Patent Abstract:
A method is disclosed for controlling an output impedance of an electronic device of the type having an impedance control terminal to which an external load is to be connected such that a predetermined value of the voltage at the impedance control terminal controls the output impedance of the device. The method is comprised of comparing a reference voltage to a voltage at the impedance control terminal. A variable count signal representing a count value is produced in response to the comparing. The impedance of a variable impedance circuit is varied in response to the count signal, wherein the impedance of the variable impedance circuit controls the voltage at the impedance control terminal. A device connected in parallel with the variable impedance circuit is periodically operated to change (increase/decrease) the impedance of the variable impedance circuit. An apparatus for performing the method is also disclosed. Because of the rules governing abstracts, this abstract should not be used to construe the claims.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 12/613,632, filed Nov. 6, 2009, U.S. Pat. No. 7,898,290, which is a continuation of U.S. application Ser. No. 11/299,888 filed Dec. 12, 2005, U.S. Pat. No. 7,626,416. These applications are incorporated by reference herein in their entirety and for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed generally to solid state devices and, more particularly, to such devices having a ZQ pad or terminal which may be used to control the output impedance of the device. 
     Semiconductor devices such as microcomputers, memories, gate arrays, among others, include input/output pins and an output circuit for transmitting data to other devices, via a bus, transmission lines formed on a board, and the like. A circuit within the semiconductor device that is responsible for transmitting data includes, for example, output buffers and drivers. For there to be optimum transmission, the impedance of the transmitting device should be matched to the impedance of the transmission network and receiving device. 
     As operational speeds of electronic devices increase, the swing of transmitted signals decreases. However, as the signal swing width of a transmitted signal decreases, external noise increases. External noise can affect the reflection characteristics of an output signal if there is an impedance mismatch at an interface. Impedance mismatches are caused by external noise or by noise on a power supply voltage, temperature and process variations, among others. If an impedance mismatch arises, the transmission speed of the data decreases, and the data from a semiconductor device may become distorted. Thus, in a case where a semiconductor device receives distorted data, problems can be caused by setup/hold failures or errors in reading received data. 
     U.S. Pat. No. 6,947,336 entitled Semiconductor Device With Impedance Control Circuit discloses a semiconductor device with an impedance control circuit capable of automatically obtaining a desired output impedance value irrespective of power supply voltage, temperature, and process variations. Disclosed therein is a semiconductor device that includes an output buffer circuit having a pull-up section comprised of pull-up transistors and a pull-down section comprised of pull-down transistors; a pad connected with an external resistor; and an output impedance control circuit that is connected to the pad and the output buffer circuit and controls an impedance of the output buffer circuit according to an impedance of an external resistor. A first transistor is connected to the pad. A first current source circuit supplies DC current to the pad, and a first-level controller is connected to the pad and controls a gate voltage of the first transistor such that the pad is established at a predetermined voltage. A second transistor is connected to a first internal node and is controlled by the first-level controller. A first variable impedance circuit is connected to the first internal node, and a second current source circuit supplies DC current to the first internal node. A first controller, responsive to a voltage variation of the first internal node, generates a first control code for controlling the first variable impedance circuit so that a voltage of the first internal node is established at the predetermined voltage. A first conversion circuit receives the first control code and converts the control code into a string of data bits. The data bits of the string are transferred in series to the output buffer circuit via a single transmission line. 
       FIG. 1  illustrates a typical prior art impedance control circuit  10 . The impedance control circuit  10  has a ZQ pad or a control pad  12  to which an external resistor  14  may be connected. The value of the voltage at the control pad  12  (V ZQP ) is input to a comparator  16  along with a reference voltage (Vref) produced by a reference generator  18 . The comparator  16  is of the type which produces up and down pulses in response to the difference in magnitude between the values of Vref and V ZQP . The up and down pulses are filtered by a filter circuit  20  and input to a counter  22 . The counter  22  produces a multi-bit, variable count signal  23  representing a count value which is responsive to the number of up and down pulses which have been counted. The count signal  23  is input to a variable impedance circuit  24 . The variable impedance circuit  24  is shown in detail in  FIG. 2A . 
