gt; of the pull-up code pu&lt;n:1&gt; is input. In an embodiment, the first transistor TR 1  to which the last significant bit pu&lt; 1 &gt; of the pull-up code pu&lt;n:1&gt; is input may be configured with one transistor, and the second transistor TR 2  to which a second least significant bit pu&lt; 2 &gt; of the pull-up code pu&lt;n:1&gt; is input may be configured with two transistors having the same size of the first transistor TR 1 , which are coupled in parallel to each other. When the two transistors are coupled in parallel to each other, the two transistors share source, drain, and gate nodes with each other. In addition, the third transistor TR 3  to which a third least significant bit pu&lt; 3 &gt; of the pull-up code pu&lt;n:1&gt; is input may be configured with four transistors having the same size of the first transistor TR 1 , which are coupled in parallel to each other. When the sizes of pull-up transistors are configured in this manner, is called a binary relationship. 
     According to the above-described configuration, if logical values of the pull-up code pu&lt;n:1&gt; are changed by ‘1,’ the pull-up resistor unit  1212  can decrease or increase resistance values in proportion to the variation. 
       FIG. 12  is a diagram illustrating the impedance calibration operation according to the present disclosure. 
     Referring to  FIG. 12 , a change in resistance of the pull-up resistor of the pull-up resistor unit  1212  described with reference to  FIG. 11  in the impedance calibration operation is illustrated. The resistance of the pull-up resistor may be controlled by on/off of the transistors having different widths as described with reference to  FIG. 11 . 
     When the impedance calibration operation starts, the first to nth transistors included in the pull-up resistor unit  1212  may all be in an off-state. In addition, the reference voltage generating unit  1220  of  FIG. 8  may generate the reference voltage Vref. In an embodiment, the reference voltage Vref may be a half of the input/output power voltage VDDQ, i.e., 0.5×VDDQ. Since the first to nth transistors included in the pull-up resistor unit  1212  may all be in the off-state, the voltage of the ZQ calibration node ZQ_cal of  FIG. 8  may be a voltage close to the ground voltage VSS. Therefore, the comparator  1213  may output a result that the voltage of the ZQ calibration node ZQ_cal is lower than the reference voltage Vref, and the pull-up counter  1211  may allow the value of the pull-up code pu&lt;n:1&gt; to be sequentially changed by one bit in synchronization with the clock signal, based on the result. The pull-up resistor unit  1212  may decrease the resistance of the pull-up resistor by controlling the on/off of the first to nth transistors of  FIG. 11  in response to the change in value of the pull-up code pu&lt;n:1&gt;. In other words, the pull-up resistor unit  1212  may allow the widths of activated pull-up transistors to be gradually increased by sequentially turning on/off the first to nth transistors of  FIG. 11  in response to the value change of the pull-up code pu&lt;n:1&gt; by one bit in synchronization with the clock signal. As the resistance of the pull-up resistor decreases, the voltage of the ZQ calibration node ZQ_cal gradually increases. 
     In addition, if the voltage of the ZQ calibration node ZQ_cal is greater than a target voltage Target, i.e., the reference voltage Vref, the comparator  1213  may output a result that the voltage of the ZQ calibration node ZQ_cal is higher than the reference voltage Vref, and the pull-up counter  1211  may allow the value of the pull-up code pu&lt;n:1&gt; to be sequentially changed by one bit in synchronization with the clock signal, based on the result. The pull-up resistor unit  1212  may increase the resistance of the pull-up resistor by controlling the on/off of the first to nth transistors of  FIG. 11  in response to the change in value of the pull-up code pu&lt;n:1&gt;. In other words, the pull-up resistor unit  1212  may allow the widths of activated pull-up transistors to be gradually decreased by sequentially turning on/off the first to nth transistors of  FIG. 11  in response to the value change of the pull-up code pu&lt;n:1&gt; by one bit in synchronization with the clock signal. As the resistance of the pull-up resistor increases, the voltage of the ZQ calibration node ZQ_cal gradually decreases. 
     According to such a feedback operation, the voltage of the ZQ calibration node ZQ_cal can be set as a voltage equal to or substantially equal to the target voltage Target, i.e., the reference voltage Vref. 
       FIG. 13  is a diagram illustrating an application example of the memory system  1000  including the memory controller  1200 . The memory system  1000  may correspond to the memory system described in  FIGS. 1 and 2 . The memory controller  1200  may correspond to the memory controller described in  FIGS. 5 to 8 . 
     Referring to  FIG. 13 , the memory system  30000  may be implemented as a cellular phone, a smart phone, a tablet PC, a personal digital assistant (PDA), or a wireless communication device. The memory system  30000  may include a nonvolatile memory device  1100  and a memory controller  1200  capable of controlling an operation of the nonvolatile memory device  1100 . The memory controller  1200  may control a data access operation of a buffer memory device  1300 , e.g., a program operation, an erase operation, a read operation, or the like under the control of a processor  3100 . 
     Data programmed in the nonvolatile memory device  1100  may be output through a display  3200  under the control of the memory controller  1200 . 
     A radio transceiver  3300  may transmit/receive radio signals through an antenna ANT. For example, the radio transceiver  3300  may convert a radio signal receive through the antenna ANT into a signal that can be processed by the processor  3100 . Therefore, the processor  3100  may process a signal output from the radio transceiver  3300  and transmit the processed signal to the memory controller  1200  or the display  3200 . The memory controller  1200  may program the signal processed by the processor  3100  in the nonvolatile memory device  1100 . 
