Abstract:
An input circuit for receiving a signal at an input on an integrated circuit, particularly a DRAM circuit, and for assessing the signal with respect to a reference voltage is provided. One embodiment provides a termination circuit for setting a termination voltage, wherein the termination circuit includes a first resistor and a second resistor connected in series between a high voltage potential and a low voltage potential, the termination voltage being tapped between the first and second resistors, a first voltage-dependent resistor element having a first resistance gradient connected in parallel with the first resistor and a second voltage-dependent resistor element having a second resistance gradient connected in parallel with the second resistor, wherein the resistance values of the first and second resistor elements are controlled by a control voltage to set the termination voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims foreign priority benefits under 35 U.S.C. §119 to co-pending German patent application number 103 15 527.9, filed Apr. 4, 2003. This related patent application is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an input circuit for receiving signals at an input on an integrated circuit, particularly at an input on a DRAM circuit, and for assessing the received signal with respect to a reference voltage. 
     2. Description of the Related Art 
     Fast data links between integrated circuits involve the use of terminated data lines in order to avoid signal reflections in an input circuit in the integrated circuit. In addition, the input circuits provide the data lines with a termination voltage around which the signals transmitted on the data line fluctuate, ideally with the same swing. The termination voltage is prescribed by the input circuit and specifies a center voltage around which the signals to be transmitted move. 
     The termination usually implemented in current DRAM standards in the input circuits is integrated directly in the integrated circuit and cannot be manipulated from the outside. Normally, the termination voltage is generated by a voltage divider made up of two resistors. Process-related fluctuations or temperature fluctuations mean that the termination voltage is not constant, as a result of which the signals received via the input circuit move around a center voltage which cannot be stipulated exactly and is prescribed by the termination circuit. 
     To assess the received signal, the respective potential level of the received signal is compared with a reference voltage which can be prescribed externally or can be set in the integrated circuit. The reference voltage cannot be aligned for different input circuits, however, but rather is normally provided for all input circuits on a general basis. It is thus conceivable that the termination voltage and the reference voltage will differ from one another, which means that the voltage swings between the high level and the reference voltage and the voltage swing between the low level and the reference voltage will be of different magnitudes. This may result in one of the high or low levels not being able to be identified reliably under all circumstances. Another drawback is that, with cyclic signals having a predetermined duty ratio, any discrepancy between the termination voltage and the reference voltage can result in a change in the duty ratio of the received signal. This is the case particularly for radiofrequency signals, which have a small edge gradient. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an input circuit in an integrated circuit for the purpose of receiving signals and reliably assessing them with respect to a reference voltage. It is also an object of the present invention to provide a method for setting a termination voltage. 
     In line with a first aspect of the present invention, an input circuit for receiving a signal at an input on an integrated circuit is provided. The input circuit assesses the signal with respect to a reference voltage. The input circuit has a termination circuit for setting a termination voltage. The termination circuit comprises a first resistor, connected in series between a high voltage potential and a low voltage potential, and a second resistor, the termination voltage being able to be tapped off between the first and second resistors. The first resistor has a first voltage-dependent resistor element, having a first resistance gradient, connected in parallel with it and the second resistor has a second voltage-dependent resistor element, having a second resistance gradient, connected in parallel with it. The resistance values of the first and second resistor elements can be controlled by a control circuit, in order to use the control voltage to set the resistance values of the first and second resistor elements and hence the termination voltage. In this way, a suitable choice of control voltage allows the termination voltage to be set such that the received signal is assessed more reliably. Process-related discrepancies between the resistance values of the first and second resistors can thus be compensated for. The control voltage controls the voltage-dependent resistor elements in different ways on account of the different resistance gradients, which means that the resistance ratio of the resistance values in the two branches of the voltage divider and hence the voltage produced thereby are altered. In one embodiment, the first and second resistance gradients have different arithmetic signs in order to obtain as large an adjustment range for the termination voltage as possible. 
     In integrated circuits, the first and/or the second voltage-dependent resistor element may be produced using a transistor. In particular, the resistance gradients with different arithmetic signs mean that a p-channel field effect transistor is suitable for the first voltage-dependent resistor element and an n-channel field effect transistor is suitable for the second voltage-dependent resistor element. 
     A control circuit may be provided for setting the control voltage which has a voltage generator circuit for producing a comparison voltage and a differential amplifier. The voltage generator circuit may be of the same physical design as the termination circuit, with the comparison voltage and the reference voltage being applied to inputs on the differential amplifier. The control voltage can be tapped off at an output on a differential amplifier and is applied to the control inputs of the first and second resistor elements in the voltage generator circuit and in the termination circuit. In this way, the termination voltage can be aligned exactly with the reference voltage, since the voltage generator circuit is essentially subject to the same process-related and temperature-related fluctuations as the termination circuits. The differential amplifier is used to generate the control voltage on the basis of the extent to which the reference voltage and the comparison voltage differ from one another. By feeding back the output of the differential amplifier to the inputs of the voltage generator circuit, the control voltage is aligned such that the voltage generator circuit and the physically identical termination circuit each generate a voltage which essentially matches the prescribed reference voltage. In this way, the termination voltage and the reference voltage can be placed at equal potentials, which means that received signals whose high and low levels move around the termination voltage can be assessed in the best possible manner. 