     In  FIG. 2A , the variable impedance circuit  24  is comprised of four P-channel MOS transistors connected in parallel. The gate of each of the transistors is responsive to one of the bits of the variable count signal  23 . Furthermore, each transistor is twice the size, i.e., has twice the drive, of the previous transistor. Thus, transistor P 2  is twice the size of transistor P 1 ; transistor P 4  is twice the size of transistor P 2 ; and transistor P 8  is twice the size of transistor P 4 . 
     In  FIGS. 3A and 3B , the voltage Vref is compared to the voltage V ZQ . At time T 0 , the voltage V ZQ  is less than the voltage Vref such that a plurality of up pulses is produced. At time T 1 , a sufficient number of up pulses has been produced so as to change the value of the variable count signal  23  so as to turn on another transistor within the variable impedance circuit  24 . For example, transistor P 1  may be turned off and transistor P 2  turned on, thereby increasing the value of voltage V ZQ  by one step. At time T 2 , the voltage V ZQ  is still less than the voltage Vref in both cases, and due to the continued counting of the up pulses, the value of the voltage V ZQ  is increased by the other step, e.g., transistor P 1  is turned on while transistor P 2  remains on. At time T 3 , the value of V ZQ  is increased by another step and now the value of V ZQ  exceeds the value of Vref. However, in case  1 , the amount of overshoot is greater than the amount of overshoot in case  2 . In both cases, however, the value of V ZQ  is recognized as greater than value of voltage Vref so that the counter  22  begins to receive down pulses from the comparator  16  for the period shown in the figure from time T 3  to time T 4 . As a result, the value of the variable count signal  23  is returned to the value of that signal at time T 2  such that the voltage V ZQ  is reduced by one step as shown at time T 4 . Thereafter, a pattern is developed in which the value of V ZQ  is increased by a step for one time period, e.g., time T 5  to time T 6 , and is then reduced by one step. By monitoring the pattern, the count of the variable count signal  23  can be locked at either the value which produces the overshoot as shown from time T 3  to time T 4  or the value which produces a value for the voltage V ZQ  as seen in the time period T 4  to T 5 . After the value for the variable count signal  23  has been locked, a similar process is carried out for a second variable impedance circuit  26  which is comprised of a plurality of N-channel MOS transistors as shown in  FIGS. 1 and 2B . 
     Returning to  FIG. 1 , the variable impedance circuit  26  is connected in series with a variable impedance circuit  24 ′ which is the same as the variable impedance circuit  24 . A voltage V ZQN  available at a node between the first variable impedance circuit  24 ′ and the second variable impedance circuit  26  is input to a comparator  28 . The output of the comparator  28  is filtered in a filter circuit  30  and input to a second counter  32 . The output of the second counter  32  varies the impedance of the variable impedance circuit  26  in a manner similar to that discussed above. When the pattern of up and down pulses becomes stable (repeatable), the value of the count of the variable signal produced by the counter  32  is locked. 
     Typically, the variable impedance circuit  24  comprised of P-channel MOS transistors is calibrated first via an enable signal input to an enable transistor  34 . Similarly, the second variable impedance circuit  26  is calibrated after a second enable transistor  36  is rendered conductive. As seen from the foregoing discussion, and particularly with respect to case  1  of  FIG. 3A  and case  2  of  FIG. 3B , the calibration of V ZQP  may have an error substantially equal to one step. Because the calibration of V ZQN  is performed after calibration of V ZQP , the voltage V ZQN  may have a two-step error. A two-step error is a substantial error, particularly for the fast corner. 
     Accordingly, an improved method and apparatus for ZQ calibration is needed which has high resolution. 
     SUMMARY OF THE PRESENT DISCLOSURE 
     According to one embodiment of the present disclosure, a method of controlling an output impedance of an electronic device of the type having an impedance control terminal to which an external load is to be connected is disclosed. In such a device, a predetermined value of the voltage at the impedance control terminal is used to control the output impedance of the device. The method is comprised of comparing a reference voltage to a voltage at the impedance control terminal. A variable count signal representing a count value is produced in response to the comparing. The impedance of a variable impedance circuit is varied in response to the count signal, wherein the impedance of the variable impedance circuit controls the voltage at the impedance control terminal. A device connected in parallel with the variable impedance circuit is periodically operated to periodically change the impedance of the variable impedance circuit. 