     Also, the radio transceiver  3300  may convert a signal output from the processor  3100  into a radio signal, and output the converted radio signal to an external device through the antenna ANT. An input device  3400  is a device capable of inputting a control signal for controlling an operation of the processor  3100  or data to be processed by the processor  3100 , and may be implemented as a pointing device such as a touch pad or a computer mount, a keypad, or a keyboard. The processor  3100  may control an operation of the display  3200  such that data output from the memory controller  1200 , data output from the radio transceiver  3300 , or data output from the input device  3400  can be output through the display  3200 . 
     In some embodiments, the memory controller  1200  capable of controlling an operation of the nonvolatile memory device  1100  may be implemented as a part of the processor  3100 , or be implemented as a chip separate from the processor  3100 . 
       FIG. 14  is a diagram illustrating another application example of the memory system  1000  including the memory controller  1200 . The memory system  1000  may correspond to the memory system described in  FIGS. 1 and 2 . The memory controller  1200  may correspond to the memory controller described in  FIGS. 5 to 8 . 
     Referring to  FIG. 14 , the memory system  40000  may be implemented as a personal computer (PC), a tablet PC, a net-book, an e-reader, a personal digital assistant (PDA), a portable multimedia player (PMP), an MP3 player, or an MP4 player. 
     The memory system  40000  may include a nonvolatile memory device  1100  and a memory controller  1200  capable of controlling a data processing operation of the nonvolatile memory device  1100 . The memory controller  1200  may control a data access operation of a buffer memory device  1300 , e.g., a program operation, an erase operation, a read operation, or the like under the control of a processor  4100 . 
     The processor  4100  may output data stored in the nonvolatile memory device  1100  through a display  4300  according to data input through an input device  4200 . For example, the input device  4200  may be implemented as a pointing device such as a touch pad or a computer mouse, a keypad, or a keyboard. 
     The processor  4100  may control overall operations of the memory system  40000 , and control an operation of the memory controller  1200 . In some embodiments, the memory controller  1200  capable of controlling an operation of the nonvolatile memory device  1100  may be implemented as a part of the processor  4100 , or be implemented as a chip separate from the processor  4100 . 
       FIG. 15  is a diagram illustrating another application example of the memory system  1000  including the memory controller  1200 . The memory system  1000  may correspond to the memory system described in  FIGS. 1 and 2 . The memory controller  1200  may correspond to the memory controller described in  FIGS. 5 to 8 . 
     Referring to  FIG. 15 , the memory system  50000  may be implemented as an image processing device, e.g., a digital camera, a mobile terminal having a digital camera attached thereto, a smart phone having a digital camera attached thereto, or a tablet PC having a digital camera attached thereto. 
     The memory system  50000  may include a nonvolatile memory device  1100  and a memory controller  1200  capable of controlling a data processing operation of the nonvolatile memory device  1100 , e.g., a program operation, an erase operation, or a read operation. The memory controller  1200  may control a data access operation of a buffer memory device  1300 , e.g., a program operation, an erase operation, a read operation, or the like under the control of a processor  5100 . 
     An image sensor  5200  of the memory system  50000  may convert an optical image into digital signals, and the converted digital signals may be transmitted to the processor  5100  or the memory controller  1200 . Under the control of the processor  5100 , the converted digital signals may be output through a display  5300 , or be stored in the nonvolatile memory device  1100  through the memory controller  1200 . In addition, data stored in the nonvolatile memory device  1100  may be output through the display  5300  under the control of the processor  5100  or the memory controller  1200 . 
     In some embodiments, the nonvolatile memory controller  1200  capable of controlling an operation of the nonvolatile memory device  1100  may be implemented as a part of the processor  5100 , or be implemented as a chip separate from the processor  5100 . 
       FIG. 16  is a diagram illustrating another application example of the memory system  1000  including the memory controller  1200 . The memory system  1000  may correspond to the memory system described in  FIGS. 1 and 2 . The memory controller  1200  may correspond to the memory controller described in  FIGS. 5 to 8 . 
     Referring to  FIG. 16 , the memory system  70000  may be implemented as a memory card or a smart card. The memory system  70000  may include a nonvolatile memory device  1100 , a memory controller  1200 , and a card interface  7100 . 
     The memory controller  1200  may control data exchange between the nonvolatile memory device  1100  and the card interface  7100 . In some embodiments, the card interface  7100  may be a secure digital (SD) card interface or a multi-media card (MMC) interface, but the present disclosure is not limited thereto. The memory controller  1200  may control a data access operation of a buffer memory device  1300 , e.g., a program operation, an erase operation, a read operation, or the like. 
     The card interface  7100  may interface data exchange between a host  60000  and the memory controller  1200  according to a protocol of the host  60000 . In some embodiments, the card interface  7100  may support a universal serial bus (USB) protocol and an inter-chip (IC)-USB protocol. Here, the card interface  7100  may mean hardware capable of supporting a protocol used by the host  60000 , software embedded in the hardware, or a signal transmission scheme. 
     According to the present disclosure, in the impedance calibration circuit of the memory system, a voltage output through the controller ZQ pad is used as a reference voltage in the buffer memory device, so that the number of pads of the memory controller can be decreased. 
     Example embodiments 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. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure as set forth in the following claims.