     In one embodiment, the comparison voltage is applied to the noninverting input, and the reference voltage is applied to the inverting input of the differential amplifier. The output of the differential amplifier may also be applied to a plurality of termination circuits, so that the termination circuits of a plurality of input circuits are actuated using the control voltage generated by the control circuit. The voltage level chosen for the reference voltage may be a value which is approximately in the center between the high level and the low level of the received signal. For receiving and assessing the signal, a signal evaluation circuit may be provided for the purpose of comparing the received signal with the reference voltage and assigning a signal value on the basis of the result of the comparison. 
     In line with a further aspect of the present invention, a method for setting a termination voltage on a termination circuit is provided. The termination voltage is set in line with a control circuit, the control voltage being chosen such that the termination voltage essentially corresponds to a prescribed reference voltage against which the received signal is detected. The method provides the advantage that, when the reference voltage and the termination voltage are provided by producing them independently of one another, the termination voltage can be aligned with the prescribed reference voltage. In this case, the control voltage is ascertained on the basis of the reference voltage by comparing the termination voltage and the reference voltage with one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  shows a conventional input circuit, based on the prior art; 
         FIG. 2  shows a possible signal profile for a termination voltage and a reference voltage; and 
         FIG. 3  is a circuit diagram illustrating a termination unit for use in an input circuit according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a conventional input circuit  1  which is usually used a plurality of times in an integrated circuit (not shown). The input circuit has a termination unit  2  and an evaluation unit  3 . Both the termination unit  2  and the evaluation unit  3  are connected to a connection pad  4  on the integrated circuit. The connection pad  4  can be used to receive, via a data line  5 , external signals which are driven onto the data line  5  by an external driver circuit  6  in line with a signal S which is to be transmitted. 
     The termination unit  2  provides a termination voltage V TT  which is obtained from the resistance ratio of a voltage divider  7  having a first resistor  8  and a second resistor  9 . The first and second resistors  8 ,  9  are connected in series between a high voltage potential V DD  and a low voltage potential GND, the termination voltage V TT  being tapped off between the first and second resistors  8 ,  9 . The termination voltage V TT  is connected to the connection pad  4 . During the transmission of signals, the driver circuit  6  operates against the resistors  8 ,  9  in the termination unit  2 . 
     The evaluation unit  3  compares the signal levels of the received signal with a reference voltage V REF  which can be prescribed externally or can be generated internally in the integrated circuit such that it can be adjusted. The evaluation unit  3  has a comparator  10  which compares the received signal with the reference voltage V REF  and assesses the signal on the basis of the arithmetic sign of the discrepancy. The reference voltage V REF  is usually selected such that it is essentially at a central potential, i.e., in the center between the high potential and the low potential of the desired signal levels. Since the signal levels are determined by the termination voltage, however, the reference voltage V REF  may differ from the ideal central potential. 
     If the reference voltage V REF  differs from the central potential, then either the high level or the low level is identified less reliably. If the received signal is a clock signal, then edge gradients which are too shallow may result in a shift in the reference voltage V REF  with respect to the central voltage involving a duty ratio which is different from that of the original signal being read at the output of the comparator  10 . Particularly in DRAM memory circuits, however, the duty ratio of prescribed clock signals is important to the manner of operation and is subject to stringent specifications. 
     Since the termination voltage V TT  is essentially stipulated by the voltage divider  7 , which means that the termination voltage V TT  cannot be changed by external influences, the reference voltage V REF  and the termination voltage V TT  can be balanced only with difficulty. 
       FIG. 2  shows the profile of a possible received signal with respect to the reference voltage V REF  and the termination voltage V TT . As shown, a discrepancy between the reference voltage V REF  and V TT  at the lower potential reduces the voltage swings between the low potential and V REF  and increases the voltage swings between the reference voltage V REF  and the high potential. In addition, the period of time for the high period TH is extended as compared with the period of time for a high period given identical termination and reference voltages. Likewise, the period of time in which the evaluation unit  3  identifies a low level is shorter than the period of time in which a low level is identified given identical termination and reference voltages. This also results in a shift in the duty ratio. 
     Particularly in DRAM circuits, because the timing of the clock signal controls the internal sequences, the edges of the clock signal need to be predetermined in line with a predetermined duty ratio. The discrepancy between reference voltage V REF  and termination voltage V TT  thus results in the duty ratio having been altered for the received cyclic signal, particularly if the edge gradient of the signal is small, e.g., on account of radiofrequency transmission. The smaller the edge gradient and the greater the discrepancy between reference voltage V REF  and termination voltage V TT , the greater, too, is the discrepancy between the duty ratio and the duty ratio of the signal transmitted to the input circuit. 
       FIG. 3  is a circuit diagram illustrating a termination unit for use in an input circuit according to one embodiment of the invention. The termination unit includes a termination circuit  20  for producing the termination voltage V TT  on the basis of a control signal TS and providing the termination voltage V TT  on the input circuit&#39;s connection pad. 