     According to another embodiment of the present disclosure, a method of controlling an output impedance of a memory device having an impedance control terminal to which an external load is to be connected is disclosed. A predetermined value of the voltage at the impedance control terminal is used to control the output impedance of the memory device. The method is comprised of comparing a reference voltage to a voltage at the impedance control terminal. A variable count signal representing a count value is produced in response to the comparing. The impedance of a first variable impedance circuit is varied in response to the count signal, and wherein the impedance of the first variable impedance circuit controls the voltage at the impedance control terminal. A device connected in parallel with the first variable impedance circuit is periodically operated to periodically change the impedance of the first variable impedance circuit. The value of the variable count signal is locked when a predetermined relationship exists between the reference voltage and the voltage at the impedance control terminal. The impedance of a second variable impedance circuit is varied until the impedance of the second variable impedance equals the external impedance. 
     Varying the impedance of the second variable impedance circuit may be comprised of comparing a voltage available at a node between the first variable impedance circuit and the second variable impedance circuit with the reference voltage. A second variable count signal representing a count value is produced in response to the comparing. The impedance of the second variable impedance circuit is varied in response to the second count signal. A second device connected in parallel with the second variable impedance circuit is periodically operated to periodically change the impedance of the second variable impedance circuit. The value of the second variable count signal is locked when a predetermined relationship exists between the reference voltage and the voltage available at a node between the first variable impedance circuit and the second variable impedance circuit. 
     Circuits are disclosed for implementing the disclosed methods. The disclosed circuits include a first variable impedance circuit comprised of a plurality of P-channel MOS transistors connected in parallel, and wherein activation of each of the plurality of P-channel MOS transistors is individually controllable based on a bit configuration of the variable count signal. More particularly, the plurality of P-channel MOS transistors includes a first PMOS transistor, a second PMOS transistor connected in parallel with the first PMOS transistor and having twice the size (twice the drive) of the first PMOS transistor, a third PMOS transistor connected in parallel with the first and the second PMOS transistors and having twice the size of the second PMOS transistor, and a fourth PMOS transistor connected in parallel with the first, second, and third PMOS transistors and having twice the size of the third PMOS transistor. The switching device may be implemented by a fifth PMOS transistor connected in parallel with the first, second, third, and fourth PMOS transistors and having half the size of the first PMOS transistor. The gate of the fifth PMOS transistor is configured to receive a driving signal from an oscillator, wherein the driving signal is a periodic pulse train having a predetermined pulse frequency. 
     The disclosed circuits also include a second variable impedance circuit comprised of a plurality of N-channel MOS transistors connected in parallel, wherein activation of each of the plurality of N-channel MOS transistors is individually controllable based on a bit configuration of the second variable count signal. More particularly, the second plurality of N-channel MOS transistors includes a first NMOS transistor, a second NMOS transistor connected in parallel with the first NMOS transistor and having twice the size of the first NMOS transistor, a third NMOS transistor connected in parallel with the first and second NMOS transistors and having twice the size of the second NMOS transistor, and a fourth NMOS transistor connected in parallel with the first, second, and third NMOS transistors and having twice the size of the third NMOS transistor. The second switching device may be implemented by a fifth NMOS transistor connected in parallel with the first, second, third, and fourth NMOS transistors and having half the size of the first NMOS transistor. The gate of the fifth NMOS transistor is configured to receive a driving signal from an oscillator, wherein the driving signal is a periodic pulse train having a predetermined pulse frequency. 