     The termination circuit  20  has a third resistor  21  and a fourth resistor  22 , which are connected in series between the high supply voltage potential V DD  and the low supply voltage potential GND. A first p-channel field effect transistor  23  is connected in parallel with the third resistor  21 , and a first n-channel field effect transistor  24  connected in parallel with the fourth resistor  22 . The control inputs of the first p-channel field effect transistor  23  and of the first n-channel field effect transistor  24  are connected to the control signal TS. The two field effect transistors  23 ,  24  have opposite resistance gradients, which means that a rising voltage for the control signal TS results in the resistance of the first p-channel field effect transistor  23  increasing and the resistance of the first n-channel field effect transistor  24  decreasing, and vice versa. 
     The control signal TS may be generated by a control unit  25 . The control unit  25  includes a voltage generator circuit  26  and a differential amplifier  27 . The voltage generator circuit generates a comparison voltage V VGL  which is applied to a noninverting input on the differential amplifier  27 . The reference voltage V REF  is applied to an inverting input on the differential amplifier  27 . 
     Like the termination circuit  20 , the voltage generator circuit  26  is integrated in the integrated circuit and may be of the same physical design, which means that the voltage generator circuit  26  and the termination circuit  20  are subject to the same process influences and temperature influences. The voltage generator circuit  26  includes a fifth resistor  28  and a sixth resistor  29 . The fifth and sixth resistors  28 ,  29  are connected in series between the high supply voltage potential V DD  and the low supply voltage potential GND. Between the fifth resistor and the sixth resistor  28 ,  29 , the comparison voltage V VGL  is tapped off. A second p-channel field effect transistor  30  is connected in parallel with the fifth resistor  28  and a second n-channel field effect transistor  31  is connected in parallel with the sixth resistor  29 . The control inputs of the second p-channel field effect transistor  30  and of the second n-channel field effect transistor  31  are connected to the control signal TS, which is tapped off at an output on the differential amplifier  27 . 
     Since the termination circuit  20  and the voltage generator circuit  26  are of the same physical design, are connected to the same voltage potentials V DD , GND, and are actuated using the same control signal TS, the termination voltage V TT  which is provided on the connection pad in the integrated circuit corresponds to the comparison voltage V VGL . The differential amplifier  27  has the task of making the comparison voltage V VGL  more like the reference voltage V REF  which is provided for the integrated circuit. 
     Accordingly, the control signal TS which can be tapped off at the output of the differential amplifier  27  has a value at which the comparison voltage V VGL  and the reference voltage V REF  essentially have the same voltage value. Since the termination circuit  20  and the voltage generator circuit  26  are of the same physical design, the reference voltage V REF  and the termination voltage V TT  at the output of the termination circuit  20  thus also have the same voltage value. Hence, the termination voltage V TT  in an input circuit in an integrated circuit can be aligned with an externally prescribed reference voltage V REF , which means that any discrepancy between termination voltage V TT  and reference voltage V REF  may be avoided. 
     The control process for the control signal TS takes place as follows: if the comparison voltage V VGL  is larger than the reference voltage V REF , then the differential amplifier  27  significantly amplifies the differential voltage (according to its gradient) and applies the amplified voltage signal to the control inputs of the second p-channel field effect transistor  30  and the second n-channel field effect transistor  31 . The high positive voltage applied thereto increases the resistance of the second p-channel field effect transistor  30  and lowers the resistance of the second n-channel field effect transistor  31 . This lowers the comparison voltage V VGL . 
     In a similar manner, a low potential (close to a low supply voltage potential for the differential amplifier  27 ) may be applied as a control signal to the control inputs of the second field effect transistors  30 ,  31 . The result of this is that the resistance of the second p-channel field effect transistor  30  is low and the resistance of the second n-channel field effect transistor  31  is high. In this case, the comparison voltage V VGL  has been increased, which means that the feedback loop is used to adjust the reference voltage V REF  and the comparison voltage V VGL  to one another. If the comparison voltage V VGL  and the reference voltage V REF  are essentially identical, then the output of the differential amplifier  27  produces a voltage potential which prescribes the appropriate operating points of the first and second field effect transistors  23 ,  24 ,  30 ,  31 , in order to obtain the desired termination voltage V TT  and comparison voltage V VGL . 
     When a plurality of input circuits are produced in one integrated circuit, it is possible for the control signal TS generated by the control unit  25  to be provided for a plurality of termination circuits  20  which are respectively provided in the input circuits in an integrated circuit. This makes it possible to align the termination voltage V TT  while saving space, since only one control unit  25  needs to be provided for the entire integrated circuit. Since the control signal TS is essentially a constant voltage signal and the control inputs of the field effect transistors  23 ,  24 ,  30 ,  31  do not represent a load for a constant voltage signal, the supply line lengths between the control unit  25  and the termination circuits  20  are essentially insignificant, which means that they do not lead to the expectation of shifts between the termination voltages V TT  in the individual termination circuits  20 . 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.