     A memory device and a system implementing the disclosed methods and circuits are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For the present disclosure to be easily understood and readily practiced, the present disclosure will now be described, for purposes of illustration and not limitation, in conjunction with the following figures, wherein: 
         FIG. 1  is a block diagram of a prior art calibration circuit; 
         FIGS. 2A and 2B  are examples of P-type and N-type variable impedance circuits, respectively; 
         FIGS. 3A and 3B  are timing diagrams illustrating when a stable condition occurs in two different cases for the calibration circuit of  FIG. 1 ; 
         FIG. 4  is a block diagram of a memory device having a calibration circuit constructed according to the teachings of the present disclosure; 
         FIG. 5  is a block diagram of a calibration circuit constructed according to the teachings of the present disclosure and which may be used in the memory device of  FIG. 4 ; 
         FIGS. 6A and 6B  are timing diagrams illustrating when a stable condition occurs and a lock signal is generated for two different cases for the calibration circuit of  FIG. 5 ; 
         FIGS. 7A and 7B  are simulations illustrating two examples of a voltage on the ZQ pad, and the up/down pulses produced as a result of the comparison of the voltage on the ZQ pad with a reference voltage; 
         FIG. 8  illustrates one example of a filter/lock circuit which may be used in the calibration circuit disclosed herein; 
         FIG. 9  illustrates one example of an up/down counter which may be used in the calibration circuit disclosed herein; 
         FIG. 10  illustrates one type of a P-type variable impedance circuit which may be used in the calibration circuit disclosed herein; 
         FIG. 11  illustrates one type of an N-type variable impedance circuit which may be used in the calibration circuit disclosed herein; and 
         FIG. 12  is a block diagram of a computer system using the memory device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  is a simplified block diagram illustrating a memory chip or memory device  40 . The memory chip  40  may be part of a DIMM (dual in-line memory module) or a PCB (printed circuit board) containing many such memory chips (not shown in  FIG. 4 ). The memory chip  40  may include a plurality of pins or ball contacts  42  located outside of chip  40  for electrically connecting the chip  40  to other system devices. Some of those pins  42  may constitute memory address pins or address bus  44 , data (DQ) pins or data bus  46 , and control pins or control bus  48 . It is evident that each of the reference numerals  44 ,  46 ,  48  designates more than one pin in the corresponding bus. Further, it is understood that the diagram in  FIG. 4  is for illustration only. That is, the pin arrangement or configuration in a typical memory chip may not be in the form shown in  FIG. 4 . 
     A processor or memory controller (not shown) may communicate with the chip  40  and perform memory read/write operations. The processor and the memory chip  40  may communicate using address signals on the address lines or address bus  44 , data signals on the data lines or data bus  46 , and control signals (e.g., a row address strobe (RAS) signal, a column address strobe (CAS) signal, a chip select (CS) signal, etc. (not shown)) on the control lines or control bus  48 . The “width” (i.e., number of pins) of address, data, and control buses may differ from one memory configuration to another. 
     The memory chip  40  may include a plurality of memory cells  50  generally arranged in an array of rows and columns. A row decode circuit  52  and a column decode circuit  54  may select the rows and columns, respectively, in the array in response to decoding an address provided on the address bus  44 . Data to/from the memory cells  50  are then transferred over the data bus  46  via sense amplifiers and a data output path (not shown). A memory controller (not shown) may provide relevant control signals (not shown) on the control bus  48  to control data communication to and from the memory chip  40  via an I/O (input/output) circuit  56 . The I/O circuit  56  may include a number of data output buffers or output drivers to receive the data bits from the memory cells  50  and to provide those data bits or data signals to the corresponding data lines in the data bus  46 . The I/O circuit  56  may also include various memory input buffers and control circuits that interact with the row and column decoders  52 ,  54 , respectively, to select the memory cells for data read/write operations. 
     The memory device  40  also includes an impedance control circuit  58  constructed according to the teachings of the present disclosure. The control circuit  58  is responsive to a ZQ pad or control terminal  60 . An external resistor  62  is connectable to the ZQ pad  60  and is used to control the output impedance of the device  40  as described below. 
     Those of ordinary skill in the art will readily recognize that memory chip  40  of  FIG. 4  is simplified to illustrate one embodiment of a memory chip.  FIG. 4  is not intended to be a detailed illustration of all of the features of a typical memory chip. Devices such as the row decode circuit  52 , column decode circuit  54 , and I/O circuit  56  may be considered to be peripheral devices or circuits as they are peripheral to the memory cells  50 . Although such peripheral devices are needed to write data to and read data from the memory cells  50 , they are not shown in detail in  FIG. 4  for the sake of clarity as they are not necessary for understanding the present disclosure. 
       FIG. 5  is a block diagram of the calibration circuit  58  constructed according to the teachings of the present disclosure and which may be used in the memory device of  FIG. 4 . The impedance control circuit  58  has a ZQ pad or a control pad  60  to which an external resistor  62  may be connected. The value of the voltage at the control pad  60  (V ZQP ) is input to a comparator  66  along with a reference voltage (Vref) produced by a reference generator  68 . The comparator  66  is of the type which produces up and down pulses in response to the difference in magnitude between the values of Vref and V ZQP . The up and down pulses are filtered by a filter/lock circuit  70  and input to a counter  72 . The counter  72  produces a multi-bit, variable count signal  73  representing a count value which is responsive to the number of up and down pulses which have been counted. The count signal  73  is input to a first variable impedance circuit  74  which may be of the type shown in  FIG. 2A . 
     A switching device, in this embodiment a PMOS transistor  75 , is connected in parallel with the first variable impedance circuit  74 . The switching device  75  is periodically operated (turned on and off). The gate of the transistor  75  may be connected to an oscillator  200  so as to provide the periodic operation. The transistor  75  may be one half the size (i.e., have one half the drive) of the transistor P 1  of  FIG. 2A . Periodically operating the transistor  75  periodically changes the impedance of the first variable impedance circuit  74 . More specifically, when the transistor  75  is conductive, the impedance of the first variable impedance circuit  74  is reduced, and when the transistor  75  is non-conductive, the impedance of the first variable impedance circuit  74  is increased. The frequency of operation of the oscillator is such that the transistor  75  is turned on and off faster than the value of the variable count signal  73  changes, e.g., twice the frequency. 
     Turning to  FIGS. 6A and 6B , the voltage Vref is compared to the voltage V ZQ . At time T 0 , the voltage V ZQ  is less than the voltage Vref such that a plurality of up pulses is produced. Also at time T 1 , the transistor  75  is rendered conductive causing a temporary increase in the voltage V ZQ  as shown by the circle  76 . However, even with that temporary increase, the value of V ZQ  is still less than the value of Vref, so the comparator continues to produce up pulses. At time T 1 , a sufficient number of up pulses has been produced so as to change the value of the variable count signal  73  so as to turn on another transistor within the variable impedance circuit  24 . For example, transistor P 1  may be turned off and transistor P 2  turned on, thereby increasing the value of voltage V ZQ  by one step. Again the transistor  75  is turned on, and again, even with this increase in voltage, the value of V ZQ  is still less than the value of Vref. As a result, the comparator continues to produce up pulses. It is not until time T 2  in  FIG. 6A  that the level of V ZQ  is such that when transistor  75  is turned on, the increase in voltage is sufficient to enable the value of V ZQ  to exceed the value of Vref. Thereafter, the pattern begins to repeat indicating that the value of the variable count signal  73  is stable and may be locked at time TL. As seen in  FIG. 6B , the turning on and off of the transistor  75  does not impact the time at which a pattern develops and the time (TL) when the lock signal is produced at time TL. The production of the lock signal is described below in conjunction with  FIG. 8 . 
     Comparing the case of  FIG. 6A  to the case of  FIG. 3A , it is seen that the pattern develops sooner, such that the value of the variable count signal  73  may be locked sooner. Comparing the case of  FIG. 6B  with the case of  FIG. 3B , it is seen that if the lock signal is produced at TL as opposed to time T 4 , a more accurate value is locked. More particularly, in  FIG. 6A  and  FIG. 6B  the value of the count of the variable count signal is locked at a point such that there is very little difference between the value V ZQ  and the value of Vref. In  FIGS. 3A and 3B , there is almost a one-step error between the value of V ZQ  and the value of Vref Thus, it is seen that the method and apparatus of the present disclosure provides for high resolution calibration. 
     Returning to  FIG. 5 , a second variable impedance circuit  76  is connected in series with another first variable impedance circuit  74 ′ which is structurally the same as the first variable impedance circuit  74 . A voltage available at a node between the another first variable impedance circuit  74 ′ and the second variable impedance circuit  76  is input to a comparator  78 . The output of the comparator  78  is filtered in a filter/lock circuit  80  and input to a second counter  82 . The output of the second counter  82  varies the impedance of the variable impedance circuit  76  as discussed above. When the pattern of up and down pulses becomes stable (repeatable), the value of the count of the variable signal produced by the counter  82  is locked. 
       FIGS. 7A and 7B  illustrate the results obtained from a simulation of the circuit shown in  FIG. 5 . It will be understood from  FIGS. 7A and 7B  that the simulation results are in line with the cases shown in  FIGS. 6A and 6B . More specifically, with the operation of the transistor  75 , a higher resolution calibration is obtained, and may be obtained more quickly. 
     Returning to  FIG. 5 , typically, the variable impedance circuit  74  comprised of P-channel MOS transistors is calibrated first via an enable signal input to the enable transistor  34 . Then the second variable impedance circuit  76  is calibrated after the second enable transistor  36  is rendered conductive. 
       FIG. 8  illustrates one example of a filter/lock circuit which may be used in the calibration circuit disclosed herein for the filter/lock circuits  70 ,  80 . The bottom portion of the figure illustrates a filter  86  which in this case is a selective, digital, low pass filter. Also disclosed in  FIG. 8  is a logic circuit  87  having a first portion  88  capable of monitoring the signals produced by the comparators  66 ,  78  and recognizing the pattern resulting from case  1  (see  FIG. 6A ) and a second portion capable of monitoring the signals produced by the comparators  66 ,  78  and recognizing the pattern resulting from case  2  (see  FIG. 6B ). In either case, the logic circuit  87  produces a lock signal. Because the filter  86  and logic circuit  87  illustrated in  FIG. 8  are merely examples of circuits that may be used to provide the recited functions, and those of ordinary skill in the art will recognize that many other circuit configurations are possible, the circuits of  FIG. 8  are not further described herein. 
       FIG. 9  illustrates one example of a counter which may be used for the counters  72 ,  82  of  FIG. 5 . The counter in  FIG. 9  receives signals from the comparator at input terminals  92  and  94  and produces the variable count signal  73  at an output terminal  96 . Because the counters  72 ,  82 , illustrated in  FIG. 9  are merely one example of a circuit that may be used to provide the recited function, and those of ordinary skill in the art will recognize that many other circuit configurations are possible, the circuit of  FIG. 9  is not further described herein. 
       FIG. 10  illustrates one type of a P-type variable impedance circuit which may be used in the calibration circuit disclosed herein while  FIG. 11  illustrates one type of an N-type variable impedance circuit which may be used in the calibration circuit disclosed herein. Each of the circuits shown in  FIGS. 10 and 11  is similar to the circuits shown in  FIGS. 2A and 2B , respectively, in that each of the circuits is comprised of a plurality of parallel connected transistors. 
       FIG. 12  is a block diagram depicting a system  118  in which one or more memory chips  40  illustrated in  FIG. 4  may be used. The system  118  may include a data processing unit or computing unit  120  that includes a processor  122  for performing various computing functions, such as executing specific software to perform specific calculations or data processing tasks. The computing unit  120  also includes a memory controller  126  that is in communication with the processor  122  through a bus  124 . The bus  124  may include an address bus (not shown), a data bus (not shown), and a control bus (not shown). The memory controller  126  is also in communication with a set of memory devices (i.e., multiple memory chips  40  of the type shown in  FIG. 4 ) through another bus  128  (which may be similar to the bus  42  shown in  FIG. 4 ). Each memory device  40  may include appropriate data storage and retrieval circuitry (not shown in  FIG. 12 ) as shown in  FIG. 4 . The processor  122  can perform a plurality of functions based on information and data stored in the memories  40 . 
     The memory controller  126  can be a microprocessor, digital signal processor, embedded processor, micro-controller, dedicated memory test chip, a tester platform, or the like. The memory controller  126  may control routine data transfer operations to/from the memories  40 , for example, when the memory devices  40  are part of an operational computing system  120 . The memory controller  126  may reside on the same motherboard (not shown) as that carrying the memory chips  40 . Various other configurations of electrical connection between the memory chips  40  and the memory controller  126  may be possible. For example, the memory controller  126  may be a remote entity communicating with the memory chips  40  via a data transfer or communications network (e.g., a LAN (local area network) of computing devices). 
     The system  118  may include one or more input devices  130  (e.g., a keyboard or a mouse) connected to the computing unit  120  to allow a user to manually input data, instructions, etc., to operate the computing unit  120 . One or more output devices  132  connected to the computing unit  120  may also be provided as part of the system  118  to display or otherwise output data generated by the processor  122 . Examples of output devices  132  include printers, video terminals, or video display units (VDUs). In one embodiment, the system  118  also includes one or more data storage devices  134  connected to the data processing unit  120  to allow the processor  122  to store data in or retrieve data from internal or external storage media (not shown). Examples of typical data storage devices  134  include drives that accept hard and floppy disks, CD-ROMs (compact disk read-only memories), and tape cassettes. 
     It is observed that although the discussion given hereinabove has been primarily with reference to memory devices, it is evident that the impedance calibration circuit discussed hereinbefore with reference to  FIGS. 5-11  may be employed, with suitable modifications which will be evident to one skilled in the art, in any other electronic device wherein output impedance may need to be adjusted. 
     While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.

Technology Classification (CPC): 7